CA2125253A1 - Metal halide discharge lamp with improved color rendition index, and method of its manufacture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

To filter light emitted, upon energization of a metal halide discharge lamp, in the short wavelength visible and adjacent UV
spectral light regions, a coating of an oxide of at least one of titanium or cerium, preferably titanium dioxide TiO2 is applied at least in the region of maximum light emission on the surface of the discharge vessel of the lamp, the coating being operated, in operation of the lamp, to have a temperature of at least .
600°C, and preferably between 750 and 900°C; the coating should have a thickness which corresponds to a coating, of TiO2, applied at a rate of up to 0.6 mg/cm2, and preferably between about 0.08 to 0.3 mg/cm2. The coating can be applied by spraying as an intermediate step in the manufacture of the discharge vessel, and can be mat, if sintered on the quartz glass of the discharge vessel at about 500°C or clear, if melted on the quartz glass of the discharge vessel at between 1,200 and 1,700°C, so that the oxide of the coating will form a doping at the outer layers of the quartz glass.

Description

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940069-shf ' ~ .
IN THE UNITED STATES PATENT ~ND TRADEMARK OFFICE
"METAL HALIDE DISCHARGE LAMP WI~H IMPROVED
COLOR RENDITION INDEX, AND METHOD OF ITS MANUFACTURE"

Reference to related U.S. application and patents, the disclosures o~ which are hereby incorporated by reference:
U.S. 4,249,~02, Krieg et al U.S. 4,985,275, Takemura et al U.S. 5,003,214, Morris et al U.S. 5,037,342~ Barthelmes and Bunk U.S. 5,051,650, Taya et al U.S. 5,214,345, Saito et al.

Reference to related literature:
"Technisch-wissenschaftliche ~bhandlungen der OSRAM-Gesellschaft", ("Technological-Scienti~ic Papers of the OSRAM
Company"), Yol. ~2, published by Springer Heidelberg, 1986, p. 11 et seq., particularly pages 14 and 15. -"Messapparatur fur die spezifische Oberflache von Metalloxid und Metallpulvern", ("Measuring Apparatus for the Specific Surface of Metal Oxide and Metal Powders"), Ann. Chem. Soc., 60 (1938) 309, and "Metall", Vol. 32 (1978), pp. 678 et. seq., article by W. Danneberg and H.H. Kuhlmann.
* * * * * * * ~ . .

FIELD OF THE INVENTION.
The present invention relates to metal halide discharge lamps and methods of making them in which the color rendition index is improved in a simple and inexpensive manner. The lamps to which the present invention relat~ are suitable for general service illumination, as well as for projection of motion picture films and slides, and also ~or illumination of scenes for motion picture film or television productions and recordings.

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6J~ l9g4 ' 2~2~2~3 BACKGROUND.
Lamps to which the present invention relate are well-known and described, for example, in U.S. Patent 5,003,214, Morris et al. In this lamp, the outer surface o~ the discharge vessel is entirely coated with a heat reflecting material, which is mat, or frosted. The light transmission in the visible spectrum is at least 90%. The pr~ferred material for such heat reflection is silicon dioxide, sioz, having layer thickness~ of between about 0.1 and 10 ~m. Small power lamps, such as known, for exampl~, from U.S. Patent 4,249,102, Krieg et al, assigned to the assignee of the present applicakion, describe lamps which are mat, and which lower the color temperature output by about ~50 K when the.
lamps have a filling providing.a warm-whi~e l~ght color of a color --temperature of abou~-3,000 K. A theoretical.alternative of titanium dioxide, Ti02~is mentioned in the claims:; there is no reference to Ti02 in the description. The coa-ting is applied by a powder coating pI;ocess with a gas flame, or by dipping.
. U.S. Patent 4,985,275, Takemura et al, describes a method to make quartæ glass bulbs for discharge lamps, particularly lamps with a xenon fill. The inner wall of a bulb tube is doped to a depth of about 10 ~m with titanium oxide which is first applied as a layer, and then di~usad into the inner surface by heat treatment. The titanium dioxide containing layer ~ompletely absorbs ultraviolet (W) radiation having a wavelength less than 200 nm.
U.S. Patent 5,051,650, Taya et al and U.S. Patent 5,214,345, Saito et al disclose that a coating of zinc oxide, or a mixture o~ zinc oxide and titanium oxide can be used to absorb W
radiation. According to these disclosures, a pure titanium dioxide layer is undesired, since it leads to an absorption within the visible spectral range, particularly th~ blue spectral :
range, which impairs the color rendition of the lamp. The primary use of the coating described, for example, in the Taya et al P~tent 5,051,650 is with fluorescent lamps which, as well known, may have bulb temperatures of a~out 50C. The coatin~ is applied by a spray process at the outside of the bulb.

