Maybe this can help you:
you can use the pcomb program to obtain the solid angle of each pixel of a picture!
for instance this will translate a radiance picture (origin.pic) into a "solid angle" picture (solid_angle.pic) where each pixel has a brightness value that matches the solid angle of the same pixel in the original picture:
pcomb -e 'lo=S(1)' origin.pic >solid_angle.pic