Selective spectral output metal halide lamp
United States Patent Lake et al. 1 Apr. 8, 1975 SELECTIVE SPECTRAL OUTPUT METAL 3.363.134 1/1968 Johnson 313/225 x HALIDE LAMP 3.43l,447 3/1969 Larson..,....,...... 3l3/229 X 3,445.719 5/1969 Thouret et al. 313/229 X 1 Inventors: i i Lake, o y; el 3,450,924 6/1969 Knochel et al 313/229 x D. Kershaw, Kirtland, both of Ohio; 3,514,659 5/1970 Gungle et a1. 313/229 X John M. Sato, Neptune City, NJ, 3,521,110 7/1970 Johnson 313/229 X [73] Asslgneezt 55:2 :21: L 5 ompdny Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm-Ernest W. Legree; Henry P. [22] Filed: Jan. 17, 1972 Truesdell; Frank L. Neuhauser [2l] Appl. N0.: 218,491 57 ABSTRACT Related US. Application Data I [63] Commumiomm an of Ser No 861. Jul 7 A high pressure metal halide vapor arc lamp provtdmg 1969 abandon; y radiation concentrated in selected spectral bands. The  
  filling comprises metal halide buffer species in addi- [52] U S Cl 3l3/l84 3 l 3/225 313/229 tion to metal halide emitter species. The buffer species [51] 6 61/18 increase the energy input to the arc, thereby augment- [58] Fieid 3 184 ing the radiation from the emitter species. The radiation from the buffer species may be negligible or is largely suppressed. A lamp emitting in the blue, green [56] References Cited and red bands comprises a limited quantity of Znl. as UNITED STATES PATENTS buffer species which serve to augment the radiation 3.219369 11/1965 Schmidt 313/229 X f Lil d T1] pre ent as emitter species, optionally $93 985 with a limited quantity of Hg as a secondary buffer. 1&#39;] NC 3.351.798 11/1967 Bauer 313/229 X 5 Claims, 2 Drawing Figures SELECTIVE SPECTRAL OUTPUT METAL HALIDE LAMP This application is a continuation-in-part of our cpending application Ser. No. 863,732, filed July 7, 1969, similarly entitled and assigned and now abandoned.  
 BACKGROUND OF THE INVENTION The invention relates to high pressure metal vapor lamps using an arc discharge in metal halide vapors to produce light or radiation.  
  Until quite recently, the mercury arc lamp, notwithstanding its yellowish-green light and poor color rendition, has been pre-eminent among high pressure arc lamps. More recently, it was discovered that a great improvement in color rendition and efficiency could be achieved by using vaporizable metal halides in the fill. A preferred filling taught by U.S. Pat. No. 3,234,421 Reiling, comprises mercury, sodium iodide, thallium iodide, and indium iodide. This lamp has emission widely scattered throughout the visible light spectrum resulting in good color rendition, and is an excellent light source for general use.  
  The principal interest in connection with metallic halide additives to mercury vapor lamps has been to improve efficiency and achieve balanced white light output. A particular problem has been to provide sufficient red radiation to achieve good color rendition. In such lamps, the electrical characteristics are essentially those of a mercury discharge.  
  In some applications involving color synthesis, emission scattered throughout the visible spectrum is undesirable. For instance in reprographic applications for making colored copies, radiation concentrated in the three primary colors, blue, green and red is desired. The three primary colors can be derived from light sources emitting continuously throughout the visible spectrum by means of filters. Thre light beams are provided. either from three separate light sources or by splitting the beam from a single source by optical means. Filters are used in each light path to eliminate everything except the desired primary color, and the three primary colors may then be recombined into a single beam. Such systems are prohibitively expensive and inefficient.  
  In some photochemical application, high energy emission in specific regions or bands is required in order to promote a chemical process, and emission in other bands must be suppressed because it may inhibit the processor produce undesirable reactions.  
  The object of the invention is to provide efficient high intensity radiation sources having spectral energy output concentrated in specific regions or balanced relative to an overall color requirement.  