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~66~I~W ~ 4 -2-- 2i2~253 THE INVENTION.
It is an object to improve the color rendition o~ metal halide discharge lamps by providing the lamp with a metal oxide coating with limited transmission in the short wave region of the visible spectrum, particularly in the blue region of less than 450 nm, to improve the characteristics of the lamp, and especially to improve the color rendition characteristics of some of such lamps which may contain sodium in the fill, and to control the W transmission of the lamps, and further to provide a simple and easy process to make such lamps.
Brie~ly, a filter coating is applied to the outer sur~ace of shortwave~egion o~ t~e at least part o~ the discharge vessel ~or fil~erlng the/v1sible spectrum ~d~la~ af~le~ W spectral components of light emitted by the lamp upon operation of the lamp. The coating, in operation of the lamp, is heated to a temperature of at least 6000C, and is made of Tio27o~ CeO2, in which the coating is applied, ~f/TiO2, at a rate of up to about 0.6 mg/cm2.
To make the lamp, the coating is applied to the outer surface of the discharge lamp in an intermediate step o~ making the discharge vessel from a glass tube, or caning, for example by powder spraying and subsequent sintering of the coatin~ to obtain a mat coating, or by powder spraying and then heating to about 1,200 to 1,700C to melt the sprayed coating on the glass surface to obtain a clear layer coating, in which the oxide will form a doping in the outer layer o~ the glass which will form the -~
discharge vessel.
The invention is generally based on the surprising discovery that the filter effect o~ a Tio2 or CeO2 coating can be used to improve the color characteristics of the emitted light, particularly the color locus and color rendition in certain lamps. These coatings were previously considered undesirable as affecting the shortwave visible spectral region, particularly in the violet and blue spectral region, below 450 nm wavelength.
1 1. MAI 1919 4 11~ 1 6. MAI 19 9 4 ~4 s.

-` 2~2~253 A specific characteristic of the oxides used facilitates obtaining the desired results. At high temperatures, the absorption edge shifts towards higher wavelength. This means that, for example, thicknesses of layers can he reduced when this effect is considered, so that the light transmission of the layer can be increased. Other characteristics of the lamp, for example, the light efficacy, namely light output per power input, is hardly decreased, and, rather, improved over prior art lamps.
The infrared (IR) and W absorbing characteristics of titanium oxide and cerium oxidehave been known already for a long time in lamp construction; however, the poor transmissivity within the visible range was previously considered as a limiting factor in their use and as disadvantageous, see, for example, the discussion in U.S. Patent 5,214,345, Saito et al.
The invention utilizes the special filter effect of70xides at high temperatures/in the shorter wavelengthPspectral region.
This effect can be observed and used both in mat coatings, as well as in clear coatings. By changing the thickness of the layer and/or the type of layer - clear or mat, respectlvely, absorption o~ shortwave radiation can be controllably changed.
This absorption does not only decrease the proportion of W
radiation, but also the shortwave proportion of the visible radiation, primarily below 450 nm, but also longer wave radiation. Thus, the color temperature of the overall light output is decreased.
In accordance with the invention, particular design color an~ maintaine~
characteristics can be obtalned/,even lf a lamp is used at low power. For example, an ordinary 70 W lamp, without any coating, can be utilized with a suitable coating with only 50 W, without -~
impairing the color characteristics of the light output.
~onversely, the concept of the coating in accordance with the ~ -present invention can be used to change the color temperatures of lamps, while maintaining the power rating.
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`-~ 212~2~3 In accordance with the present invention, it is possible to readily control the color temperature of a lamp within a certain color rendition index, ~or example, to decrease the color temperature by ab ut 500 K ithi tChllriht~ihght warm-white (WDL),which corresponds to a color temperature of about 2,600 - 3,300 K. Further, it is possible to change a lamp which has a different color temperature to one of a lower temperature, for example, to decrease a lampgo~NDL (neutral white), corresponding to a color temperature of about 3,600 -4,500 Kgto/WDL, and use, for both of those lamps, the same fill ~-system. Color temperatures can even be lowered by more than 1,200 K by the coating according to the invention.
The foregoing has far-reaching consequences, particularly when sodium halides are contained within the fill, which, as well known, cause difficulties. The "Technisch-wissenschaftliche Abhandlungen der OSRAM-Gesellschaft", Vol. 12, published by ~ ~
Springer, Heidelberg, lg86, p. 11 et seq., particularly pages 14 ~ -and 15, describe that sodium ions have a tendency to diffuse through the quartz glass, ordinarily used as the glass bulb material in metal halide discharge lamps. The reason is the generatio ~ electrons ~rom frame portions in the outer bulb by the photo effect. Sodium containing fills can be used, therefore, only by using complex measures in order to obtain acceptable -lifetime. Up to now, only when comparatively high color temperatures were considered, about 5,300 K, corresponding to the light color D, it was possible to eliminate the use o~ NaI, and rather use a cesium iodide in thP fill. It had not been possible -heretofore, however, to eliminate sodium iodide when lower color temperatures were desired, particularly in the warmer light ~ ~
colors WDL and NDL, corresponding, respectively, to color ~--temperatures of about 2,600 K to 4,500 K.
It is now possible, in accordance with khe invention, to -~
provide metal halide discharge lamps with NDL, that is neutral 1 1. MAI l 9 9 1~ _5 ~( 1 6 ~1~1 1994 .. . .. .. ... . . . . . . .
. - . . - - - -. , `-- 2:1252~3 light colors utilizing fills containing cesium iodide, CsI, while eliminating sodium iodide. This is a breakthrough in the development of metal halide lamps. Fills which are known and which are to provide light colors D, corresponding, generally, to average daylight, can ba used, with the coating in accordance with the invention, for lower color tempsra~ures, for example, light color NDL. Typical fills include iodides of Cs, Tl, and the metals Dy, Ho and Tm.
Particular advantages are o~tained when fills including sodium are used for WDL and, possibly NDL lamps. Use of the coating in accordance with the present invention is particularly important, since the light of short wavelengths, causing affects the content the photo effect wh~ch /sodium/,is largely absorbed by the coating, thus increasing the lifetime of the lamp. The loss in sodium is delayed, so that t~e~ l is hardly used up.Consequently, ~oertion of sodium in the fill, even at lower color temperature, cann~We reduced. Thus, the rare earth halides, particularly of lam~?s having : .
Dy, Ho, Tm,can be used, which, previously, were used only ln/high color temperatures. This greatly improves the color rendition of the lamp.
The coating can, basically, be used in two ways, namely either by mat coatings in which the absorption in the -~
blue spectral range is primary, or by clear coatings which additionally have particularly effective W absorption.
Use of mat coatings off.ers-addi~ï'ona'l~''advan1tages. Such -,-~
coatings, besides the filtering effect, have light dispersing -~
characteristics. Such coatings can be applied, as will be described below, to the outer surface of the discharge vessel, effectively resistant against removal by wiping or surface contacts. A typi~al rate of application, with reference to Tio is, pre~erably, between about 0.05 and 0.3 mg/cmZ. This corres~onds, roughly estimated, to layer thicknesses of between 0.2 to 1.3 ~m. Corresponding application rates for Ce containing 4 11. MAIl99~ -6-, S 5Ç~
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~1 2~2~3 layers can be ob~ained by comparison of the atomic weights and will be about 0.1 to 0.~5 mg/cm2, which is valid bo~h for CeOz as well as for Ce203.
Mat layers have the particular characteristics of multiple reflection, which, in effect, increases the path length o~ the light through the layer. This increases the operating temperature of the arc tube which, in turn, increases the halide vapor pressure, and, in turn, increases the light output. This increased light output compensates, largely the additional absorption, which increases through the thickness o~ the applied mat layer. A mat layer, further, improves the uniformity of emitted light radiation, and at the same time the mixture of the color of the light being radiated. This means that various zones within the discharge arc may generate different hu es of colors which, then, by multiple dispersion, are intermixed. This characteristic is particularly important when used in luminaires or light fixtures.
Increasing the vapor pressure, due to increased operating temperatures, lowers the color temperature of the lamp, while increasing the light output. Thus, by suita~ly controlling the ~-characteristics of the mat layer, the mat layer can be used to improve the overall light output, when it is so applied that the improvement in light output due to increased temperature exceeds the losses due to absorption in the filter coating. A few experiments can determine the appropriate coating layer/in any ~pecific bulb or vessel.
Another mode of applying the filter layer is use of clear layers which, in end result, effectively results in doping of the outer layers of the quartz glass vessel:-The-doping dthrelsnnsertoUrf~sce7 This avoids three serious difficulties of the lamps of the type described in U.S. Patent 4,985,275, Takemura et al, in that the manufacture is simplified since the outer surface o~ the tube from which the discharge vessal is formed is easily accessible, the wall o~ the discharge vessel ls~uni~orm~y~eated ~nVop~ration, - since the radiation first passes through the wall of the vessel and is 1 MA119 9 ~ , S~4 1 6. M~119 9 ~ ~7~

