What is a blackbody, a graybody, a realbody?
A blackbody is a perfect radiator. It has zero transmittance and zero reflectance. According to Kirchhoffs law, then, the emissivity of a blackbody is one. Blackbodies were first defined for visible light radiation. In visible light, something that doesnt reflect or transmit anything looks black. Hence the name. A graybody has an emissivity less than one that is constant over wavelength. A realbody has an emissivity that varies with wavelength. IR cameras sense infrared radiant energy over a waveband. To get temperature, they compare results explained above with a calibration table generated using blackbody sources. The implicit assumption is the target is a graybody. Most of the time this is true, or close enough to get meaningful results. For highly accurate measurements, the thermographer should understand the spectral (wavelength) nature of the target. Max Planck is credited for developing the mathematical model for blackbody radiation curves. The accompanying figure shows the magn
