2002 Osram HMI 18000W

In the late 1960s, Osram of West Germany developed the first modern sources or artificial daylight for stage and studio lighting applications. To this end, the Germans employed short-arc mercury lamps filled with rare earth halide additives so as to generate light with a nearly continuous spectrum across the visible range. What became the HMI family of double-ended arc lamps were an instant success because they produce artificial daylight way more efficiently than filtered incandescent lamps and reduced the heat output considerably. Also, the arc contained within the quartz envellope burns in a far more stable manner, and more silently, than open flaming arcs that were used until then in high-intensity floodlights.

Following the success of the original four HMI lamps released by Osram in 1969 (rated from 575 to 4000 W), the need for more powerful sources of this kind became evident. This demand led Osram to develop and release 6000 and 12,000 W models in 1982 and 1984, respectively. These powerful sources effectively ended the use of flaming arcs in large outdoor film sets. It is only a decade later, in 1992, that still more powerful HMI lamps were released in order to reduce the number of projectors needed in certain applications. That year, Osram introduced the 18,000 and 24,000 W models, although the latter was quickly abandoned, only to be re-introduced in 2009.

For a number of years the HMI 18000W, shown here, was thus the most intense source of artificial daylight available. With an output of 1,7 million lumens, it emits more light than 30 kW xenon short-arc lamps due to the more efficient arc burning in metal halide vapor. Interestingly, this particular lamp is filled with a very simple salt chemistry which consists only of a dysprosium-cesium halide mixture. Yet, the lamp is characterized by a lumen efficacy of 94 lm/W combined with a CRI of 95 Ra8. Such impressive performance is realized thanks to the combination of a very high arc power load of 3.7 kW per centimeter and a high cold-spot temperature (above 1000 K) that enables a high dysprosium vapor pressure in the hot plasma. Naturally, the extreme thermal and chemical stresses put on the quartz burner results in a short service life which is limited to 250 h only, long enough for the intended application.