Note: Descriptions are shown in the official language in which they were submitted.
2 1 94724
M E T A L H A LID E L A M P
TECHNICAL FIELD
This invention relates to high int~.~ily discharge (HID) lamps and more particularly
5 to such lamps having increased efflcacy.
BACKGROUND ART
HID lamps are among the most efficient light sources currently available. When first
introduced in 1934 as mecury lamps, such lamps had an arc length of 158 mm, a bore
diameter of 33 mm and a power input of 400 W, resulting in a power loading of 2
10 W/cm2. Efficacy was about 40 lumens per watt (LPW) and the color rendering index
(CRI) was less than 20.
With the introduction in 1939 of the quartz arc tube, bore diameter was reduced to 22
mm and the arc length to 70 mm. For a 400 W input, the power loading reached 6
W/cm2. An efficacy of 50 LPW was achieved and the life was in~,leased to 6000
hours.
By the early 1960~s irnproved quartz m~ .f~ ing techniques made possible the
introduction of metal alides to ~l~gm~nt the sparse Ille.~ spectrum. This resulted
20 in lamps with additives of scandium and sodiurn which provided for LPW of 80 and
CRI of 65 for a 400 W power input. These larnps had a bore ~i~metçr of 20 mm andan arc length of 45 mm to satisfS the vaporization le~ e,llcilts of the metal halide
additives. Power loading incleased to 12 W/cm2 for the 400 W input power.
25 Still more recent activity in this are~a has included the development of lithium iodide
~nh~nre~nents to the scandiurn-sodium che~ try (see, for exarnple, U.S. Patent No.
5,057,743, Keeffe et al.). These lamps fi~her improved the CRl to 75 at a correlated
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color temperature (CCT) of 3200K with LPW of 85 and average wall loadings of
15.5 W/cm2 in low wattage lamps.
These lamps are noted for their long life and good color rendering properties. These
5 lamps comprise arc discharge envelopes of quartz having complex fills of metalhalides, mercury and an inert gas. The arc discharge chamber is usually enclosedwithin an outer envelope of a hard glass such as an aluminosilicate having good UV
absorbing plOp.,- Lies. However, in an era of high energy costs, it would be
advantageous to provide a metal halide HID lamp having even better and more
10 efficient lllmin~ting qualities.
DISCLOSURE OF INVENTION
lt is, th~,iefore, an object of the present invention to obviate the disadvantages of the
prior art.
15 lt is another object of the invention to provide a metal halide HID lamp having a color
rendering index greater than 75; an efficacy greater than 90 LPW and a CCT of
between 3500 and 4040K.
These objects are accompli~he.i in one aspect of the invention, by the provision of a
20 metal halide HID lamp which comprises an outer glass envelope having a pair of
electrical conductors extçn-ling into the interior thereof. A quartz discharge tube is
disposed within the outer envelope and includes a pair of spaced electrodes which are
electrically con~ ecl to the electrical con~ucto~ for creating an electrical discharge
during operation of the lamp. The discharge tube has an arc chamber with the
25 configuration of a prolate spheroid having a major cross-sectional diameter and a
given arc ~i~t~n~e as measured by the linear ~ict~n~e between the interior te~rnin~tions
of the electrodes, the ratio of the major ~ ~eter to the arc ~i~t~n~e being less than I
and greater than 0.9. An arc gene.dling and su~t~inin~ medium is contained within
the arc chamber and includes the halides of sodium, scandium, lithium, thulium and
30 thallium, a fill gas selected from argon and xenon, and a given quantity of mercury to
2! 94724
achieve a desired lamp voltage. The lamp is operated with a wall loading greater than
1 7 Wlcm~.
Lamps produced as above have fulfilled these design requirements and additionally
have shown better lamp-to-lamp uniformity, improved color consistency, long life,
5 reduced near W emission, elimin~tion of the troublesome end paint, reduced arcmantle color separation and m~nllfacturing cost savings by reducing the amount of
chemical fill n~cess~ry for the lamp dose.
BRIEF DESCRIPTION OF THE DRAWINGS
10 Fig. I is a pe.s~.,ti~e view of a lamp in accordance with an aspect of the invention;
Fig. 2 is a graph of CRI vs. Life of lamps employing the ~h.omi~ry of the invention
and prior art lamps;
15 Fig. 3 is a graph of CCT vs. Life of lamps employing the chPmictry of the invention
and prior art lamps;
Fig. 4 is a graph of the lu nen m~ tf .~ e of lamps employing the chemistry of the
invention and prior art lamps;
Fig. 5 is a cross-sectional view of an embodiment of an _rc tube in accordance with an
aSpect of the invention; and
Fig. 6 is a graph of core te.~ dl~lres at peak current for various prior art çhemi~tries
25 and the ch~.ni~try of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
For a better underst~n~ing of the present invention, together with other and further
objects, advantages and capabilities thereof, reference is made to the following
21 94724
disclosure and appended claims taken in conjunction with the above-described
drawings.
