Title of Invention	A TUNABLE OPTICAL FITER
Abstract	The invention disclosed in this application relates to a tunable optical filter useful for selecting different bands of wavelengths in the UV, Visible and IR region without the necessity of changing the optical element, which comprises of a ferrofluid-based emulsion sandwiched between two transparent optical sheets, the thickness of the gap between the transparent optical sheets being at least 100 microns thereby forming a cell , the cell being placed inside a solenoid which is provided with a socket for connecting to a variable direct current source so as to facilitate changing the magnetic field of the cell The tunable optical filter of the present invention is shown in Fig 1 below

Title of Invention

A TUNABLE OPTICAL FITER

Abstract

The invention disclosed in this application relates to a tunable optical filter useful for selecting different bands of wavelengths in the UV, Visible and IR region without the necessity of changing the optical element, which comprises of a ferrofluid-based emulsion sandwiched between two transparent optical sheets, the thickness of the gap between the transparent optical sheets being at least 100 microns thereby forming a cell , the cell being placed inside a solenoid which is provided with a socket for connecting to a variable direct current source so as to facilitate changing the magnetic field of the cell The tunable optical filter of the present invention is shown in Fig 1 below

Full Text	FORM 2 THE P ATENTS ACT, 1970 COMPLETE SPECIFICATION [ SECTION 10] A TUNABLE OPTICAL FILTER Department of Atomic Energy ,a department of the Govt of India having its office at Anushakthi Bhavan , Chathrapathy Shivaji Maharaj Marg, Mumbai, 400001, Maharashtra, India The following specification particularly describes and ascertains the nature of this invention and the manner in which is to be performed 1 The invention disclosed in this application relates to a tunable optical filter. The tunable optical filter of the present invention is useful for selecting different bands of wave lengths from a white light, without the necessity for changing the optical element. Accordingly the filter of the present invention is also useful for obtaining different colors, which will find applications in industries such as Optical, Opto-electronics, Entertainment, Laser, Spectral radiometry, Color separation etc. Optical filters are used to select wavelengths (colors) of light. A filter works by excluding all but a limited set of wavelengths. An optical filter is specified by a wavelength number in nanometers. There are three general types of filters, long pass, short pass and band pass. The wavelengths that are allowed through a filter are called the pass band. Long pass band filters let all the wavelengths longer than the wavelength number through. These types of filters are used as barrier filters in fluorescence. They will let the fluorescent emitted light through and not the exciter light. Short pass band filters let light that is lower in wavelength through. Short pass band filters are sometimes used as exciter filters for fluorescence. A filter can transmit or reflect a specified range of wavelengths. Such a filter designed for spectroscopy is called a spectroscopic filter. Bandpass filters transmit light only within a defined spectral band ranging from less than one to many nanometers wide. They are used in a great variety of applications including spectral photometry, medical diagnostics, chemical analysis, colorimetry, and astronomy. When used in spectral photometry, bandpass filters are preferred over monochromators because of their higher transmission and better signal-to-noise ratio. The wavelength number is referred to as the center wavelength of a pass band filter. The center wavelength is the wavelength that allows the most light through within the range of wavelengths that will be passed through. Frequently, this will be the center wavelength of the filter. A pass band filter is rated by center wavelength and the pass band or width. The pass band can be 'quite'narrow and can be uneven., An uneven pass band would have more width on one side than the other. 2 An interference filter is usually a pass band filter. It is specified by the center wavelength and the width. Manufacturers of interference filters provide plots of the intensity versus wavelength. This plot is the best way to understand the characteristics of a band pass filter. Interference filters use very thin glass and coatings of dielectric material such as SiO2, TiO2. ZrO2, TaO2, ZnS, MgF2, Al etc or their combinations to set up constructive and destructive interference in the filter. This interference creates the filter effect. Optical filters have been made by sputtering or by other methods of depositing materials of differing refractive index onto a transparent substrate. Such structures work well for low-power light application; however, for high-power illumination the deposited films peel-off or crack. Such a structure is found in U.S. Pat. No. 3,962,062 in which the films are deposited by sputtering. Electrically insulated integrated circuits (U.S. Pat. No. 3,976,511) have been formed by bombarding the surface of a substrate by ions and then heating the resultant bombarded surface sufficiently to react the ions with the substrate. The structure is further formed by depositing a layer onto the ion-bombarded structure and then forming desired patterns of electrically insulating layers thereon. U.S. Pat. No. 3,976,512 also illustrates ion bombardment in forming integrated circuits. FIG. 1 of the drawing accompanying this specification illustrates a high-resistivity silicon substrate *C' with one layer 4B' of nitrogen ions implanted therein a short distance from and parallel with the upper surface of the substrate. An ion accelerator is used to accelerate nitrogen ions 'A' which are uniformly incident on the surface of the silicon substrate, fn the buried implanted layer, a substrate temperature of about 700.degree. C. efficiently promotes the formation of the second-phase material (ion layer), silicon nitride, whose refractive index is 2.0. Thus silicon, with an index of refraction of 3.43, has a layer of silicon nitride with an index of refraction of 2.0 buried beneath the surface. The large difference in the index of refraction of the main body and the ion-implanted layer will cause light to be multiply reflected between the Sis N4 layer and the upper surface of the substrate. These multiple reflections lead to interference maxima and minima in the transmitted light as a function of wavelength. 3 Another type of filter which is currently used is of the same type as explained above with multiplayer coating, which are being used in high power laser devices. A narrow-band interference filter comprises of alternating reflective layers arranged in a series in the form of layers on a substrate (S) where the H layers have a high index of refraction and the L layers have a low index of refraction and the same optical thickness as the H layers is disclosed in a US patent No.4 009 453. Such filter is specifically useful for applications' involving lasers with high output power. The schematic of the multilayered filter is shown in figure 2. Here, the metal layers M are interposed in each alternating reflective layer system, so that one obtains the layer sequence 1 of (SHL HML HMLL MHL MHL HA), m the above example, S is the glass substrate with an index of refraction 1.5 and the layer H is ZnS and the layer L is MgF2 and the metal layer is Al. Color imaging systems benefit from the use of precision optical filters which control the spectral properties of light and color separation to exacting tolerances. In addition, system performance improves with the elimination of wavelengths outside the visible spectrum. Image capture is enhanced when the prime colors of light are precisely separated or trimmed before reaching the detector. Image reproduction, where colors are recombined, also benefits from the use of precision filters. The result is that image quality improves when a system's optics deliver precise color separation, high color signal-to-noise, and a wide dynamic range. Color imaging and measurement systems utilize color separation filters as well as prisms to differentiate the prime color wavelengths. Image capture systems divide light into red, green, and blue before reaching the detector. Image delivery systems recombine the red, green, and blue to form an image. In either case, system performance can be enhanced by carefully controlling the wavelengths and the areas of overlap. Systems, which use prisms to control the spectrum, can have enhanced performance using color separation filters. The above described presently available pass band or color optical filters are useful only for selecting a given central wavelength. If different wavelengths have-to be selected, different +. - - filters have.to be used for each desired central wavelength In other words such filters have no 4 tunability option. Such filters are, therefore, not only cumbersome to use but also expensive because of the requirement of special high cost coatings such as those mentioned earlier. In spectroscopic and interferometric experiments involving different wavelengths of light, we need to use different types of interference filters to eliminate stray light entering the detector head. In those experiments, we need to replace the filters, whenever the wavelength of the light source is changed, e.g.: when we use He-Ne laser instead of Ar-ion laser (or when we change the color from red to green). This is often cumbersome, especially when the wavelength is changed continuously