Use of the single-wavelength approximation in radiometric diffraction loss calculations

Estimating the effects of diffraction is essential in modern radiometric experiments. The majority of the tools used for this date back to the pioneering work by W. Blevin and W. Steel. These were analytical in nature, obtained by aggressive use of approximate techniques applied to the Fresnel diffraction integral; further, blackbodies were treated as uniform sources that could be described by a single characteristic wavelength, enabling diffraction effects to be determined through a single monochromatic calculation. This requires diffraction effects to change linearly with wavelength. The domain over which this is satisfied to the error tolerance required by contemporary radiometry is unclear. This paper investigates the single wavelength technique and establishes criteria for its use.