A hole in a cavity kept at a constant temperature is supposed to approximate the behaviour of a black body. If an external heat source is keeping the body at a constant temperature wouldn't the heat permeate through the walls of the cavity and be radiated into the centre meaning that there is radiation inside the cavity whether there is radiation coming in through the hole. My understanding was that you put some radiation through the hole and measure the spectrum of radiation that comes out as a result which should approximate a black body spectrum which is modelled by the equation $$U(\nu ,T)=\frac{8\pi h \nu^3}{c^3}\frac{1}{e^{\frac{h\nu}{kT}}-1}$$ My question is does the theory take into account the radiation that is already present inside the cavity, if so where and how?

• Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber from en.wikipedia.org/wiki/Black_body – user179430 Dec 29 '17 at 12:01