 SUMMARY OF THE INVENTION In accordance with our invention, a high intensity arc lamp radiating in selected spectral bands or regions has a filling comprising metal halides of which at least one is a buffer species and another is an emitter species. Both buffer and emitter species participate in the discharge but the buffer is the primary determinant of the electrical characteristics of the discharge while the emitter is the primary determinant of the spectral characteristicss of the discharge. The buffer controls the electrical characteristics of the discharge primarily by its effect on the thermal characteristics. The buffer augments the radiation from the emitter species and may also reduce electrophoretic or chemical processes tending to deplete the emitter species or attack the envelope wall. The buffer species is selected for its specific ability in respect of these functions and not primarily for its radiative contribution to the desired spectral output. The emitter species is selected for its specific spectral characteristics.  
  In a preferred embodiment of the invention wherein the spectral output is concentrated in the blue, green and red bands with energy levels of approximately 1:2:2, the amount of Lil is at least 0.2 mg/cc of envelope volume, the amount of Znl is from .02 to 1.5 mg/cc, the amount of T1] is from .02 to 3 mg/cc. Optionally there may be provided Hg in an amount less than 1 mg/cc.  
 DETAILED DESCRIPTION In general the preferred buffer species are metal halides having the following characteristics:  
 1. High vapor pressure;  
 2. High molecular weight;  
 3. High ratio of halogen to metal in the halide molecule;  
 4. High ionization potential relative to the emitter species.  
  In ranking materials, a figure of merit indicative of relative desirability as buffer species is obtained by multiplying the molecular weight of the material, by its vapor pressure at the operating temperature (600 C), by the number of halogen atoms in the metal halide molecule and is given in Table I below.  
 TABLE I Molcc Ionization ular Approx. V.P. Figure of Potential Wgt. at 600 Ctorr Merit X 10&#34; Electron volts Sbl 503 2 X 10 3000 8.5 Asl; 456 2 X 10 2700 10.5 Hg 200 5 X 10 1000 10.4 Bil 590 5 X 10 900 8.0 1111;, 496 3 X 10&#34; 450 5.8 lnl 242 3 X 10 5.8 Znl 319 10 6 9.4 Cdlg 366 50 4 9.0 Se 79 240 2 9.7 P61; 461 20 2 7.4 Tll 255 35 1 6.1 501;, 426 7 X 10 10&#34; 6.7 Cal 294 4 x 10- 2 x 10- 6.1 Csl 250 5 X 10 10&#34; 3.9 Dyl 544 2 X 10 3 X 6.8 Nal 150 2 X 10 3 X 10&#34; 5.1 Sml 531 2 X 10 3 X 10 6.6 Lal 520 10 2 X 10 5.6 Srl 342 10* 7 X 10 5.7 Lil 134 5 X l0 7 X 10 5 5.4 Bal 391 10 10 5.2  
  The list is not exhaustive nor does it determine finally the preferability of one buffer over another because other considerations may apply to specific material combinations. Among halides, the iodides are generally preferred because they are less reactive and iodine has the greatest atomic weight among the halogens.  
  In general, materials suitable as buffer species have a figure of merit greater than 1.0 while materials which may be useful as emitter species have a figure of merit not exceeding 1.0. The materials classified as buffer species in the table have not been used as primary emitters except for lnl and InI which have an exceptionally low ionization potential. However in the list of materi als where the figure of merit is less than 1.0 are found some of the most desirable emitter species currently used in commercial mercury metal halide lamps, for instance Scl Dyl Nal and Lil. These are used in lamps where Hg provides the buffer action. The buffer species may have some characteristic radiation which is useful and desired in a particular application. In such case the buffer species determines the desired thermal and electrical characteristics for itself as well as for other emitter species present.  
  By using two or more buffer materials, the spectral output of the discharge may be greatly increased even though one or more of the buffers contributes little or no direct radiation.  
  In selecting the buffer species, an important consideration is the ionization potential of the buffers relative to the emitters and to each other. We have found it desirable to have buffer species whose ionization potential is substantially higher than that of any of the emitter species. Also where there is more than one buffer species, the ionization potential of the secondary buffer species should be higher than that of the primary buffer species.  
 DESCRIPTION OF DRAWING FIG. 1 illustrates a high pressure tubular lamp embodying the invention.  
 FIG. 2 shows the spectral output of the lamp of FIG.  
 PREFERRED EMBODIMENTS A lamp embodying the invention and illustrating the foregoing principles is designed to radiate predominantly in the blue, green and red bands defined as follows:  
 Blue 430 500 nm. Green 520 540, Red 650- 700.  
 and with energy levels within these bands in the approximate ratio l:2:2. In this application, radiation in the region between the green and red bands is particularly undesirable and must be kept as low as possible. Mercury in a thermal arc radiates quite strongly at 546 nm which is within the undesirable region and therefore cannot be used as primary buffer species for this lamp.  
