||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. This design, perhaps the most popular ME lamp that was produced, has the arc tube housed within a black enamelled steel box and the cap is the standard 5-pin plug - once the principal mains electricity socket in use in the UK. Light is emitted via a circular window of ordinary Pyrex glass, but a second box-type lamp was also made having a quartz window which was a more suitable alternative when the UV output of the lamp was also required. The arc tube is filled with an argon-neon mixture for easy ignition, and a third auxiliary starting electrode penetrates the side of the arc tube. The arc tube and its end seals to molybdenum ribbon is hand-made throughout.
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 pin-base design a lamp having a pre-focussed base was also sold, and a number of lower-cost types in glass outer jackets were made available as well. Today, for new applications they have been totally superseded by the double ended bare arc tube designs such as Osram's HBO range.