In this case, is theta between L1 and N as opposed to L1 and p0?
tbell
@a_s Yep, since the question is "how much light hits the surface at P". The direction of observation has no effect on that. Once we start considering "how much light gets reflected toward $P_0$, the angle to $P_0$ will become relevant."
irradiance at point p?
Yes
In this case, is theta between L1 and N as opposed to L1 and p0?
@a_s Yep, since the question is "how much light hits the surface at P". The direction of observation has no effect on that. Once we start considering "how much light gets reflected toward $P_0$, the angle to $P_0$ will become relevant."