with a tunable type of laser. In order to avoid the problems associated with the necessity to change the filters, we have undertaken this project to develop a tunable type of optical filter. The main objective of the present invention; therefore, is to provide a tunable optical filter useful for obtaining of different wave lengths of light from a white light source, without the necessity for changing the optical element. Another objective of the present invention is to provide a tunable optical filter which is useful for selecting the wavelengths in the ultraviolet, visible and infrared (IR) regions. Yet, another objective of the present invention is to provide a tunable optical filter in which it is not required to change the optical element for choosing different wavelengths. Another objective of the present invention is to provide a tunable optical filter in which the desired band and central wavelength can be selected electronically, without changing the filters. Yet another objective of the present invention is to provide a tunable optical filter in which the spectral distribution can be easily controlled by adjusting the pofydispersity of the emulsion used 5 Still another objective of the present invention is to provide a tunable optical filter in which the intensity of the transmitted light can be controlled by changing the concentration of the emulsion used. Further objective of the present invention is to provide a tunable optical filter which is simple to operate and less expensive as compared to existing filters The invention has been developed based on the fact that the magnitude of the magnetic dipole moment of a magnetic emulsion, increases with the strength of the applied magnetic field until saturation is reached. This leads to chaining of the droplets. At iow concentration, one droplet thick chains are well separated and oriented along the field direction. Due to the presence of the one-dimensional ordered structure, Bragg reflection or scattering takes place from the emulsion. Here, the Bragg reflected wavelength corresponds to the spacing between the droplets. The condition for forming a linear chain is that the repulsive force between the droplets must exactly balance the attractive force between the droplets induced by the applied magnetic field. The dominant force in a field induced droplet chain is the dipole- dipole attraction. The Van der walls contribution also becomes significant at short distances. For perfectly aligned particles with a separation d' the first order Bragg condition leads to 2d = λ0/n. Where "n" is the refractive index of the suspending medium (n=1.33 for water) and λo is the wavelength of the light Bragg scattered at an angle of 180 degrees. The peak position moves toward smaller wavelength as the field is increased. The magnitude of the induced dipole can be controlled by the applied field. We explored the above property of controlling the spacing between the monodispersed emulsion droplets for developing the optical filter of the present invention where the pass band of wavelength can be chosen by choosing the appropriate magnetic filed. Using an appropriate emulsion of given diameter, the desired band can be selected by applying a given magnetic field. 6 Accordingly we employed magnetic emulsions of magnetic material such as γ-Fe2O3, magnetite and combinations thereof, in a carrier medium of octane, stabilized with an inner surfactant and is emulsified with water in the presence of an ionic(cationic or anionic) or nortcomc surfactant with particles of suitable sizes and concentrations, sandwiched between two optical quality transparent plates ( e.g: glass, quarts, mylar etc.) which is hereinafter referred to as ferrofluid. The ferrofluid is sandwitched between the two transparent plates thereby forming a cell which is placed inside a non-magnetic casing (eg:AI), the cell is placed inside a compact solenoid, which is connected to a current source. The magnetic field is varied by changing the current. The reflected light from the ferrofluid cell is measured as a function of applied field strength by employing a spectrograph. As the ferrofluid droplets are super paramagnetic in nature, an applied magnetic field induces a magnetic dipole in each drop, causing them to form chains. Without external field, these droplets have no permanent magnetic moments because of the random orientation of the magnetic grains within the droplets, due to thermal motion. An external magnetic field orients these magnetic grains slightly toward the field direction, which results in a dipole moment in each droplet. In order to study the characteristic features of the new tunable optical filter, the filter is illuminated by a white light source and the reflected light from the filter assembly is sent through an optical fiber. The out put from the fiber assembly is finally entered inside a monochromator equipped with a diode array. The diode array output is processed by a computer by means of an interface card, which gives the reflected/transmitted light intensity as a function of wavelength. The tunable pass band optical filter useful for selecting different wavelengths of light of the present invention is shown in Fig 3. The magnetic emulsion (1) is sandwiched between two optical quality transparent plates (2) thereby forming a cell with is placed inside a compact solenoid (3). The spacing between the transparent glass plates is controlled by a mylar washer (4) of thickness 200 microns. The leads of the solenoid which is connected to a socket (5 )and is subsequently connected to a current source (6). According to the present invention there is provided a tunable pass band optical filter useful for selecting.different bands of wavelengths in the UV, Visible and 1R region without the 7 necessity for changing the optical element, which comprises of a ferrofluid-based emulsion sandwiched between two transparent optical sheets, the thickness of the gap between the transparent optical sheets being at least 100 microns thereby forming a cell , the cell being placed inside a solenoid which is provided with a socket for connecting to a variable direct current source so as to facilitate changing the magnetic field of the cell In a preferred embodiment of the present invention the optical transparent sheets used may be selected from glass, quarts, mylar and the like. The magnetic material used may be selected from magnetic material such as iron, nickel, cobalt, γ-Fe2O3 magnetite and combinations' thereof and the like The carrier fluid in which the magnetic grains are dispersed may be n-octane, cyclohexane, n-dodecane, n-tetradecane, n-hexadecane, n-octadecane, or kerosene and the like.. The magnetic material may also be stabilized using an inner surfactant such as oleic acid, linoleic acid, olive oil and the like. The ferro magnetic material may also be emulsified with water and an ionic or nonionic surfactant with particles of suitable sizes and concentrations. The ferro magnetic material may also be emulsified with oil and an ionic or nonionic surfactant with particles of suitable sizes and concentrations. The ionic surfactant used may be selected from anionic surfactants such as polyoxyethylene, alkylphenyl ether sulfates, polyoxyethylene styrenated phenyi ether sulfates, alkylpho phates, polyoxyethylene alkyi ether phosphates, polyoxyethylene alkylphenyl ether phosphates, fatty acid salts, alkylbenzene sulfonates, alkyl sulfonates, alkyi naphthalene sulfonate alphha -olefin sulfonates, dialkyl sulfosuccinates, alpha -sulfonated fatty acid salts,N-acyl-N-methyIlaurate, alkylsulfates, sulfated lipids, polyoxyethylene alkyl ether sulfates and Naphthalene sulfonate formaldehyde condensates and the like. The cationic surfactants may be alkyltrimethyl ammoniu salts, primary to tertiary aliphatic amine salts, dialkyldimethyl ammonium salts, trialkylbenzy ammonium salts, alkyl pyridmium salts, tetraalkyl ammonium salts, and polyethylene polyaVnine fatty acid amide salts and the like. 8 The non ionic surfactant selected from -polyoxyethylene polyoxypropylene glycols, polyoxyethylene polyoxypropylene alkyl ethers, polyoxycthylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene alk^iphenyl ethers, polyoxyethylene polysryrylphenyl ethers, polyhydric alcohol fatty acid partial esters, sorbitan fatty acid esters, glycerol fatty acid esters, deca-glycerol fatty acid esters, polyglycerol fatty acid esters, propylene glycol pentaerythritol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, polyoxyethylene polyhydric alcohol fatty acid partial esters, polyoxyethylene fatty acid esters, polyglycerol fatty acid esters, polyoxyethylenated castor oil, fatty acid diethanolamifles, polyoxyethylene alkylamines, triethanolamine fatty acid partial esters, trialkylamine Oxides, and polyoxyalkylene group containing organopolysiloxanes and the like TRe emufsion may 6e staoil'ized" tor ennancihg tne nfe time by incorporating a macromolecule, which may be a non-ionic, cationic or anfonic polymer or a macromolecule which may be a di-block or tri-block polymer or a macromo\|ecule-surfactant mixture. The emulsion droplets employed may be a direct, inverted and multiple emulsion which are very well known in the field. The magnetic suspension, which may be a solid-liquid dispersion, solid-air dispersion or liquid-air dispersion. The thickness of the gap between the plates/sheets can be adjusted by employing spacers. For a given current, the spacing between the droplets is fixed in such a manner that a given band of light with a central wavelength of X (-2d/n) would be reflected. The central wavelength can be continuously tunable by changing the current, employing a polymer stabilized