|Title of Invention||
AN IMPROVED MERCURY VAPOR DISCHARGE LAMP
|Abstract||The invention relates to an improved mercury vapor discharge lamp (10) comprising a light-transmissive envelope (12) haying an inner surface, means for providing a discharge (18), a discharge-sustaining fill (22) of mercury and inert gas sealed inside said envelope (12), and a single composite layer (14) coated inside said envelope (12), said composite layer comprising at least one halophosphor (32), three rare earth phosphors (34), and colloidal alumina particles (36) in a heterogeneous mixture.|
|Full Text||FILD OF THE INVENTION
The present invention relates generally to a fluorescent lamp and more particularly to a
fluorescent lamp having an improved composite phosphor layer.
BACKGROUND OF THE INVENTION
There are two types of phosphors used in fluorescent lamps: relatively inexpensive
halophosphors, and relatively expensive rare earth phosphors. Halophosphors, though
commonly used due to their low cost, exhibit poor color rendering properties and lower
lumens compared with more expensive rare earth phosphors. Rare earth phosphors, for
example blended into a triphosphor layer as is known in the art, exhibit excellent color
rendering properties and high lumens but are used sparingly due to their high cost
The fluorescent lighting industry has adopted a dual-coating technology for producing
certain medium performance lamps incorporating both halophosphors and rare earth
triphosphors. "Medium performance" as used herein means performance (in terms) of
color rendering properties and lumens) intermediate between Chat of inexpensive
halophosphors and expensive rare earth triphosphors. The dual-coating technology
involves applying halophosphors and rare earth triphosphors as discrete coating layers
with the more expensive triphosphor layer placed in the well-utili2ed second coat next to
the arc discharge. Medium performance fluorescent lamps produced using this dual-
coating technique have become quite popular and account for between 70%-90% of
fluorescent lamp sales worldwise.
Despite the popularity of this dual-coating technology, the application of phosphors as
discrete layers presents many significant manufacturing problems. Initially, the expensive
triphosphor layer is very thin, often less man a monolayer of particles, contributing to
significant variations in thickness and uniformity of the triphosphor layer during the
application process. Such variations result in increased variations in the color rendering
index (CRI) and lamp brightness which are strongly related to the triphosphor layer
Other manufacturing difficulties include a narrow range of acceptable coating additives
(such as dispersants and surfactants) as well as elevated coating and production costs.
Each coating step increases production losses and requires significant equipment and
In addition to two discrete phosphor layers, fluorescent lamps of the prior art require a.
third discrete boundary layer of alumina particles coated directly onto the glass tube
beneath the phosphor layers. This third layer of alumina prevents UV emission from the
fluorescent lamp by reflecting unconverted UV radiation back toward the interior of the
lamp where it is subsequently converted to visible light by the phosphors. The alumina
layer also minimizes mercury loss due to reaction with the glass tube. The addition of this
third coating layer further increases production losses due to equipment and labor usage.
OBJECT OP THE INVENTION
It is therefore an object of the invention to propose an improved mercury vapor discharge
lamp that combines halophosphors, rare earth phosphors or triphosphors and alumina
particles into a single blended composite coating mat can be applied as a single layer in a
single step in the production of medium performance fluorescent lamps.
SUMMARY OF THB INVENTION
Accordingly there is provided an improved mercury vapor discharge lamp provided
comprising a light-transuiissive envelope having an inner surface, means for providing a
discharge, a discharge sustaining fill of mercury and an inert gas sealed inside the
envelope, and a single composite layer coated on the inner surface of the envelope. The
composite layer is provided having at least one type of halophosphor, at least three types
of rare earth phosphors, and colloidal alumina particles in a heterogeneous mixture
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows diagrammatically, and partially in section, a fluorescent lamp having a
single composite phosphor layer according to the present invention.
Figure 2 shows a cross-section of a composite phosphor-containing layer of the present
invention coated on the inner surface of a glass envelope of a fluorescent lamp.
Figure 3 graphically shows experimental results of initial lumen performance as a
function of both coating weight and halofraction (weight % halophosphor relative to rare
earth triphoshor) for fluorescent lamps according to the present invention.
Figure 4 graphically shows experimental results of CRI as a function of both coating
weight and halofraction for fluorescent lamps according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
In the description mat follows, when a preferred range, such as 5 to 25 (or 5-25), is given,
mis means preferably at least 5, and separately and independently, preferably not more
than 25. When a range is given in terms of a weight percent (weight %) for a single
component of a composite mixture, this means mat the single component is present by
weight in the composite mixture in the stated proportion relative to the sum total weight
of all components of the composite mixture.
