Development of a method to measure shape of soot particles by using polarized thermal emissions

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Widely used Kirchhoff's law of thermal radiation is based on geometric optics approximation, in which typical linear dimension of object is much larger than the wavelength. General theory of thermal radiation beyond the Kirchhoff's law is required for interpretation of thermal radiation (incandescence) from heated aerosol particles because of break down of geometric optics approximation. S.M.Rytov (1953) derived general theory of thermal radiation from object of arbitrary size and shape, based on electrodynamics and fluctuation-dissipation theorem. However, no experimental evidences have been reported so far because of technical difficulties and low attention from practitioners.We have found significant change of directional dependence of thermal radiation from small particles according to their shape. The observed directional dependences were successfully explained by the Rytov theory. This seems to be the first experimental evidence of Rytov theory since 1953.