ferrofluid emulsion, the central wavelength can be varied up to 300 In order order to achieve tunability in the lower wavelength region, emulsions with smaller droplets are suitable. Similarly, for longer wavelength region, droplets of larger sizes are to be employed. In order, to calibrate the tunable filter assembly, we have filled the cell assembly with ferrofluid.of a given droplet diameter (first column in the table 1). The ferrofluid used in the above experiment was made of Y-Fe2O.3 stabilized with an inner surfactant of oleic acid and 9 dispersed in a carrier medium of octane. The our surfactant was sodium dodecyl sulphate. The emulsion was subsequently stabilized with polyvinyl alcohol of molecular weight 1555 000 at a concentration of 0.5%. The filter assembly is kept inside the solenoid assembly, where the magnetic field can be varied from 0 to 200 gauss. By varying the magnetic field strength from 0 to 200 Gauss, we have measured the corresponding Bragg peak central wavelength. The minimum and maximum value for different droplet radius is then estimated. Table 1 given below shows the approximate values of minimum and maximum values of central wavelength obtained for emulsions with different droplet radii ( where h = d - 2a). Here the approximate , L inter droplet spacing varies from lOOnm (minimum field) to close contact (maximum field). From the Tablel, it is clear that 4 different filters made of emulsion droplets of 100, 200, 300 and 400 nm would be capable of covering a wide tunable wavelength range of 266 to 1330 nm. Tablel Droplet Diameter (nm) Tunable wavelength (nm) 100 266 -532 120 319 -585 140 . 372 -638 160 425 -691 180 478 -744 200 532 -798 220 585 -851 240 638 -904 260 691 -957 280 744 -1010 300 798 -1064 320 851 -1117 340 904 -1170 360 957 -1223 380 1010-1276 400 . 1064-1330 10 The invention is described in detail in the Examples given below which are provided only to illustrate the invention and therefore should not be construed to limit the scope of the invention Example 1 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier mediuni' of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %. The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 run. The magnetic filed at the center of the solenoid is adjusted to 280 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition, The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 4a shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 590 run. The percentage of reflectivity in this case is about 12% Example 2 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe20.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 % The concentration of the ferrofluid 11 used in the above filter assembly was 0.001%, and the droplet diameter was 184 nm.. The magnetic filed at the center of the solenoid is adjusted to 210 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 4b shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 635 nm. The percentage of reflectivity in this case is about 12% Example 3 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated'by a white light source. The ferrofluid used in the above experiment was made of y-FejO.s stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 nm. The magnetic filed at the center of the'solenoid is adjusted to 157 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 4c shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 665 nm. The percentage of reflectivity in this case is about 12% Example 4 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a 12 white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 run. The polydispersity of the emulsion in this case was about 10 %. The magnetic filed at the center of the solenoid is adjusted to 105 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 4d shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 69.0 nm. The percentage of reflectivity in this case is>about 12% Example 5 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184 nm. The polydispersity of the emulsion in this case was about 10 %. The magnetic filed at the center of the solenoid is adjusted to 258 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 5a shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 600 nm. The percentage of reflectivity in this case is about 77 % 13 Example 6 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. .The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184 nm. The polydispersity of the emulsion in this case was about 10 %. The magnetic filed at the center of the solenoid is adjusted to 194 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition/The reflected. light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 5b shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 625 nm. The percentage of reflectivity in this case is about 77 % Example 7 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. . The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184 nm The magnetic filed at the center of the solenoid is adjusted to 140 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. 14 Figure 5c shows the Bragg reflected light from the filter unit The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 645 nm. The percentage of reflectivity in this case is about 77 % Example 8 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184nm. The magnetic filed at the center of the solenoid is adjusted to 98 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 5d shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 710 nm. The percentage of reflectivity in this case is about 77 % The results of the Examples 1-8 show that the pass band of the reflected light is tunable over a wide range of wavelengths by adjusting the current to the solenoid or the magnetic field. The intensity of the reflected band of light depends on the concentration of the ferrofluid emulsion employed. Here, the polydispersity of the emulsion dictate the FWHM. High polydispersity values give rise to higher the FWHM. Example 9 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a 15 white light source. The ferrofluid used in the above experiment was made of γ-Fe2C3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 210 nm. Here, the Ferro fluid is stabilized with poly-vinyl alcohol of average molecular weight 40000. The polydispersity of the emission in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelengt^ Figure 6a shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 576-621 nm. The percentage of reflectivity in this case is about 15 % Example 10 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and disperseq in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a Concentration of 0.02 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 202 nm. Here, the Ferro fluid is stabilized poly-vinyl alcohol of average molecular weight 40000. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varried from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelenth. 16 Figure 6b shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in tin above case is around 18nm and the central wavelength varies from 536-587 nm. Tin percentage of reflectivity in (his case is about 15 % Example 11 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid which is powered by a variable direct current source. The filter assembly is illuminated by : white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3. stabilized with an inner surfactant of oieic acid and dispersed in a carrier medium of octane The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-vinyl alcohol of average molecular weight 40000.Th Figure 6c shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 505-561 nm. The percentage of reflectivity in this case is about 15 % Example 12 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a while light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. 17 The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-viny] alcohol of average molecular weight 40000.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 180 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 6d shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 481-550 nm. The percentage of reflectivity in this case is about 15 % Example 13 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-vinyl alcohol of average molecular weight 40000.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 176 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectyograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 6e shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the 18 above case is around 18nm and the central wavelength varies from 451-509 nm. The percentage of reflectivity in this case is about 15 % The results of Examples 9-13 show that the tunable range of wavelength can be varied by changing the droplet size of the ferrofluid emulsion used. Also the emulsion droplets stabilized with poly vinyl alcohol can stabilize and augment the tunable range. Example 14 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 0OO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.OOI%, and the droplet diameter was 210 nm. The polydispersity of the emulsion in this case vas. about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The ;pectrograph output gives the percentage of reflectivity as a function of wavelength. figure 7a shows the central wavelength of the Bragg reflected light as a function of applied leld strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 772-879 nm. The sercentage of reflectivity in this case is about 13 % Example 15 The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, vhich is powered by a variable direct current source. The filter assembly is illuminated by a 19 white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with "an inner surfactant of oleic acid and dispersed in a carrier medium of octane, The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 OOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 202 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 7b shows the central wavelength of the Bragg reflected light as a function,, of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around I8nm ana1 tne central1 wavelength vanes from 713-716 nm. Tne percentage of reflectivity in this case is about 13 % . Example 16 The tunable pass band optical filter assembly shown in figure 3 is kept inside .the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 DOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 3.001%, and the droplet diameter was 190 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 22Q Gauss, oy adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. 