Radiance is a physically based rendering package written largely by Greg Ward,
initially at LBNL, EPFL, then SGI, now running Anyhere Software. It is a
physically-based, image-generating, backward raytracer with very a powerful
rendering engine. It is used worldwide for lighting analysis and can generate
accurate values for radiance/luminance (W/sr.m^2,cd/m^2) and
irradiance/illuminance (W/m,Lux).