Osira ME/H Compact Source Mercury

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 /H suffix indicates that the lamp is for burning in the horizontal position only.

This particular lamp is more commonly known under its Air Ministry reference '5L/457 Bombing Teacher'. It was a war time development for the Royal Air Force, who required a lamp of extreme high brightness for use with a projection system from an aircraft. A short arc mercury lamp was chosen over Xenon because of its higher efficiency. To aid in rapidly replacing these lamps, which had a short lifetime, they are equipped with two pre-focussed caps. The main cap which supplies current is a standard P40s type, and is pre-focussed as normal. A second brass shell is cemented over the crown of the bulb, and a small indent is drilled in its end face. An imaginary line drawn between this and the centre of the main cap intercepts perfectly the axis of the electrodes, providing a reliable vibration-resistant design.

The arc burns between electrodes of solid tungsten to provide a compact source of extremely high brightness. Electrodes have flat ends on account of the high current, and a small overwound tungsten coil acts as a heat radiator. Seals are of the hand-made vacuum-shrunk type to double molybdenum foils capable of handling the high current. The arc tube is filled with mercury and argon and was ignited with a Tesla coil. A very rugged mounting was required to prevent arc tube movement during vibration. Metal cups supporting either end are bolted to a steel channel, clamped to the stem and at the other end to a dimple in the bulb.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 500 Watts
Lamp Current: 8.2 Amps
Lamp Voltage: 70 Volts Tesla coil for ignition
Cap: P40s Prefocus 38mm shell at bulb crown
Bulb Finish: Clear Borosilicate
Bulb Type: T-65
Overall Length: 260 mm
Light Centre Length: 120 mm
Electrodes: Solid tungsten with flat ends
Inner / Outer Atmosphere: Inner: Ar | Hg Outer: Nitrogen
Luminous Flux: Not disclosed
Brightness: Not disclosed
Colour Temperature & CRI: Not disclosed
Chromaticity Co-ordinates: Not disclosed
Burning Position: Horizontal
Rated Life: No life claim
Warm Up / Re-strike Time: 2 minutes 5 minutes
Factory: Wembley United Kingdom
Date of Manufacture: February 1945 Date Code 38
Original / Present Value: Not known
References: 1) The Ministry of Defence, Defence Standard No. 62-10, 1994 Edition. Published by Her Majesty's Stationery Office.
2) The War Office, Defence List of Electric Lamps for the use of Government Departments, 1953 Edition.