A system, comprising a processor and a memory. The memory stores instructions executable by the processor to determine, from an image including a portion of a surface of a human body, a reflected light intensity from the body surface portion, determine, a skin reflectance of the body surface portion based on a location of the body surface, a light source, and an image sensor location, and to determine, for the body surface portion, an incoming radiance, based on the skin reflectance and the reflected light intensity.