2~ 2525i3 absorbed then and not, already, at the inside o~ ~he wall of the vessel and, additionally, reaction of the coating wi~h the fill is eliminated. This is particularly important when ~ills including sodium are used which always present problems.
~ypical layers which are clear are applied at a rate of between about 0.05 and 0.6 mg/cm2, corresponding, roughly estimated, to layer thicXnesses of 0.2 to 2.6 ~m, if titanium dioxide is used. In specific cases, thicker layers, that is, a higher rate of application can be used. The preferred maximum rate of application in mat layers is about 0.4 mg/cm2. It is determined by increasing absorption as the layer thic~ness increases. In clear layers, the upper limit is determined ;~
essentially by the doping characteristics. The minimum rate of application is determined by loss of effective filtering effect.
The filter layers in accordance with the invention can be used both in single-ended metal halide lamps, as well as in double-ended metal halide lamps, independently of their power rating. Frequently, an additional outer envelope or ~ulb is used -to decrease heat losses. The thickness of the layer is affected, ~ ~
in specific instances, by the operating temperature at the outer ~ --surface of the discharge vessel. A minimum temperature of the layer of 600~C is desirable in order to obtain a high filter .. .. . _ . _ .. ..... .
effect - even when using very thin layers - by taking advantage of a temperature shift of the absorption edge of the filter, since these thin layers would be only weakly absorbent in the shortwave portion of the visible spectral region without the phenomenon o~ temperature shift. A practical upper li~it .......... ..
for the layers at the time of this invention is at about 980C, sinca, above this value, the quartz glass of the vessel has a tendency to devitrify.
The invention has an additional advantage: it is not necessary, under some circumstances, to provide the usual heat damming, or heat retention coatings at the end portions of the vessels, made of Zro2 or a similar material, in dependence on the distribution of the coating in accordance with the invention, and 1 1. MAI 19 9 ~ ,? 5~ ~4 ~ 9~ 8-~ ~5. ~

