Some Aspects of Radiometric Measurements Involving Gaussian Laser Beams

The use of laser beams for the calibration of one radiometer against another presents certain problems due to the gaussian intensity profile and coherence of such beams. The errors introduced by a decentering of the radiometers with respect to each other or with respect to the gaussian laser beams are analyzed in detail. The calibration of radiometers for irradiance measurements is of special importance in radiometry; the formulae required for making such calibrations are given, and the influence on the measurement accuracy of certain factors such as radiometer surface uniformity, relative radiometer aperture sizes, and gaussian beam waist diameter, are discussed. Internal reflections in radiometer windows can cause problems due to the coherence of laser beams. These problems are shown to be potentially more serious with large diameter beams than with small diameter beams. However, it is shown that, in either case, the inclusion of a very small wedge angle in the window (0.1° typically) is usually sufficient to eliminate such problems.