1979 Westinghouse C250-S50/DX4

The development of the high-pressure sodium lamp during the early 1960s led to a very successful lineage of highly efficient light sources. In less than twenty years this technology replaced high-pressure mercury lamps in most of their applications. However, the poor light color quality of sodium lamps soon became a limitation and it was felt that a whiter light color was needed for the illumination of main streets and outdoor public areas.

Introduced in 1977, the Ceramalux-4 shown here is the first color-improved high-pressure sodium lamp made commercially available in the West. Although GE first discovered that increasing the sodium vapor pressure has a beneficial impact on the color quality of the light emitted from HPS lamps, it was Matsushita which developed the first color-improved lamps using this knowledge. The Japanese presented the first of such lamps to the CIE in 1971 and they released their first commercial (K-Hica) lamps in the second half of the 1970s.

Westinghouse followed suit in the mid-1970s after R. Bhallah successfully strengthened the end seals of HPS burners by applying a silicon pre-coat to the niobium caps. This innovation permitted a durable lamp operation with a cold-spot temperature 100 °C higher than what was previously possible. This critically enabled the higher sodium vapor pressure needed in order to broaden the emission spectrum and increase the light color quality. While Matsushita aimed at the production of white light with a CRI above 80 Ra8, Westinghouse limited the color rendering to 65 Ra8 in order to maintain a high lumen efficacy and a long service life using the primitive cap-sealed burner design of their Ceramalux lamps. To this end, and in order to maintain a full electrical compatibility with standard HPS lamps, the electrode gap length was shortened from 70 mm to 52 mm while the burner was widened to 8 mm. The latter change ensured that the burner temperature does not exceed 1200 °C so as to limit the sublimation of alumina in vacuum. Finally, both extremities of the burner are wrapped with metal foils in order to raise the cold-spot temperature to a suitably high level.

It is remarkable that with its Ceramalux-4 Westinghouse managed to obtain a light color temperature of 2400 K combined with a lumen efficacy of 100 lm/W. The company developed this lamp platform further and in the middle of 1980 it upgraded the end seals to a wire feedthrough design and two new 150 and 200 W models were released. In this process the company changed the burner properties presumably to extend its service life, resulting in a light color temperature lowered to 2200 K. However, the upgraded Ceramalux-4 was plagued by reliability issues related to its end seals and its production ceased in 1983 shortly after Philips took over Westinghouse's lighting operations. It is only 8 years later that North American Philips re-introduced color-improved HPS lamps in its HID lamp portfolio. The new Ceramalux Comfort lamps were designed with roughly the same light technical properties as the last Ceramalux-4 lamps (2200 K and 92 lm/W for the 250 W model) but featured Philips's far more reliable monolithic thermo-compressive end seals, resulting in a 15 kh mean service life. These lamps were eventually phased out for good in 2012 because of a limited demand on the market stemming from their low lumen efficacy relative to standard high-pressure sodium lamps.