4 . 0 Modeling Diffraction in the Monte - Carlo Ray - Trace Environment

In the Theory of Heat Radiation, Max Plank [1959] states that “so far as their physical properties are concerned, heat rays are identical with light rays of the same wavelength”. His book, which contains much of the basis upon which modern radiation heat transfer is based, does not consider the phenomenon of diffraction “on account of its complicated nature”. Plank goes on to say “[i]t will be assumed that the linear dimensions of all parts of space considered, as well as the radii of curvature of all surfaces under consideration, are large compared with the wavelengths of the rays considered. With this assumption we may, without appreciable error, entirely neglect the influence of diffraction caused by the bounding surfaces, and everywhere apply the ordinary laws of reflection and refraction of light”. Many of the instruments modeled by the Thermal Radiation Group do not adhere to this restriction, rather some of the instrument dimensions are on the order of the wavelength of the entering radiation. In such cases, general methods used to model radiation heat transfer must be supplemented with some means of dealing with diffraction in the development of the complete radiative model of an instrument.