CERAMIC METAL HALIDE LAMPS

Abstract: A ceramic metal halide high pressure discharge lamp has an arc gap separating the discharge electrodes which is not less than 10mm and a wall loading which is at least 50W cm⁻². The relatively small arc gap enables a relatively low operating voltage (in the range from 80V to 130V) to be used.

Description: This invention relates to high pressure ceramic metal halide (CMH) discharge lamps. More particularly the invention is concerned with providing a commercially acceptable CMH discharge lamp, that is to say, a discharge lamp comprising suitable metal halide vapours hermetically sealed within a discharge arc tube of a light-transmitting ceramic material and designed to operate with a lamp voltage drop of between 80 to 130 volts. With a voltage drop in this range the lamp could be powered by from a standard 220-240V supply using a commercially available starting circuit, such as a standard wire wound choke ballast or a series capacitor circuit. Moreover, in order to be commercially acceptable it is preferable that the lamp should be capable of operating in the horizontal mode. A CMH lamp is proposed in a paper by Brown et al entitled "Tin Sodium Halide Lamps in Ceramic Envelopes" published IES conference 9 August, 1981 describing what is hereinafter referred to as the TSH lamp. This lamp, which was developed by optimising efficacy is found to be inconvenient from an operational standpoint, not least because it is designed to operate with a transformer ballast requiring a lamp voltage of 200-240V. This would mitigate against the lamp being put into general commercial use. Furthermore, because of the limitations of conventional sealing techniques, the end seals of a TSH lamp attain a temperature of around 600°C, and the effective cool spot temperature for the lamp would be no greater than about 700°C. In practice, an effective cool spot temperature somewhat higher than 700°C would be desirable. Also, the TSH lamp is limited to a vertical mode operation. The effective cool spot temperature referred to throughout this specification is measured by comparing the lamp spectrum obtained during normal operation with the spectra obtained when one end of the lamp arc tube is maintained, in a bath of molten indium, at different test temperatures. The effective cool spot temperature is deemed to be that test temperature giving the closest match of spectra. An investigation into an experimental ceramic metal halide lamp is reported in the paper "New Structure of Ceramic Metal Halide Lamps - their characteristics" by M Kitagawa, S Yamayaki and K Tabatsu of the Iwasaki Electric Co. presented at the I.E.S/conference in Japan, March, 1985. No details are given of the electrical characteristics of the experimental lamp and experimental lamps were life tested to 1,000 hours only, under an input of 150W, before being analysed. According to the report damage to the ends of the lamp when operating at temperature between 800 and 900°C was very severe. It is therefore an object of the present invention to provide an improved Ceramic Metal Halide lamp which alleviates at least some of the afore-mentioned problems. According to the present invention there is provided a ceramic metal halide high pressure discharge lamp adapted to operate at a seal operating temperature of up to 1000°C, the lamp comprising an arc tube made of a light-transmissive ceramic material, containing a fill comprising a metal halide dose, mercury and a rare gas for starting, and an end closure member hermetically sealing each end of the arc tube and supporting a respective discharge electrode, wherein the arc gap separating said discharge electrodes is not greater than 10mm and the wall loading on the arc tube is at least 50W cm⁻². The wall loading is defined here as the ratio of lamp input power to the internal surface area of that part of the arc tube surrounding the arc gap. In the case of a cylindrical arc tube the relevant internal surface area is evaluated as the inner circumference of the arc tube multiplied by the arc gap. As stated hereinbefore a main requirement of a commercially acceptable ceramic metal halide lamp is the requirement of a voltage drop of between 80 to 130 volts. Assuming that the mercury pressure is fixed for high efficancy (70 lumens/W to 100 lumen/W, for example) the desired voltage drop imposes a required Watts per mm of arc gap value which in turn determines the wall loading for a given diameter of arc tube. The afore-mentioned papers by Brown et al and Kitagawa et al describe CMH lamps having maximum wall loadings of about 40W cm⁻² and 37W cm⁻² respectively. The inventors have discovered that contrary to expectation a ceramic arc tube can operate satisfactorily at significantly greater wall loadings - ­ that is greater than 50W cm⁻², and typically in the range from 80W cm⁻² to 160 cm⁻². A wall loading in the range from 90W cm⁻² to 100W cm⁻² is preferred and a wall loading of approximately 90W cm⁻² is found to be particularly satisfactory. The