CA1207005A

CA1207005A - Long life, warm color metal halide arc discharge lamp

Info

Publication number: CA1207005A
Application number: CA000420345A
Authority: CA
Inventors: Elliot F. Wyner; W. Calvin Gungle
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1982-03-01
Filing date: 1983-01-27
Publication date: 1986-07-02
Also published as: EP0087745A1; JPS58158854A

Abstract

LONG LIFE, WARM COLOR METAL HALIDE ARC DISCHARGE LAMP

ABSTRACT

A warm color, long life metal halide arc discharge lamp employs a phosphor mixture on the interior surface of the outer jacket. The mixture comprises from 40-60% manganese activated magnesium fluorogermanate and 40-60% europium activated yttrium orthovanadate. The color temperature of the lamp is between 3200 -3300°K and the C.R.I. is 74.9.

Description

D-24,212 ~C37(~5 -LONG LIFE, WARM C0LOR METAL HALIDE ARC DISCHA~GE LAMP

TECHNICAL FIELD

This invention relates to metal halide arc dischar9e lamps and more particularly to such larnps which provide a warm color temperature approximating that of incandescent lamps.

BAC ~ROUND ART

Metal halide arc discharge lamps comprise an arc tube containin9 mercury and a plurality of metal halides, usually the iodides of, for example, sodium and scandium, and a starting gas such as argon.
These lamps are well known and are frequently employed in Commercia establishments because of their long life (about 10,000 to 20,000 hours depending on wattage) and their high efficacy. The efficacy of lamps is usually measured in lumens/watt, and the metal halide arc lamps have efficacies in the neighborhood of 80 to 125 lumens/watt, again, depending on the wattage. In spite of all the favorable characteristics of these lamps, their use is sometimes contra-indicated where good color rendition is necessary because of their relatively high color temperature, i.e., over 4400D Kelvin (K). Changes in color temperature to lower values necessitateS ar increased ratio of red radiation to blue radiation without a significant change in green radiation.
Attempts, not altogether successful~ to improve the color of visible radiation have included adding other metals~ such as lithium iodide, to the arc stream, and phosphor coating the interior of the outer jacket that surrounds the arc tube. The latter technique has frequently been employed with high pressure mercury discharge lamps. (See, for example, U.S. Patent Nos. 3,825,792 and 4,241,276.) Phosphor coated metal halide lamps also are available but with color temperatures of only 4000D~.
Another recent attempt to lower the color temperature of such tubes involved increasing the arc tube loading. This raises the 12~7~)5 temperature of the arc tube wall which, in turn, increases -the pressure obtainable in the gaseous phase, especially that of the sodium and mercury. The effect on the discharge is twofold. First, the core temperature decreases which reduces atomic mercury radiation creation and giYes the lamp a lower color temperature.
Second~ the sodium is pressure broadened further into the red ~hich further reduces the col~r temperature. However, penalties are suf,ered when the arc tube is too highly loaded. For ~xample, such a lamp would have a loadi~g of 20.7 wattslcm2 wherea5 conventiona metal halide lamps have a loading of 12.4 watts/cm2. The high loading apparently requires removing the lamp starting pro~e from the arc tube. This necessitates a costly external starting circuit employin~ a probe in close proximity to the arc tube. This conditi~n potentially leads to sodium electrolysis through the arc tube wall. Additionally, the increased ~all temperatures accelerate life limiting reactions of th~ additives with the arc tube wall.
Such reactions can cause catastrophic failure within 4000 hours; an unacceptable condition.
It is, therefore, an object of this invention to obviate or substantially reduce the disadvantages of the prior art.
It is another object of the invention to enhance metal halide arc discharge lamps.
Yet another object of the invention is the provision of a metal halide arc discharge lamp having a low color tempera-ture and a long life.
According to the present invention there is provided in a hi~h pressure metal halide arc discharge lamp of the type having an arc tube containing therein an inert gas, the halides of sodium and scandium, and mercury, said arc tube providing a light output having a color temperature greater than 4400 Kelvin and a spectral response which includes radiation in the ultraviolet and blue regions of the spectrum but being sub-stantially de~icient in the red region of said spectrum, saidarc tube being disposed within an outer jacket having a lumin-escent coating on ~ne inner wall thereof, the improvèment wherein said luminescent coating comprises a first phosphor which absorbs radiation in the blue region of said spectrum and emits radiation in the red region, and a second phosphor ~21:3700S

which absorbs radiation in the ultraviolet region of said spectrum and emits radiation in the red region.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: -FIG. 1 is a diagrammatic view, partially in section, of5 a lamp according to this embodiment; and FIG. 2 is a combined spectral distribution curve illus-trating the spectra of an uncoated prior art lamp and the coated lamp of the embodiment.

