Planck described blackbody radiation as quantized resonances in a cavity.
It was not until five years after Planck made his heuristic assumption of abstract elements of energy or of action that Albert Einstein conceived of really existing quanta of light in 1905 as a revolutionary explanation of black-body radiation, of photoluminescence, of the photoelectric effect, and of the ionization of gases by ultraviolet light. … Contrary to Planck's beliefs of the time, Einstein proposed a model and formula whereby light was emitted, absorbed, and propagated in free space in energy quanta localized in points of space.
Source (Highlighting by me)
The German Wikipedia concerning the quanta of blackbody radiation:
A blackbody (also: blackbody radiator, Planckian radiator, ideal blackbody) is an idealized thermal radiation source. … At the same time, it emits electromagnetic radiation as thermal radiation …
The attempt to describe the blackbody radiation theoretically has contributed essentially to the emergence of quantum physics. ... Only the assumption of Max Planck in 1900 that matter can absorb and emit the radiation energy only in the form of certain energy quanta could solve this riddle.
What in a blackbody emits the quanta?