  The foregoing requirements are met by a filling composed essentially of the halides, preferably the iodides, of zinc, lithium and thallium wherein Z1112 is the buffer species and Lil and Tll are the emitter species. Preferably there is also included some mercury metal as a secondary buffer and an inert starting gas such as argon at a pressure below 100 torr. The zinc augments the radiation from all three predominant lithium lines but its own radiation is very low, less than of the whole. The mercury radiation in the visible spectrum is almost completely suppressed; however the mercury increases the lamp efficiency and also causes a reduction in the reignition voltage at each cycle.  
  Referring to FIG. 1, the lamp comprises an arc tube 1 of quartz or fused silica of about 8 millimeters inside diameter and 10 millimeters outside diameter having sealed therein at opposite ends a pair of arcing electrodes 2, 2 defining an arc gap of about 10 centimeters. The volume of the lamp is about 6 cc. The electrode inleads 3 have intermediate thin molybdenum foil sections 4 hermetically sealed through full diameter pinch seals 5 at the ends of the tube. The electrodes each comprise a double layer tungsten wire helix 6 wrapped around a tungsten core wire 7, and may be conventionally activated by thorium oxide applied as a coating on the turns of the helix or filling the space or interstices between turns. In order to heat the end chamber and thus increase the vapor pressure of the metal iodides, a quartz sleeve 8 extending forward up to the electrode tip is fitted over the end of the lamp. The space between the tube wall and the sleeve is filled with a refractory white insulation 9 of quartz fibers.  
  According to a feature of the invention, we have found that with unactivated tungsten electrodes there is an increase in total light output within the three color bands and less variation in segregation of the various species along the length of the lamp. With activated electrodes, we have measured variations in radiant output of up to 15% within a particular band from end to end of the lamp. This is probably due to the variation in end chamber temperature consequent upon lack of uniformity in activation and resultant work function. By using unactivated electrodes consisting of the previously described tungsten wire helix without activating material of any kind, segregation of species and variation in output along the length of the lamp is reduced to less than 5%. The explanation for the increased total output (about 10%) appears to be that the greater electrode voltage drop in the absence of activation entails additional dissipation of power at the electrodes result ing in increased end chamber temperature and this in turn means more complete vaporization of the various species present. With unactivated electrodes, the arc attachment is diffuse and not concentrated at a hot spot.  
 PARAMETERS OF THE INVENTION According to our invention, in order to achieve a high proportion of red it is necessary to have, per cubic centimeter of envelope volume, at least 0.2 milligrams of lithium iodide which is the source of the red radiation. LiI has the lowest vapor pressure of the ingredients of the filling and a saturated vapor is desired. Therefore, Lil is provided in an amount such that if it were the only salt present, an unvaporized excess would remain as a liquid. In fact, the unvaporized excess of Lil forms a liquid phase wherein, is dissolved TH and ZnI While there is at least in theory no upper limit to the quantity of LiI which may be added, there is no practical benefit in going beyond 2 milligrams per cubic centimeter and the material is wasted.  
  The amount of zinc iodide required is in the range of .02 to 15 milligrams per cubic centimeter of envelope volume. If less than .02 mg/cc of Znl is provided, the Lil radiation is not appreciably enhanced. Also, the chemical buffering effect of Znl is insufficient to protect the quartz envelope walls and prevent the depletion of Li from the arc tube. If the quantity of ZnI is greater than 1.5 mg/cc, the volage gradient becomes excessive when practical levels of T1 are present. For instance the voltage gradient might be 15 volts per centimeter resulting in an arc drop of 525 volts across a 35 centimeter long lamp. A minimum current of 2 to 3 amperes is necessary for reignition and are stability. The  
 temperature limitations of quartz require that the dissipation per centimeter of arc length not exceed about 45 watts in the illustrated lamp and thus the resulting limit of about 15 volts per centimeter of gradient imide buffer species, specifically Znl in accordance with the invention, in a lamp designed to emit in specified spectral bands may be seen from a comparison of the characteristics of the six lamps listed in Table II below.  
