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Large-area blackbody emissivity variation with observation angle

Calibration of wide-angle (100°+ field of view) long wave infrared cameras with commercially available large-area blackbody calibration targets poses problems. Typically the emissivity of blackbody sources is specified on axis and up to angles of approximately 20°. For wide-angle camera calibration the emissivity needs to be known out to 60° or greater. Presented is a technique that uses the known on-axis emissivity for the blackbody and changes in radiance with angle to determine the angle-dependent emissivity. Four commercial blackbodies with various surface structures were measured. The emissivity was found to be significantly angle dependent beyond 30°, dropping to 0.95 or less by 60°.