Thermal Detection

Abstract

We now consider the detection of optical and infrared radiation by means of the temperature change of the detecting element, rather than by use of the direct photoexcitation process. Because detectors of this type have no inherent long-wavelength cutoff and the sensitivity limitation is mainly due to the thermal background power, we shall first investigate in detail the fluctuations of the thermal radiation field. To this point, we have assumed that the thermal background may be treated as a constant power striking the detector, but we have noted that this assumption is valid only for Bose-Einstein state occupancy much less than unity. In the first part of the chapter, we shall derive the effective fluctuation of the radiation field in the general case in which f, the occupancy factor, may have any value. Our treatment also yields the basis for thermal or Johnson noise.