Note: Descriptions are shown in the official language in which they were submitted.
D-24,212 ~C37(~5
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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
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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
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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
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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.