inventors have recognised that the larger wall loading permits use of a relative small arc gap commensurate with the afore-mentioned desired voltage drop (i.e. 80V -130V). The inventors find that the arc gap should not be greater than 10mm and preferably not less than 5mm. In an example an arc gap of 7.3mm was used. Moreover, whereas the TSH lamp has an aspect ratio of about 2.2, a lamp in accordance with the present invention may have a significantly lower aspect ratio, typically in the range from 0.7 to 1.4., aspect ratio being defined as the ratio of arc gap to arc tube internal diameter. The lower aspect ratio achievable with the present invention reduces damage to the arc tube wall due to bowing of the discharge arc allowing the tube to be operate in a horizontal mode. Also the reduced aspect ratio leads to a higher arc tube wall temperature, and so a higher effective cool spot temperature which, as described hereinbefore, is generally desirable. With a configuration in accordance with the present invention a seal operating temperature of at least 700°C and up to 1000°C is envisaged. The increase of at least 100 centigrade degree over the seal operating temperature of 600°C reported by Brown et al for the TSH lamp enables a less corrosive, less volatile, dose comprising indium, thallium and sodium to be used instead of the more agressive dose of tin and sodium used in the TSH lamp. In order that the invention may be carried into effect particular embodiments thereof are now described, by way of example only, by reference to the accompanying drawings of which Figure 1 illustrates a known TSH lamp as described in the afore-mentioned paper by Brown et al, Figures 2 and 3 shown longitudinal, cross-sectional views through two CMH lamps in accordance with the present invention and Figures 4 and 5 show circuits for use with the CMH lamps of Figures 2 and 3. Referring to figure 1 a known TSH lamp comprising a polycrystalline alumina arc tube 10 has an overall length of approximately 40.4mm and an arc gap of approximately 16.4mm. The metal halide dose of tin sodium halide plus mercury and rare gas for starting is hermetically sealed within ceramic arc tube 10 by means of electrically conductive cermet end closure members 11. These are seen to cover ends 12 of the arc tube almost entirely and are sealed to the arc tube by means of a suitable metal halide resistant sealing material 13. Figure 2 illustrates an example of a CMH lamp in accordance with the present invention. In contrast to the lamp of Figure 1 the arc tube 14, which is also made of polycrystalline alumina, is much shorter - 27.8mm in overall length - and has a smaller arc gap of only 7.3 mm. With an input power of 150W and an arc tube internal diameter of 7.35mm the wall loading is approximately 90W/cm², and with the reduced arc gap the required voltage drop of 80 to 130 volts can be achieved. Arc tube 14 is hermetically sealed by electrically conductive cermet end closure members 15 sealed to the ends of the arc tube 14 by suitable metal halide resistant sealing material (not shown) which is melted and seals between the interfaces of the cermet member 15 and arc tube 14. With a backspacing of approximately 7mm, represented by reference numeral 16, arc gap 7.3mm and arc tube internal diameter 7.35mm the embodiment of figure 2 achieves an effective aspect ratio of about 1. This configuration enables the lamp to operate at an increased seal temperature of 770°C. The higher operating temperature allows the less aggressive dose of sodium, thallium and indium to be used instead of the more aggressive tin, sodium halide dose which, in turn, leads to reduced electrode corrosion and an increased operating lifetime. Moreover, with the geometry shown, bowing of the discharge arc is much reduced or eliminated allowing the lamp to be used in the horizontal mode. This substantially increases the commercial viability of the lamp. In addition, the relatively high seal (and so effective cool spot) temperatures which can be achieved with this configuration give rise to a significant, and beneficial, broadening of the major spectral lines, namely the lines at 589nm, 535nm and 451nm produced by sodium, indium and thallium halides respectively. Furthermore, the inventors have found that a substantial continium, the origin of which is not fully understood, accounts for up to 30% of the visible light which is produced by the lamp and contributes to the production of an excellent colour rendering index, Ra, of 80 in this example. In order to reduce end losses by radiation due to the high emittance of the cermet end members, end members 15 are of much reduced size as compared with the cermet end caps used in the TSH lamp. In the example of Figure 2 the cermet end members 15 are 3.8mm diameter and cover just over 50% of the end area of the arc tube representing a major reduction in size as compared with the cermet end caps used in the TSH lamp illustrated in figure 1 wherein it can be seen the end caps of end