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Tungsten Halogen Lamps

in Tungsten Halogen Lamps, a trace of halogen usually iodine is added to the normal argon gas filling. At the bulb wall this combines with tungsten evaporated from the filament to form tungsten iodide which is carried back to the filament by convection currents. At the very high temperature near the filament the tungsten separates from the iodide and is redeposited onto the coil. The halogen is then released to repeat the cycle.

The blackening which is normally caused by a deposit of evaporated tungsten on the wall of the bulb is virtually eliminated and a high efficacy is maintained throughout the life of the lamp. A Minimum bulb wall temperature of 250oC is needed to maintain the cycle and consequently a smaller envelope made from quartz is required. This causes much higher gas pressure in operation so that the life of halogen lamps can be double that of conventional tungsten filament lamps. The filament can be operated at a higher temperature and efficiency giving whiter light at a higher level.

Linear lamps must be operated horizontally within ±15o. At steeper angles the halogen vapour migrates to the lower end leaving the upper end starved and there is rapid bulb blackening in that area. At a much reduced voltage the cycle may not operate but the blackening rate will be very low anyway.

Seal temperature is limited to a maximum of 350oC to prevent failure of the seal due to foil expansion.

Contamination of the outside of the bulb e.g. the deposit of grease by handling must be avoided. because of the extremely high temperature at which the quartz operates, if any dirt of grease is present on the outside surface of the bulb it develops large numbers of very fine cracks (devitrification) which obscure the light and eventually cause failure of the quartz envelope. If the lamp is accidentally handled with bare hands it should be cleaned with methylated spirit. The protective paper or plastic sleeve should be used when handling quartz lamps.

Crompton Dichroic lamps are made from U.V. stop quartz with a cover glass cemented to the face of the lamp.

This prevents accidental damage to the lamp caused by handling of the quartz capsule and exceeds the safety requirement for UV emission. In addition, the front glass contains any lamp fragments in the event of capsule failure at the end of its life. These latter two points provide a lamp that fully complies with BS EN60598.

Source : Crompton Lamps