My understanding of ray tracing is that it maps out luminance for a scene. Now with ray tracing (backwards ray tracing that starts from the camera outwards) does each ray have a luminance value associated with it? In the case of a perfectly diffuse surface, can one simply take the mean of all of the secondary rays luminance to arrive at a luminance value for the primary ray or is this assumption wrong? If L_{0} are the primary rays luminance, and L_{1} ,L_{2} ,L_{3}, L_{4} ,L_{5} are the secondary rays that are created due to the diffuse surface. (see Diagram for explanation).
Interesting question- in radiometric terms, rays in a ray trace are associated with radiance- a conserved quantity with units of W/(m^2*sr)- the quantity tells you how much power is propagating in a given direction. Luminance is the photometric equivalent to radiance- radiance that has been corrected for the spectral response of normal human eyes.