20 Figure 7c shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 652-759 nm. The percentage of reflectivity in this case is about 13 % . Example 17 The tunable pass band optical filter assembly shown in figure 3 is kept insi.de the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of yγ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 OOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 180 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 7d shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 631-716 nm. The percentage of reflectivity in this case is about 13 % . The results of Examples 14-17 show that the tunable range of wavelength can be further varied by using ferrofluid droplets, stabilized with a neutral polymer like poly vinyl alcohol with higher molecular weight. 21 Example 18 In order to compare the band selection capability of the new filter, we have also performed the experiments with a commercial interference filters from M/s ORIEL, USA. The central wavelength of the filter is 690 nm Here, the filter assembly is illuminated by a white light source and the reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 8 shows the Bragg reflected light intensity as a function of wavelength One would see that the band of the commercial filter is broader compared to the filter of the preset invention. Also, in the case of the commercial filter, it could be possible to get only one band of wavelength and there is no possibility to change the pass band wavelength Example 19 In order to compare the band selection capability of the new filter, we have also performed the experiments with another commercial interference filters from M/s ORIEL, USA. The central wavelength of the filter is 530 nm Here, the filter assembly is illuminated by a white light source and the reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength. Figure 9 shows the. Bragg reflected light intensity as a function of wavelength One would see that the band of the commercial filter is broader compared to the filter of the preset invention. Also, in the case of the commercial filter, it could be possible to get only one band of wavelength and there is no possibility to change the pass bajid wavelength Example 20 In order to illustrate the color changing property, the filter assembly is illuminated by a white light source and the current to the solenoid is adjusted from minimum to maximum. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.4 mM. The droplet sizes used in the above 22 example were 160 nm and 180 nm. The concentration of the ferrofluid used in the above example was 0.1% by volume. The color changes seen in the filter assembly is recorded by using a digital camera at different magnetic field strength. We have seen the entire colors in the VIBGYOR range. This shows that it is possible to get all colors in the visible spectra, by controlling the magnetic field. For a given current, the spacing between the droplets is given by d = nX/2. Therefore, the color seen on the filter assembly depends on the first order Bragg peak wavelength, In order to achieve tunability in the lower wavelength region, emulsions with smaller droplets are suitable. Similarly, for longer wavelength region, droplets of larger sizes are to be employed. The main advantages of the invention. A single filter can be used for a range of central wavelength, where the desired central wavelength region is tuned by external magnetic field. • The device is suitable for selecting wavelength in the ultraviolet, visible and infrared (IR) regions. • There is no need for changing the optical element for different wavelength regions. • Tuning can be easily achieved by changing the field strength. • The spectral distribution can be easily controlled by adjusting the polydispersity of the emulsion • The intensity of the transmitted light can be controlled by changing the emulsion concentration • Simplicity to operate and less expensive compared to the existing filters 23 We Claim : 1. A tunable optical filter useful for selecting different bands of wavelengths in the UV, Visible and IR region without the necessity of changing the optical element, which comprises of a ferrofluid-based emulsion sandwiched between two transparent optical sheets, the thickness of the gap between the transparent optical sheets being at least 100 microns thereby forming a cell , the cell being placed inside a solenoid which is provided with a socket for connecting to a variable direct current source so as to facilitate changing the magnetic field of the cell 2. A tunable optical filter as claimed in claim 1 wherein the ferrofluid emulsion is made of magnetic material may be selected from iron, nickel, cobalt, y-Fe2O.3, magnetite and combinations thereof and the like, 3. A tunable optical filter as claimed in claim 1 and 2 wherein the carrier fluid for suspending the magnetic material may be selected from n-octane, cyclohexane, n-dodecane, n- tetradecane, n-hexadecane, n-octadecane, or kerosene and the like. 4. A tunable optical filter as claimed in claims 1 & 2 wherein the magnetic material may also be stabilized using an inner surfactant such as oleic acid, linoleic acid, olive oil and the like 5. A tunable optical filter as claimed in claims 1 wherein the optical transparent sheets used may be selected from glass, quarts, mylar and the like. 6. A tunable optical filter as claimed in claims 1 wherein the ferro magnetic material is emulsified with water and a ionic or nonionic surfactant wiih particles of suitable sizes and concentrations. 7. A tunable optical filter as claimed in claim 1 & 6 wherein the ionic (anionic) surfactant used is selected from polyoxyethylene, alkylphenyl ether sulfates, polyoxyethylene 24 styrenated phenyl ether sulfates, alkylphosphates, polyoxyethylene alkyl ether phosphates, polyoxyethylene alkylphenyl ether phosphates, fatty acid salts, alkylbenzene sulfonates, alkyl sulfonates, alkyl naphthalene sulfonates, alpha -olefin sulfonates, dialkyl sulfosuccinates, alpha -sulfonated fatty acid salts,N-acyl-N-methylJaurate, alkylsulfates, sulfated lipids, polyoxyethylene alkyl ether sulfates and naphthalene sulfonate formaldehyde condensates and the like. 8. A tunable optical filter as claimed in claim 1 & 6 wherein the ionic (cationic) surfactant is selected from alkyltrimethyl ammonium salts, primary to tertiary aliphatic amine salts, dialkyldimethyl ammonium salts, trialkylbenzyl ammonium salts, alkyl pyridinium salts, tetraalkyl ammonium salts, and polyethylene polyamine fatty acid amide salts and the like. 9. A tunable optical filter as claimed in claim 1 & 6 wherein the non-ionic surfactant is selected from polyoxyethylene polyoxypropylene glycols, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polystyrylphenyl ethers, polyhydric alcohol fatty acid partial esters, sorbitan fatty acid esters, glycerol fatty acid esters, deca-glycerol fatty acid esters, polyglycerol fatty acid esters, propylene glycol pentaerythritol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, polyoxyethylene polyhydric alcohol fatty acid partial esters, polyoxyethylene fatty acid esters, polyglycerol fatty acid esters, polyoxyethylenated castor oil, fatty acid diethanolamides, polyoxyethylene alkylamines, triethanolamine fatty acid partial esters, trialkylamine oxides, and polyoxyalkylene group containing organopolysiloxanes and the like 10. A tunable optical filter as claimed in claims 1 to 6 wherein the emulsion is stabilized for enhancing its life time by incorporating therein a macromolecule, which is non-ionic, cationic or anionic polymer, a macromolecule or a di-block or tri-block polymer or a macromolecule-surfactant mixture. 25 11. A tunable optical filter as claimed in claims 1 to 10 wherein the emulsion droplets employed is direct, inverted and multiple emulsion which are very well known in the field. 12. A tunable optical filter as claimed in claims 1 to 10 wherein the magnetic suspension beinii a solid-liquid dispersion, solid-air dispersion or liquid-air dispersion which are very well known in the field,. 13. A tunable optical filter as claimed in claims I wherein the thickness of the gap between the plates / sheets is effected by employing spacers 14. A tunable optical filter as claimed in claims 1 wherein the solenoid is replaced by a ring magnet. 15. A tunable optical filter as claimed in claim 1 wherein for a given current, the spacing between the droplets is fixed in such a manner that a given band of light with a centra' wavelength of λ (=2d/n) would be reflected. 16. A tunable optical filter useful for selection of different wave lengths of light substantially as herein described with reference to the Examples 1 to 17 and the Fig 3 of the drawing accompanying this specification Dated this 3rd day of June 2002 26

