WO2012063205A2

WO2012063205A2 - Thorium-free quartz metal halide lamps

Info

Publication number: WO2012063205A2
Application number: PCT/IB2011/054991
Authority: WO
Inventors: Junming Tu; Robinson Randy
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-11-10
Filing date: 2011-11-09
Publication date: 2012-05-18
Also published as: WO2012063205A3

Abstract

A thorium-free quartz metal halide lamp including an outer lamp envelope (120) defining an envelope space (122); a base (130) sealably connected to the outer lamp envelope (120); a discharge vessel (110) defining an inner discharge space (112), the discharge vessel (110) being disposed in the envelope space (122); at least two electrodes (114) projecting into the inner discharge space (112), the at least two electrodes (114) being thorium-free; and filling material disposed in the inner discharge space (112), the filling material including sodium iodide and at least one rare earth iodide, the filling material being thorium-free.

Description

Thorium-Free Quartz Metal Halide Lamps

[0001 ] The technical field of this disclosure relates to high-pressure gas discharge lamps, particularly, this disclosure relates to thorium-free quartz metal halide lamps.

[0002] High-pressure gas discharge lamps are widely used for outdoor and indoor lighting, sports lighting, parking lots, and other lighting applications. Quartz metal halide lamps typically include a quartz discharge vessel having thorium-doped tungsten electrodes and chemical fillings including thorium iodide.

[0003] Thorium-doped electrodes are used in quartz metal halide lamps because the thorium-doped electrode has a low work function (2.5eV), which is the minimum energy to emit electrons, and the electrons in the thorium are fairly easily dislodged, which allows the electrodes to emit electrons at lower electrode temperatures and quickly make the transition from the glow-to-arc to thermionic electron emission. The short glow-to-arc transition time helps reduce electrode sputtering during the warm-up phase and thus helps reduce wall blackening. Pure tungsten electrodes without thorium doping can be used, but the pure tungsten electrodes are operated at higher temperatures due to their high work function (4.5 eV) and the glow-to-arc transition time is long. This results in more tungsten sputtering during the warm-up phase and tungsten evaporation during normal operation, causing wall blackening. As a consequence, the lumen maintenance is reduced: around 65% after 1,000 hours for pure tungsten electrodes compared to around 90% after 1,000 hours for thorium- doped electrodes.

[0004] Unfortunately, one problem with thorium is that it is radioactive. Radioactive materials present consumer concerns, face complicated shipping regulations, and require careful disposal. Lamp designers and manufacturers are under pressure to eliminate thorium completely from quartz metal halide lamps in order to create a more environmentally friendly lamp. One approach has been to remove thorium from the electrodes, but thorium iodide remains in the chemical fillings. Another problem with the removal of thorium is maintaining lamp operating characteristics, such as power factor, which are consistent with the operating characteristics of lamps including thorium.

[0005] Therefore, it would be desirable to have a thorium-free quartz metal halide lamp that can overcome the aforementioned and other disadvantages.

[0006] One aspect of the present invention provides a high-pressure gas discharge lamp including an outer lamp envelope defining an envelope space; a base sealably connected to the outer lamp envelope; a discharge vessel defining an inner discharge space, the discharge vessel being disposed in the envelope space; at least two electrodes projecting into the inner discharge space, the at least two electrodes being thorium-free; and filling material disposed in the inner discharge space, the filling material including sodium iodide and at least one rare earth iodide, the filling material being thorium-free.

[0007] Another aspect of the present invention provides a discharge vessel for a high- pressure gas discharge lamp including a discharge vessel wall defining an inner discharge space; at least two electrodes projecting into the inner discharge space, the at least two electrodes being thorium-free; and filling material disposed in the inner discharge space, the filling material including sodium iodide and at least one rare earth iodide, the filling material being thorium-free.

[0008] The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof.

[0009] FIG. 1 is a side view of a high-pressure gas discharge lamp in accordance with the present invention.

[00010] FIG. 2 is a side view of a discharge vessel for a high-pressure gas discharge lamp in accordance with the present invention.

[00011 ] FIG. 3 is a graph of a spectrum from a high-pressure gas discharge lamp in accordance with the present invention. [00012] FIG. 4 is a graph of a spectrum from a high-pressure gas discharge lamp for an existing Na-Sc metal halide protected lamp MP320W with thorium.

[00013] FIGS. 1 and 2, in which like elements share like reference numbers, are side views of a high-pressure gas discharge lamp and a discharge vessel for a high-pressure gas discharge lamp, respectively. The high-pressure gas discharge lamp 100 includes a discharge vessel 110 mechanically supported within the envelope space 122 of an outer lamp envelope 120 and electrically coupled to a base 130. The base 130 is sealably connected to the outer lamp envelope 120. The envelope space 122 can contain an inert gas, such as nitrogen or the like. The discharge vessel 110 can be made of a material such as a ceramic or a fused quartz glass. The discharge vessel 110 has a discharge vessel wall 111 which defines an inner discharge space 112 within the discharge vessel 110 for receiving filling material disposed in the inner discharge space 112. The filling material includes sodium iodide and at least one rare earth iodide, and the filling material is thorium-free. The inner discharge space 112 can also include mercury to obtain the desired lamp voltage and an inert fill gas, such as argon or the like. At least two electrodes 114 project into the inner discharge space 112. In this example, the two electrodes 114 are electrically connected to the base 130. The electrodes 114 are thorium-free and can be made of a material such as tungsten, for example. Those skilled in the art will appreciate that the particular mechanical and electrical configurations of the lamp 100 can be selected as desired for a particular application.

[00014] The operating point of the discharge vessel 110 can be selected to provide the desired wall loading, which is defined herein as the inner Watts power per inner wall surface area, typically expressed in Watts per square centimeter. When the wall loading is increased over the wall loading used for existing thorium-containing sodium-scandium (Na-Sc) metal halide lamps, the wall temperature for the discharge vessel 110 and the arc temperature increase, resulting in a higher salt pool temperature and higher vapor pressures of the filling material. When the discharge vessel 110 is thorium free and scandium free, the wall loading can be selected to be between 110 percent and 120 percent of the wall loading for an existing thorium-containing Na-Sc lamp. When the discharge vessel 110 is thorium free and contains scandium, such as scandium iodide, the wall loading can be selected to be up to 110 percent of the wall loading for an existing thorium-containing Na-Sc lamp, keeping the wall temperature below the value at which the scandium reacts with the discharge vessel wall and reduces lumen maintenance. For example, a Philips MH400/U thorium-containing Na-Sc lamp, which normally operates with a wall loading of about 16 W/cm , can operate between 17.6 and 19.2 W/cm when the lamp is thorium free and scandium free, and up to 17.6

W/cm when the lamp is thorium free and contains scandium.

[00015] Experimental results for medium to high wattage lamps show that the wall loading correlates with the salt pool operating temperature of the filling material in the discharge vessel. A Philips MH400/U thorium-containing Na-Sc lamp operating at a 100 percent wall loading has a salt pool operating temperature of 725 to 750 degrees centigrade.

A thorium free and scandium free lamp operating at a 110 to 120 percent wall loading has a salt pool operating temperature of 775 to 800 degrees centigrade. A thorium free and scandium containing lamp operating at a 100 to 110 percent wall loading has a salt pool operating temperature of 750 to 775 degrees centigrade. The salt pool operating temperature can be selected by the geometry of the discharge vessel and/or optional lamp features, such as an optional shroud, optional end coatings, or the like.

[00016] Optional features of the lamp 100 can also be used to enhance vapor pressures of the filling material. In one exemplary embodiment, the lamp 100 has an optional shroud 116 disposed about the discharge vessel 110. The shroud 116 can be made of transparent ceramic or quartz glass. In one exemplary embodiment, the lamp 100 has optional end coatings 118 on one or both ends of the discharge vessel 110. The heat reflective end coatings 118 can be made of aluminum oxide (A1203), zirconium oxide (Zr02), or the like applied to the outer surface of the discharge vessel 110. The shroud 116 and/or the end coating 118 can be used to increase the salt pool temperature, i.e., to increase the temperature of the filling material (sodium iodide and at least one rare earth iodide) in the discharge vessel 110. This increases the vapor pressure of the filling material.

[00017] The filling material disposed in the inner discharge space 112 of the discharge vessel 110 receives electrical current through the electrodes 114 and ignites in an arc providing light from the lamp 100. The filling material is thorium-free. The filling material includes sodium iodide and at least one rare earth iodide. Rare earth iodides are defined herein to include iodides of yttrium, scandium, lanthanum, cerium, praseodymium, promethium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium. In one exemplary embodiment, the filling material includes erbium iodide in the range of 0.0 to 3.0 mole percent. Those skilled in the art will appreciate that different combinations of sodium iodide and rare earth iodides, plus additional additives, can be used as desired for particular applications and to produce light with desired characteristics. The filling material can have a low work function to allow the electrons to disassociate from the rare earth iodide at a lower temperature, speeding up the transition from glow to arc operation. The short transition reduces electrode sputtering during the lamp warm-up phase and thus reduces wall blackening.

[00018] In one exemplary embodiment, the filling material is scandium-free in addition being thorium-free. Removal of scandium avoids any quartz- scandium reaction between the discharge vessel and the filling material. Such a reaction can reduce lumen maintenance due to blackening of the discharge vessel, and is more pronounced with increased wall loadings above the standard Na-Sc lamp wall loading. Thus, scandium-free filling material can be used when operating at wall loadings above the standard Na-Sc lamp wall loading, such as wall loadings of 110 to 120 percent of a standard Na-Sc lamp wall loading.

[00019] In one exemplary embodiment, the work function of the at least one rare earth iodide or iodides is less than 3.3 electron Volts. This is the case for iodides of cerium, praseodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium. A low work function allows the electrons to disassociate from the rare earth iodide at a lower temperature, speeding up the transition from the glow-to-arc to thermionic operation. This reduces the tungsten sputtering and improves lumen

maintenance. A low work function also reduces the electrode temperature during normal lamp operation, which reduces the tungsten evaporation and wall blackening.

[00020] The filling material can include cerium iodide as the rare earth iodide with the sodium iodide. In one exemplary embodiment, the filling material includes cerium iodide in addition to the sodium iodide. In another exemplary embodiment, the filling material includes cerium iodide and holmium iodide, in addition to the sodium iodide. In another exemplary embodiment, the filling material includes cerium iodide and an iodide selected from dysprosium iodide, holmium iodide, thulium iodide, and combinations thereof, in addition to the sodium iodide. In one exemplary embodiment, the cerium iodide is in the range of 0.2 to 20 mole percent. Cerium can be used to increase lamp lumen output because it has emission lines in the visible portion of the spectrum.

[00021 ] The filling material can include scandium iodide as the rare earth iodide with the sodium iodide. In one exemplary embodiment, the filling material includes an iodide selected from cerium iodide, yttrium iodide, holmium iodide, and combinations thereof, in addition to the scandium iodide and the sodium iodide. In one example, the cerium iodide/yttrium iodide/holmium iodide is 5 to 15 weight percent of the filling material. In another exemplary embodiment, the filling material includes an iodide selected from the group consisting of dysprosium iodide, holmium iodide, thulium iodide, and combinations thereof, in addition to the scandium iodide and the sodium iodide. In one example, the dysprosium iodide/holmium iodide/thulium iodide is 15 to 50 weight percent of the filling material. To reduce the quartz- scandium reaction between the discharge vessel and the filling material and preserve acceptable lumen maintenance, the wall loading of the discharge vessel can be maintained at less than 110 percent of a standard Na-Sc lamp wall loading, such as from 100 to 110 percent of a standard Na-Sc lamp wall loading.

[00022] The filling material and/or inner discharge space can optionally include additional materials to obtain particular lamp performance as desired. In one exemplary embodiment, the filling material also includes an optional color point adjuster to adjust the color of the light from the high-pressure gas discharge lamp. Examples of color point adjusters include lithium iodide, indium iodide, thallium iodide, and combinations thereof. In one exemplary embodiment, the color point adjuster is thallium iodide at less than 8 mole percent of the filling material. In another exemplary embodiment, the color point adjuster is lithium iodide at less than 7 mole percent of the filling material.

