I am curious about what the major factors are to cause such difference. Is it because of the absorbance of the surface material?
@wcyjames yes I think it has something to do with the absorbance of the surface material since part of the radiance loss are converted to heat maybe?
@wcyjames In physics, atoms of the material would absorb the energy of photons to reach a high energy state from a low energy state. This is basically where the loss of energy occurs and thus the incidsent and exiting surface radiance differ.