Figure 1 shows a representative low pressure mercury vapor discharge fluorescent lamp
10, which is generally well know in the art The fluorescent lamp 10 has a light-
transmissive glass tube or envelope 12 which has a circular cross-section. The inner
surface of the glass envelope is provided with a single composite phosphor-containing
layer 14 according to the present invention.
The lamp is hermetically sealed by bases 20 attached at both ends, and a pair of spaced
electrode structures 18 (which are means for providing a discharge) are respectively
mounted on the bases 20. A discharge-sustaining fill 22 of mercury and an inert gas is
sealed inside the glass tube. The inert gas is typically argon or a mixture of argon and
other noble gases at low pressure which, in combination with a small quantity of
mercury, provide the low vapor pressure manner of operation.
The invented composite phosphor-containing layer 14 is preferably utilized in a low
pressure mercury vapor discharge lamp, but may also be used in a high pressure mercury
vapor discharge lamp. It may be used in fluorescent lamps having electrodes as are
known in the art, as well as in electrodeless fluorescent lamps as are known in the art,
where the means for providing a discharge is a structure which provides high frequency
electromagnetic energy or radiation.
With further reference to Fig. 2, the invented phosphor-containing layer 14 comprises
halophosphors 32, rare earth phosphors 34, and colloidal alumina particles 36, all blended
together in a heterogeneous mixture of substantially uniform composition as shown in
Figure 2. Preferably, the rare earth phosphors 34 comprise a blended triphosphor system
as is known in the art, such as a blend comprising red, blue, and green color-emitting
rare earth phosphors as disclosed in U.S. Pats. Nos. 5,045,752, 4,088,923, 4,335,330,
4,847,533, 4,806,824, 3,937,998, and 4,431,941. Less preferably, rare earth phosphor
blends comprising other numbers of rare earth phosphors, such as systems with 4 or 5
rare earth phosphors, may be used.
The halophosphor particles 32 in the phosphor-containing layer 14 may comprise, for
example, mixture of calcium halophosphate activated with antimony and manganese.
Preferably, manganese is 0.5-5, more preferably 1-4, more preferably 1.5-3.5, more
preferably 2-3, more preferably 2.2, mole percent of the halophosphor mixture.
Preferably, antimony is 0.2-5, more preferably 0.5-4, more preferably 0.8-3, more
preferably 1-2.5, more preferably 1-2, more preferably 1.6, mole percent of the
halophosphor mixture. Alternatively, other halophosphor particles known in the art may
be used. The halophosphor particles are provided having a narrow particle size
distribution and substantially uniform shape, without complex structural features that
would tend to reflect ultraviolet (UV) radiation away from the phosphor particles.
Narrow particle size distribution and minimization of complex structural features
preferably are achieved via air- or wet-size classification techniques as arc commonly
known in the art, though any suitable size classification technique may be used. The
halophosphor particles 32 are provided preferably about 10, less preferably between 9-11,
less preferably between 8-12, less preferably between 7-13 micrometers in diameter, with
a minimum of fines (particles having a diameter of about 5 micrometers or less),
preferably not more than 5, more preferably 4, more preferably 3, more preferably 2,
more preferably 1, more preferably 0.5, percent fines.
The rare earth phosphor particles 34 (preferably a mixture of triphosphors as is known in
the art) are likewise provided having a narrow particle size distribution and uniform
shape via size classification techniques, having a minimum of complex structural features
that would tend to reflect UV radiation away from the phosphor particles. Preferably, the
rare earth phosphor particles are provided having a size distribution between 3-5, less
preferably 3-6, less preferably 2-6, less preferably 1-6 micrometers in diameter.
The phosphor-containing layer 14 is 0.05-40, more preferably 0.1-30, more preferably
0.2-20, more preferably 0.3-20, more preferably 0.4-15, more preferably 0.5-10, more
preferably 1-10, more preferably 2-8, weight percent alumina. The alumina particles in
the phosphor-containing layer 14 are of a range of particle sizes, preferably 10-1000,
more preferably 12-800, more preferably 14-600, more preferably 16-400, more
preferably 18-300, more preferably 20-200, more preferably 30-150, more preferably 50-
100, nanometers in diameter, and are uniformly size distributed throughout the phosphor-
containing layer 14. The alumina particles beneficially reflect UV radiation toward
phosphor particles where it may be utilized, leading to improved phosphor utilization and
more efficient production of visible light. In this manner, the alumina particles 36
minimize UV emission from the fluorescent lamp 10 and maximize the utilization of the
rare earth triphosphors 34, achieving maximum lamp efficiency with a lower proportion
of expensive rare earth phosphors 34.