2~2~2~3 ~ the type of fill. Eliminating such heat damming regions simplifies the manufacture and improves the light output ~ radiation characteristics of the lamp.
Preferably, the coating is applied to the entire outer surface of the discharge vessel. If heat damming portions are provided, it can be applied between opposite edges o~ the heat damming regions, which, normally, are cup, or hollow shell shaped. The Tio2 layer, however, can also be, in a simple manufacturing step, applied without problems over a ZrO2 heat damming layer.
The coating can be applied, basically, in various ways.
The coating can be applied on a discharge vessel which ~as filled and sealed. To do so, a suspension of an oxide powder of titanium, or cerium, respectively, with a binder of nitro cellulose is made. The primary grain distribution of the powder have its center should / at 30 nm corresponding to a BET surface of 50 mZ/g.
The "BET" surface is based on the theory of physical multi-layer adsorption developed by S. Brunauer, P.H. Emmett and E.
Teller. This theory utilizes simplified assumptions with respect to the adsorption process which, however, as far as the effect is concerned within the usually pertinent regions, compensate each other. The theory is well-known and has been published in the Ann. Chem. Soc., 60 (1938) 309, and "Metall", Vol. 32 (1978), pp. 678 et. seq., article by W. Danneberg and ~.H. Kuhlmann, Z5 "Messapparatur fur die spezifische Oberflache von Metalloxid und Metallpulvern" (Measuring Apparatus for the Specific Surface o~
Metal Oxide and Metal Powders).
It is extremely diffi lt t the th~kn/ess ~hi Accordingly, rates of application of layers are usually used as the controlling parameter when making the layers. Generally, the thickness of the layers is in the order of between about 0.2 to 2.6 ~m, which corresponds to a rate of application, for titanium oxide, of 0.05 to 0.6 mg/cm2.
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The finished discharge vessel is dipped into the suspension, or, alternatively, sprayed with the suspension. The dischar~e vessel is then fired at high temperature, during which the binder vaporizes. This provides mat coatings which, however, are not especially resistant to abrasion, or wiping.
In accordance with a pre~erred embodiment, the oxide powder is applied, without any binder, by a powder spray process on the discharge vessel, in a flame spray process in which the powder is directly applied on the vessel. This does not require a firing and mat coatings are generated which are highly resistant to removal by wiping, or abrading contact.
The most preferred process, particularly when the lamps are made automatically, - as disclosed, for example, in U.S. Patent 5,037,342, Barthelmes, assigned to the assignee of the present application, in which the discharge vessel is formed from smooth tubes or caning by compression and shaped blowing, is: the coating of the outer surface is applied as an intermediate step before the finished discharge vessel is made. The application of the coating at the outside, thus, can be easily integrated into an automatic production sequence.
The coating can be applied at various operating steps in the -~
produc~ion process. For example, the smooth tubular portion, or caning, can be first coated. This is done, as in the finished discharge vessel, by spraying, pressure spraying, dipping, application by printing, or powder spraying, particularly flame powder spraying. Care should be taken that the ends of the tubes are uncoated, since, later on, molybdenum foil pinch seals are to be applied at the ends.
Subsequent compression-bulging, and blowing of the tube at high temperature, up to about 2,000C, implicitly and inherently results in burning-in of the outer layer and the discharge vessel will thus be coated with a clear layer.

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In an alternative process, the discharge vessel is first formed by compression and blowing o~ the smooth tube and the so preformed tube is coated, particularly by spraying or powder spraying, but only in the region of the preformed portion of the blank. Preferably, the ooating is applied at a time while the blank is still hot, that is, directly after pre-shaping of the blank. To obtain a mat coating, the blank, as coated, is sintered at about 500C. To make a clear coating, the blank, as coated, is heated to a temperature of 1,200 to 1,500C, which melts the outer layer, so that the oxide layer at the outer surface diffuses into the blank, so that the quartz glass will receive a gradual doping. Thereafter, if desired, the final blow molding of the vessel can be carried out. Subsequently, the blank is further handled and supplied with the appropriate fill, and sealed, as described in the referenced Barthelmes et al Patent 5,037,342j for instance.
The processes in accordance with the present invention permit application of very thin layers which, nevertheless, are highly effective. The discharge space itself is free from TiO2 or Ce203, or other titanium or cerium oxide compounds. The characteristics of the quartæ glass are not affected, and correspond to those of undoped, or uncoated quartz glass. This is particularly important when used in metal halide discharge lamps. The coating, or doping, respectively, is isolated from the discharge space, the electrodes, and the fill therein, by the thickness of the quartz gla~s wall of the discharge vessel.
DRAWINGS.
Fig. 1 is a highly schematic side view of a single ended metal halide discharge lamp having the coating in accordance with the present invention;
Fig. ~ is a highly schematic side view o~ a double-ended metal halide discharge lamp within an outer envelope having the coating in accordance with the present invention;

1 1. MAIl9 ~ 16.MAIl9S4 ~ 1~.S, ~ 21252~3 Fig. 3 is a graph illustrating the color loci as a function of layer-thickness for lamps in accordance with Fig. 2;
Fig. 4, collectively, shows various characteristics tordinate) as a function of layer -thi~kAess (abscissa) for lamps according to Fig. 2, wherein Fig. 4a illustrates the W A proportion;
Fig. 4b shows the spectral range up to 545 nm;
radiation emitted ln the Fig. 4c illustrates the proportlon oftred spectral range;

Fig. 4d illustrates the X coordinate of the color locus;
Fig. 4e illustrates the Y coordinate:- of the color locus;
Fig. 4f illustrates the light output in lumens per watt; -Fig. 5 illustrates light transmissivity, in percent, of various coatings as a function of wavelength in nm (abscissa), and at various temperatures;
Fig. 6, collectively, shows graphs comparing the spectrums (abscissa) of a discharge vessel with and without coating~ in which s Fig. 6a is for a mat coating, and Fig. 6b is for a clear coating;
Fig. 7, collectively, shows a plurality of graphs -of output light for various shortwave radiation regions (abscissa) with clear and mat coatings, with respect to coating layer thicknesses expressed as weight of layer/pger squarP ~-centimeters ~mg/cm2), in which Fig. 7a shows the portion of radiation less than 545 nm wavelength; -~
Fig. 7b shows the portion with respect to the W A range;
Fig. 7c shows the portion with respect to the W B range, and Fig. 7d shows the portion with respect to the UV C range.
DETAILED DESCRIPTION.
Referring first to Fig. 1: a single ended metal halide -~
discharge lamp 1 is shown, for a power rating of 150 W, and light for light color WDL (warm-white). The lamp 1 has a discharge -~
vessel 2 o~ quart~ glass which is surrounded by an outer bulb or 1 1. MAI l ~ S
1 6. MAI 19 9 ~ q~ -12-. .