Referring now to the drawings with greater particularity, the rnetal halide arc discharge lamp o~ FIG. 1 comprises a mercury containing arc tube 1 ~Jithin a glass outer jacket 2. The arc tube 1 contains an inert starting gas, e.g., argon, and at least the halides of sodium and-scandium. Generally~ the halides are the iodides. The arc tube 1 is supported in the usual mount 3 and has the usual electrical connections to base 4.
The dotted line spectrum of FIG. 2 represents the light output of the arc tube 1, when operating. As can be seen therefrom the spectrum has strong peaks in the blue region but is relatively deficient in the red. Not shown in the spectrum is the strong peak in the ultraviolet (at 365 nm) which occurs from the mercury discharge. Such an arc tube, when operated in a clear glass BT37 jacket at 400 watts, has a light output of about 38410 lumens; a color temperature of about 4465K; and a color rendering index (C.~.I.) of 63Ø

~7~(35 The lamp is formed by applying to the interior surface of jacket 2 a phosphor coating S. Tne phosphor coating 5 comprises a mixture of a first phosphor which absorbs radiation in the blue region and emits in the red; and a second phosphor which absorbs radiation in the ultraviolet and also emits in the red. The mixture comprises from 40 weight percent to 60 weight percent of the first phosphor with the balance being the second phosphor. The preferred range is 50% of each phosphor.
In a preferred embodiment the first phosphor is a manganese activated magnesium fluorogermanate (Mg4(F) GeO6 : Mn) (Sylvania type 236 or 2361) and the second phosphor is europium activated yttrium orthovanadate (YV04 : Eu) (Sylvania type 2390 or 2391).
The preferred mixture includes 1.5 grams of each phosphor.
The spectrum of the lamp employing phosphor coating 5 is shown by the solid line in FIG. 2. The differences between the two spectra are clearly apparent and show the additional emission peaks in the red region (at about 620 nm and 658 nm) with a concurrent suppression of the blue emission. The overall result provides a lamp color temperature of about 3266K and a marked improvement in color rendering with a C.R.I. of 74.9. There is a slight but tolerable drop in lumens to about 36820.
Thus, there is provided a metal halide arc discharge lamp ha~ing a desirable, low color temperature and high C.R.I. The result is accomplished without disturbing the complicated, but proven, chemistry of a conventional metal halide arc tube, and the typical life expectancies of such lamps are maintained.
`~hile there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope o~
the invention as defined by the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a high pressure metal halide arc discharge lamp of the type having an arc tube containing therein an inert gas, the halides of sodium and scandium, and mercury, said arc tube providing a light output having a color temperature greater than 4400° Kelvin and a spectral response which includes radiation in the ultraviolet and blue regions of the spectrum but being substantially deficient in the red region of said spectrum, said arc tube being disposed within an outer jacket having a luminescent coating on the inner wall thereof, the improvement wherein said luminescent coating comprises:
a first phosphor which absorbs radiation in the blue region of said spectrum and emits radiation in the red region, and a second phosphor which absorbs radiation in the ultraviolet region of said spectrum and emits radiation in the red region.

2. The improvement according to Claim 1 wherein said first phosphor of said luminescent coating is a manganese-activated phosphor and said second phosphor is a europium-activated phosphor .

3. The improvement according to Claim 2 wherein said first phosphor includes magnesium fluorogermanate and said second phosphor includes yttrium vanadate.

4. The improvement according to Claim 3 wherein said first phosphor comprises about forty percent by weight to about sixty percent by weight of said luminescent coating and said second phosphor comprises the balance thereof.

5. The improvement according to Claim 3 wherein said first phosphor comprises about fifty percent by weight of said phosphor coating.

6. The improvement according to Claim 2 wherein said second phosphor includes yttrium orthovanadate.

7. The improvement according to Claim 1 wherein said lamp does not include a non-luminescent coating on the interior surface of said outer jacket.