The three principal components of the phosphor-containing layer 14 (halophosphor
particles, rare earth phosphor particles, and colloidal alumina particles as described
above) preferably are packed to a maximum bulk density in a substantially nested
configuration based upon the three modes of particle size characteristic of the three
different types of particles. Specifically, the small alumina particles, having colloidal
size or dimension, fill in the void spaces (pores, crevices and cavities) between the rare
earth phosphor particles which are several orders of magnitude larger in dimension or
diameter than the alumina particles. The rare earth triphosphor particles, in turn, are
tightly packed against the larger halophosphor particles to achieve maximum filling of
the void space between the larger halophosphor particles, thereby achieving maximum
qensity in the phosphor-containing layer 14. The resulting composite mixture is
preferably of uniform bulk density, particle composition and size distribution.
The lamp of the present invention is made without a discrete or separate boundary layer
of alumina particles as known in the prior art, and is made without a second coating of
phosphors or a second phosphor-containing layer. In addition to greatly reducing labor
and equipment costs compared with the three-coat design of the prior art, the single-coat
composite phosphor-containing layer 14 of the present invention significantly reduces the
variability in performance characteristics. An experiment was performed comparing an
F40T12 SP35 fluorescent lamp of the prior art having discrete halophosphor and rare
earth triphosphor layers, with a similar lamp having a single composite phosphor-
containing layer 14 according to the present invention. The color rendering index and
lumens after 100 hours were measured for both lamps. The results are tabulated below.
Lamp________CRI Lumens, 100 hours
Avg. S.Dev. Avg. S.Dev.
SP35 Dual-Coat 71.3 2.4 2750 50
SP35 Single-Coat 74.0 0.2 2750 25
As seen above, the single-coat lamp exhibited comparable average performance relative
to the dual-coat lamp. However, the variability in both CRI and lumens were
significantly decreased in the single-coat design. The single-coat lamp exhibited only a
0.27% standard deviation in CRI, compared with 3.37% for the dual-coat lamp,
approximately corresponding to a 12-fold decrease in CRI variability. Further, the
variability in 100-hour lumens was reduced by half for the single-coat lamp. Such a
significant reduction in CRI variability, as well as lumen variability, was surprising and
unexpected. Reduction in variability of both CRI and lumens is key to providing
customer satisfaction and coating cost control.
The relative proportion of halophosphors to rare earth phosphors in the phosphor-
containing layer 14 is determined by cost, lumen, color and CRI constraints relative to a
particular application. For example, relative compositions in the range of 50-99, 50-95,
50-90, 50-85, 50-80, 50-75, 50-70, 50-65, or 50-60 weight percent halophosphor (with
the balance being rare earth phosphors and colloidal alumina) may be used. A relative
composition of between 50-70 weight percent halophosphor and between .5-10 weight
percent colloidal alumina has been found to be sufficient in achieving medium
performance in General Electric's F40T12 SP35 and SP41 fluorescent lamps. The
phosphor-containing layer 14 is preferably 5-50, more preferably 10-50, more preferably
20-40, more preferably 30-40, more preferably 30-35, weight percent rare earth
The composite phosphor-containing layer 14 is provided having a coating weight
preferably between 2-10, more preferably 3-8, more preferably 4-6, more preferably
3.40-7.00 mg/cm2. Coating weights outside the above range may be used to enhance
lamp performance for a particular application. A principal advantage of the present
invention is that a lamp comprising a single composite phosphor-containing layer 14 can
be tuned to achieve the desired CRI for a particular application. In the dual-coat design
of the prior art, CRI is a strong function of coating weight making it extremely difficult to
tune a lamp to a desired CRI without compromising lumens. In the single-coat design,
however, coating weight and the proportion of halophosphors to rare earth triphosphors
can be tuned to provide a lamp having specific performance characteristics for both CRI
The invented lamp preferably has a CRI of at least 62, preferably 65, preferably 68,
preferably 70, preferably 72, preferably 73. The invented lamp preferably has a lumen
output of at least 77.5, preferably 78, preferably 78.5, preferably 79, preferably 79.5,
preferably 80, lumens/watt. For example, for a 40-watt lamp according to the present
invention, lumen output is preferably at least 3100, preferably 3120, preferably 3140,
