A radiance source includes a housing having an interior wall, wherein at least a spherical portion of the interior wall of the housing is spherical, an interior volume, and an exit port. A light source is disposed within the interior volume of the housing. A calibration structure blocks and reflects a light ray that would otherwise travel directly from the light source to the exit port without reflecting from the interior wall. The calibration structure has a calibration body having a curved back surface facing the light source and a curved front surface facing the exit port. There is an optically diffuse, lambertian reflecting surface on at least the spherical portion of the interior wall of the housing, the back surface of the calibration body, and the front surface of the calibration body.