The amount of energy absorbed when a stream of external radiation enters a rectangulargroove cavity has been calculated for a variety of surface conditions, cavity depths, and incident energy distributions. The surfaces of the cavity are either diffuse reflectors or specular reflectors, and a wide range of reflectivity values has been considered. The incoming radiation is either diffusely distributed across the cavity opening or else arrives in a bundle of parallel rays. The results show that for diffuse incoming radiation, a specular cavity absorbs more effectively than does a diffuse cavity. For incoming radiation in a parallel ray bundle, the comparison depends on the angle of inclination of the rays. 1963 by ASME.