The emissivity εc of cylindrical cavities with different bases (flat, perpendicular to the axis; flat, inclined to the axis; conical) is derived from measurements of the cavity reflectance ρc. A general formula for the calculation of ρc = 1 - εc is given, which holds for high values of the ratio length L of the cylinder to radius R of the cylinder. For a flat base perpendicular to the axis only the value of the reflectance ρo of the wall material is needed; in both other cases one must refer to the measurements.
G. Bauer and K. Bischoff, "Evaluation of the Emissivity of a Cavity Source by Reflection Measurements," Appl. Opt. 10, 2639-2643 (1971)