Full Text

FORM 2
THE P ATENTS ACT, 1970

COMPLETE SPECIFICATION

[ SECTION 10]

A TUNABLE OPTICAL FILTER
Department of Atomic Energy ,a department of the Govt of India having its office at Anushakthi Bhavan , Chathrapathy Shivaji Maharaj Marg, Mumbai, 400001,
Maharashtra, India
The following specification particularly describes and
ascertains the nature of this invention and the manner in
which is to be performed
1

The invention disclosed in this application relates to a tunable optical filter. The tunable optical filter of the present invention is useful for selecting different bands of wave lengths from a white light, without the necessity for changing the optical element. Accordingly the filter of the present invention is also useful for obtaining different colors, which will find applications in industries such as Optical, Opto-electronics, Entertainment, Laser, Spectral radiometry, Color separation etc.
Optical filters are used to select wavelengths (colors) of light. A filter works by excluding all but a limited set of wavelengths. An optical filter is specified by a wavelength number in nanometers. There are three general types of filters, long pass, short pass and band pass. The wavelengths that are allowed through a filter are called the pass band.
Long pass band filters let all the wavelengths longer than the wavelength number through. These types of filters are used as barrier filters in fluorescence. They will let the fluorescent emitted light through and not the exciter light.
Short pass band filters let light that is lower in wavelength through. Short pass band filters are sometimes used as exciter filters for fluorescence. A filter can transmit or reflect a specified range of wavelengths. Such a filter designed for spectroscopy is called a spectroscopic filter.
Bandpass filters transmit light only within a defined spectral band ranging from less than one to many nanometers wide. They are used in a great variety of applications including spectral photometry, medical diagnostics, chemical analysis, colorimetry, and astronomy. When used in spectral photometry, bandpass filters are preferred over monochromators because of their higher transmission and better signal-to-noise ratio. The wavelength number is referred to as the center wavelength of a pass band filter. The center wavelength is the wavelength that allows the most light through within the range of wavelengths that will be passed through. Frequently, this will be the center wavelength of the filter. A pass band filter is rated by center wavelength and the pass band or width. The pass band can be 'quite'narrow and can be uneven., An uneven pass band would have more width on one side than the other.
2
An interference filter is usually a pass band filter. It is specified by the center wavelength and the width. Manufacturers of interference filters provide plots of the intensity versus wavelength. This plot is the best way to understand the characteristics of a band pass filter. Interference filters use very thin glass and coatings of dielectric material such as SiO2, TiO2. ZrO2, TaO2, ZnS, MgF2, Al etc or their combinations to set up constructive and destructive interference in the filter. This interference creates the filter effect.
Optical filters have been made by sputtering or by other methods of depositing materials of differing refractive index onto a transparent substrate. Such structures work well for low-power light application; however, for high-power illumination the deposited films peel-off or crack. Such a structure is found in U.S. Pat. No. 3,962,062 in which the films are deposited by sputtering. Electrically insulated integrated circuits (U.S. Pat. No. 3,976,511) have been formed by bombarding the surface of a substrate by ions and then heating the resultant bombarded surface sufficiently to react the ions with the substrate. The structure is further formed by depositing a layer onto the ion-bombarded structure and then forming desired patterns of electrically insulating layers thereon. U.S. Pat. No. 3,976,512 also illustrates ion bombardment in forming integrated circuits.
FIG. 1 of the drawing accompanying this specification illustrates a high-resistivity silicon substrate *C' with one layer 4B' of nitrogen ions implanted therein a short distance from and parallel with the upper surface of the substrate. An ion accelerator is used to accelerate nitrogen ions 'A' which are uniformly incident on the surface of the silicon substrate, fn the buried implanted layer, a substrate temperature of about 700.degree. C. efficiently promotes the formation of the second-phase material (ion layer), silicon nitride, whose refractive index is 2.0. Thus silicon, with an index of refraction of 3.43, has a layer of silicon nitride with an index of refraction of 2.0 buried beneath the surface. The large difference in the index of refraction of the main body and the ion-implanted layer will cause light to be multiply reflected between the Sis N4 layer and the upper surface of the substrate. These multiple reflections lead to interference maxima and minima in the transmitted light as a function of wavelength.
3
Another type of filter which is currently used is of the same type as explained above with multiplayer coating, which are being used in high power laser devices. A narrow-band
interference filter comprises of alternating reflective layers arranged in a series in the form of

layers on a substrate (S) where the H layers have a high index of refraction and the L layers have a low index of refraction and the same optical thickness as the H layers is disclosed in a US patent No.4 009 453. Such filter is specifically useful for applications' involving lasers with high output power. The schematic of the multilayered filter is shown in figure 2. Here, the metal layers M are interposed in each alternating reflective layer system, so that one obtains the layer sequence 1 of (SHL HML HMLL MHL MHL HA), m the above example, S is the glass substrate with an index of refraction 1.5 and the layer H is ZnS and the layer L is MgF2 and the metal layer is Al.
Color imaging systems benefit from the use of precision optical filters which control the spectral properties of light and color separation to exacting tolerances. In addition, system performance improves with the elimination of wavelengths outside the visible spectrum. Image capture is enhanced when the prime colors of light are precisely separated or trimmed before reaching the detector. Image reproduction, where colors are recombined, also benefits from the use of precision filters. The result is that image quality improves when a system's optics deliver precise color separation, high color signal-to-noise, and a wide dynamic range.
Color imaging and measurement systems utilize color separation filters as well as prisms to differentiate the prime color wavelengths. Image capture systems divide light into red, green, and blue before reaching the detector. Image delivery systems recombine the red, green, and blue to form an image. In either case, system performance can be enhanced by carefully controlling the wavelengths and the areas of overlap. Systems, which use prisms to control the spectrum, can have enhanced performance using color separation filters.
The above described presently available pass band or color optical filters are useful only for selecting a given central wavelength. If different wavelengths have-to be selected, different
+. - -
filters have.to be used for each desired central wavelength In other words such filters have no

4
tunability option. Such filters are, therefore, not only cumbersome to use but also expensive because of the requirement of special high cost coatings such as those mentioned earlier.
In spectroscopic and interferometric experiments involving different wavelengths of light, we need to use different types of interference filters to eliminate stray light entering the detector head. In those experiments, we need to replace the filters, whenever the wavelength of the light source is changed, e.g.: when we use He-Ne laser instead of Ar-ion laser (or when we change the color from red to green). This is often cumbersome, especially when the wavelength is changed continuously with a tunable type of laser. In order to avoid the problems associated with the necessity to change the filters, we have undertaken this project to develop a tunable type of optical filter.
The main objective of the present invention; therefore, is to provide a tunable optical filter useful for obtaining of different wave lengths of light from a white light source, without the necessity for changing the optical element.
Another objective of the present invention is to provide a tunable optical filter which is useful for selecting the wavelengths in the ultraviolet, visible and infrared (IR) regions.
Yet, another objective of the present invention is to provide a tunable optical filter in which it is not required to change the optical element for choosing different wavelengths.
Another objective of the present invention is to provide a tunable optical filter in which the desired band and central wavelength can be selected electronically, without changing the filters.
Yet another objective of the present invention is to provide a tunable optical filter in which the spectral distribution can be easily controlled by adjusting the pofydispersity of the emulsion used

5
Still another objective of the present invention is to provide a tunable optical filter in which the intensity of the transmitted light can be controlled by changing the concentration of the
emulsion used.

