1941 Osram HWA 500 (250 W / 220 V)

The lamp featured here is the first self-ballasted mercury lamp produced by Osram, who first released this model in 1939, at the start of the Second World War. The HWA500 is built around a quartz mercury burner of the earliest type, with graded-glass end seals whose design was invented in 1935 at Philips, in the Netherlands. Osram began to use this burner constructions two years later with a slightly modified structure where the ignition electrode is located parallel to the burner’s axis rather than sideways.

The current that flows through the discharge tube is limited by a tungsten filament in a nitrogen-argon atmosphere typical of the Nitra incandescent lamps Osram manufactured at the time, with the filament optimized so its life expectancy matches that of the mercury burner. The lamp is designed with a 50/50 flux ratio between the mercury discharge and the filament, a common practice in blended-light lamps made in Europe at the time and which was also first devised by Philips out of consideration for the overall color of the emitted light.

The glass material of the lamp’s bulb contains a high level of iron III oxide, which gives the lamp a slight bottle-green color. This critically ensures that all shortwave radiations from the mercury discharge are effectively absorbed so as to limit the amount of UV emitted by the lamp. The bulb was etched on its inside surface with hydrofluoric acid in order to limit glare from the bright mercury discharge and to ensure a proper color mixing of the two light sources. An interesting feature of this lamp is its peculiar bulb shape terminated by a long neck. This geometry ensured that the HWA could be used in incandescent lamp luminaires which have a deeply recessed socket. Instead of using standard GLS lamp bulbs to ensure such compatibility, Osram designed a specific jacket for its blended light lamps, likely because new bulbs had to be made with the special UV-blocking iron-doped glass. The peculiar conical shape of the HWA was chosen certainly so as to improve the lamp’s photometry for optimum performances when used in downlighters, a common type of luminaire used in industrial lighting applications.

The HWA500 came initially with an E40 base so as to replace high-wattage filament lamps rated above 300 W and commonly used (at the time) in industrial lighting applications. A second version, shown here, was made available later with a skirted E27 base for a use mostly in domestic settings and in any other cases where the large E40 base was not installed. This early type of blended-light mercury lamp was eventually phased out in the early 1950s in favor of a simplified lamp construction. Osram dispensed with the use of the special UV-absorbing iron glass and it replaced the conical-shaped bulb by a standard A-shaped one. However, it kept the lamp inside frosted since this is a critical feature which is instrumental to the proper blending of the emitted light. HWA lamps remained in production until 1963 and were replaced by the fluorescent-coated HWL.