How Does Solid Angle Affect Power Calculation in Radiometry?

[Maximilian2]

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?

 

[lippyka]

The solid angle is determined by the size of your collecting area. The larger the collecting area, the greater the solid angle. Therefore, if you take two points on the same collecting area, the solid angle will be the same regardless of the point you take.