Errors In Passive Infrared Imaging Systems Due To Reflected Ambient Flux

AbstractPassive infrared imaging systems produce a signal in which the amplitude at a particular spot is related to the radiance of the corresponding spot in the scene viewed. The differences in signal levels in different areas of the image is usually interpreted in terms of radiance temperature differences in the scene viewed, and may be converted to true radiance temperatures if the scene includes an object whose radiance temperature is known.The radiance temperatures are usually converted to true temperatures by correcting for the emittance of objects in the scene. This would be correct in the absence of reflected ambient flux. However, for scenes at ambient temperatures, ambient flux is always present in significant amounts. Temperature errors due to reflected ambient flux are discussed from a theoretical standpoint, and a procedure for experimentally evaluating the ambient flux is suggested.IntroductionInfrared thermography produces a monochromatic image in which the brightness of an area in the cathode-ray tube image is related to the apparent radiance of the corresponding area in the scene viewed. These apparent radiances are frequently interpreted in terms of the radiance temperature of the area in the scene, or more commonly, in terms of the temperature differences between different areas in the scene, which can be converted into true temperatures, or temperature differences, if the emittances of the materials viewed are known. Such interpretation is subject to errors, which are frequently over­ looked. The principal errors are due to atmospheric absorptance and the reflected radiance of the areas viewed. Only the latter error will be discussed in this paper.The problem is easier to deal with when the measurements are made in an enclosed area, with walls at a reasonably uniform temperature. These conditions apply both to infrared thermography in an enclosed area, and optical pyrometer temperature measurements inside a furnace, but this discussion will be confined to infrared thermography in a room.Blackbody radiatorsFirst, we will define a blackbody radiator and discuss its properties. A blackbody is a surface having the properties of (1) absorbing all incident radiant energy; (2) emitting the maximum possible concentration of spectral exitance (or radiance) for any body at its temperature at all wavelengths. The basic physical laws of radiation were derived for blackbody radiators. They are expressed as the Planck equation, which relates the spectral exitance (or radiance) of a blackbody to its temperature and the wavelength of the emitted radiation; the Stefan-Boltzmann equation, which relates the spectrally total exitance (or radiance) of a blackbody to its temperature; Lambert's law, which in essence states that the radiance of a blackbody radiator is independent of direction; and Kirchhoff's law, which states that, for a body in thermal equilibrium, heated and cooled by radiation only, the radiant energy absorbed is equal to that emitted.Blackbody cavitiesNo true blackbody surface exists in nature, but a completely enclosed cavity with opaque walls at a uniform temperature is a true blackbody. If a small aperture is made in the wall of the cavity, the blackbody radiation can be observed with only a very slight degradation of the quality of the blackbody. Since the opaque walls of the cavity are at a uniform constant temperature, they are in thermal equilibrium, and reemit all incident radiant energy they absorb, and since the walls are opaque, all incident radiant energy that is not absorbed is reflected. Hence the emittance and reflectance of the walls sum to one, and the exitance of any area on the wall is equal to the irradiance on that area, regardless of the emittance at that point.