What is the radiant exitance?
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;
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.