Title of Invention

"A PHOTOREFRACTIVE NIGHT VISION DEVICE"

Abstract This invention relates to a photorefractive night vision device comprising a housing having an objective glass (1) and an image intensifier tube (3) t disposed therein, said image intensifier adapted to amplify the in coherent light passing through the objective glass characterised by, an optical element (5) provided in the path of said in coherent light for converting the in coherent light into coherent images, a laser diode (8) adapted to read out a coherent beam from element (5), a non linear element (10) adapted to receive two coherent image beams from said non linear element (8) first from said optical element (5) and another through a pair of mirrors (7) and (9). Fig1
Full Text A PHOTOREFRACTIVE NIGHT VISION DEVICE
Field of Invention
The present invention relates to Photorefractive Night Vision Device for lev; light level imaging.
i
PrJ or Art
Passive low light level viewing devices are used for imaging during dusk & dawn, star-lit conditions, partial moonlight situations and over-cast conditions. These devices consist of an optical glass aperture, image intensifier tube and an eye piece. The image intensifier tubes (II tube) are being used because of their capability of low light level amplification. Such devices have application in imaging under the low light level conditions. These devices operate in the visible to near IR regions of the electro- magnetic spectrum.
The devices known in the art have the disadvantage that for increasing the viewing range of the device, large input aperture is required to ensure enough numbers of photon from the scene or object for amplification by the II tube. The large optical aperture to meet the above requirement, make the device heavy and difficult to manufacture.
Another disadvantage of the passive low light ievel viewing device known in the art, is that the performance of II tube used in these devices for low light level amplification is limited by its gain, resolution and signal-to-noise ratio, which not only impairs the image quality at the output end but also affects the imaging capability of the device.
The primary object of the present invention is to propose a photorefrective night vision device with enhanced range for low light level imaging without increasing the size of input aperture.
Another object of the present invention is to propose a photorefractive night vision device with capability to provide improved quality image without increasing the size of the aperture or the overall weight"of device.

Further object of the present invention is to provide a photorefractive night vision device based on both electronic as well as optical amplification of low light level as compared to the known devices which are based on only electronic amplification.
Statement of Invention
According to this invention there is provided a photorefractive night vision device comprising a housing having an objective glass (1) and an image intensifier adapted to amplify the in coherent light passing through the objective glass, an optical element (5) provided in the path of said in coherent light for converting the in coherent light into coherent images, a laser diode (R) adapted to read out a coherent beam from element (51, a non linear element (10) adapted to receive one of the coherent image beams from said non linear element (8).
Tn accordance with the present invention, the photorefractive night vision device enables imaging using electronic j and optical ampli.fi cation _of the low light level input signal. The process involves the use of an Optically Addressed Spatial Light Modulator (OASLM) for incoherent to coherent image conversion and a high gain Photo-Refractive Crystal (PRC^ for amplification of low light level input signal form a small aperture TT tube. A few photons from low
I
light level scene are collected by a small aperture objective glass and are amplified by the TT tube to a certain level. The output of ; the TT tube which is an incoherent image, is converted into coherent image by using OASLM and a small laser source for ; read-out. The coherent image so achieved is then finally amplified using a photorefractive crystal (PRC). The amplified image can be directly viewed through eyepiece or may;be displayed on a TV monitor.
According to the present invention, the angle between the light beams in the PRC and orientation of the PRC in the crystal plane is optimised to attain high signal-to-noise ratio. This optimisation of angle improves the output imnge (|iui I i l.y uiiH enhancoy UK-- viewing ruuijo of t:he device.

The foregoing features, objects and advantages of the
present invention will be more apparant from the following description read in conjunction with the accompanying figure which is intended to illustrate an embodiment of the present invention and is not intended to limit in any way the scope of the present invention.
Description of Figures
The accompanying figure shows the layout diagram of the complete assembly of the photorefractive device proposed in the disclosed invention.Description of the present invention with respect to
drawings :
In accordance with the present invention• the photorefractive Night. Vision Device comprises of a small aperture objective glass (i) for collecting the: few available photons from the scene. The objective glass (i) is designed such that it is free from the geometrical aberrations and could form an unaberrated image of the input scene oh the photocathode (2) of the II tube (3) . The II tube amplifies the few available photons collected by the Objective Glass (1) to a certain level. The II tube consists of a photocathode (2) at the input and a phosphor screen (4) at the output end. The few photons at the output of the Objective Glass (1) focussed on the photocathode (2) of the II tube. Photoelectrons are generated in the II tube
(3) which on amplification, hit the phospor screen (4) and finally emits the incoherent visible light. The Objective Glass (1) and the II tube (3) are assembled in a mechanical housing, such that the focal plane of the Objective Glass
(1) super imposes that of the photo-cathode (2) of II tube.
The incoherrent light so available at the output end of the II tube is allowed to write the image of the input screen on the optically addressed spatial light modulator
{OASLM) (5) . The OASLM (5) in our invention has been used as an optical element which converts incoherent image into coherent image. The image written on the OASLM is read-out by a coherent beam of small laser Diode (8) through mirrors
(6) and (7). The direction of the read-out beam is shown by arrows in the figure.
One of the coherent image beam generated by the laser diode (8) falls on OASLM (5) which generated a coherent image beam which then falls on to the photorefractive crystal (PRC) {10}. Another beam from laser diode (8) through mirrors (7) and (9) is also allowed to fall on the PRC (10) such that it could interfere with the image bearing beam coming from the OASLM as coherent image beam. The PRC (10) is basically a nonlinear material which undergoes local refrective index modulation on ^illumination by a laser light. PRC (10) is capable of recording volume" phase hologram within its volume with high diffraction efficiency at relatively low input optical powers. Therefore, when two optical beams i.e one image bearing beam coming from the OASLM (5) and falling on PRC (10) and another beam coming through the mirror (7) and (9) are allowed to couple in tliovolume of 'the PRC (10). The beam coupling leads to energy exchange between the two beams. This phenomenon is referred to as nonreciprocal/energy transfer from one optical beam to another mutually coherrent beam. This energy exchange between the beams results into the amplification of one beam at the cost of the another beam. Since for such type of energy exchange, one beam is taken as strong and another beam as a week beam, the week beam is kept as image bearing beam in our invention.
The output amplified image bearing beam which is output of PRC (10) , is viewed through eye-piece (12) and/or is -displayed on the ,TV monitor (14) by user (17) mirror
(13), and a beam splatter (11) which splits the light beam into two directions, namely in the direction of eye piece
(12) and in the direction of TV monitor (14).
It is to be understood that the above description of the present invention is susceptible to considerable modifications, change and adaptations by those skilled in the art. Such modifications are intended to be considered to be within the scope of the present invention, which is set forth by the following claims :-



