Well, that is, by far, the easiest part of the whole thing. See the attached gh file.
Assuming this is for an actual project, what you are attempting to do is not a trivial simulation. I seriously doubt if you can just assume a lambertian distribution for a LED screen. I would suggest having a discussion with the screen manufacturer about the its brightness profile and then backing that up with some onsite measurements with a luminance meter.
Regards,
Sarith
GlowBoard.gh (972.4 KB)