Maybe a bit of a stupid question, but I'm imagining that my room is effectively a cavity with no leakage of light and minimal reflection. If I get a spectrometer and hold it out will it have the Planck shape?
I'm trying to rationalize how certain thought experiments allow you to consider being in a radiation that has a blackbody spectrum (i.e. Einstein Coefficients)
Your room is never in such a state, due to transparent window, reflecting walls, etc.
In a hypothetical situation where everything inside the room is at the same temperature for a long enough time that everything inside, including the radiation and the inner side of the walls comes to mutual thermodynamic equilibrium, then radiation coming off the walls would have black body spectrum. The white walls make the process of equilibration slow, putting some non-reflective black paint or soot on them would make it faster. That's why Lummer and Pringsheim made their metallic cavities inner walls black.
Mmmm... . if you are sitting in your bedroom and are in thermal equilibrium, you are dead.
