Planck's law describes the spectrum of blackbody radiation, which depends solely on the object's temperature. Max Planck's study of blackbody radiation marked the beginning of quantum mechanics. Both cavities are filled with blackbody radiation in equilibrium with the antenna and resistor. The black hole acts as a perfect blackbody radiating at this temperature. A reference source, such as blackbody radiation, is defined as having a CRI of 100. The (blackbody) colors of stars are sometimes confused with the colors of the spectrum. The output is similar to a blackbody.