# What is the radiant exitance?

Dec 17, 2014

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 $5.67 \times {10}^{-} 8 W {m}^{-} 2 {K}^{-} 4$ and $T$ is the temperature of the emitting body in Kelvin. For the Sun, $T = 5 , 777 K$, the radiant exitance is;

$R = \left(5.67 \times {10}^{-} 8\right) {\left(5 , 777\right)}^{4} = 3.58 \times {10}^{8} W {m}^{-} 2$

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.