I'm having trobule trying to work out the solution to a quite simple radiometry problem i've found in the book Physically Based Rendering. The problem is this one a) Compute the irradiance at a point due to a unit-radius disk h units directly above its normal with constant outgoing radiance 10 J/m2 sr. Do the computation twice, once as an integral over solid angle and once as an integral over area. b) Similarly, compute the irradiance at a point due to a square quadrilateral with the same outgoing radiance that has sides of length 1 and is 1 unit directly above the point in the direction of its surface normal. Can someone explain the solution to these problems? Thank you in advance.