preferably 3160, preferably 3180, preferably 3200, lumens. The invented phosphor-
containing layer 14 is preferably used in medium performance SP-type lamps, for
example SP30, SP35, SP41, SP50, or SP65 fluorescent lamps. Optionally, the invented
phosphor-containing layer may be utilized in other medium performance lamps known in
the art, as well as in high performance lamps, for example General Electric's SPX-type
Referring to Figures 3 and 4, experiments were conducted with 9 specially prepared
F40T12 mercury vapor discharge fluorescent lamps having coating weights and
halophosphor proportions (halofractions) as shown in the following table. The colloidal
alumina content in the composite coating was fixed at 5 weight percent for all lamps. All
coating weights are in mg/cm2, and rare earth triphosphors made up the balance of the
Lamp Coating Weight % Halophosphor
1 3.40 85
2 5.20 85
3 7.00 85
4 3.40 70
5 5.20 70
6 7.00 70
7 3.40 55
8 5.20 55
9 7.00 55
Figure 3 shows the lumens resulting from each of the 9 F40T12 lamps and, via computer
simulation, interpolated lumen performance within the entire range of halofractions
tested. As can be seen from the figure, the present invention allows ease of lumen design
by varying either halofraction or coating weight.
Figure 4 was generated in similar manner to Figure 3, and shows CRI as a function of
halofraction and coating weight within the experimental range. As the figure indicates,
CRI is virtually independent of coating weight in the single-coat phosphor-containing
layer 14 of the present invention. This coating-weight independence is a significant
advance over the dual-coated phosphor layers of the prior art, where CRI is strongly
dependent upon coating weight. Coating-weight independence allows lumen output to be
extremely finely tuned by varying coating weight without sacrificing CRI. Consequently,
a lamp utilizing a single phosphor-containing layer according to the present invention has
the advantage of precise tunability to a specific application without sacrificing other
untuned performance characteristics.
A composite phosphor-containing layer 14 as described above eliminates the need for a
separate alumina barrier layer coating on the glass envelope 12 as required by the prior
art. In the present invention, the phosphor-containing layer 14 is coated on the interior
surface of the glass envelope 12, in direct contact therewith. In addition, by blending
halophosphors and rare earth triphosphors into a single heterogeneous mixture of
substantially uniform composition, the dual-coating technology of the prior art is
replaced with a single phosphor coating that is effective in providing similar medium
performance in fluorescent lamps at greatly reduced production and equipment cost. The
composite phosphor-containing layer 14 of the present invention effectively combines a
three-step process, requiring three discrete coating applications, into a single coating that
is applied in a single step.
The composite phosphor-containing layer 14 is prepared as a codispersion of
halophosphors and rare earth triphosphors in an aqueous vehicle containing colloidal
alumina as described above. The rheological properties of this coating formulation are
controlled during the production and application processes in the following manner. The
colloidal alumina particles which are provided in a range of particle sizes, e.g. 20-200
nanometers as described above, beneficially induce mild electrostatic stabilization of the
halophosphor and rare earth triphosphor particles of different size, thereby inhibiting
ordering by size which could lead to color flooding in the finished lamp product. The use
of colloidal alumina in this manner is preferable to the use of polyelectrolyte dispersants
which can induce particle ordering by size. Additionally, the coating formulation is kept
slightly acidic, ideally between pH 5-7, to assure the colloidal alumina exhibits sufficient
surface charge to act as an effective mild dispersant in the phosphor dispersion.
Preferably, hydrochloric or nitric acid is used to maintain suitable coating formulation
pH, though any suitable acidic reagent can be used. A preferably nonionic thickener,
preferably polyethylene oxide having a molecular weight in the range of 200,000 to
1,000,000 gm/mol is used in the formulation as a viscosity controlling additive.
Surfactant additives are also preferably nonionic, and are added to control coating
leveling and improve wetting of the glass tube 10. Surfactants are preferably selected
from the class of nonylphenyl ethoxylates, though any suitable nonionic surfactant can be
used. Acrylic-based thickeners and dispersants as are commonly used in the prior art are
avoided, thereby eliminating the well known problem of ammonia emissions in the
manufacturing environment associated with ammonia-neutralized acrylics.
