Evaluation of Blackbody Cavity Emissivity in the Infrared Using Total Integrated Scatter Measurements

Abstract

Deviations from ideal blackbody (BB) behavior can be characterized by a BB’s effective emissivity. The cavity emissivity is most often obtained through a model, given a particular set of input parameters associated with the BB cavity geometry and surface optical properties. It can also be measured directly (radiance) or indirectly (reflectance). A study of BB cavity emissivity using the reflectance method is presented. Several types and designs of blackbody cavities, including those from fixed-point and water bath BBs, using our infrared total integrated scatter (ITIS) instrument for emissivity evaluation are examined. The emissivity is characterized as a function of position on the output aperture, as well as a function of output angle. The measurements have revealed emissivity values, both significantly greater than, and in confirmation of, modeling predictions. For instance, the emissivities of three fixed point BB cavity designs were found to vary significantly despite modeling predictions in the design process of similar behavior. Also, other BB cavities that exhibited poor emissivity performance were re-painted and re-machined, in one case more than once, before the predicted performance was achieved.