How to Calculate the Effective Area of a Theoretical Antenna?

Picture of Black-Body Radiation

Thermodynamics-Black Body Radiation-EM-Energy-Equilibrium-Electromagnetism

Example pf Spectral Density (Fluorescent Light)

Reference: https://en.wikipedia.org/wiki/Aperture_(antenna)#Derivation_of_antenna_aperture_from_thermodynamic_considerations

https://en.wikipedia.org/wiki/Black-body_radiation

https://en.wikipedia.org/wiki/Spectral_density