# Brightness and solid angle

• I
I have a doubt regarding the role of the solid angle when calculating the power(W) with the brightness of the source I'm observing with area Asource. I was given the definition:

with A the Aantenna. If now I take as an example the picture below to calculate W, we conisder as solid angle the one respect to the center of our "collecting area"(antenna Aant), if I take another point on this area the solid angle that subtends the source is different and shouldn't be different the integration terms?