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I have a problem in which I'm asked to calculate the radiancy of a black body which consists of a hole in a body, and they ask me for the angular distribution of the radiation that comes out of it.

I have already calculated the radiancy, now, for the angular distribution, I don't know how to proceed. The exercise says: $$\frac{dR_T}{d\Omega}=R_T\frac{\cos\Omega}{\pi}$$ But at the end, it just multiplies the numerical value $R_T$ times $\cos\Omega$. Can someone explain how this is done and why?

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