Infrared detectors

You can easily detect infrared radiation with your hand but there are much more sensitive methods.
As was shown by Leslie with his cube in 1804, blackened surfaces are better detectors of heat than shiny ones are, and this fact is used in several of the detectors described below.

(i) A liquid-in-glass thermometer with a blackened bulb

Heat is absorbed by the bulb and the liquid level rises.

(ii) The ether thermoscope

This consists of a tube with a glass bulb at either end; one is clear and the other is blackened (Figure 1). The tube is partly filled with ether and therefore both bulbs contain a mixture of air and ether vapour. When infrared radiation falls on the apparatus more is absorbed by the blackened bulb than by the shiny one, and the pressure inside this bulb rises and pushes the ether along the tube.

(iii) Crookes radiometer

A vane mounted on a vertical pivot is enclosed in a glass bulb filled with air at low pressure. One side of each part of the vane is blackened and the other is silvered (Figure 2). When infrared radiation falls on the radiometer the black surfaces absorb more energy than the shiny ones and so become hotter. The air molecules hitting one of these blackened surfaces will gain energy and rebound with an increased velocity so pushing the vane round. (There is a constant temperature gradient across the vane). The black surfaces are the trailing surfaces in this case. This piece of apparatus has almost become an executive toy and can be bought in a number of gift shops.

If the pressure of air in the bulb is reduced nearly to that of a vacuum the vane will begin to rotate in the opposite direction. This is because of the actual pressure of radiation on the shiny surfaces; the quanta of radiation rebound strongly from these surfaces so pushing the vane round. The shiny surfaces are now the trailing surfaces.

(iv) The bolometer

This instrument was invented by Langley in 1881. It is simply a blackened strip of platinum and the radiation falling on it is measured by the resulting change in the resistance of the strip. This is measured by connecting the strip into one arm of a Wheatstone bridge.

(v) The thermopile

The principle of the thermocouple was used by Nobili and Melloni in 1830 to measure the intensity of radiation.
A series of thermocouple junctions were connected in series, so that the final e.m.f. generated was much larger than that due to one junction (Figure 3). They called this arrangement a thermopile.














The junctions onto which radiation is to fall are blackened. The thermocouple has been used to measure the radiation from the planets and hence their temperatures may be found.

(vi) The transistor

A phototransistor can be used to measure the intensity of radiation, because the leakage current through it increases with temperature. Such a device is very simple to use.

(vii) The disappearing filament pyrometer

This consists of a telescope that has a lamp filament in the focal plane of the eyepiece. Radiation from the source is focused on to the filament. The lamp filament and the source are viewed through a red filter and the temperature of the filament is adjusted by altering the current through it until the filament disappears. The instrument is calibrated by comparison with a source of known temperature.

(viii) The thermistor

One type of this semiconductor device has a negative temperature coefficient of resistance — its resistance decreases with increasing temperature.