Question

What is the radiant exitance?

Answer 1

Radiant exitance is the amount of light, emitted by an area of surface of a radiating body. In other words, its the radiant flux on the surface that is radiating. The SI units are Watts / meter^2.

Radiant exitance is commonly used in astronomy when talking about stars. It can be determined using the Stefan-Boltzmann equation;

`R = sigma T^4`

where `sigma` is the Stefan-Boltzmann constant, equal to `5.67 xx 10^-8 W m^-2 K^-4` and `T` is the temperature of the emitting body in Kelvin. For the Sun, `T = 5,777 K`, the radiant exitance is;

`R = (5.67 xx 10^-8)(5,777)^4 = 3.58 xx 10^8 W m^-2`

That means that one square meter of the sun is giving off as much light as about 4 million light bulbs!

If you multiply the radiant exitance by the entire surface area of the body, you can find the luminosity, or the total amount of power given off.