The ratio between the emissive power and the absorptive power is the same for all bodies at the same temperature".
v], in order to deal with dimensionless emissivity, since Max Planck had already utilized the needed symbol when expressing emissive power.
16 is stating: The emissive power of an arbitrary cavity at thermal equilibrium is equal to the emissivity of the material which makes up the cavity multiplied by a function.
Conversely, when referring to emissive power, E, he was, in fact, referring to this quantity, even in modern terms.
It's all to do with emissivity, the term that describes the ratio of emissive power
of a surface at a given temperature to that of a black body at the same temperature and with the same surroundings.
1 was presented in this form , the reflectivity term was viewed as reducing the emissive power from arbitrary cavities.
The cavity will now possess an emissive power, E, which might differ substantially from that set forth by Kirchhoff for all cavities.
The emissive power might still not be equal to the Kirchhoff function in this case, depending on the amount of photons that are available from reflection.
1 can be expressed in terms of emissive power in the following form:
While Stefan's law might appear to hold over narrow spectral ranges within the infrared, such band-like emissions fall far short of producing the emissive power
expected at all frequencies, through the application of the 4th power relationship.
Stewart's formulation leads to the realization that the emissive power of any object depends on its temperature, its nature, and on the frequency of observation.
Stewart's treatment, un like Kirchhoff's, does not lead to universality [1, 8, 9, 14] but, rather, shows that the emissive power of an object is dependent on its nature, its temperature, and the frequency of observation.