member 11 cover substantially the whole of the end area of arc tube 10. Since the sealing material has to be heated to its melting temperature of 1500-1600°C it has been found surprisingly that the large amount of heat, which has to be applied through the much reduced size of the cermet end member in order to achieve the melting temperature, does not result in cracking of arc tube 14 even though extremely short sealing times are required in order to prevent evaporation of the dose. Figure 3 is an illustration of a 100W CMH lamp in accordance with the present invention. The overall length of polycrystalline alumina arc tube 17 has been reduced to 21.3mm but the arc gap has been maintained at 7.3mm. Back spacing 18 has been reduced slightly so that the operating temperature of the ends is slightly higher at 850°C. As with the embodiment of figure 2 the diameter of electrically conducting cermet end members 19 is about 50% of the diameter of end caps 11 of the TSH lamps of figure 1. With the geometry shown the wall loading is about 60W/cm². Table 1 sets out a comparison of the afore-mentioned TSH lamp and five examples (1-5) of CMH lamp in accordance with the present invention. Use of a highly volatile metal halide doses, of course, present problems in sealing the ends of the ceramic arc tube and in order to provide satisfactory seals for ceramic metal halide lamps to both operate at about 700°C and up to 1,000°C the finished seals must have a softening temperature not less than about 1300°C. One method of achieving a satisfactory seal is that disclosed in our European patent application No. 86306645.2 which discloses a method of rapidly sealing the arc tube complete with dose by first forming a glassy seal from a MgO, Al₂O₃, SiO₂, sealing composition and then converting it to a highly refractory crystalline seal. The success of this method is probably dependent entirely on the speed at which the complete sealing process is carried out and a sealing process lasting about 60 seconds is found to be particularly useful. Figure 4 is a diagram of an electric circuit designed to operate a nominal 150W CMH lamp 20 having a nominal voltage drop of 100V from a 240V supply. As can be seen from figure 4 a simple series inductor ballast 21 having an impedance of 110 ohms at 1.8A and a power factor of 0.06 is connected to an igniter 22 generating pulses of between 3 to 5 kV. An alternative circuit is shown in figure 5 which includes a series capacitor 22, of about 14 F having a working voltage of 450V. This circuit has the advantage of greater stability against supply voltage variation, a reduced tendency to flicker and improved starting characteristics. It will be understood that the ceramic arc tube used in CMH lamps in accordance with this invention need not necessarily be cylindrical. Alternatively arc tubes having a bulbous form could be used. Furthermore, it will be understood by those skilled in the art that a CMH arc lamp according to the present invention can be incorporated in a variety of outer envelopes to provide a finished product.

Claims: 1. A ceramic metal halide high pressure discharge lamp adapted to operate at a seal operating temperature of up to 1000°C, the lamp comprising an arc tube made of a light-transmissive ceramic material and containing a fill comprising a metal halide dose, mercury and a rare gas for starting, and an end closure member hermetically sealing each end of the arc tube and supporting a respective discharge electrode, wherein the arc gap separating said discharge electrodes is not greater than 10mm and the wall loading on the arc tube is at least 50W cm⁻². 2. A ceramic metal halide high pressure discharge lamp according to Claim 1 wherein the arc gap is in the range from 5mm to 10mm. 3. A ceramic metal halide high pressure discharge lamp according to Claim 1 or Claim 2 wherein the wall loading on the arc tube is from 80W cm⁻² to 160W cm⁻². 4. A ceramic metal halide high pressure discharge lamp according to Claim 3 wherein the wall loading on the arc tube is from 90W cm⁻² to 100W cm⁻². 5. A ceramic metal halide high pressure discharge lamp according to any one of Claims 1 to 4 wherein the arc tube is cylindrical. 6. A ceramic metal halide high pressure discharge lamp according to Claim 5 wherein the arc tube has an aspect ratio in the range from 0.7 to 1.4. 7. A ceramic metal halide high pressure discharge lamp according to any one of the preceding Claims wherein said metal halide dose comprises indium, thallium, sodium and a halogen. 8. A ceramic metal halide high pressure discharge lamp according to Claim 7 wherein said halogen is iodine or bromine. 9. A ceramic metal halide high pressure discharge lamp according to any one of the preceding Claims wherein said end closure member is made of an electrically conductive cermet material.

Inventors: BROWN, KENNETH EDWARD, CAUNT, GARY KENNETH, HICK, KEVIN, RAMBALDINI, BRUCE HAYWOOD SHARP
Applicants: THORN EMI PLC
Application year: 1988
Main classification: H01J61
IPC codes: H01J61