WE CLAIM:
1. A photorefractive night vision device comprising
a housing having an objective glass (1) and an image
intensifier tube (3) disposed therein, said image intensifier adapted to amplify the in coherent light
passing through the objective glass, an optical element (5) provided in the path of said in coherent light for converting the in coherent light into coherent images, a laser diode (8) adapted to read out a coherent beam from element (5), a non linear element (10) adapted to receive one of the coherent image beams from said non linear element (8) .
2. A photorefractlve night vision device as claimed
in claim 1 wherein said image intensifier tube (3) has aa
photocathode (2) at the inlet end.
3. A photorefractive night vision device as
claimed in claim 1 wherein said image intensifier tube
(3) has a phospor screen (4) at the outlet end.
4. A photorefractive night vision device as claimed
in claim 1 wherein said optical element (5) is an
optionally addressed special light modulator.
ri . A photorefracrive niqht vision device as claimed in claim I wherein mirrors are provided—between said tube and optical element (S) .A photorefractive night vision device as claimed
in claim 1 wherein said non-linear element (10) is a
photorefractive crystal.
6. A photorefractive night vision device,as claimed
in claim 6 wherein said photorefractive crystal (10)
receives a first optical image from said said optical
element (5) and another through a pair of mirror (7) and
(9) .
8. A photorefractive night vision device as claimed
in claims 1 to 7, wherein the focal plane of the
objective glass ( 1.) superimposes the photocathode (2) of
the TT Tube (3).
9. A photorefractive night vision device as claimed
in claims 1 to 8, wherein the mirrors (6), (7) and (9)
are positioned in such a way that the path difference between the beam from (7), (9) to (10) and the beam (5)
to (10) falls within the cohedrence length of the laser diode (8) .
10. A photorefractive night vision device as claimed
in claims 1 to 9, wherein said photorefractive crystal
(10) is of high gain non-linear crystal with large
electro-optic co-efficient of the order of IfiOO pvm (pico
voltmeter) .
11. A photorefractive night vision device as claimed
in claims 1 to 10 wherein said optically addressed
special light modulator (OASLM) (5) is as an interphas i rig
element to convert incoherent optical beam to coherent
optical beam.A photorefractive night vision device as claimed
in claims 1 to 11 wherein said OASLM (5) device has the
aperture compatible with the size of the output image of
TI tube (3) .
12. A photorefractive night vision device as claimed
in claims 1 to 12 comprising an eyepiece (12) for viewing
the output amplified image bearing beam from element (10)
and/or a TV monitor (14).
13. A photorefractive night vision device as claimed
in claims 1 to 13 comprising a user mirror (13) and a
beam splitter (11) for splitting the light beam in the
direction of the eyepiece (12) and TV monitor (14).
14. A photorefractive night vision device substant
ially as herein described and illustrated.

Documents:

238-del-1997-abstract.pdf

238-del-1997-claims.pdf

238-del-1997-correspondence-others.pdf

238-del-1997-correspondence-po.pdf

238-del-1997-description (complete).pdf

238-del-1997-drawings.pdf

238-del-1997-form-1.pdf

238-del-1997-form-19.pdf

238-del-1997-form-2.pdf

238-del-1997-form-26.pdf

238-del-1997-form-3.pdf

238-del-1997-gpa.pdf

abstract.jpg


Patent Number 220143
Indian Patent Application Number 238/DEL/1997
PG Journal Number 28/2008
Publication Date 11-Jul-2008
Grant Date 15-May-2008
Date of Filing 30-Jan-1997
Name of Patentee THE CHIEF CONTROLLER RESEARCH & DEVELOPMENT
Applicant Address
Inventors:
# Inventor's Name Inventor's Address
1 DR. OM PRAKASH NIJHAWAN
2 DR. PRAKASH CHANDRA MEHTA
3 SHRI KUNWAR SINGH DHARMSAKTU
PCT International Classification Number G02B 5/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA