Question

What is the radiant exitance ?

Answer 1

Radiant flux, also known as radiant power, is the radiant energy reflected, transferred, or obtained per unit time. The spectral flux, also known as spectral power, is the radiant flux per unit frequency or wavelength, based on whether the spectrum is viewed as a function of frequency or wavelength.

Complete answer:
Radiant exitance is the volume of light released by an area of the radiating body’s surface. Alternatively, the radiant existence is the radiating flux on the radiating surface. Watts per square meter are the SI units of Radiant exitance. The radiant existence is said to be $1$ $W$ per ${m^2}$, if an optical power of $1$ $W$ is radiated from an area of $1$ ${m^2}$.

Irradiance is a synonymous concept that uses the same units but refers to received radiation.
When discussing stars, the term "radiant existence" is often used. The Stefan-Boltzmann equation can be used to calculate the radiant exitance. The Stefan- Boltzmann equation is :
$R = \sigma {T^4}$
where $T$ is the temperature of the emitting body in Kelvin and $\sigma$ is the Stefan- Boltzmann constant, which is equal to $5.67 \times {10^{ - 8}}\,W{m^{ - 2}}{K^{ - 4}}$.

Note: Existence is a measurement of a surface's potential to produce radiation. The luminosity, or overall volume of power given off, is calculated by multiplying the radiant exitance by the entire surface region of the body.