Note: Descriptions are shown in the official language in which they were submitted.
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LOW ~ATTAGE METAL HALID LAMP
TECHNICAL Fl~LD:
This invention relate~ to low ~att,age ~etal halide la~p~
and more particularly to ~e configuration and chemical fill of
low wattage metal halide lamps.
BACKGROUND ART:
Generally, metal halide disc~arge lamps av~ilable in todays
market are of the intermediate or high wattage variety,
i.e.. 175 to 1500 ~atts for example. Al~o, these h;gher
wattage metal halide lamps have t~e higher efficacy which is
directly related to efficiency and conveniently defined as the
ratio of total lumen output to input power as expre~,sed in
lumens per watt. Moreover, it is known that the efficacy of a
lamp tend~ to decrease as the wattage decreases. Thus, it ha~
been generally presupposed that low wattage metal halide lamp6,
wattages of 100-watts or le~s~ would be entirely unsatisfactory
in ~o far as efficacy is concerned.
Another common practice in intermediate and rela~iYely ~igh
wattage metal halide lamps is ~he provision of an iner~ fill
gas in the outer envelope surrounding the arc tube.
Accordingly, it was believed that the inert fill ga~ would
prevent oxidation of the metal parts located in the outer
envelope and increases the breakdown voltage whereby arcing
would be inhibited. However, it was ~ound that the undesired
loss of heat due to on~ection currents made po66ible by the
presence of the inert ga~ fiignificantly reduced the eficacy of
the di~charge lamp.
Known attempts ~o reduce this undesired heat loss due to
conve~tion current~ include the utilization of a gla~s cylinder
surrounding the arc tube within ~he sealed ou~*r envelope.
However, structures which include ~ f ill gas in the outer
envelope do have undesired convection currents. ~oreover~
these convection curren~ and ~he accompanying ~eat loss are
~, ~
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pre~ent e~en though a gla~ cylinder like arrangement i6
employed.
Further, the 6mallest known dome~ti~ commerci~lized metal
halide di~charge lamp i~ a Sylvania 175 watt l~mp for~ed for
horizontal operation only and having a color tempera~ure of
about 3000 X. Thi~ l~mp ha~ a pho~p~or-coated outer envelope
which tran~for~ W radiation into visible red radi~tion
thereby creating a relatively warm color appearance. However,
the lamp undesirably re~uire~ a relatively large and cu~ber~ome
luminaire for adequate light di~tribution control, in addition,
ha~ ~e disadvantage of being operable in a horizontal po~ition
only.
OBJ~CTS AND 5UMMARY OF THE INVENT ON:
An object of the present invention i~ to o~ercome the
dif f icultie~ of the prior art. Another object of the invention
i~ to provicle an improved me~al halide discharge lamp having
relatively low wattage and reduced heat lo~es due to
convection current6. Still another object of the invention is
to provide a low wat~age hig~ efficiency metal halide di~charge
lamp of ~mall ~ize ana having a color temp~rature of abou~
3000K. A further object of the invention i~ to provide a
chemical fill for a low wat~a~e relatively high efficacy metal
halide di~charge lamp.
The~e and other object~, advantage6 ~nd capabilitie~ are
achieved in one a~pect of the invention by a low wattage metal
halide di~charge lamp havi~g an outer glass envelope with a
pair of electrical conductor~ ~ealed into and pa~sing
therethrough, an arc tu~e dispo~ed wit~i~ the envelope and t~e
arc tube having a pair of ~paced electrode~ with each of the
elec~rode~ electrically connectea to one of the pair of
electrical conductors and a chemical fill within the arc tube
including ~odiu~ and ~candium iodide~ of a molar ratio in ~he
range of about 20:1 to 28:1.
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BRIEF DESC_IPTION OF THE DRAWINGS:
FIG. 1 i6 a cro~-6ectional vie~ of a low wattage metal
halide di~charge lamp ~f the invention;
PIG. 2 is chromaticity coordinate chart for molar ratio~ of
~odium iodide to ~candium iodide;
~ IG. ~ is a chart illu6trating the color temperature (Tc)
and color rendering index ~CRI) for molor ratio~ of ~odium
iodide and 6candium iodide: and
~ IG. 4 is a comparison chart illustrating the lumens per
watt of low wattage metal ~alide discharge lamp~ at variou6
~odium to ~candium iodide molar ra~io~.
BEST I~ODE FOR CARRYING OUT THE INVENTION:
~ or a better understanding of the pre~ent invention.
together with other and furt~er o~jects, adva~tages and
capabilitie~ thereof, reference is made to the followin~
disclo&ure and appended claims in conjunction with the
accompanying drawings.
