1978 Tungsram HgMI 250/D

The HGMI 250/D shown here was introduced around 1978 and is the last standard metal halide lamp released by Tungsram during the 1970s. These 250 W lamps have always been a challenge to develop and produce in Europe because their smaller arc tube makes it difficult to obtain stable and consistent light-technical properties when run on the standard series-choke ballast. Osram was the first company in the early 1970s to successfully develop a 250 W metal halide lamp. To this end the Germans limited the spread in lamp voltage by shortening the electrode gap length to 2.5 cm and by keeping the mercury vapor pressure as low as possible. As a result the discharge power load increased to 90 W/cm and an elaborate fill chemistry based on a mixture of thallium, dysprosium, holmium, and thulium iodides was introduced in order to realize a high lumen efficacy of 80 lm/W with a relatively low cold-spot temperature.

Tungsram followed suit during the second half of the 1970s by using Osram’s design approach. Interestingly, the Hungarians shortened the electrode gap distance to 2.0 cm, which increased the arc power load to 113 W/cm. Because the resulting arc temperature is higher, a higher burner cold-spot temperature is required in order to achieve good lamp characteristics. Such design choice thus resulted in a more compact arc tube built with three quartz parts so as to better control its geometry during the production process. This way Tungsram managed to obtain a well balanced 6000 K light color with a CRI of 90 Ra8, combined with with the same 80 lm/W efficacy as Osram’s original 250 W daylight HQI-T.

Because of their relatively low mercury vapor pressure, both lamps were designed for an operation at 100 V on 250 W high-pressure sodium lamp ballasts. However, while Osram’s lamp could be operated at all positions, the higher cold-spot temperature requirement of the HGMI 250/D restricts its optimum operation to the horizontal position ±45°.

Tungsram upgraded its 250 W daylight HGMI first with a bulged bulb which replaced the original tubular one in 1984. This change reduced the lamp temperature and limited the rate of hydrogen effusion from the glass bulb, thereby increasing the lamp’s service life. The next change occurred around 1985 when the Hungarians adopted a new burner design with a much longer arc length and built from a single quartz tube. Finally, after GE took over Tungsram’s lighting operations in December 1989, a pair of Al-Zr getters was added in the lamp in order to better control the vacuum around the burner. This type of lamp is still produced in Hungary and its sale in Europe continues to this day.

Interestingly, while Osram changed the design of its HQI-T 250W/D during the 1970s and lowered the light color temperature to 5200 K, Tungsram kept a CCT of 6000 K for its 250 W HGMI and then HgMIF lamps. This choice was certainly motivated by the need to keep a consistent light color across Tungsram’s range of daylight metal halide lamps so different source wattages can be used seamlessly in lighting installations.