Thermography is a technique of obtaining an image of the heat distribution over the surface of an object. The usual method is to use a special television camera with an infrared sensitive detector and a lens which transmits infrared radiation. Such cameras can operate at normal video rates.
Temperature variations in the subject are then displayed as shades of grey or can be converted into pseudo-colour image. Temperature variations as small as 0.1°C can be detected.
The two main fields of application are:
a) to look at the heat distribution in hot specimens such as furnace walls, insulated structures, electronic circuits, etc in steady state – generally known as passive thermography;
b) to provide a pulsed source of heat on one side of a specimen and examine the other side for non-uniformities in infrared emission which would correspond to internal inhomogeneities or large flaws – generally known as active thermography.
The heat pulse may be supplied by a laser and the method is used to examine CFRP laminates and honeycomb bonded panels.
One limitation of thermography is that the infrared emission depends also on the surface condition of the specimen (surface emissivity).
Thermographic images can also be obtained by coating the specimen surface with heat-sensitive (cholesteric) liquid crystals before applying the heat source to the opposite side.