Quote Originally Posted by Stephen Benskin View Post
According to the Encyclopedia of Photography, "The luminance of an extended source or surface is invariant with viewing distance. Luminance is also invariant within a lossless optical system because changes in image size are balanced by inverse changes in solid angle. The luminance of a Lambertian diffuser is invariant with viewing angle because the reduced power reflected off-axis is balanced by a reduction in projected area."
That's it.