**Irradiance**is the radiant flux received by the detector area. The unit of

**irradiance**is W/m

^{2}.

**Irradiance**is denoted by E

_{e,λ},

φ

_{s}is the received radiant flux on the detector surface and A

_{D}is the detector area or surface. