The present disclosure relates to high intensity discharge lamps and, more particularly, to a discharge lamp configured with heat insulators at either ends of the arc tube.
Due to the ever-increasing interest in energy conserving lighting systems, metal halide lamps with higher and higher lamp efficacy are desired. Besides the optimization of metal halide fill chemistry in the arc tube, thermal control of the arc tube is also important. For metal halide lamps, the lamp performance is directly related to the vapor pressure of metal halide fill at normal working condition. The cold spot temperature of the arc tube controls the vapor pressures of the metal halide fills. The higher the cold spot temperature the higher the vapor pressures of the metal halide fills inside the arc tube. With higher vapor pressure the lamps can have better performance with higher lamp efficacy and better color rendering properties. Usually the cold spot is located behind the arc tube electrodes at the two ends of an arc tube.
Therefore, it is desirable to provide a metal halide lamp with higher cold spot temperatures in such a way that the maximum temperature of the arc tube will not exceed the limit of working temperature of the arc tube material. The metal halide vapor pressures inside the arc tube should be increased at a given wattage, thereby improving lamp efficacy and color performance. In addition, the cold spot temperatures of an arc tube should be increased without blocking the visible light output produced by the arc tube so all the increase of efficacy due to higher cold spot temperature can be transported outside of the lamp. The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A high intensity discharge lamp is provided with an arc tube having an improved heat insulator. The heat insulator is formed proximate to at least one end of the arc tube but absent around an outer middle portion of the arc tube. In addition, the heat insulator is made of a material transmissive of visible light but not transmissive of thermal radiation. By adding a heat insulator to the cold spot area of the arc tube, radiation heat lose from that area will be significantly reduced and the temperature in that area will be increased. Moreover, by choosing a material that will not get oxidized and will not block the visible light, higher lamp efficacy can be achieved.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
An end cap 16 is provided at one end of the outer envelope. A pair of lead wires 18 extends from the end cap 16 and into the inner cavity formed by the outer envelope 14. The pair of lead wires 18 in turn electrically connects to current feedthrough members 19 at each end of the arc tube 12. A getter 13 for trapping impurity gases may be welded to one of the lead wires.
A cylindrical electrode sleeve 23 (also referred to as a capillary tube) extends longitudinally outward from each end of the arc tube. Each cylindrical electrode sleeve 23 provides a through hole for a current feedthrough member 19 to extend from outside of the arc tube into an inner cavity of the arc tube. To seal the inner cavity of the arc tube, a sealing frit is placed into the end of the electrode sleeve 23 to fill any gap around the current feedthrough member 19. While a basic construction for the discharge lamp has been discussed above, it is readily understood that variations from this design are contemplated by this disclosure.
Typically, the cold spot in a high intensity discharge lamp is located at the ends of the arc tube or at the capillary portion of the arc tube. Due to the location of the thermal discharge, the ends of the arc tube and its capillaries are cooler so that salt additives condense at this location. If the temperature of this location can be raised by collecting and reradiating the thermal energy loss from the hot current feedthrough members, lamp additive vapor pressure will increase inside the arc tube producing better performance. By adding a heat insulator to the cold spot area, radiation heat lose from that area will be significantly reduced and the temperature of that area will be increased. Moreover, by choosing a material that will not get oxidized (e.g., metal) due to lamp process conditions and will not block the visible light, higher lamp efficacy can be achieved. Accordingly, this disclosure employs a heat insulator made of a material which is highly transmissive in the visible light region, but not transmissive of infrared thermal radiation. While the following description is provided with reference to quartz, it is understood that other materials, such as aluminosilicate or borosilicate, may also be used for the heat insulator.
In one exemplary embodiment, two straight quartz tubes 22A, 22B are employed as heat insulators as shown in
In another aspect of this embodiment, the heat insulators preferably do not encircle the entire capillary portion of the arc tube. With reference to
As above, the heat insulators 32A, 32B are made of a material which is highly transmissive in the visible light region, but not transmissive of infrared thermal radiation. In addition, the heat insulators 32A, 32B preferably do not extend to the end of the capillary portion and thus are not adjacent to where the sealing frit 21 occurs within the electrode sleeve 23. This structure will increase the temperature of the metal halide chemicals inside the capillary, thereby enabling the metal halide lamps to be designed with lower amount of metal halide chemical fill inside the arc tube. Lower metal halide chemical fill will reduce the impurity inside the arc tube and will also reduce the chemical reactions inside the arc tube between the metal halide fill and the arc tube wall material.
With the application of heat insulators to the two ends of an arc tube, the temperature difference between the maximum temperature of the arc tube and the cold spot temperature of the arc tube can be reduced so that the distribution of operating temperature over the body of the arc tube is more isothermal. There are desirable benefits derived from the more isothermal operation of the arc tube. Usually most lamp characteristics, such as luminous efficacy and color performance, improve significantly with higher cold spot temperature without increase in the maximum temperature. Most metal halide lamps have their maximum temperatures very close to the limit of the arc tube materials for good maintenance and long life. Further increase the maximum temperature will reduce the lamp life. For a fixed maximum arc tube temperature, a relatively higher cold spot temperature will improve color rendition because more of the metal halide additive is in the vapor phase. Also the reduction of the maximum arc tube temperature will reduce the chemical reaction between the metal halide additives and the alumina arc tube body. Because the temperature differentials are reduced, thermal stresses within the arc tube wall will be reduced. This will eliminate the cracking failure of alumina arc tubes during lamp life due to the thermal stress.
Experimental data also proved the effect of such heat insulators. In the table below, photometric data of two ceramic metal halide lamps with different arc tube geometries and chemical fills are presented with and without the quartz heat insulators.
Arc tube wall temperature measurement using infrared imaging techniques shows that with the quartz heat insulators the arc tube has more uniform temperature with reduced maximum temperature near the center of the arc tube and increased minimum temperature near the capillaries. With a more isothermal arc tube, the metal halide lamp will have better performance at different orientations. In addition, there is significant lamp efficacy increase when the quartz heat shield is applied to the arc tube of the lamps. Color rendering indexes of the two lamps are also improved. These improvements of lamp performance are due to the higher cold spot temperatures that will lead to higher metal halide vapor pressure inside the arc tubes.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.