Further objective of the present invention is to provide a tunable optical filter which is simple to operate and less expensive as compared to existing filters
The invention has been developed based on the fact that the magnitude of the magnetic dipole moment of a magnetic emulsion, increases with the strength of the applied magnetic field until saturation is reached. This leads to chaining of the droplets. At iow concentration, one droplet thick chains are well separated and oriented along the field direction. Due to the presence of the one-dimensional ordered structure, Bragg reflection or scattering takes place from the emulsion. Here, the Bragg reflected wavelength corresponds to the spacing between the droplets.
The condition for forming a linear chain is that the repulsive force between the droplets must exactly balance the attractive force between the droplets induced by the applied magnetic field. The dominant force in a field induced droplet chain is the dipole- dipole attraction. The Van der walls contribution also becomes significant at short distances. For perfectly aligned particles with a separation d' the first order Bragg condition leads to 2d = λ0/n. Where "n" is the refractive index of the suspending medium (n=1.33 for water) and λo is the wavelength of the light Bragg scattered at an angle of 180 degrees. The peak position moves toward smaller wavelength as the field is increased. The magnitude of the induced dipole can be controlled by the applied field.
We explored the above property of controlling the spacing between the monodispersed emulsion droplets for developing the optical filter of the present invention where the pass band of wavelength can be chosen by choosing the appropriate magnetic filed. Using an appropriate emulsion of given diameter, the desired band can be selected by applying a given magnetic field.
6
Accordingly we employed magnetic emulsions of magnetic material such as γ-Fe2O3, magnetite and combinations thereof, in a carrier medium of octane, stabilized with an inner surfactant and is emulsified with water in the presence of an ionic(cationic or anionic) or nortcomc surfactant with particles of suitable sizes and concentrations, sandwiched between two optical quality transparent plates ( e.g: glass, quarts, mylar etc.) which is hereinafter referred to as ferrofluid. The ferrofluid is sandwitched between the two transparent plates thereby forming a cell which is placed inside a non-magnetic casing (eg:AI), the cell is placed inside a compact solenoid, which is connected to a current source. The magnetic field is varied by changing the current. The reflected light from the ferrofluid cell is measured as a function of applied field strength by employing a spectrograph. As the ferrofluid droplets are super paramagnetic in nature, an applied magnetic field induces a magnetic dipole in each drop, causing them to form chains. Without external field, these droplets have no permanent magnetic moments because of the random orientation of the magnetic grains within the droplets, due to thermal motion. An external magnetic field orients these magnetic grains slightly toward the field direction, which results in a dipole moment in each droplet.
In order to study the characteristic features of the new tunable optical filter, the filter is illuminated by a white light source and the reflected light from the filter assembly is sent through an optical fiber. The out put from the fiber assembly is finally entered inside a monochromator equipped with a diode array. The diode array output is processed by a computer by means of an interface card, which gives the reflected/transmitted light intensity as a function of wavelength.
The tunable pass band optical filter useful for selecting different wavelengths of light of the present invention is shown in Fig 3. The magnetic emulsion (1) is sandwiched between two optical quality transparent plates (2) thereby forming a cell with is placed inside a compact solenoid (3). The spacing between the transparent glass plates is controlled by a mylar washer (4) of thickness 200 microns. The leads of the solenoid which is connected to a socket (5 )and is subsequently connected to a current source (6).
According to the present invention there is provided a tunable pass band optical filter useful for selecting.different bands of wavelengths in the UV, Visible and 1R region without the
7
necessity for changing the optical element, which comprises of a ferrofluid-based emulsion sandwiched between two transparent optical sheets, the thickness of the gap between the transparent optical sheets being at least 100 microns thereby forming a cell , the cell being placed inside a solenoid which is provided with a socket for connecting to a variable direct current source so as to facilitate changing the magnetic field of the cell
In a preferred embodiment of the present invention the optical transparent sheets used may be selected from glass, quarts, mylar and the like. The magnetic material used may be selected from magnetic material such as iron, nickel, cobalt, γ-Fe2O3 magnetite and combinations' thereof and the like The carrier fluid in which the magnetic grains are dispersed may be n-octane, cyclohexane, n-dodecane, n-tetradecane, n-hexadecane, n-octadecane, or kerosene and the like.. The magnetic material may also be stabilized using an inner surfactant such as oleic acid, linoleic acid, olive oil and the like. The ferro magnetic material may also be emulsified with water and an ionic or nonionic surfactant with particles of suitable sizes and concentrations.
The ferro magnetic material may also be emulsified with oil and an ionic or nonionic surfactant with particles of suitable sizes and concentrations. The ionic surfactant used may be selected from anionic surfactants such as polyoxyethylene, alkylphenyl ether sulfates, polyoxyethylene styrenated phenyi ether sulfates, alkylpho phates, polyoxyethylene alkyi ether phosphates, polyoxyethylene alkylphenyl ether phosphates, fatty acid salts, alkylbenzene sulfonates, alkyl sulfonates, alkyi naphthalene sulfonate alphha -olefin sulfonates, dialkyl sulfosuccinates, alpha -sulfonated fatty acid salts,N-acyl-N-methyIlaurate, alkylsulfates, sulfated lipids, polyoxyethylene alkyl ether sulfates and Naphthalene sulfonate formaldehyde condensates and the like.
The cationic surfactants may be alkyltrimethyl ammoniu salts, primary to tertiary aliphatic amine salts, dialkyldimethyl ammonium salts, trialkylbenzy ammonium salts, alkyl pyridmium salts, tetraalkyl ammonium salts, and polyethylene polyaVnine fatty acid amide salts and the like.
8
The non ionic surfactant selected from -polyoxyethylene polyoxypropylene glycols, polyoxyethylene polyoxypropylene alkyl ethers, polyoxycthylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene alk^iphenyl ethers, polyoxyethylene polysryrylphenyl ethers, polyhydric alcohol fatty acid partial esters, sorbitan fatty acid esters, glycerol fatty acid esters, deca-glycerol fatty acid esters, polyglycerol fatty acid esters, propylene glycol pentaerythritol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, polyoxyethylene polyhydric alcohol fatty acid partial esters, polyoxyethylene fatty acid esters, polyglycerol fatty acid esters, polyoxyethylenated castor oil, fatty acid diethanolamifles, polyoxyethylene alkylamines, triethanolamine fatty acid partial esters, trialkylamine Oxides, and polyoxyalkylene group containing organopolysiloxanes and the like
TRe emufsion may 6e staoil'ized" tor ennancihg tne nfe time by incorporating a macromolecule, which may be a non-ionic, cationic or anfonic polymer or a macromolecule which may be a di-block or tri-block polymer or a macromo|ecule-surfactant mixture.
The emulsion droplets employed may be a direct, inverted and multiple emulsion which are very well known in the field. The magnetic suspension, which may be a solid-liquid dispersion, solid-air dispersion or liquid-air dispersion.
The thickness of the gap between the plates/sheets can be adjusted by employing spacers. For a given current, the spacing between the droplets is fixed in such a manner that a given band of light with a central wavelength of X (-2d/n) would be reflected. The central wavelength can be continuously tunable by changing the current, employing a polymer stabilized ferrofluid emulsion, the central wavelength can be varied up to 300 In order order to achieve tunability in the lower wavelength region, emulsions with smaller droplets are suitable. Similarly, for longer wavelength region, droplets of larger sizes are to be employed.
In order, to calibrate the tunable filter assembly, we have filled the cell assembly with ferrofluid.of a given droplet diameter (first column in the table 1). The ferrofluid used in the above experiment was made of Y-Fe2O.3 stabilized with an inner surfactant of oleic acid and
9

dispersed in a carrier medium of octane. The our surfactant was sodium dodecyl sulphate. The emulsion was subsequently stabilized with polyvinyl alcohol of molecular weight 1555 000 at a concentration of 0.5%. The filter assembly is kept inside the solenoid assembly, where the magnetic field can be varied from 0 to 200 gauss. By varying the magnetic field strength from
0 to 200 Gauss, we have measured the corresponding Bragg peak central wavelength. The
minimum and maximum value for different droplet radius is then estimated. Table 1 given
below shows the approximate values of minimum and maximum values of central wavelength
obtained for emulsions with different droplet radii ( where h = d - 2a). Here the approximate
, L
inter droplet spacing varies from lOOnm (minimum field) to close contact (maximum field).

From the Tablel, it is clear that 4 different filters made of emulsion droplets of 100, 200, 300
and 400 nm would be capable of covering a wide tunable wavelength range of 266 to 1330
nm.
Tablel

Droplet Diameter (nm) Tunable wavelength (nm)
100 266 -532
120 319 -585
140 . 372 -638
160 425 -691
180 478 -744
200 532 -798
220 585 -851
240 638 -904
260 691 -957
280 744 -1010
300 798 -1064
320 851 -1117
340 904 -1170
360 957 -1223
380 1010-1276
400 . 1064-1330
10

The invention is described in detail in the Examples given below which are provided only to illustrate the invention and therefore should not be construed to limit the scope of the invention
Example 1
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a
white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3
stabilized with an inner surfactant of oleic acid and dispersed in a carrier mediuni' of octane.
The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %. The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 run. The magnetic filed at the center of the solenoid is adjusted to 280 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition, The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 4a shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 590 run. The percentage of reflectivity in this case is about 12%
Example 2
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe20.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 % The concentration of the ferrofluid
11

used in the above filter assembly was 0.001%, and the droplet diameter was 184 nm.. The magnetic filed at the center of the solenoid is adjusted to 210 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage
of reflectivity as a function of wavelength.

Figure 4b shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 635 nm. The percentage of reflectivity in this case is about 12%
Example 3
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated'by a white light source. The ferrofluid used in the above experiment was made of y-FejO.s stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 nm. The magnetic filed at the center of the'solenoid is adjusted to 157 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 4c shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 665 nm. The percentage of reflectivity in this case is about 12%
Example 4
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a
12

white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 184 run. The polydispersity of the emulsion in this case was about 10 %. The magnetic filed at the center of the solenoid is adjusted to 105 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 4d shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 69.0 nm. The percentage of reflectivity in this case is>about 12%
Example 5
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the
droplet diameter was 184 nm. The polydispersity of the emulsion in this case was about 10 %.

The magnetic filed at the center of the solenoid is adjusted to 258 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 5a shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 600 nm. The percentage of reflectivity in this case is about 77 %
13

Example 6
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. .The ferrofluid used in the above experiment was made of y-Fe2O.3
stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane.