 poses the previously mentioned upper limit on zinc io- 5 Lamps Nos. 4, 5 and 5X embody the invention, Lamps dide. Nos. 5 and 5X being preferred embodiments. In the The quantity of thallium iodide should be between c lumn under the heading Dose mg/cc, the number .02 and 3 milligrams per u i ntlm h lo r following the chemical compound is the weight thereof llmlt f -02 mg/C 1S Impose y the practlcal necessity in milligrams per cubic centimeter. Lamps 1 to 5 had for green in a lamp contalnmg blue and red and WhlCh a 10 cm arc l th d were approximately the same is requiredtohave an overall white appearance. When h i l di si as the lamp previously described the upper ll 0f 3 g/ 1S CXCeeded, the Voltage g and illustrated in FIG. 1 which has a volume of about dleht 89 p and the p become? too g It 6 cc. Lamp 5X had a 35 cm arc length. The output radi- Comes tact almost P y a thalhum dlschafge ation is given in watts in the three bands and has been cause Tl controls the arc temperature of the discharge; measured by integrating the intensity Over the b dif the Pressure of Tl too high, the are temperature widths previously specified and comparing it to a stancreases and the radiation from the other species presdard l li d i wants Output f r h same hamelyled and blue t L1 and Zn, decreases and bands. The red efficiency is the ratio of output radiathe PTOPOTtIOh of green lhcrehses Y more tion in the red band measured in watts to the input to th lumps accordlhg t0 t lhvehtlohi 2 15 the P 20 the lamp likewise measured in watts. The wattage input t buffet and mercury 15 hot heeded-.Howeverahmlevels at which the lamps were operated resulted in lied q y of mercury y be Provided to achleve minimum cold spot temperatures of about 600 C, and Certain characterlsticswhen mercury in an amount the specified doses are sufficient to assure an unvaporhot exceeding about mg/cc l5 addedi the relghttloh ized excess of metal halide, that is Lil liquid in which voltage is reduced by a relatively large factor, as much 25 are di l d T1] d Z I TABLE II INPUT Radiation: Watts Red amp No. Dose mg/cc Watts Volts Amps Blue Green Red Eff.  
 1 Lil 3.8 400 67 t 7.2 1.8 13.1 3.3%  
 2 Lil 1.33 400 77 6.8 1.4 10.0 8.5 2.1%  
 Tll L33 3 Lil lnl 400 94 5.3 24.0 l 1.3 3.3%  
 4 Lil 1.67 400 101 5.1 9.0 15.0 17.5 4.4%  
  Tll 1.67 Znl .83 5 Lil 1.67 350 125 3.5 9.2 15.6 17.0 4.9%  
 Tll 1.67 Znl .83 Hg .83  
 5x Lil .57 1450 320 4.4 25 55 55 4.2%  
 Tll .9l Znl .63 Hg .06  
 as threefold. There is substantially no characteristic The unexpected advantages achieved in accordance mercury radiation present in either the visible or the ultraviolet, and the volt ampere characteristic of the lamp remains substantially unchanged. This level of mercury is advantageous in lamps wherein it is desired to exclude all line radiation of mercury from the spectrum.  
  Where mercury line radiation is not objectionable, a larger quantity of mercury may be added up to about 1 mg/cc. Because the ionization potential of mercury is substantially higher than that of zinc, the Znl will still control the voltage gradient. However the mercury will decrease thermal losses to the wall and increase the lifetime of charge carriers. This means improved lamp power factor, lower current, and lower electrode losses at the same power input.  
 ADVANTAGES OF METAL HALIDE BUFFER SPECIES The effects and advantages of including a metal halwith our invention by adding Znl as a buffer to a lamp containing Lil and Tll are apparent upon comparison of lamps Nos. 1, 2 and 3 not containing Znl with lamps Nos. 4, 5 and 5X having an addition of Znl within the stated limits.  
 Lamps Nos. 1, 2 and 3 Lamp No. 1 containing only Lil emits in the red band but has the characteristically low voltage of a nonthermal discharge and requires a high operating current to produce substantial red radiation. The high operating current results in large electrode losses and consequently low power input to the arc itself, the efficiency in producing red radiation being only about 3.3%.  
  Lamp N0. 2 containing Lil and Tll, both emitter species, shows slightly improved electrical characteristics, namely higher voltage and lower current resulting in lower electrode losses. The improved electrical characteristics are due to the fact that TlI with a figure of merit of 1 can act as a buffer for Lil whose figure of merit is 7 X 10*. However a decrease in red radiation and a drop in red efficiency to 2.1% results because the discharge is now tending towards a thermal arc and the electron temperature has decreased.  
