when light hits a surface at an angle. it is darker if it hits it at higher angle(facing away) than at a lower angle(facing towards). 1 Is there a formula for the brightness.The relationship of angle to brightness does not look linear. http://www.vam.ac.uk/vastatic/microsites/1384_leonardo/animated_illustrations/view/?movie=light1
Is this only because the greater the angle, the greater the surface area that the beam interacts with? Or are there other factors?
There is also an inverse-square dependence, which is negligible if (for example) a point-like source is very far from the surface.