Referring now to the drawings with greater particularity, there is shown in Fig. I a
metal halide arc discharge lamp 10 including a lamp envelope 12 and an arc tube 14
mounted within the envelope by mounting frame 16. The arc tube may be positionedwithin a shroud 20 which can also be supported by the mounting frame 16. Electrical
energy is coupled to the arc tube 14 through a base 22, a lamp stem 24 and electrical
leads 26 and 28. The arc tube contains a chemical fill or dose of materials to provide
light when an arc is initiated therein, as will be explained hereinafter. The shroud 20
comprises a cylindrical tube of light tr~ncmiccive, heat resistant material such as
quartz.
As noted, in this particular inct~n~e, the mounting frame 16 SuppOl~ both the arc tube
l 5 and the shroud within the lamp envelope 12. The mounting frame 16 includes a metal
support rod 30 att~h~l to lamp stem 24 by a strap 31. The support rod engages aninward projection 32 in the upper end of the lamp envelope 12. The support rod 30 in
its central portion is parallel to a central axis of the arc tube 14 and shroud 20. The
mounting means 16 further includes an upper clip 40 and a lower clip 42 which secure
both arc tube 14 and shroud 20 to support rod 30. The clips 40 and 42 are ~tt~ hed to
the support rod 30, p~efe~àbly by welding.
It has been discovered that when a metal halide chemical composition comprised of
the iodides of thulium, scandium, sodium and lithium is iluOll~olaled within an arc
tube 14, and that arc tube is o~.àted at power loa~ing~ heretofore found to be
excessive for prior art chemistries, ull~,A~,t~d advantages ensue. For example, it has
been found that when such a ch~ ry, in which the mole fractions of the iodides of
Tm, Sc, Na and Li are 0.316, 0.020, 0.474 and 0.190, re~e~,Li~/ely, is dosed into a low
wattage (i.e., 75 W) cylindrical arc tube which is then Opc~àtcd at 100 W, so that the
actual wall loading is 100/75 x 15.5 = 20.67 W/cm2, the pc.Ço~ ce is unexpectedly
improved while no deleterious effects owing to the elevated loading, which would
2 1 94 724
have been present with prior art chemistries, are found. The CRI performance of such
a larnp is shown in Fig. 2 wherein plot A is a larnp in accordance with the aspect of
the invention described above, showing an initial value of 87 while m~int~ining a
value of 84 at 10,000 hours. Plots B, C, D and E depict prior art larnps. Similarly, the
5 CCT is remArk~bly stable, as shown by plot A of Fig. 3, holding the initial value of
4600K + 100K over the 10,000 hours. Plots B, C, D and E, again depict prior art
lamps. The lurnen m~intçn~nce, (Fig. 4) likewise, is rem~rk~bly constant following
an initial drop which is believed to be caused by tl-ngcten evaporation from theelectrodes caused by ope.dling the lamps above their rated wattage. Again, in Fig.4,
10 plot A le~l~s~,nts a larnp in accordance with an aspect of the invention and plots B, C,
D and E represent prior art larnps. Plots C and D illustrate lamps operated at aconventional average low wattage power loading of 15.5 W/cm2, while plot E
represents a prior art lamp with a conventional fill operated at 33% over the design
power loading. Although this latter lamp has a life of over 10,000 hours, its
15 m~ nAnce is not as good as lamps yle~ed with the improved chPmictry and, thus,
it is not a viable alternative absent the improved chemical fill of this invention.
All of the above to tbe contrary noturithct~n~ing, it has been discovered that even
further p~lçorlllance gains can be made with an optimized chemical dose and by
20 increasing the power loading to even higher levels by de~ g from the cylindrical
shape of t_e prior art arc tuhes and shaping the arc tube to conform more closely with
the configuration of the arc discharge. The arc discharge is known to take a more or
less ellipsoidal shape and variations of such shapes have been previously proposed for
arc tubes. See, for t;A~ll~le, U.S. Patent No. 4,020,377, which dicc-.cses isothermal
25 arc tubes of elliptical configuration. However, the specific configuration of arc tube
14 shown in Fig. 5 provides a definite advance in the art and is p~efe.l~d. This shape
is a prolate spheroid generated by rotating an ellipse with semi-axes "a" and "b",
where a>b, about the major axis, "a". If "a" and "b" are internal ~imencions, then the
internal surface area is given as
/
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ARCSIN e~
A=2~ b2~ab e
(a2 _ b2 )ll2
wh~rc e =
so that the average power loading is P/A, where P is the input power.