` - 212~253 envelope 3 of hard glass. The envelope 3 also is single ended and pinch sealed at one end. The space between the discharge vessal 2 and the envelope 3 is evacuated. A getter 14', electrically unconnected, is located in that space.
Two bent-over electrodes 4, facing each other, axe located within the discharge space o~ the discharge vessel 2. Foils 5 within the pinch seal 10 connect the electrodes ~ with current supply leads 6 passing through the interior space of the outer bulb 3. The current supply leads 6 are bondPd, for example by lo welding, to foils7 in the pinch seal of the outer ~ulb. The foils 7, in turn, are connected to external current supply leads 8, for connection of the lamp to a suitable source of electrical energy.
The discharge vessel 2 is filled with a fill including a system of sodium and rare earth, with metal halides as follows:
40% NaI, 20~ TmI3, 15% DyI3, 20% TlI and 5% HfI4. All percentages by weight.
In accordance with a feature of the invention, the discharge vessel 2 is almost entirely coated at the outer surface thereof with a mat coating 9 of Tioz. The operating temperature of the discharge vessel, when the lamp is energized, is such that the temperature of the coating 9 is at about 930C.
The Table 1, below, clearly shows the influence of the Tio2 coating, by comparison of the light output and the characteristics with an uncoated lamp, otherwise identical to the lamp shown in Fig. 1 without an outer bulb, example a; a coated exam~le lamp in accordance with the invention without the outer bulb 3,/~, and a coated lamp with the outer bulb ~,/.As can be seen from the Table, the color rendition index (CRI~ Ra is improved ~rom 41 to 70, with an improved light output, which increases from just over 12,000 lumens (lm) to about 13,000 lumens, as clearly seen by comparing ~ows a and b in Table 1. When using the discharge vassel 2, coated with a coating 9 within an outer bulb 3, the CRI
Ra, as well as the light output is improved even further,see test row c.

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150 W Lamps~W L/Single-Ended . _ ~.. . --Test - Light Color Ra Tioz Outer Sample Output Temp. (K) Layer;Bulb (lm) Weight (mg/cm2) I
_ . .. __ _ . __ _ _~
a 12,250 5,750 41 none no I
_~ _ __ __ ._ . Il b 13,000 3,450 70 0.30 no I
~ .... . _ I
c 13,300 2,950 91 0.30 yes ¦
10 ' _ - ---.- ._= ~
The freely operating discharge vessel, test a, provides -~
poor color~characteristics with high color temperature due to convection cooling. By coating, for example by dipping the lamp, the color temperature drops sharply, coupled with an improvement in color rendition, Ra, as clearly seen in test b. ~-~
Encapsulating the lamp within an outer envelope 3 further improves the thermal charac~eristics of the lamp and this further thermal improvement results in light outpu~s a~/c~arac~eristics which, heretofore, were unattaina~le.
The filtering effect in the shortwave regions of the ; -spectrum is particularly noticeable below wavelengths of 450 nm; -~
minor effects can be noticedj - although to a much slighter extent,upto560 nm. Conversely, however, an increase in radiation output in the long wave range is obtained, which is particularly -;-noticeable i~n the red portion of the emitted light spectrum, which increases from 5~, of an uncoated lamp/to about 16.4% of red portion -(test b) A further improvement is obtained to 24.5% of the red portion when the discharge vessel 2 is lacated within an outer bulb 3, corresponding to--;- test c in Table 1.
Fig. 2 illustratas a 70 W lamp 11 which has a double-ended pinch seal discharge vessel 12 of quartz glass, surrounded by an 9~ 3~ ~ ;

1 6.MAIl99~, 212~2~3 outer bulb or envelope 13 which, likewise, is double-ended. The space between the discharge vessel 12 and the outer bulb 13 is evacuated. The electrodes 14,15 are melt sealed by foils 16,17 in the discharge vessel 12, and connected via current supply leads 18,19, and sealing foils 20,21 in the outer bulb l~, pinch sealed therein, to electrical terminals of ceramic bases 22,23.
Short connecting stubs connect the electrically conductive portions in the bases 22,23 to the pinch sealed foils 20,21. One of the pinch seals o~ tha discharge vessel 12 has, additionally, a wire melt sealed therein, electrically unconnected to the electrodes, which carries a getter material 24.
The ends 25,26 of the discharge vessel12 are coated with a heat reflective coating of ZrO2 in form of half shells extending about the ends 25,26 of the discharge vessel 12 and over at least a portion of the pinch seal, in the form of generally spherical caps. The spacingqof the two caps from each other is about 9 mm.
In accordance with a feature of the invention, the central portion of the discharge vessel betwaen the caps of ZrO2 is coated with a mat Tio2 coating 27a. The separating line between the ZrO2 coatings and the Tio2 is shown by broken lines 25a,26a, spaced by distance d, in Fig. 2. The separating line is not visible as such by mere visual inspection.
In accordance with an alternative feature of the invention, the heat retention coating or heat reflecting coatings of ZrO2 can be omitted entirely and, instead, a coating of Tio2 is placed over the entire discharge vessel up to a portion of the pinch seal, as illustrated in Fig. 2. The technical data of such a second of example lamp are shown in the / row / c of Table 3 below. This alternative has the advantage of simplified manufacture, while rendition giving up some improvement in color/characteristics.
In accordance with another feature of the invention, which is also relatively simple to make, the discharge vessel first has the heat damming or heat retention coating of 2rO2 applied, and IAI1994 /~ ~S.9 1 6.~119 ... . - .. ` ~ , .
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then a coating of Tio2 is applied thereover. Again, in the drawing, the Figure will be identical to Fig. 2. The Tio2 coating is now extended to the coating portions 27b and 27c beyond the central coating 27a on the central portion 27, and over the heat damming or heat retention coatings at the ~nds 25,26, includlng a portion o~ the pinch seals at the end of the discharge vessel 12.
Fig. 3 is a graph of the plurality o~ color loci, drawn for a lamp in accordance with this embodiment with a spacing d of the caps 25,26, that is, of the lines 25a,26a/of 9 mm. The thickness o~ the Tio2 layer varies between 0, that is, no coating at ~1-measuring point a and 0.3 mg/cm2 at measuring point e. As can be clearly seen, the original color temperature of the uncoated lamp, having a - -; color temperature of 3,800 K/is lowered to about 3,000 K. The broken line of Fig. 3 connects the various loci. The optimal color locus, which is on the Planck curve P, shown as a ~ull line, and which corresponds to a color temperature of 3,300 K, is obtained with a coating of ~:
about 0.08 mg! cmZ, and corresponds to the color coordinates X = 0.417 and Y = 0.396.
Table 2 summarizes the data from which the curves ofFig. 3 ~;-were derived, in terms of measuxing points and coating thickness, expressed ~s theweight of the layer per unit area (herein ofte~ referred to -as the rate of application of coating) in milligrams per centimeter squared. --.
Measuring Point Layer weight ~: ~mg/cm2) .... _ . . Il b 0.10 ~ I
.... ~ O. 19 I ,:
0.24 11 _ e ___ _ __ -l l. MA119 ~ lt.
' w ~ ~, 6 ~ 9g~t ~ ~.q4 -16--- .

