||ME mercury lamps have air-cooled quartz arc tubes loaded above 100 watts per centimetre, and this 250W lamp operates at a mercury vapour pressure of around 30 atmospheres. The /D suffix indicates that the lamp is for burning in the vertical base down position only, as the shape of the arc tube is often specially profiled for this orientation.
The arc burns between electrodes of solid tungsten to provide a compact light source of extremely high brightness. The arc tube and its end seals to molybdenum ribbon is hand-made throughout. A third auxiliary electrode penetrates the arc tube side, which is filled with an argon-neon mixture for easy starting. This very compact ME lamp was intended for substitution into standard film and slide projection equipment, being equipped with the ordinary P28s pre-focussed cap and having the same light centre length as the incandescent projector lamps it replaces. The sides of the arc tube are coated with opaque platinum paint and the whole is enclosed in an oval shaped metal tube to mask off stray light, leaving only light from the centre of the arc to enter the optical system.
ME/D lamps were once very popular light sources for monochrome slide and film projectors, film printers, and a great many still live on today in technical instruments such as projection microscopes and profile projectors. In addition to this projection design, lamps were also made available in clear tubular glass outer bulbs, and in larger metal boxes for equipment not requiring such a small lamp. Today, for new applications they have been totally superseded by the double ended bare arc tube designs such as Osram's HBO range.