The arc tube 14 is made from quartz or other suitable translucent, high telllpelalu~e
material and has a cavity 44 with t mgctçn electrodes 46 sealed therein by means of
the usual molybdenum foils 48 in a press seal 50. The arc tube cavity 44 ha a major
cross-sectional diameter Z and an arc ~ict~n~e Y, as measured by the linear distance
15 between the interior terrninations of the electrodes 46, and the ratio Z/Y is less than I
and greater than 0.9. In a preferred form of the invention, Z equals 0.440 "(1.1176
cm); Y equals 0.472 " (1.1988 cm); and Z/Y equals 0.932. Electrode pencllalion is 3
mm and the electrode tips are not at the foci of the prolate spheroid arc tube 14. This
arc tube has a volume of 1.167 cm3 and an interanl surfacc area of 5.55 cm2.
The permissiblc a~ .agc power loading for this shape is higher than for conventional
shapes for the prior art sodium-scandium-lithium c~emi~t~y, owing to the more
unifonn heat load upon tne conformably tailored shape. Moreover, the permissiblewall loading is un~Ape~;ledly higher for the thulium-scandium-sodium-lithium-
25 iodides- thallium/mercury chemical dose. For example, while low wattage (i.e., 75
W) conventional cylindrical shapes cont~ining sodium-scandium-lithium iodides ~-e
limited to 15.5 W/cm2 for long life operation, and ellipsoidal shapes con-~ining this
ch~rnist~y have been s~lccessfi~lly operated at 17.8 W/cm2, ellipsoidal shapes
con-~ining thulium-scandium-sodium-lithium-iodides with thallium/mercury have
30 shown good photometric results at wall loadings as high as 26.7 W/cm2. This result
occurs because of the lowering of core te...pe.~ture for the latter chemistry, as shown
2 ! 9 ~ 724
in Fig.6. Therein, it will be seen that the core temperature for the chemistry including
thalliunvthulium is some 850K cooler than that with thulium alone, and some 900K
cooler than the previous standard without either the thallium or thulium.
5 In Table I, lamps design~te~ K and L represent prior art lamps having the design~ted
fills, and M and N replesent lamps having the fills of the instant invention. From the
table, it will be a~pa,~,"t that merely inc~easing the wall loading of the prior art lamps
will not achieve the benefits of the instant invention. Additional increases in LPW
were shown between lamp M and lamp N when the fill gas of 100 Torr argon was
10 replaced with 100 Torr xenon. The preferred mercury dose is that quantity which
provides the desired lamp voltage. For example, to achieve a lamp voltage of 95 volts
in the arc tube described above, a mercury dose of 15 mg is required with 0.3 mg of
thallium. The plef~ d method of dosing the thallium is as an ~m~lg~m with about
0.5 to 2 wgt. percent of the ~ ,.cur~; however, dosing may also be accomplished as
I S thallium iodide.
The scandium is preferably added in the form of the iodide and as a 0.13 mg metallic
chip to getter residual oxygen impurities and scavenge any excess iodine released by
the metal iodide salts.
21 94724 $.
TABLE I
Wall Loading - 17.8 W/cm2 Wall Loading - 26.7 W/cm2
Th _ 925C Tc _ 790C Th - 990C Tc - 880C
Chemistry LPW CRI CCT LPW CRI CCT
N~ISc/T i/Tm (no paint)
K 24:1:10:16 MR 83 83 4350 79 92 3800
Hg & 100 Torr Ar
L 24:1:10:16 MR 91 81 4210 88 91 3610
2% TllHg & 100 Torr Ar
M 48:1:10:16 MR 92 78 3900 94 88 3560
2% Tl/Hg & 100 Torr Ar
N 48:1:10:16 MR 98 75 4040 102 85 3670
2% Tl/Hg & 100 Torr Xe
The chemical dose described herein is more tolerant of elevated power loadings
heretofore found to be detrimental to long lamp life and provides for good life
~ectal1cy, LPW greater than 90; and a CCT of between 3500 and 4040 K.
While there have been shown and described what are at present considered the
~lcf~ d embo~ of the invention, it will be appa.. llt to those skilled in the art
that various changes and modifications can be made herein without departing from the
scope of the invention as defined by the appended claims.
