Shortwave IR Lamps

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	SHORT WAVE I.R. LAMPS A shortwave I.R. lamp consists of a coiled tungsten filament in a quartz tube. A combination of a halogen gas and Argon is present in the tube. The diameter of the filament is less than the diameter of the quartz tube. So, spacers/support rings are used to keep the filament in the centre of the tube. This prevents the filament from coming in direct contact with the tube. Pinching of the tube is also done and this prevents the filament from sagging under its weight when installed in a vertical position. A reflective coating of Zirconium can be done on half the circumference of the tube (or as desired). This helps in focusing the heat on the object to be heated. This coating is applied in thin layers and multiple layers are applied. This achieves good adhesion and the coating does not come off. The nominal working life of I.R. lamps is 5000 hours. Care must be taken that they are not subjected to severe vibrations and mechanical shocks.

	What goes on inside an I.R. lamp?


	The Halogen Cycle The halogen cycle describes a complex chemical interaction between tungsten, oxygen and a halide that makes tungsten halogen lamps possible. Incandescent lamps operate by using an electric current to heat a filament so that it glows. The material that evaporates from the hot filament builds up on the inner bulb-wall and darkens the lamp. This "lamp blackening" becomes even more severe when the filament is situated near the bulb-wall, as in thin tubular lamps. The halogen cycle prevents lamp blackening and extends the service life of the bulb. The diameter of the filament is less than the diameter of the quartz tube. So, spacers/support rings are used to keep the filament in the centre of the tube. This prevents the filament from coming in direct contact with the tube. Pinching of the tube is also done and this prevents the filament from sagging under its weight when installed in a vertical position. A reflective coating of Zirconium can be done on half the circumference of the tube (or as desired). This helps in focusing the heat on the object to be heated. This coating is applied in thin layers and multiple layers are applied. This achieves good adhesion and the coating does not come off. The nominal working life of I.R. lamps is 5000 hours. Care must be taken that they are not subjected to severe vibrations and mechanical shocks.

	The cycle works as follows:

	Tungsten atoms evaporate from the hot filament and diffuse toward the cooler bulb wall. The filament temperature is about 3030º Celsius (or about 5480º Fahrenheit). The temperature at the bulb wall is about 730º C (or about 1340º F). Tungsten, oxygen and halogen atoms combine on or near the bulb-wall to form tungsten oxyhalide molecules. Bromine is now the most common halogen. Chlorine is used in some special photocopying lamps that operate only for brief intervals. Tungsten oxyhalides remain in a vapour phase at the bulb-wall temperatures and this vapour moves toward the hot filament. A combination of diffusion and convection currents are responsible for the movement. High temperatures near the filament break the tungsten oxyhalide molecules apart. The oxygen and halogen atoms move back toward the bulb wall and the tungsten atoms are re-deposited on the filament. The cycle then repeats.

	Inrush Currents The initial current through a filament is called the inrush current. This current can be as high as x12 times the normal operating current , but this will only last for around 20 milliseconds. Hot resistance depends on the temperature of the filament and this varies with lamp design. Cold resistance of a filament is generally regarded to be that of room temperature. To reduce the inrush current a low voltage can be applied to the lamp before the normal voltage.

	Dimming Lamps can be dimmed as per normal incandescent lamps, but caution must be taken. A 5% under voltage will extend life by 80%, the watts will fall by 8% and if the bulb wall falls below 250oC the halogen cycle will breakdown. The best applications are when the lamp is designed for the operational voltage.


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