Referring to FIG. 1 of the drawings, a low wattage metal
halide a c discharge lamp 5 importantly include an evacuated
outer envelope 7. Thi~ evacuated outer envelope 7 i~
her~e~ically æealed to a gla6s stem member 9 having an external
base member 11 affixed thereto. A pair of elec~rical
conductors 13 and 15 ar~ sealed into and pas~ through the ~tem
membe~ 9 and provide acces~ for e~ergization of the di~charge
lamp 5 by an external ~ource (no~ 6hown).
~ i~hin the vacuum of the evacuated outer envelope 7, a
support membe~ 17 i~ affixed to o~e of ~be ~lectrical
conductors 13 and extend~ ~ub6tantially parallel to the
longitudinal axi~ of t~e lamp 5 and forms a circular
configuration 19 near the upper portion of the enYelope 7.
Thi~ circular configuration 19 in conjunction with the upper
portion of the envelope 7 tend6 to maintain ths 6upport member
17 in proper alignment and resi~tant ~o de~ormation cau6ed by
external sho~k.
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~ irfit ~trap ~ember 21 is welded to the ~upport member 17
and e~tends therefrom in a direction normal to the longitudinal
axi~ and the direction of the ~upport member 17. A domed
guart2 ~leeve or ~emperature equalizing mean~ 23 ha~ a pair of
sppo~i~ely di~posed notche6 Z5 and 27 on the and thereof 28
opposite to the dome portion. The~e notche6 25 and 27 are
formed to ~lip over the first 6trap member 21 which 6erve~ to
~upport the domed quartz ~leeve 23. Also, a sub~ta~tially
circular ~haped ~trap 29 ~urround~ the domed quartz sleeve 23
near the domed portion thereof and i~ attached to the ~upport
me~bar 17.
Within the temperature equalizing ~eans or domed quar~z
sleeve 23 i~ an arc tube 31 having a chemical fill including
elemental ~candium and mercury, ~odium~ 6candium and ce6ium
iodides and an inert gas. The arc tube 31 ha~ a pinch &e~l at
opposite end6 t~ereof, 33 and 35 re~pectively. Metal foil
members 37 and 39 are ~ealed into the press ~eals 33 and 35 and
elec~rical conductor& 41 and 93 are attac~ed to the foil
member~ 37 and 39 and extend outwardly from the pre~6 ~eal~ 33
and 35. A flexi~le support member 45 i6 affixed to one of the
electrical conductor6 41 and to the ~upport member 17. Al~o,
lead 47 is afixed to the other electrical conductor 43 which
pa~se~ through the domed portion of t~e domed quart2 sleeve
23. ~oreov~r, a flexible spring-lika member 49 connects the
lead 47 to the other one 15 of the pair of electrical
conduc~ors 13 and 15. A pair of getter~ 51 and 53 are affixed
to the electrical csnductor6 13 and 15 and ~ar~e to provide and
maintai~ the vacuum within the e~acuated outer enYelope 7 and
the domed quartz ~leeve 23.
Referring to the arc tube ~1, a preferred configura~ion,
suitable or u~e in a metal halide lamp of a si2e in the ra~ge
of about 40 to 150 watt~ for example, would have an inner
diameter of abou~ lOmm and an arc leng~h ~etween the electrode~
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41 and 43 of a~out 14~m. Also. it may be noted tbat ~ach of
the end~ of the arc tube 31 immediately adjacent and including
the pres~ seal~ 33 and 35 i6 coated wit~ a ~hite zirconium
oxide paint in order to provide a wall temperature of i~crea~ed
unifor~ity. Moreover, it ha~ been found that a wall loading in
the range of about 14 to 17 watt6~m2 i~ preferable and now
attainable in ~etal ~alide la~ps of a 6ize in the range o
ab~ut 40 to 150-watt~, and of t~e abo~e-mentionsd configuration.
Referring to the chemical fill o-f the above-mentio~ed
dixcharge lamp configuration, the comparison graph of FIG. 2
illu~trate~ ~ariou6 ratio~ of ~odium and ~candium-iodides as
plo~ed on chromaticity coordinate~ (x and y) of a standard
chro~aticity chart. As can readily be seen on the graph, the
molar ratio of sodium to scandium iodide which most closely
approaches the hig~ly desirable black body (BB) cur~e
representatiYe of the output of an incandescent lamp i6 a molar
ratio in the range of about 20:1 to 28:1. More specifically, a
sodium to ~candium iodide molars ratio of about 24:1 appear~ to
be a highly desirable fill condition for metal halide di~charge
lamps. Moreover, i~ is to be noted that ~he above-mentioned
highly desirable sodium to scandium iodide molar ratios a~e
biased toward the red side ~below the BB curve~ which i~
preferable in terms of general illumination application~.