  Lamp No. 3 contains lnl which has a figure of merit of 450 and is a relatively good buffer. Its addition to the lithium iodide discharge raises the voltage gradient substantially and reduces the electrode losses. Indium is a strong blue emitter and due to its low ionization and excitation potential, it tends to dominate any discharge into which it is introduced. The low ionization potential of indium produces a fairly low electron temperature so that the red radiation of the lithium does not increase as much as might be expected by reason of the improved electrical characteristics of the arc.  
  If the indium iodide content were reduced to provide the desired blue to red ratio of approximately 1 to 2, the arc voltage would revert very nearly to that of Lamp No. 1 containing only Lil. Thus ll&#39;llg cannot effectively be used as a buffer with Lil for the present purpose, and it is virtually relegated to an emitter species. Lamps Nos. 4, 5 and 5X Increase in Red and Green Radiation In these lamps, the unexpected effectiveness of ZnI- serving as a buffer to increase the characteristic radiant output of the lithium and thallium present is immediately apparent. Considering the red efficiency from lithium iodide in lamp No. 4, it is increased to 4.4% which is more than double the red efficiency in lamp No. 2 using the same emitter species Lil and T11 but without the Zlllg buffer species. Considering the green efficiency from thallium iodide, it is increased to 15% by comparison with 10% in lamp No. 2, an increase of 50% The same startling increase in the red efficiency resulting from lithium radiation and in the green efficiency resulting from thallium radiation is observed in lamps Nos. 5 and 5X both of which contain ZnI- Blue Radiation When Znl is used as a buffer, its high ionization potential insures that it will not produce radiation which will dominate the spectral output. Therefore the quantity can be adjusted to achieve the desired electrical and thermal characteristics and at the same time to produce the desired blue radiation by the characteristic emission of zinc in the blue spectral band. Lithium Depletion Another unexpected benefit from the presence of Znl is its ability to prevent chemical attack of the quartz arc tube by the lithium. Lithium is the lightest of the alkali metals and when lithium iodide is used alone in a quartz arc tube, it tends to become depleted rapidly, possibly by diffusion of lithium through the quartz. The presence of Znl provides a chemical protective effect which greatly reduces the rate of loss of Lil and extends the useful life of the lamp as much as a hundredfold, from a few hours to a thousand hours or more. Coincidence of Colors in Arc A desirable feature of the lamps according to the invention is the physical coincidence of the red, green and blue colors in the arc. In the usual metal halide lamps containing mercury and various metal halide additives, the core of the arc is generally surrounded by a reddish sheath of larger diameter. This means that the red radiation cannot be focused in exactly the same way as the other radiation and this fact introduces serious problems in reprographic applications. This shortcoming is eliminated by the use of Znl combined with Lil and T11 according to the invention.  
 Mercury Addition Less Than 0.25 mg/cc In lamps Nos. 5 and 5X, some mercury is added. In lamp No. 5X wherein the added mercury is less than 0.25 mg/cc, the effect is to reduce the reignition voltage without producing any mercury radiation, either in the visible or in the ultraviolet. The substantially complete elimination of mercury radiation is a desirable characteristic for certain lamp applications.  
 Mercury Addition Less Than 1 mg/cc In lamp No. 5 containing a larger amount of mercury but not exceeding 1 mg/cc, mercury serves as a secondary buffer backing up Znl as primary buffer and the lamp characteristics are further improved. The ionization potential of mercury is substantially higher than that of zinc so that at the limited mercury vapor pressures achieved, Znl still controls the voltage gradient. However the mercury decreases the thermal losses to the wall and increases the lifetime of charge carriers. This results in improved lamp power factor, namely lower current at the same power input resulting in lower electrode losses. Thus, comparing lamps Nos. 4 and 5, the latter achieves substantially the same spectral output only at a lower power input and at higher efficiency. The loading or input wattage to lamp No. 5 has been reduced 10% by comparison with lamp N0. 4, but the decreased thermal losses has improved the red radiation efficiency by 10% and the spectral output is undiminished. Thepresence of mercury in lamp No. 5 does not cause any appreciable radiation in the visible but it does cause some ultraviolet mercury radiation. Overall Efficiency When lamp No. 5 containing Znl as a buffer and a limited quantity of mercury is compared to lamp No. 2 containing only the emitter species Lil and T11, the useful output is 2.3 times greater overall and is approximately balanced in the desired 1:2:2 ratio as between blue, green and red. The vapor pressure at 350 watts input with 600 C cold spot temperature are Znl 30 to 50 torr; Tll, 7 to 15 torr; and Lil, 0.04 to 0.1 torr.  