` -`` 212~253 Fig. 4, collectively, shows the relationship of various parameters of a lamp in accordance with Fig~ 2 as a function of weight of the layer, representing thickness of the Tio2 layer, in each instance, represented by the abscissa.
Figs. 4a and 4b illustrate that it is possible to reduce the portion of W -A and shortwave visible radia~lon/already at coating layers having thicknesses corresponding to coating weights of 0.10 mg/cm2 of titanium dioxide to a substantial extent. The red proportion, as shown in Fig. 4c, and the x and y coordinates of the color locus, Figs. 4d and 4e, respectively, are increased.
Fig. 4f illustrates the relationship of light efficacy with respect to the titanium dioxide layer. This light ef~icacy decreases at higher weights of the layer, corresponding to higher thicknesses, especially beyond 0.15 mg/cm2. At the optimum value of 0.08 mg/cm2, the drop in lightefficacy is practically negligible. The ordinate in Fig. 4e represents lumens per watt.
~able 3 shows results o~ measurements with a system of a lamp of Fig. 2, with and without Tioz layers and for various spacings of the caps 27b,27c. The fill used in the lamp of Fig.
2, on which Table 3 is based, is known as such, and uses a sodium rare earth fill, in which the rare earthsused are Ho, Tm and Dy, and additionally contains Tl. Only iodine is used as the halogen. The fill contains, by weight, the following metal halides: NaI 32.5~, DyI3 19.5%, HoI3 19.5%, TmI3 1905%
and TlI 9.0%. Normally, the light color neutral white, NDL, is obtained, corrPsponding to a color temperature of between about 4,200 --4,500 K if no Tio2 coating is used. By changing the spacing d of the end caps 25,26, that is, by spreading the separating lines 25a,26a, it is possible to somewhat vary the color temperature, compare measurements in Table 3,~examples a and b. Upon application of a Tio2 coating having a weight of 0.19 mg/cm2, the same ~ill system can be used to obtain the light 11. MAI1994 _l7_ ~)~ 1 6. MAI 1 9 9 ~7,5`.~4 12~2~3 color warm white, WDL, with a color temperature of about 3,050 K, which results in a drop o~ 1,200 K - compare first and second measurements of the rows a and b, respectively. Another group of measurements was made, test row c, in which the heat retention coatings were entirely replaced by a Tio2 coating.
This causes a similar drop in the color temperature. The operating temperature of the coating remains constant at about 930C. In the Table 3,example c, the heat da~aring regions made of ZrO
(first row) are fully replaced by an overall coating of TiO2 (second row). -~
Fig. 5 illustra~es~e e~fect of the coatings on the spectral distribution of light output, in which the abscissa represents~light output wavelengths in nanometers, and the ordinate, transmissivity of the vessel, with clear, or mat coatings, respectively. Fig. 5 is drawn with respect to a predetermined coating thickness of Tio2 of 0.3 mg/cm2, and for temperatures of the layer of 25C and 930C, respectively, for mat, or clear coatings, which are separately shown for the high tempera- ~-ture. The graphs of Fig. 5 are derlved, indlrectly, from the spectrum of the lamp.
The effect for clear coatings which comprise a layer of CeO2 and Tio2 in proportion of 4:1 by weight -i is also shown, both for room t~mperature and 800C.
The differential characteristics of clear and mat layers can be demonstrated by this example. The discharge vessel was completely coated with Tio2, without any heat damming caps.
The lamp characteristics change, as can be seen from Table 4, in dependence of whether the coating of Tio2 is clear, or mat.
The drop in color temperature is substantially less with a clear coating than with a mat coating. The color temperature differences ~Tn between an uncoated lamp and a clear coated lamp is about -200 K; for a mat layer ~Tn = -1000 K. It is believed t~at the foregoing effect is due to the shift in the absorption edge which terminates in test a at about 450 nm for a clear fired layer, whereas, for a mat layer, which is not fired, the ~-~)~ 1 6. MAI 19 9 ~ 5~f ,q4 21252~3 absorptioend~8nds at about 550 nm. Consequently, the influence o~
the mat layer on the spectral radiation power is more intense, as seen by comparing Figs. 6a and 6b. Fig. 6 shows the spectral distribution, in which the abscissa represents wavelengths in S nanomet2rs, and the ordinate light output in milliwatts~nm. Fig.
6a illustrates the spectrum ~or a mat layer, and Fig. 6b for a clear layer. The color locus is shifted, as seen in Table 4, to higher x and y values in the mat layer. The light flux ~(ln lumens) decreases. The color rendition Ra remains practically unchanged in both instances, and is good, that is, Ra = 80 for theclearlayer and 86 for the mat layer.
The influence of the different absorption edge when the layers are clear or mat, respectively, cah also be clearly seen when comparing the portion of radiation at various wavelengths.
Fig. 7, collectively, shows the portions of radiations in watt for various wavelengths, as a function of layer thickness, as represented by rate of application of the layer, in mg/cm2. In Fig. 7a, wavelengths are shown for less than 545 nm. Fig. 7b illustrates the relationship for wavelengths in the ultraviolet-A
range, Fig. 7c for the W -B range and Fig. 7d for the W -C range.
The ordinates in Fig. 7 are represented as light power output in watts. In Figs. 7a through 7d, the mat layers are shown by a solid line and the clear layers by a broken line.
As can be seen by comparing Figs. 7~ through 7d, the two layers behave essentially identically in the ultraviolet (W) rangheW~ mat layer is much more e~fective in tha blue spectral region, see Fig. 7a, than the clear layer, which is to be expected because the former's absorption edge reaches further into the longer wavelength region of the spectrum. The filters in accordance with the present invention permit, typically, reduction of the shortwave radiation portion by 20 to 30%.
Table 5 illustrates another use for the coating in accordance with the present invention. A lamp, as described in 1~ ( 1 6.MAIl99~ 9-?. ~
5`.q(f 5 ~

