RLOD#12 (2020.05.16) Early-1980s Philips MHW 70W D143

The late 1970s and early 1980s saw the development of compact low-wattage metal halide sources as an energy-efficient alternative to incandescent lamps. The main developer of the technology in Europe was Osram of (then) West Germany, who formulated a tin halide fill chemistry with sodium, thallium, indium and lithium additives for its warm-white lamps rated between 35 and 150 W. Philips began its own research work in that area in the early 1980s using a sodium-thallium-dysprosium-tin halide fill chemistry with the intention of realizing better light color and output. With Osram’s salt mix, tin halide dominates the discharge’s optical characteristics, resulting in a significant infrared output and in a light color rendering limited to 80 Ra8. Philips thus replaced part of the tin dose with dysprosium in order to curb the infrared emission and to improve the spectral distribution of the emitted light.

The MHW 70W D143 shown here is representative of early low-wattage metal halide lamp prototypes developed, built and tested at the company’s central lamp research laboratory in Eindhoven, the Netherlands. Interestingly, this lamp has a transverse burner, certainly to enable an operation in the correct position (horizontal arc tube) in the lab’s ageing racks. What is more intriguing is the presence of an external exhaust tip on the outer bulb. This was probably necessary for the testing and adjustment of a gaseous atmosphere around the burner in order to optimize its thermal balance. A pair of zirconium-aluminum getters, attached to the lamp stem, is placed next to the burner so as to keep the lamp free of hydrogen. Philips never used its Na-Tl-Dy-Sn fill chemistry in commercial lamps, certainly because of issues related to chemical stability (quartz corrosion and/or electrode beavering), and they eventually used Osram’s design for their mass-produced MHWs released in 1986 (see there).