[00023] In one exemplary embodiment, the filling material also includes an optional arc fattener to broaden the arc and reduce arc wandering. An example of an arc fattener is cesium iodide. In one exemplary embodiment, the arc fattener is cesium iodide at less than 5 mole percent of the filling material. Those skilled in the art will appreciate that in some embodiments it is desirable to limit the amount of cesium iodide to maintain the desired lumen output from the lamp.

[00024] In one exemplary embodiment, an optional iodide getter is disposed in the inner discharge space to pick up free iodine remaining from sodium loss by sodium ion migration from the inner discharge space. Examples of iodide getters include zinc metal, scandium metal, and combinations thereof. In one exemplary embodiment, the iodide getter is 0.2 mg of zinc metal. In another exemplary embodiment, the iodide getter is 0.2 mg of scandium metal.

[00025] EXAMPLES

[00026] Three sets of experimental results are provided to illustrate the efficacy of the thorium-free quartz metal halide lamps. The results demonstrate that thorium-free quartz metal halide lamps can provide light output similar to that of standard Na-Sc lamps with thorium. In some cases, the color rendering index (CRI) and/or the correlated color temperature (CCT) are higher for the thorium-free quartz metal halide lamps than for the standard Na-Sc lamps with thorium. The filling material can be selected to provide the desired light output. The notation "std" in the tables indicates a standard or existing Na-Sc lamp with thorium, with representative values provided for comparison to the experimental values.

[00027] EXAMPLE 1

[00028] In this example, the base filling material for each discharge vessel included cerium iodide as the rare earth iodide with the sodium iodide. The lamp does not contain scandium. The lamps were medium wattage quartz metal halide lamps (320W) with an

ED28 outer lamp envelope filled to 175 torr with nitrogen. The discharge vessel was made of fused quartz glass having a wall loading of 20 W/cm , a wall thickness of 1.0 mm, and filled to 80 torr with argon. The wall loading is 11 percent higher than an existing MP320W lamp containing the standard Na-Sc filling. The electrodes were pure tungsten electrodes without thorium with a separation distance of 34.5 mm. The chemical filling for each discharge vessel had a composition in mg as listed in Table 1A below.

[00029] The test results are presented in Table IB below. FIG. 3 is a graph of a spectrum from a high-pressure gas discharge lamp for discharge vessel ID 23403 at 100 hours burn time. FIG. 4 is a graph of a spectrum from a high-pressure gas discharge lamp for an existing Na-Sc metal halide protected lamp MP320W with thorium for comparison with

FIG. 3. Typical values for an existing Na-Sc metal halide protected lamp MP320W with thorium are provided in Table IB for comparison. The test results also showed that inspection of discharge vessel ID 23403 at 1000 hours burn time revealed minimal wall blackening around the electrode, which indicates very low tungsten sputtering. Table 1A

Table IB

[00030] EXAMPLE 2

[00031 ] In this example, the base filling material for each discharge vessel included cerium iodide as the rare earth iodide with the sodium iodide. The lamp does not contain scandium. The lamps were medium wattage quartz metal halide lamps (320W) with an ED28 outer lamp envelope filled to 175 torr with nitrogen. The discharge vessel was made of fused quartz glass having a wall loading of 20 W/ cm , a wall thickness of 1.0 mm, and filled to 80 torr with argon. The wall loading is 11 percent higher than the existing MP320W lamp containing the standard Na-Sc filling. The electrodes were pure tungsten electrodes without thorium with a separation distance of 34.5 mm. The lamp used a quartz shroud and an end coating made with aluminum oxide material. The chemical filling for each discharge vessel had a composition in mg as listed in Table 2A below.

[00032] The test results are presented in Table 2B below. Typical values for a standard Na-Sc MP320W lamp with thorium are provided for comparison.

