# Radiant Exitance and Luminous Exitance

Radiant Exitance is defined as the radiant flux emitted from a source per unit area surface of that source.
Radiant flux is the source quantity.
It is denoted as Me,λ and its unit is W/m2.

One important thing is that radiance is related to the source whereas the irradiance is related to the detector. Both have same unit i.e. W/m2.

By applying Plank’s Law in the specific blackbody radiation, the radiant Exitance can be expressed as

Now we can define the Luminous Exitance and we can get Luminous Exitance by using the radiometric to photometric conversion equation.

Where, Km is the constant which is called maximum spectral luminous efficacy and its value is 683 lm/W.
So, the Luminous Exitance can be defined as the Luminous flux emitted from a source per unit surface area of that source.
The unit of Luminous Exitance is lm/m2.

## Radiant Intensity and Luminous Intensity

The energy flux per unit solid angle in certain direction is called radiant intensity.
If the intensity is denoted by

Here, ω is the solid angel in steradian and φλ is the energy flux at λ wavelength.

A filled detector with its active area is placed at r1 distance from a source and another detector with a different area is placed r2 in such a manner that they are in same solid angle.

The received flux by each other must be same.
But in photometric, the luminous flux per unit solid angle in a given direction is called Luminous intensity. Its unit is Candela (Cd) or Lumen/steradian (lm/sr)
The relation between the photometric and the radiometric intensity is given by

Ie, λ is measured in W/sr and Iv is measured in lm/sr.