Hey rqyytrf.

The usual way of looking at maximum intensity for something whether it be flux or something else is to look at orthogonality.

In normal 3D space, something is orthogonal to something else if x_a*x_b + y_a*y_b + z_a*z_b = 0 for some vectors (x_a,y_a,z_a) and (x_b,y_b,z_b).

Basically if the normal vector is the reverse of the incoming vector, then you get maximum intensity.

you can apply this idea across an un-even surface and for that, you want to look at a flux integral.