While the invention has been described with reference to a preferred embodiment, it will
be understood by those skilled in the art that various changes may be made and
equivalents may be substituted for elements thereof without departing from the scope of
the invention. In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without departing from the essential
scope thereof. Therefore, it is intended that the invention not be limited to the particular
embodiment disclosed as the best mode contemplated for carrying out this invention, but
that the invention will include all embodiments falling within the scope of the appended
1. An improved mercury vapor discharge lamp (10) comprising a light-
transmissive envelope (12) having an inner surface, means for providing a
discharge (18), a discharge-sustaining fill (22) of mercury and inert gas
sealed inside said envelope (12), and a single composite layer (14) coated
inside said envelope (12), said composite layer comprising at least one
halophosphor (32), at least three rare earth phosphors (34), and colloidal
alumina particles (36) in a heterogeneous mixture.
2. The lamp (10) as claimed in claim 1, wherein said heterogeneous mixture
is of uniform composition.
3. The lamp (10) as claimed in claim 1, wherein said means for providing a
discharge (18) comprise electrodes.
4. The lamp (10) as claimed in claim 1, wherein said means for providing a
discharge (18) comprises a structure that provides high frequency
5. The lamp (10) as claimed in claim 1, wherein said composite layer (14) is
0.05 to 40 weight percent colloidal alumina particles (36).
6. The lamp (10) as claimed in claim 1, wherein said composite layer (14) is
50 to 99 weight percent halophosphors (32).
7. The lamp (10) as claimed in claim 1, wherein said composite layer (14) is
5 to 50 weight percent rare earth phosphors (34).
8. The lamp (10) as claimed in claim 1, wherein said colloidal alumina
particles (36) present in said composite layer (14) have a range of mean
diameters in the range of 10 to 1000 nm.
9. An improved mercury vapor discharge lamp (10) as claimed in claim 1,
wherein said halophosphor particles (36) present in said composite layer
(14) have a mean diameters in the range of 7-13 nm.
10. The lamp (10) as claimed in claim 9, wherein said halophosphor particles
(32) comprise not more than 5% fines having a mean diameter of 5 mm
11. The lamp (10) as claimed in claim 9, wherein said halophosphor particles
(32) comprise calcium halophosphate activated with antimony and
12. An improved mercury vapor discharge lamp (10) as claimed in claim 1,
wherein said composite layer (14) is packed in a nested particle
configuration, wherein said colloidal alumina particles (36) substantially fill
in the void spaces between said rare earth phosphors (34), said rare
earth phosphors (34) substantially filing in the void spaces between
adjacent halophosphors (32).
13. The lamp (10) as claimed in claim 1, wherein said rare earth phosphors
(34) present in said composite layer (14) are of substantially uniform
surface configuration having a mean diameter in the range of 1-6 mm.
14. The lamp (10) as claimed in claim 12, wherein said rare earth phosphors
(34) comprise a blended triphosphor system comprising red, blue and
green color-emitting phosphors.
15. The lamp (10) as claimed in claim 1, wherein said composite layer (14) is
formed as a coating formulation comprising a dispersion of halophosphors
(32), rare earth phosphors (34), and colloidal alumina particles (36) in an
aqueous vehicle further comprising nonionic thickeners and nonionic
16. The lamp (10) as claimed in claim 1, comprising a CRI of at least 62.
17. The lamp (10) as claimed in claim 1, comprising a lumen output of at
least 77.5 lumens/watt.
The invention relates to an improved mercury vapor discharge lamp (10)
comprising a light-transmissive envelope (12) haying an inner surface, means for
providing a discharge (18), a discharge-sustaining fill (22) of mercury and inert
gas sealed inside said envelope (12), and a single composite layer (14) coated
inside said envelope (12), said composite layer comprising at least one
halophosphor (32), three rare earth phosphors (34), and colloidal alumina
particles (36) in a heterogeneous mixture.
|Indian Patent Application Number||IN/PCT/2002/746/KOL|
|PG Journal Number||40/2009|
|Date of Filing||03-Jun-2002|
|Name of Patentee||GENERAL ELECTRIC COMPANY|
|Applicant Address||1, RIVER ROAD, SCHENECTADY, NEW YORK|
|PCT International Classification Number||H01J 61/44, 61/46|
|PCT International Application Number||PCT/US2001/29670|
|PCT International Filing date||2001-09-21|