The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM.The

concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184 nm. The polydispersity of the emulsion in this case was about 10
%. The magnetic filed at the center of the solenoid is adjusted to 194 Gauss, by adjusting the

current to the solenoid. At this stage the filter, assembly is in activated condition/The reflected.

light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 5b shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 625 nm. The percentage of reflectivity in this case is about 77 %
Example 7
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. . The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184 nm The magnetic filed at the center of the solenoid is adjusted to 140 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
14

Figure 5c shows the Bragg reflected light from the filter unit The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 645 nm. The percentage of reflectivity in this case is about 77 %
Example 8
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of y-Fe2O.3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.8 mM. The polydispersity of the emulsion in this case was about 10 %.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.01%, and the droplet diameter was 184nm. The magnetic filed at the center of the solenoid is adjusted to 98 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 5d shows the Bragg reflected light from the filter unit. The full width at half maximum (FWHM) in the above case is around 20nm, with the central wavelength around 710 nm. The percentage of reflectivity in this case is about 77 %
The results of the Examples 1-8 show that the pass band of the reflected light is tunable over a wide range of wavelengths by adjusting the current to the solenoid or the magnetic field. The intensity of the reflected band of light depends on the concentration of the ferrofluid emulsion employed. Here, the polydispersity of the emulsion dictate the FWHM. High polydispersity values give rise to higher the FWHM.
Example 9
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a
15

white light source. The ferrofluid used in the above experiment was made of γ-Fe2C3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 210 nm. Here, the Ferro fluid is stabilized with poly-vinyl alcohol of average molecular weight 40000. The polydispersity of the emission in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output
gives the percentage of reflectivity as a function of wavelengt^

Figure 6a shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 576-621 nm. The percentage of reflectivity in this case is about 15 %
Example 10
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and disperseq in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a Concentration of 0.02 mM.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 202 nm. Here, the Ferro fluid is stabilized poly-vinyl alcohol of average molecular weight 40000. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varried from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelenth.
16

Figure 6b shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in tin above case is around 18nm and the central wavelength varies from 536-587 nm. Tin percentage of reflectivity in (his case is about 15 %
Example 11
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid which is powered by a variable direct current source. The filter assembly is illuminated by : white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3. stabilized with an inner surfactant of oieic acid and dispersed in a carrier medium of octane The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-vinyl alcohol of average molecular weight 40000.Th Figure 6c shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 505-561 nm. The percentage of reflectivity in this case is about 15 %
Example 12
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a while light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane.
17

The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-viny] alcohol of average molecular weight 40000.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and

the droplet diameter was 180 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 6d shows the central wavelength of the Bragg reflected light as a function of applied
field strength from the above filter assembly. The full width at half maximum (FWHM) in the
above case is around 18nm and the central wavelength varies from 481-550 nm. The percentage of reflectivity in this case is about 15 %
Example 13
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with - poly-vinyl alcohol of average molecular weight 40000.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 176 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectyograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 6e shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the
18

above case is around 18nm and the central wavelength varies from 451-509 nm. The percentage of reflectivity in this case is about 15 %
The results of Examples 9-13 show that the tunable range of wavelength can be varied by changing the droplet size of the ferrofluid emulsion used. Also the emulsion droplets stabilized
with poly vinyl alcohol can stabilize and augment the tunable range.

Example 14

The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 0OO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.OOI%, and the droplet diameter was 210 nm. The polydispersity of the emulsion in this case vas. about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The ;pectrograph output gives the percentage of reflectivity as a function of wavelength.
figure 7a shows the central wavelength of the Bragg reflected light as a function of applied leld strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 772-879 nm. The sercentage of reflectivity in this case is about 13 %
Example 15
The tunable pass band optical filter assembly shown in figure 3 is kept inside the solenoid, vhich is powered by a variable direct current source. The filter assembly is illuminated by a
19

white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with "an inner surfactant of oleic acid and dispersed in a carrier medium of octane, The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 OOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 202 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 7b shows the central wavelength of the Bragg reflected light as a function,, of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around I8nm ana1 tne central1 wavelength vanes from 713-716 nm. Tne percentage of reflectivity in this case is about 13 % .
Example 16
The tunable pass band optical filter assembly shown in figure 3 is kept inside .the solenoid, which is powered by a variable direct current source. The filter assembly is illuminated by a white light source. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 DOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 3.001%, and the droplet diameter was 190 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 22Q Gauss, oy adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
20

Figure 7c shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 652-759 nm. The percentage of reflectivity in this case is about 13 % .
Example 17
The tunable pass band optical filter assembly shown in figure 3 is kept insi.de the solenoid,
which is powered by a variable direct current source. The filter assembly is illuminated by a

white light source. The ferrofluid used in the above experiment was made of yγ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.02 mM. Here, the Ferro fluid is stabilized with a polymer - poly-vinyl alcohol of average molecular weight 115 OOO.The concentration of the ferrofluid emulsion used in the above filter assembly was 0.001%, and the droplet diameter was 180 nm. The polydispersity of the emulsion in this case was about 8 %. The magnetic filed at the center of the solenoid is varied from 0 to 220 Gauss, by adjusting the current to the solenoid. At this stage the filter, assembly is in activated condition. The reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 7d shows the central wavelength of the Bragg reflected light as a function of applied field strength from the above filter assembly. The full width at half maximum (FWHM) in the above case is around 18nm and the central wavelength varies from 631-716 nm. The percentage of reflectivity in this case is about 13 % .
The results of Examples 14-17 show that the tunable range of wavelength can be further varied by using ferrofluid droplets, stabilized with a neutral polymer like poly vinyl alcohol with higher molecular weight.
21

Example 18
In order to compare the band selection capability of the new filter, we have also performed the experiments with a commercial interference filters from M/s ORIEL, USA. The central wavelength of the filter is 690 nm Here, the filter assembly is illuminated by a white light source and the reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 8 shows the Bragg reflected light intensity as a function of wavelength One would see that the band of the commercial filter is broader compared to the filter of the preset invention. Also, in the case of the commercial filter, it could be possible to get only one band of wavelength and there is no possibility to change the pass band wavelength
Example 19
In order to compare the band selection capability of the new filter, we have also performed the experiments with another commercial interference filters from M/s ORIEL, USA. The central wavelength of the filter is 530 nm Here, the filter assembly is illuminated by a white light source and the reflected light from the filter assembly is directed to a spectrograph. The spectrograph output gives the percentage of reflectivity as a function of wavelength.
Figure 9 shows the. Bragg reflected light intensity as a function of wavelength One would see that the band of the commercial filter is broader compared to the filter of the preset invention. Also, in the case of the commercial filter, it could be possible to get only one band of wavelength and there is no possibility to change the pass bajid wavelength
Example 20
In order to illustrate the color changing property, the filter assembly is illuminated by a white light source and the current to the solenoid is adjusted from minimum to maximum. The ferrofluid used in the above experiment was made of γ-Fe2O3 stabilized with an inner surfactant of oleic acid and dispersed in a carrier medium of octane. The outer surfactant was sodium dodecyl sulphate at a concentration of 0.4 mM. The droplet sizes used in the above
22

example were 160 nm and 180 nm. The concentration of the ferrofluid used in the above example was 0.1% by volume. The color changes seen in the filter assembly is recorded by using a digital camera at different magnetic field strength. We have seen the entire colors in the VIBGYOR range. This shows that it is possible to get all colors in the visible spectra, by controlling the magnetic field. For a given current, the spacing between the droplets is given
by d = nX/2. Therefore, the color seen on the filter assembly depends on the first order Bragg

peak wavelength,

In order to achieve tunability in the lower wavelength region, emulsions with smaller droplets
are suitable. Similarly, for longer wavelength region, droplets of larger sizes are to be

employed.
The main advantages of the invention.
A single filter can be used for a range of central wavelength, where the desired central wavelength region is tuned by external magnetic field.
• The device is suitable for selecting wavelength in the ultraviolet, visible and infrared
(IR) regions.
• There is no need for changing the optical element for different wavelength regions.
• Tuning can be easily achieved by changing the field strength.
• The spectral distribution can be easily controlled by adjusting the polydispersity of the
emulsion
• The intensity of the transmitted light can be controlled by changing the emulsion
concentration
• Simplicity to operate and less expensive compared to the existing filters
23