.. -:
Fig. 2, for a 70 W lamp, is filled with a well-known WDL fill, as described in connection with the lamp in Fig. 1. The coating is used, essentially, only to improve the lifetime of the lamp, by decreasing the sodium loss due to the filter effect of the coating for shortwave radiation. In dependence on t ~ c~néss~oPf Y
the coating layer, the lifetime of the lamp can be increased from, originally, 6,000 hours by up to 50%. Improvement in the Ra value, however, is then not obtainable. This use is particularly important commercially because extremely low color temperatures, 2,700 K for example, can be obtained which, heretofor~, had been believed to be unattainable with lamps of this type.
It is believed that the improvement in lifetime is based on two effects: at the beginning of operation of the lamp, it is particularly important to effectively shield the W -C radiation, since the energy of the ~v-C radia-tion exceeds the electron work -function for the molybdenum current supplies, which is 4.15 eV.
Diffusion, which cannot be avoided, of sodium into the outer bulb, causes deposition of sodium ions on the molybdenum current supply . This decreases ~eye~ectron emission work to 2.2 eV, YhiC~bCo~e~4~ ~mt. Thus, during operation of the lamp, and particularly towards the end of the life of the lamp, it is equally important to absorb the longer /-g~radiation up into the blue spectral region . This has become posslble/by coating the lamp, in accordance with the present invention, without affecting other chaxacteristics thereof.
In accordance with a particularly interesting application of the present invantion, the coating of the lamp is used with fill systems which include the well-known sodium scandium or sodium tin fill systems. In such fill systems, the wall temperature should ~e between about 700-750C, so that the absorption edge of the coating reaches not so farinto the longer ~ng portion of the spectral region (see Fig~ 5).
~j` b 1 i.MAI1994 16 MAI199~ -20-q~t -.:: : , . : :: - - - :.- ~- ~. -2~2~2~3 Various fill systems, particularly arranged for use of the . which ~eaulre different wall temperatures coatlng of the present inventloh can be used/,and accordingly show different absorption characteristics of the coatings~
Additional parameters for fin~ tuning are the thickness/o~ ~he layer, and the use of either clear or mat coatings.
Cerium oxide, Ce203, under some circumstances also CeOz, is similar in many respects to Tio2 in its use. The coating thicknesses with such materials can be essentially similar for those given for Tio2. The rate of application, that is, the layer thicknessPs ~y weight must, however, be doubled or tripled, respectively. Analogous considerations obtain for a mixture of the two types of coatings, namely using cerium oxides and titanium oxides, together.
Various changes and modifications may be made and any features disclosed herein may be used with any of the others, within the scope of the inventive concept.

1 1. M~l 19 9 1 6. MAI l 9 9 ~, ~;~ -4 ~ ::

-21- ~

. TABLE 3 70 W Lam~/NDL~Double Ended Light Color Color Rz Tio2 ¦ Cap 5Output Locus Temp. Layer ~ Spacing _ _ ~ x ¦ Y (K) _ _ I
a)jS853¦ .361¦.370¦ 4500 ¦ 721 no ¦ 10.5 m~ :
5570 ! .439I.41OI _3050 ! 82l Yes I 1 5 ~ .
lOb~ 6141¦ .3691 .364 ¦ 4200 ¦ 7g1 no ¦ g mm _ _ _ _ 54571 .437l .~ 3050 87l yes ~ mm c)17181l .385l.36J1- 3800 ! 851 no 1 9 ¦ 5342¦ .437¦.426l 3150 L 80¦ _YeS~ I_ no HQI-TS TY~e 70 W/NDL~Double-Ended . i ~ T~ - Temp. _ _ L X y (K) Ra 0(1m) -:
a) Wi~h Tioz Layer ~le~rl.40 8 401 3500 80 5900 Uncoated 1.391 377 3700 ¦ 81 1 65Q0 Diffarence ¦.011 024. _-200 ¦ -1 -600 b) With Tio Layer Mat ~455 .420 2800 ¦ 86 5300 . ~Uncoate~ 386 3800 T 6 1 6500 L Difference 069¦ .054 ¦-1000 ¦ 0 -1200 ~, -22_ :

212~i253 .

