1994 Philips CDM-T 150W

The first commercial ceramic metal halide lamps released by Philips in the autumn of 1994 where compact light sources intended for commercial and retail lighting applications. Typical of MH lamps of this type made at that time, the CDMs were built with a standard-grade fused silica jacket that lets a considerable amount of UVs through, requiring luminaires to be fitted with a UV-absorbing protective front glass to limit exposure to shortwave radiation.

Around that time Osram upgraded their low-wattage quartz metal halide HQI-TS lamps with an outer jacket doped with UV-absorbing elements, which solved both problems of hazardous radiation exposure from unprotected luminaires and the generation of ozone, especially during the run up phase when a significant part of the emitted light lies in the UV-C domain. The experimental CDM-T 150W shown here is an early attempt from Philips to upgrade their ceramic metal halide lamps with a UV-block jacket. This lamp was made specifically to evaluate the impact of the increased optical absorption in the outer bulb on the burner's operation. The reasons for this double-jacket construction are two fold: simplicity - prototypes could be made quickly from standard production-grade lamps - and trapping air around the lamp emulates an operation in reflector luminaires. Blocking the natural convection flow of air around a free-burning compact MH lamp has a similar effect on its thermal balance as the back reflection of light in luminaires, an interesting equivalence that was discovered by Osram when they developed their first HQI-TS lamps in the late 1960s and which greatly facilitates life-testing procedures under realistic operating conditions.

Tests in burning racks showed that the arc tube in CDM lamps runs significantly hotter with the new jacket, whose higher temperature causes an increased IR emission towards the burner. The results are a noticeable shift of the light color point due to the increased salt vapor pressure (a 3000-K CCT salt mix is used here), and the development of microcracks at the junction between the extended plugs and the side discs due to too much local stresses, eventually leading to small leakages there that shorten the lamp life considerably. Philips had no choice but to apply some changes to the design of its ceramic burners to allow an optimal and durable operation in UV-block jacketed CDM lamps. The upgraded lamp were released in 1995 and, of course, were not fitted with an outer bulb as shown here.