1994 Osram HQI-TS 70W/WDL Plus

The fields of commercial and retail lighting saw a major technology breakthrough in 1982 when Osram first introduced its compact double-ended metal halide lamps. The smallest model, rated at 70 W, was designed to produce a warm-white light color with a good color quality at four times the efficacy of linear tungsten halogen lamps of similar output. In effect the HQI-TS 70W/WDL could replace 300 W halogen lamps in their applications while delivering the same amount of light, which thus enabled massive energy savings. This was a particularly welcome addition in the range of lighting products available in Europe, where the cost of electricity had increased significantly during the energy crises of the 1970s.

In order to achieve a high lumen efficacy in a low-wattage discharge lamp and obtain a light color that would meet the stringent requirements of retail lighting applications, Osram developed a new precision-shaped quartz burner with narrow end seals. Warm-white light at 3000 K was obtained thanks to a new (at the time) fill chemistry with a tin halide base, supplemented with iodides of indium, thallium, sodium, and lithium. A high color rendering index close to 80 Ra8 was obtained via the combined optical properties of tin’s molecular emission which covers the whole visible spectrum, and an enhanced light output in the red coming from lithium and from Na-Hg quasimolecules. Because the latter aspect of the lamp’s light output is particular critical for the lighting of people, the mercury vapor pressure in the burner is particularly high (16 bar) in order to properly broaden sodium’s optical emission towards the red end of the spectrum.

Although the performances of the compact metal halide lamp were quite impressive compared to those of incandescent lamps, in the early 1990s it was felt that the characteristics of the HQI-TS 70W/WDL could be improved further. The use of a tin-halide-based fill chemistry came with a set of challenges and limitations, such as the corrosion of electrodes and the blackening of the small burner caused by the high halogen partial pressure, and a lumen efficacy limited by the strong optical output in the infrared from tin molecules in the discharge.

In order to mitigate these problems and improve the lamp’s lumen maintenance and raise its output, Osram simply replaced the fill chemistry by one consisting of a mixture of sodium, thallium, and scandium iodides and bromides. This particular formulation is superior to the tin-based salt mix since scandium radiates much less infrared energy than tin halide, while the halogen partial pressure in the burner is much lower, thereby reducing the rate of electrode corrosion and burner end-blackening. However, because scandium has a much higher chemical reactivity than than tin, the burner design had to be changed (optimized temperature profile and thicker quartz wall), its processing had to be improved (less impurities) while the iodine/bromine ratio of its vapor atmosphere had to be adjusted precisely so as to prevent deleterious material transports from taking place. These were the requirements to ensure a stable lamp operation for more than 6000 h.

Interestingly, this development was done entirely in Germany, before Osram acquired Sylvania in the USA (in January 1993) and gained full access to its technical expertise on metal halide lamps with a scandium-based chemistry. Nevertheless, the improved design was successfully implemented and the Germans eventually released the new lamp in 1992 in the form of the present HQI-TS 70W/WDL Plus. The change in salt fill chemistry not only raised the light output by 20 % to 6 klm, the emitted light color quality was also improved significantly with the R9 (red rendering) increased from -90 to +15 and an average CRI improved from 76 to 85 Ra8. Moreover, the lamp color shift through life was reduced from 300 K to 100 K only - later re-rated at 200 K to account for variations in the thermal balance of luminaires in the field.

These improvements were so significant that Osram actually offered both versions of its HQI-TS 70W/WDL for sale; the older tin-filled lamp became the low-cost alternative, while the much improved scandium-filled lamp targeted higher-end lighting installations and thus commended a higher price. This situation changed dramatically in the fall of 1994 when Philips released its far superior CDM ceramic metal halide lamps, and Osram could not prevent the Dutch from capturing the high-end retail lighting market. As a result, all compact low-wattage quartz metal halide lamps fell into the low-cost segment of this market, which no longer justified the presence of more expensive quartz lamps such as the HQI-TS 70W/WDL Plus. Osram eventually discontinued this particular model at the end of the 1990s.