ABSTRACT
All humans “glow” with 10-micron infrared radiation which follows the same optical principles as visible light. In physics, the word “heat” can be either a noun or a verb: a warm body will heat (verb) a cooler body, increasing the heat (noun) of the cooler body. Objects at high temperatures emit radiation with shorter wavelengths, and physiologic temperatures are no exception. For bodies having physiologic surface temperatures, the radiated energy peaks within the 7 to 14 μm octave of the electromagnetic spectrum. Emissivity is an important concept in medical thermography. Biologic “emissivity” is the ability of the skin to emit or absorb radiation within the 9 to 10-micron infrared range. The range of emissivity runs from 1.0 (the ideal emitter/absorber) to 0.0 (the ideal reflector). Human skin exhibits a uniform emissivity of 0.98, making it an excellent thermal radiator. Other ways the body loses heat are conduction, convection, and evaporation. Thermography detects heat radiation, and this is explained graphically via the laws of Planck and Wien. The effects of background temperature, humidity, imaging distance, and the imaging angle are discussed.
