Measurements in Infrared Thermography

Abstract

s experiment, resulting in the discovery of infrared radiation, had fundamental significance for the rise and development of research in the infrared spectrum. In his experiment Herschel (1800a-d, 1830) observed heat effects associated with different spectral ranges of the Sun's radiation. He placed blackened containers of sensitive mercury thermometers along the spectrum obtained by splitting the Sun's radiation in a glass prism. The energy of incident rays was absorbed by the containers and the thermometers indicated temperatures higher than the ambient temperature. On investigating the results of his experiment, Herschel discovered that the readings of the thermometers located beyond the red end of the spectrum were higher than those of the thermometers located within the visible range. The experiment proved that the full spectrum of the Sun's radiation is wider than the visible range, and that directly beyond the red end there exist rays, weakly refracted by the prism, invisible to the naked eye.Herschel named them 'invisible rays' or the 'invisible thermometrical spectrum'. Somewhat later they were commonly named 'infrared rays'. Further experiments carried out by other scientists showed that the infrared rays -likewise visible rays -are subject to reflection, refraction and absorption. Experiments on interference and polarization proved that infrared radiation has the same nature as visible radiation. However, it was Herschel who first determined the point of maximum heat effect and stated that it is located beyond the visible range of the spectrum.Although Herschel is considered as a pioneer in infrared research, it is difficult to imagine contemporary infrared thermography without the basic laws of radiative heat transfer. They are formulated and described below. Basic Laws of Radiative Heat TransferAt temperatures higher than 0 K, the absolute zero temperature, each body emits heat (thermal) radiation. The intensity of this radiation depends on wavelength l and the body's temperature (Gerashenko et al. 1989).