Mid-1990s Philips SDW-R 50W PAR20

Following the successful introduction of the PAR20 Halogena around 1993, Philips developed metal halide and white sodium variants of the compact lamp in order to get similar benefits, i.e., small size and efficient control of light, in highly efficient lamps. The main challenge in designing PAR20 HID light sources consisted in fitting a double-ended burner in the small lamp, a rather difficult task given the need for the HID arctube to be protected from ambient air. To that end the burner was sealed in a compact double-ended quartz jacket since PAR20 lamps do not have a gastight construction. Because the length of the jacketed HID source exceeded that of halogen capsule used in the original Halogena PAR20, the HID version had to be fitted with a protruding front window made via a glass-pressing method.

The SDW-R 50W PAR20 shown here is the result of such development. While the lamp has the same wattage as the Halogena variant, its sodium burner produces 3.8-times more light at a lower color temperature (2500 vs. 2900 K). Because of this lamp configuration, heat is not evacuated as effectively as in the standard SDW-T. So, in order to keep the burner temperature within tolerances, its quartz jacket is filled with nitrogen so as to dissipate heat via thermal conduction and convection processes. In this case there is no getter used in the lamp.

The SDW PAR20 concept was not developed into a commercial product because its protruding front glass precludes an effective control of the projected light beam. The Halogena PAR20 relies on a lenticulated front window to homogenize its beam pattern, something which was of course not possible with the SDW variant. However, the development of the compact single-ended ceramic metal halide lamp in the first half of the 1990s solved the capsule length problem and this enabled Philips to release a proper metal-halide PAR20 lamp with a lenticulated front glass in 1995. Such design was not applied to SDW lamps because of the longer sodium burner which still caused a geometrical mismatch with this particular lamp platform.