Table 2A

Table 2B

[00033] EXAMPLE 3

[00034] In this example, the base filling material for each discharge vessel included scandium iodide as the rare earth iodide with the sodium iodide. The lamps were medium wattage quartz metal halide lamps (400W) with an ED37 outer lamp envelope filled to 300 torr with nitrogen. The discharge vessel was made of fused quartz glass having a wall loading of 16.1 W/cm , a wall thickness of 1.25 mm, and filled to 80 torr with argon. The wall loading is 10 percent higher than the existing MP400W lamp containing the standard Na-Sc filling. The lamp used a quartz shroud and an end coating made with aluminum oxide material. The electrodes were pure tungsten electrodes without thorium with a separation distance of 39.5 mm. The chemical filling for each discharge vessel had a composition in mg as listed in Table 3A below.

[00035] The test results are presented in Table 3B below. Typical values for a standard

Na-Sc MP400W lamp with thorium are provided for comparison.

Table 3A

Table 3B

[00036] It is important to note that FIGS. 1-3 illustrate specific applications and embodiments of the present invention, and are not intended to limit the scope of the present disclosure or claims to that which is presented therein. While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims

1. A high-pressure gas discharge lamp, the lamp comprising:

an outer lamp envelope (120) defining an envelope space (122);

a base (130) sealably connected to the outer lamp envelope (120);

a discharge vessel (110) defining an inner discharge space (112), the discharge vessel (110) being disposed in the envelope space (122);

at least two electrodes (114) projecting into the inner discharge space (112), the at least two electrodes (114) being thorium-free; and

filling material disposed in the inner discharge space (112), the filling material including sodium iodide and at least one rare earth iodide, the filling material being thorium- free.

2. The lamp of claim 1 , wherein the filling material is further scandium-free.

3. The lamp of claim 2, wherein salt pool operating temperature of the filling material is in a range of 775 to 800 degrees centigrade.

4. The lamp of claim 1 , wherein the work function of the at least one rare earth iodide is less than 3.3 electron Volts.

5. The lamp of claim 1, wherein the at least one rare earth iodide comprises cerium iodide.

6. The lamp of claim 1 , wherein the at least one rare earth iodide comprises cerium iodide and holmium iodide.

7. The lamp of claim 1, wherein the at least one rare earth iodide comprises cerium iodide and an iodide selected from the group consisting of dysprosium iodide, holmium iodide, thulium iodide, and combinations thereof.

8. The lamp of claim 1, wherein the at least one rare earth iodide comprises scandium iodide and an iodide selected from the group consisting of cerium iodide, yttrium iodide, holmium iodide, and combinations thereof.

9. The lamp of claim 1, wherein the at least one rare earth iodide comprises scandium iodide and an iodide selected from the group consisting of dysprosium iodide, holmium iodide, thulium iodide, and combinations thereof.

10. The lamp of claim 1, wherein the at least one rare earth iodide comprises scandium iodide and salt pool operating temperature of the filling material is in a range of 750 to 775 degrees centigrade.

11. The lamp of claim 1 , wherein the filling material further comprises a color point adjuster selected from the group consisting of lithium iodide, indium iodide, thallium iodide, and combinations thereof.

12. The lamp of claim 1, wherein the filling material further comprises an arc fattener.

13. The lamp of claim 10, wherein the arc fattener is cesium iodide.

14. The lamp of claim 1, further comprising an iodide getter disposed in the inner discharge space.

15. The lamp of claim 12, wherein the iodide getter is selected from the group consisting of zinc metal, scandium metal, and combinations thereof.

16. The lamp of claim 1, further comprising a shroud (116) disposed about the discharge vessel (110).

17. The lamp of claim 1, further comprising end coatings (118) disposed on the discharge vessel (110).

18. The lamp of claim 1, wherein the at least two electrodes (114) are pure tungsten.

19. A discharge vessel for a high-pressure gas discharge lamp, the discharge vessel comprising:

a discharge vessel wall (111) defining an inner discharge space (112); at least two electrodes (114) projecting into the inner discharge space (112), the at least two electrodes (114) being thorium-free; and

20. The discharge vessel of claim 19, wherein the filling material is further scandium- free.

21. The discharge vessel of claim 19, wherein the at least one rare earth iodide comprises cerium iodide.

22. The discharge vessel of claim 19, wherein the at least one rare earth iodide comprises scandium iodide.