Understanding Infrared Thermography Reading 3

Characteristics of the Target Surface
Target surfaces are separated into three categories; blackbodies, graybodies
and nongraybodies (also called real bodies, selective radiators or spectral
bodies).
The target surfaces shown in Figure 1.6 are all perfect radiators (or
blackbodies). A blackbody radiator is defined as a theoretical surface having
unity emissivity at all wavelengths and absorbing all of the radiant energy
impinging upon it.
Emissivity, in turn, is defined as the ratio of the radiant energy emitted from a
surface to the energy emitted from a blackbody surface at the same
temperature. Blackbody radiators are theoretical and do not exist in practice.
The surface of most solids are graybodies, that is, surfaces with high
emissivities that are fairly constant with wavelength. Figure 1.8 shows the
comparative spectral distribution of energy emitted by a blackbody, a
graybody and a nongraybody, all at the same temperature (300 K).
Charlie Chong/ Fion Zhang