70 W Lam~/WDL/Double-Ended S Light ~ ~ Color Ra T~02- Layer Lifetime Outpu~ x Y Temp~ ~ Weight ~ours (lm) _ t~) . _ ~Std.) 6500 .42~ .402 3200 83 _ 6000 , . .. ~. ~ .
6600 .430 424 3270 80 0.1 mg/cm 8100 5600 .470 433 2723 ¦ 81 0. 23 mg/cm 8700 - .- .

Claims (23)

1. A metal halide discharge lamp having a discharge vessel (2,12) of quartz glass;
a fill in the discharge vessel including an ignition gas, mercury, and a metal halide;
two electrodes (4,14,15) located in the vessel and being gas tightly retained therein; and a coating (9,27a,27b) at the outer surface of at least part of the discharge vessel, wherein, in accordance with the invention, the coating comprises a filter coating acting in the short wavelength portion (below 545 nm) of the visible spectrum emitted by the lamp in operation upon energization of the electrodes, said coating comprising an oxide of at least one of titanium or cerium;
the coating, in operation of the lamp, being heated to a temperature of at least 600 C; and the coating has a coating thickness which results when coating the outer surface of at least part of the discharge vessel at the rate of up to 0.6 mg/cm2 by applying titanium oxide or, by applying a cerium oxide, at a rate of up to about 1.3 mg/cm2.

2. The lamp of claim 1, wherein the coating comprises titanium dioxide (TiO2).

3. The lamp of claim 1, wherein the coating is applied so as to filter radiation at wavelengths below 450 nm.

4. The lamp of claim 1, wherein the coating is mat, and the weight of the coating, with respect to TiO2, is up to 0.4 mg/cm2.

5. The lamp of claim 1, wherein the weight of the coating is at least 0.05 mg/cm2.

6. The lamp of claim 1, wherein the coating covers the entire outer surface of the discharge vessel.

7. The lamp of claim 1, further including heat damming or heat retention coatings applied to portions of the outer surface of the discharge vessel.

8. The lamp of claim 1, further including an outer bulb or envelope (3;13) surrounding the discharge vessel.

9. The lamp of claim 8, wherein, to obtain a color temperature of light output, upon energization of the lamp, of about 3,600 to 4,500 K, corresponding to light of NDL, or neutral white characteristic, the metal halide include cesium as the alkaline metal of a metal halide composition.

10. The lamp of claim 8, wherein, in order to obtain a color temperature of light output, upon energization of the lamp in the order of about 2,600 to 3,400 corresponding to the light color of WDL, or warm white characteristic, the metal halide composition includes rare earth metals and furthermore sodium as alkaline metal.

11. The lamp of claim 1, wherein the coating is mat and has light diffusing characteristics.

12. The lamp of claim 1, wherein the coating is clear and, in part, forms a doping of at least part of the outer surface of the discharge vessel.

13. The lamp of claim 1, wherein the coating comprises a cerium oxide.

14. The lamp of claim 1, wherein the coating comprises a mixture of titanium oxide and a cerium oxide.

15. The lamp of claim 1, wherein said filter coating is applied on the discharge vessel, at least in the region of the arc emitting the light from the lamp, upon energization of the electrodes (4;14,15).

16. The lamp of claim 15, including additional heat damming or heat retention coatings applied on the discharge vessel in regions remote from the major portion of light emitted from the arc between the electrodes, upon energization of the electrodes.

17. The lamp of claim 1, wherein the coating improves the color rendition of the lamp.

18. The lamp of claim 1, wherein the coating improves the lifetime of the lamp.

19. A metal halide discharge lamp having a discharge vessel (2,12) of quartz glass;
a fill in the discharge vessel including an ignition gas, mercury, and a metal halide;
two electrodes (4,14,15) located in the vessel and being gas tightly retained therein; and a coating (9,27a,27b) at the outer surface of at least part of the discharge vessel, wherein, in accordance with the invention, the coating comprises a filter coating acting in the short wavelength portion (below 545 nm) of the visible spectrum emitted by the lamp in operation upon energization of the electrodes, said coating comprising an oxide of at least one of titanium or cerium;
the coating, in operation of the lamp, being heated to a temperature of at least 600°C; and said coating having a coating thickness between about 0.2 to 2.6 micrometers (µm).

20. The lamp of claim 19, wherein the coating improves the color rendition of the lamp.

21. A method to make the lamp as claimed in claim 1, wherein the lamp manufacturing method includes the step of forming the discharge vessel by shaping a quartz glass tube, and including, in accordance with the invention, the step of applying the coating of the oxide of at least one of titanium or cerium during manufacture of the discharge vessel from the glass tube.

22. The method of claim 21, wherein said coating step comprises applying the coating by powder spraying and then sintering the coating on the sprayed surface at a temperature of about 500°C, to form a mat layer or coating.

23. The method of claim 22, wherein said coating step comprises applying the coating by powder spraying and the melting-on the coating on the sprayed surface at a temperature of about 1,200 to 1,700°C to obtain a clear coating in which the oxide diffuses into the outer regions of the wall of the glass tube and behaves as a doping of the glass material.