Electric lamp and discharge devices – With luminescent solid or liquid material – With gaseous discharge medium
Reexamination Certificate
1998-09-28
2001-11-06
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
With gaseous discharge medium
C313S486000, C313S635000
Reexamination Certificate
active
06313578
ABSTRACT:
TECHNICAL FIELD
This invention is related to phosphor coatings for gas discharge lamps. In particular, this invention is related to the binder materials used in phosphor coatings for gas discharge lamps.
BACKGROUND ART
Examples of phosphor coating methods for gas discharge lamps are described in U.S. Pat. Nos. 3,551,180, 2,987,414, 4,340,512, and in European Patent Application EP 0 479 300 A1. Generally, the phosphor particles contained in the coating layer do not adhere well by themselves to the glass envelopes used in gas discharge lamps without the aid of a suitable binding agent. In fluorescent lamps, the binding agent consists of a polymeric material, such as polyethelyene oxide, and a finely divided, high surface area, aluminum oxide. A generally preferred aluminum oxide binder is Aluminum Oxide C (available from Degussa) which is a gamma aluminum oxide, &ggr;-Al
2
O
3
, having a particle size of about 20 nm. An aqueous coating suspension containing these binding agents and an ultraviolet (UV) stimuable phosphor is applied to the interior surface of the glass envelope and dried to form a phosphor coating layer. The polymeric binder is removed from the coating layer during a subsequent high temperature lamp baking operation. The aluminum oxide binder is not removed during the subsequent processing steps and remains in the phosphor coating layer of the finished lamp.
The presence of aluminum oxide in the phosphor layer does not pose a significant problem for fluorescent lamps. For example, &ggr;-Al
2
O
3
is nearly transparent to the 254 nm resonance radiation generated by the mercury discharge. (The optical gap or &agr;-Al
2
O
3
is in the vicinity of 200 nm and shifts to longer wavelengths, >185 nm, for &ggr;-Al
2
O
3
.) Thus, the aluminum oxide improves the adherence of the coating layer without absorbing the UV radiation used to excite the phosphor. This situation changes however for other types of gas discharge lamps which utilize UV radiation occurring in the vacuum ultraviolet (VUV), region, less than about 170 nm (e.g., Xe excimer and neon gas discharge lamps). In those lamps, the aluminum oxide in the phosphor coating absorbs VUV radiation emitted from the gas discharge. Unlike the phosphor in the coating, the VUV radiation absorbed by the &ggr;-Al
2
O
3
is not converted to visible radiation but is instead dissipated through non-radiative loss processes associated with the bulk material. Hence, in VUV applications, the presence of aluminum oxide in the phosphor coating causes a reduction in lamp efficacy. If the aluminum oxide binder is removed from the phosphor coating, the lamp efficacy increases but the coating easily falls off the lamp envelope. Thus, it would be an advantage to have a binding agent which provides adherence characteristics similar to finely dividied, high surface area, aluminum oxide without causing a significant reduction in lamp efficacy.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the disadvantages of the prior art.
It is an object of the invention to provide a phosphor coating having an inorganic binder which exhibits adherence characteristics comparable to Aluminum Oxide C.
It is a further object of the invention to provide a gas discharge lamp having increased efficacy.
In accordance with one object of the invention, there is provided a phosphor coating for a gas discharge lamp comprising a primary phosphor and an inorganic binder, the inorganic binder comprising a nano-size phosphor, and the primary phosphor emitting visible light upon stimulation by VUV radiation.
In accordance with another object of the invention, there is provided a gas discharge lamp comprising an envelope formed of a light transmissive material, the envelope having a wall defining an enclosed volume, the enclosed volume containing a gas fill, the gas being capable of emitting VUV radiation when stimulated, and a phosphor coating comprising a primary phosphor and an inorganic binder, the inorganic binder comprising a nano-size phosphor, and the primary phosphor emitting visible light upon stimulation by VUV radiation.
In accordance with still another aspect of the invention, the nano-size phosphor in the phosphor coating has a total VUV absorbance over a range of VUV wavelengths which is less than the total VUV absorbance of gamma aluminum oxide over the same wavelength range.
REFERENCES:
patent: 2987414 (1961-06-01), Martyny
patent: 3551180 (1970-12-01), Margolis
patent: 4340512 (1982-07-01), Schreurs
patent: 5455489 (1995-10-01), Bhargava
patent: 5460701 (1995-10-01), Parker et al.
patent: 5518808 (1996-05-01), Bruno et al.
patent: 5565741 (1996-10-01), Jennato et al.
patent: 5650691 (1997-07-01), Jansma
patent: 5923118 (1999-07-01), Jennato et al.
patent: 0 479 300 A1 (1992-04-01), None
Evans Jean M.
Mishra Kailash C.
Peters Thomas E.
Rothwell, Jr. Harold
Clark Robert F.
Day Michael H.
Osram Sylvania Inc.
Williams Joseph
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