The Rayleigh–Jeans Law is an approximation of the Planck’s law for a blackbody that states that, under certain conditions, emitted radiance is directly proportional to the blackbody temperature. Such an approximation, fits quite well with measurements of radiation emitted by sources at around 300K of temperature (like, in average, for the Earth) at wavelengths higher than 1mm (microwaves).. Wien’s approximation can be used to describe the emission spectrum of a high temperature blackbody n the VIS-NIR spectral range lengths. The estimated errors is less than 2% at wavlengths less that 5microns when a blackbody at around 6000K (like the Sun photosphere) is considered.
The Rayleigh–Jeans approximation is widely used in the processing of satellite images collected by passive MW sensors. Its extension to the thermal infrared spectral range (TIR) is also used for calibrating TIR satellite images (in this case linearity can be guaranteed just by steps on different brigthness temperature intervals).