What is Infrared?                                

Infrared (IR) radiation exists around us at all times. Any object having a temperature above absolute zero (-273 ^oC) emits infrared radiation.

IR is the electromagnetic radiation that has a wavelength longer than visible light but shorter than microwave radiation. IR wavelengths are between 700nm and 1mm, and can be further divided into short-wave infrared, medium-wave infrared and long-wave infrared.

There are 3 basic methods of transferring heat into an object – Conduction, Convection and Radiation

Conduction is quite simply the transfer of heat through direct contact between the energy source and the object being heated.

Convection is the heating of a gas or liquid by a heat source, which then transfers the energy to the object. e.g an oven.

Radiation occurs when the emitted IR radiation is absorbed by a cooler body. The absorption of IR causes the temperature of the cooler body to increase. Infrared radiation is the means by which heat energy reaches us from the Sun.

Electromagnetic Spectrum                                       

Long wave is least sensitive to colour and is readily absorbed by water.

Medium wave is also insensitive to colour and readily absorbed by water and many plastics and paints.

Short wave is more penetrating than Long wave and is good for heating metals, but can pass through clear materials.

Short wave IR 760 - 1600 nm , Typical temp 2200°C

Medium wave IR 1600 - 4000 nm, Typical temp 950°C - 1600°C

Long wave IR 4000 - 1 nm, Typical temp 700°C or less.

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 250^oC the halogen cycle will breakdown. The best applications are when the lamp is designed for the operational voltage.

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 cycle works as follows -

1: 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).

2: 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.

3: 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.
 

4: 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.

To view a movie on The Halogen Cycle >click here< (courtesy of James Hooker)

Applications