The main problem with the conventional incandescent gas-filled lamp is that it loses particles of tungsten from the filament when the lamp has been in service over long periods (that is, lamp blackening). This evaporation process is reduced by enclosing the filament in a small quartz envelope. The envelope can be made smaller because quartz will operate at a much higher temperature than glass, and the increased pressure on the filament slows down the rate of evaporation.
In addition to reducing the dimensions of the lamp, a halogen element is also added to the gas of the lamp. Iodine, chlorine or bromine elements may be used to produce a reversible chemical reaction between tungsten and the halogen. In effect, a regenerative cycle of events occurs where some of the tungsten is evaporated from the filament towards the wall of the lamp and mixes with the halogen to form a metal halide. However, instead of tungsten depositing itself upon the wall of the lamp, the tungsten halide reverses direction towards the hot filament where they dissociate, causing tungsten to be deposited back on the filament, while the halogen is available to commence the regenerative process again. To maintain this regenerative cycle a lamp wall temperature above 250 C is required. Although the tungsten is not always deposited back to the part of the filament it came from, a significant improvement is made in the lumen output and life of the lamp compared to GLS lamps of equivalent wattage. Fig 1 and Fig 2 show linear and single ended tungsten halogen lamps.
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| Fig. 1 linear tungsten halogen lamp |
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| Fig. 2 Single ended tungsten halogen lamp |
Extra low voltage tungsten halogen lamps combined with precision faced glass reflectors (spotlights) are used extensively for commercial display lighting and domestic applications. The compact 12 volt lamp is supplied from adjacent 230 V o 12 V transformer or via a remotely positioned transformer. Several lamps can also be supplied from a 12 V track system and a suitably rated transformer.
Tungsten halogen lamp data
. efficacy ranges between 18 and 24 lumens per watt (mains voltage)
. good colour rendering
. operation position – certain low wattage lamps may by operated in any position while linear may have horizontal operation only (reference should be made to the manufacturer)
. suitable for dimmer circuits
. no control gear required – connected directly to the source of voltage
. more expensive than GL lamps
. precautions –
Care should be taken not to contaminate the lamp surface with greasy fingers, resulting in fine cracks when in operation
Contaminated lamps can be cleaned with a soft cloth and methylated spirit
Under no circumstances must the lamp touched, either directly or indirectly, when in operation.
. average life 2000 or 4000 hours depending on wattage and type
. many applications including – floodlighting, display, security, exhibitions, photographic, general domestic, commercial and industrial uses
. lamp designation - TH