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Metal-halide lamps use more ceramics

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The main advantage with the ceramic discharge-tube technology is that it helps lamps to maintain a more stable color throughout their life (metal-halide lamps with quartz burners generally show color drift over time), and the spectral variation between individual lamps is virtually eliminated. This is due to the fact that the ceramic material prevents metal salts from wandering through the walls of the discharge capsule.

Lower wattages
The new ceramic lamps are only offered in lower wattages (35, 70, and 150W), and they are available in single-ended, double-ended, and screw-base PAR-reflector versions (the range varies from manufacturer to manufacturer and is increasing). The lower wattages may have been chosen because these lamps are intended for applications where their somewhat higher price is easier to justify and where people are especially sensitive to the color appearance of the light. Such applications include display lighting in shop windows, lighting in offices, and outdoor applications in parks, city centers, etc.

Improved color rendering
Ceramic metal-halide (MH) lamps tend to have better color rendering and a more balanced spectrum than most standard metal-halide lamps. Philips' new 4 200-K lamp is better still, with a Ra-index above 90. (It should be noted, however, that there are also quartz MH lamps that offer extremely good color rendering, but these lamps tend to be in the very white spectrum such as Japan-based Iwasaki's 6 000K, Ra 96 metal-halide daylight lamp.) Ceramics can withstand higher stress than quartz, and the ceramic discharge tube allows the operating temperature to be about 200°C higher compared with a quartz lamp. With the ceramics, more metallic ions can be contained in the discharge tube, which, according to Osram, results in a smoother spectrum and higher luminous efficiency. Ceramic lamps have a typical efficacy (including ballast losses) of 70-80 lm/W for 35-W versions, 75-80+ lm/W for 70-W versions, and 75-85 lm/W for 150-W versions. A standard metal-halide lamp in the same wattage range has a system efficacy that is 10-20 % lower. (Note that this information is based on data provided by manufacturers.)

Confusing lumen depreciation
The information on lumen depreciation is confusing, however. Philips has reported a lumen depreciation of about 10-15 percent over the life of the lamp (on an electronic ballast), whereas, according to GE's Technical Catalogue, lumen depreciation for the GE ceramic lamp can be as high as 50 percent after 6 000 hours. According to GE, these figures were obtained from a lamp operated on a conventional ballast. Whether these differences can be explained by the fact that different ballasts were used, by different technological approaches or quality assurance protocols, or even by differences in measurements and data reporting remains to be seen. Based on data from GE's catalogue, the more efficient ceramic lamp will, in fact, be less efficient than their standard metal halide lamps after ~4 000 hours of operation!

The ceramic metal-halide lamps have a rated life of 6 000-9 000 hours. Although the ceramic metal-halide lamps seem to offer a solution to the problems with color drift and color variation between individual lamps, the ramp-up time is still slow, and most lamp types don't allow instant restrike.

Same ballasts
The new lamps can be operated on a conventional ballast for standard MH lamps. But Osram, Philips, and some independent ballast manufacturers now offer electronic ballasts for these lamps. Philips' ballast operates at a frequency below 200 Hz, and the electrical wave-shape is square which helps to reduce the sense of flickering often associated with metal-halide lamps operated on conventional ballasts. Osram's ballast operates at 22 kHz and is more like a standard electronic ballast. As a rule, an electronic ballast will reduce losses by about 10 watts per lamp, and by even more for the 150-W version.

Nils Borg