We Claim :
1. A tunable optical filter useful for selecting different bands of wavelengths in the UV,
Visible and IR region without the necessity of changing the optical element, which
comprises of a ferrofluid-based emulsion sandwiched between two transparent optical
sheets, the thickness of the gap between the transparent optical sheets being at least 100
microns thereby forming a cell , the cell being placed inside a solenoid which is provided
with a socket for connecting to a variable direct current source so as to facilitate changing
the magnetic field of the cell
2. A tunable optical filter as claimed in claim 1 wherein the ferrofluid emulsion is made of
magnetic material may be selected from iron, nickel, cobalt, y-Fe2O.3, magnetite and
combinations thereof and the like,
3. A tunable optical filter as claimed in claim 1 and 2 wherein the carrier fluid for suspending
the magnetic material may be selected from n-octane, cyclohexane, n-dodecane, n-
tetradecane, n-hexadecane, n-octadecane, or kerosene and the like.
4. A tunable optical filter as claimed in claims 1 & 2 wherein the magnetic material may also
be stabilized using an inner surfactant such as oleic acid, linoleic acid, olive oil and the
like
5. A tunable optical filter as claimed in claims 1 wherein the optical transparent sheets used
may be selected from glass, quarts, mylar and the like.
6. A tunable optical filter as claimed in claims 1 wherein the ferro magnetic material is
emulsified with water and a ionic or nonionic surfactant wiih particles of suitable sizes and
concentrations.
7. A tunable optical filter as claimed in claim 1 & 6 wherein the ionic (anionic) surfactant
used is selected from polyoxyethylene, alkylphenyl ether sulfates, polyoxyethylene
24

styrenated phenyl ether sulfates, alkylphosphates, polyoxyethylene alkyl ether phosphates, polyoxyethylene alkylphenyl ether phosphates, fatty acid salts, alkylbenzene sulfonates, alkyl sulfonates, alkyl naphthalene sulfonates, alpha -olefin sulfonates, dialkyl sulfosuccinates, alpha -sulfonated fatty acid salts,N-acyl-N-methylJaurate, alkylsulfates, sulfated lipids, polyoxyethylene alkyl ether sulfates and naphthalene sulfonate formaldehyde condensates and the like.
8. A tunable optical filter as claimed in claim 1 & 6 wherein the ionic (cationic) surfactant is
selected from alkyltrimethyl ammonium salts, primary to tertiary aliphatic amine salts,
dialkyldimethyl ammonium salts, trialkylbenzyl ammonium salts, alkyl pyridinium salts,
tetraalkyl ammonium salts, and polyethylene polyamine fatty acid amide salts and the like.
9. A tunable optical filter as claimed in claim 1 & 6 wherein the non-ionic surfactant is
selected from polyoxyethylene polyoxypropylene glycols, polyoxyethylene
polyoxypropylene alkyl ethers, polyoxyethylene alkyl ethers, polyoxyethylene alkenyl
ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polystyrylphenyl ethers,
polyhydric alcohol fatty acid partial esters, sorbitan fatty acid esters, glycerol fatty acid
esters, deca-glycerol fatty acid esters, polyglycerol fatty acid esters, propylene glycol
pentaerythritol fatty acid esters, polyoxyethylene sorbitan fatty acid esters,
polyoxyethylene glycerol fatty acid esters, polyoxyethylene polyhydric alcohol fatty acid
partial esters, polyoxyethylene fatty acid esters, polyglycerol fatty acid esters,
polyoxyethylenated castor oil, fatty acid diethanolamides, polyoxyethylene alkylamines,
triethanolamine fatty acid partial esters, trialkylamine oxides, and polyoxyalkylene group
containing organopolysiloxanes and the like
10. A tunable optical filter as claimed in claims 1 to 6 wherein the emulsion is stabilized for
enhancing its life time by incorporating therein a macromolecule, which is non-ionic,
cationic or anionic polymer, a macromolecule or a di-block or tri-block polymer or a
macromolecule-surfactant mixture.
25

11. A tunable optical filter as claimed in claims 1 to 10 wherein the emulsion droplets
employed is direct, inverted and multiple emulsion which are very well known in the field.
12. A tunable optical filter as claimed in claims 1 to 10 wherein the magnetic suspension
beinii a solid-liquid dispersion, solid-air dispersion or liquid-air dispersion which are very
well known in the field,.
13. A tunable optical filter as claimed in claims I wherein the thickness of the gap between the
plates / sheets is effected by employing spacers
14. A tunable optical filter as claimed in claims 1 wherein the solenoid is replaced by a ring
magnet.
15. A tunable optical filter as claimed in claim 1 wherein for a given current, the spacing
between the droplets is fixed in such a manner that a given band of light with a centra'
wavelength of λ (=2d/n) would be reflected.
16. A tunable optical filter useful for selection of different wave lengths of light substantially
as herein described with reference to the Examples 1 to 17 and the Fig 3 of the drawing
accompanying this specification
Dated this 3rd day of June 2002

26

Documents:

501-mum-2002-abstract(5-6-2002).pdf

501-mum-2002-abstract(granted)-(17-10-2005).pdf

501-mum-2002-cancelled pages(19-1-2005).pdf

501-mum-2002-claims(5-6-2002).pdf

501-mum-2002-claims(amended)-(18-5-2004).pdf

501-mum-2002-claims(amended)-(19-1-2005).pdf

501-mum-2002-claims(granted)-(17-10-2005).pdf

501-mum-2002-correspondence(13-6-2005).pdf

501-mum-2002-correspondence(ipo)-(4-1-2005).pdf

501-mum-2002-description(complete)-(5-6-2002).pdf

501-mum-2002-description(granted)-(17-10-2005).pdf

501-mum-2002-drawing(26-8-2002).pdf

501-mum-2002-drawing(5-6-2002).pdf

501-mum-2002-drawing(granted)-(17-10-2005).pdf

501-mum-2002-form 1(5-6-2002).pdf

501-mum-2002-form 19(18-5-2004).pdf

501-mum-2002-form 19(30-10-2003).pdf

501-mum-2002-form 2(complete)-(5-6-2002).pdf

501-mum-2002-form 2(granted)-(17-10-2005).pdf

501-mum-2002-form 2(title page)-(5-6-2002).pdf

501-mum-2002-form 2(title page)-(granted)-(17-10-2005).pdf

501-mum-2002-form 3(5-6-2002).pdf

501-mum-2002-general power of attorney(30-10-2003).pdf

501-mum-2002-power of attorney(5-6-2002).pdf

501-MUM-2002-POWER OF AUTHORITY(5-6-2002).pdf

501-mum-2002-specification(amended)-(17-1-2005).pdf

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Patent Number

196063

Indian Patent Application Number

501/MUM/2002

PG Journal Number

24/2010

Publication Date

11-Jun-2010

Grant Date

Date of Filing

05-Jun-2002

Name of Patentee

DEPARTMENT OF ATOMIC ENERGY

Applicant Address

ANUSHAKTHI BHAVAN, CHATHRAPATHY SHIVAJI MAHARAJ MARG, MUMBAI, 400001 MAHARASHTRA, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	JOHN PHILIP	METALLURGY & MATERIALS GROUP, NON-DESTRUCTIVE TESTING 7 EVALUATION SECTION, DIVISION FOR POST IRRADIATION EXAMINATION & NON- TESTING DEVELOPMENT, INDRA GANDHI CENTER FOR ATOMIC RESERCH, KALPAKKAM-TAMIL NADU,603102, INDIA

PCT International Classification Number

N/A

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA