||In the Autumn of 1994 Philips unveiled a radically new kind of light source, and announced the first successful design of metal halide lamp employing a ceramic discharge tube in place of the usual quartz type.
Ceramics have several advantages over quartz as an arc tube material, most notably that they can better withstand the corrosive nature of the metal halide salts. Thus the arc tube wall temperature can be raised, bringing with it an increase in the vapour pressures of the halides and a consequent gain in luminous efficacy. Colour rendering properties are also improved. Colour stability from lamp-to-lamp is enhanced in this high temperature state, and the greater consistency in shape of the ceramic tubes also minimises colour variations.
The CDM-R lamps are designed along different principles than the rest of the CDM family, a design change being necessary on account of the higher burning temperature of the standard arc tube when assembled into this reflector, which has a characteristic of overheating its arctube. Ordinarily this can be compensated for with a change in the mercury dose weight of the arc tube and other minor changes, but for the 70W version the situation is more severe and the arc tube still burns too hot. The 70W CDM-R lamp is unique in that its arc tube is enclosed within a gas filled outer jacket, whereas all other products in the CDM family have vacuum outer jackets. The nitrogen gas filling cools the arc tube significantly by increasing conducted and convected gas losses. No getter is necessary in the outer jackets of these lamps. Furthermore since it is gas filled, this lamp does not suffer the same troublesome end of life failure mechanisms as 70W CDM-TC, with in which arcing in the vacuum outer jacket occurrs.