A measurement technique for determining the directional spectral emittance of blackbody cavities was developed and subsequently applied to several specific cavity geometries. The emittance was deduced from cavity reflectance measurements performed with a laser energy source (10.6 µ) and an integrating hemi-ellipsoid. It was demonstrated that this technique is capable of providing highly precise emittance values with a resolution of five significant figures for high emittance cavities. The emittances of four specific cavity configurations were measured. One of these, an elongated off-axis cone with an entrance lip, gave emittances greater than 0.99999 when coated with nominally specularly reflecting or nominally diffusely reflecting black paints. The emittances of this cavity were on the order of 0.95 in the absence of a coating, the actual emittances of the cavity surfaces being approximately 0.05. Three other cavities with length-to-diameter ratios of three were also studied. These include a cylinder, cone, and off-axis cone having internal surfaces which were coated with black paints. Although the measured emittances were not so high as those for the 12.45-L/D off-axis cone, the level of blackness that was determined is sufficient for most engineering applications.
R. P. Heinisch and R. N. Schmidt, "Development and Application of an Instrument for the Measurement of Directional Emittance of Blackbody Cavities," Appl. Opt. 9, 1920-1925 (1970)