Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or's emitted from a particular area, and falls within a given solid angle in a specified direction. They're used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance's watts per steradian per square metre (W·sr-1·m-2). Description Radiance characterizes total emission or reflection, while spectral radiance characteriz… (More on Radiance)