Al60, the color temperatures ~Tc) and general color
rendering index (CRI) for the above-mentioned lamps having
various molar ratios is illustrated in FIG. 3. As can be &een,
the previou~ly-mentioned de~irable molar ranse of about 20:1 to
28 :1 c~f ~odium to 6candium iodide6 provide6 a desired color
temperature of about 3000K within a range of not ~ore or les~
t~an about 200K. ~oreover, the 60dium to ~candium iodide
molar ratio of about 24:~ appears to very clo~ely approac~ the
de~ired 3000K color temperature.
84~ 81
Fur~her, t~e compari~on graph of FIG. 3 al60 illus~ra~e6
~he cons~ant color xendering index ~C~I) at variou~ molar
ra~ios of 60dium to 6candium iodide. Again, it can be seen
that a molar ratio of about 24:1 appro~iches a hi~hly de~irable
value of about 65.0 on t~e CRI index w.ith the ran~e of molar
ratios of about 20:1 to 28~1 not varyil~g rom t~e preferred
value of 65.0 by not ~ore or le~6 than about 2.0 indice~.
~ lso, the lamp ~fficiency a~ the above-mentionecl ~arying
molar ratio~ of ~odium to scandium iod-;2es i~ illustrated in
FIG. ~. Herein it can be ~een tha~ a sodium to scandium iodide
molar ratio of about 24~1 provide~ a desirable efficiency of
about 100 lumen~ per watt (LP~). Moreover, thi6 de~irable 100
LPW capability remain~ sub~tantially co~6tant over a sodium to
~candium iodide molar ratio in the range of about 20:1 to 2~:1.
Further, tests were run to determine an optimum dosage of
sodium to scandium iodide at a molar ratio of about 24:1.
Employing a lamp having an arc tube volume of about 1 cm~ and
varying to ~alt dosage from about 8 to 20 mg, it was found ~hat
a dosage of about 12 mg/cm3 at the preferred molar ratio of
about 24~1 yielded the highest lumen~ per watt ~nd color
re~dering index (CRI). At do~ayes of a weight less than about
12 ~g, it wa~ found that the desired color rendering index
(CRI) could not be ~ustained while dosage~ greater than abou~
12 mg resulted in poor lumen mainte~ance and a 6hortened lamp
life. ~oreover, it was found that a dosage of about
12 mg~cm3 wa~ m~st appropriate for lamp~ in the range of
about 40 to 150 watts.
Additionally, it ha6 been found tnat the incorpora~ion o$
cesium iodide into ~he lamp dosage enhance~ ~he starting time
of the di~charge lamp. Upon the inclufiion of ce~ium iodide in
~e range of about 0.3 to 1.0 mg i~ the above-men~ioned lamp6,
it was found that the lamp ~tar~ing could be reduced from an
average of about one to one and one-half (1~ 2) minute~
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witbout the ce~ium iodide to virtual instanta~eou6 ~tarting
(less than 1-6ec) w~en a do~age of about 0.5 ~g was employed.
Lesser amounts of cesiu~ iodide provide much 6~aller
improvements in starting time while greater amount~ tended ~o
degrade lamp e~fi~iency and warm color ~haracteristics. Thus 7
~he preferred dosage for a metal halide dischdrge lamp ha~i~g a
volume of about 1 cm3 and a wattage in the range of about 4~
'co 150 watt6 includes about 12mg of sodium, scandium and cesium
iodides in the molar ratio of about 24 :1 : O. 6.
~ n addition to the a~ove-mentioned components, it has been
determined that a weight dosage of about 100 micrograms/cm3
of elemental scandium, elemental mercury in accordance with the
formula:
N (Hg) (mg~cm3~ = 7.7 D 1~7
where D = arc tube diameter in millimeter~
and argon gas at a pressure of abou~ 100 torr are most
appropriate ~o the above men~ioned metal halide lamps of about
40 to 150 watts. Thus, a di~charge lamp of enhanced color
temperature, starting ~apability, extended life and efficiency
i~ at~ainable i~ wattage~ less than preYiously known.
~ hile there has been shown and described what is at presen~
considered the preferred embodiments of this invention, it will
be obvious to those skillea in ~he art t~at variou6 change6 and
modifications may b~ made therein without departin~ from the
invention as defined by the appended claims.
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