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Posted 05 May 2013 - 02:08 AM

The best explanation I have found is here:




The key is understanding that measured radiance increases as the viewing angle increases

(due to the the projected area term in the radiance equation)


(Intuitively this is due to the same amount of light being compressed into a smaller area - thus it is "brighter")


A Lambertian surface cancels this out by using another cosine term


This means if you are working with Lambertian surfaces you can

- drop the cosine term from the BRDF

- drop the projected area term from radiance

(as these 2 cancel out)


and you end up with typical shader code where the diffuse BRDF is just albedo



Posted 14 April 2013 - 09:05 AM

If you have an emitter and a receiver
and you move the receiver by rotating it around the emitter
(so only the view angle has changed)




(blue represents the solid angle subtended by receiver)

(magenta represents the projected area of the emitter)


am I correct in saying:

- the radiance will increase (cosine term in radiance equation)
- the intensity will decrease (lambert's cosine law)
- the projected area of the emitter will decrease

and the net result is constant radiance at the receiver?

and the incident flux remains constant also ...