  Taking lamp No. 5 as typical of lamps according to the invention which include Znl as buffer species and Lil and T11 as emitter species, the radiant output in the selected bands is more than doubled. The spectral output of this lamp is shown in FIG. 2 from which the concentration of radiation within the selected bands and the low level outside of these bands is apparent. With an input of 350 watts the radiation in the selected blue, green and red spectral bands is 42 watts for an efficiency of 12%. This is a high level of efficiency for a specialized lamp, and several times higher than the efficiency achieved by the use of multiple light beams with filters.  
  Considering the overall efficiency of lamp No. 5 within the visible range extending from 4,000 A to 7,000 A. with an input of 350 watts the output radiation is watts for an overall efficiency of 20%. By comparison, in a conventional high pressure mercury vapor lamp having an input of 350 watts, the radiation in the visible range is 57.5 watts for an overall efficiency of 16.5%. Thus the overall efficiency within the visible range of lamp No. 5 exceeds that of the conventional mercury vapor lamp by better than 20%.  
 OTHER EMBODIMENTS the same physical dimensions as the lamp previously l teristics are required.  
  What we claim as new and desire to secure by Letters Patent of the United States is:  
  1. A high intensity lamp providing radiation concentrated in selected spectral bands comprising:  
 an hermetically sealed light-transmissive envelope;  
 a pair of arc electrodes sealed therein and defining an arc gap; an inert starting gas at a pressure of a few torr described and illustrated in FIG. 1. therein;  
 TABLE III Lamp Dose (mg) INPUT Radiation: Watts No. Buffer Emitter Watts Volts Amps Blue Green Red 5A Znl (5), LiI( I). 300 127 3.! 6.4 l2.l l2.l  
 Hg() Tll( l0) 6 Cdl Lil( I0), 300 110 4.1 5.4 11.8 l2.4  
 Tll( l0) 7 Sbl;.(5) Lil( 10). 300 168 2.5 3.7 9.9 9.0  
 Tll( l0) 8 Phl 10) Lil( I0). 300 98 4.2 2.7 7.8 6.9  
 Tll( l0) 9 SCI 5 LiI( I0). 300 I42 2.7 2 2 13.7 10.]  
 Hg(5) Tll( l0) l0 A51 LiI( 10), 300 165 2.2 3.6 12.2 I 1.4  
 Hgl5) TlI( I0) Lamp No. 5A is the same as lamp No. 5, the preferred embodiment previously given in Table II, only operated at 300 watts instead of 350 in order to permit direct comparison. In lamps Nos. 6 through 10, the emitters are lithium iodide and thallium iodide, and the emitter dosage is the same as in lamp No. 5; however different buffers selected from Table I are used with the results indicated.  
  It will be observed that in lamps Nos. 6 through 10, at least one of the blue, green or red radiation bands of the lithium or thallium is considerably enhanced by comparison with lamp No. 2 containing only lithium iodide and thallium iodide emitters without buffers. However, the relative output in the blue, green, and red bands are no longer approximately in the 122:2 ratio in every case. Thus the various combinations of Table III may be used where lamps having other spectral characand a charge therein comprising per cubic centimeter of envelope volume from about .02 to 2.0 milligrams of Lil. from about .02 to 1.5 milligrams of ZnI- and from about .02 to 3.0 milligrams of TH.  
  2. A lamp as in claim 1 containing in addition not over about 0.25 milligrams of mercury per cubic centimeter of envelope volume.  
  3. A lamp as in claim 1 containing in addition not over about 1 milligram of mercury per cubic centimeter of envelope volume.  
  4. A lamp as in claim 1 wherein the electrodes consist of unactivated tungsten.  
  5. A lamp as in claim 1 containing per cubic centimeter of envelope volume about .57 mg. of Lil, about .91 mg. of TI], about .63 mg. of Znl and in addition about .06 mg. of Hg.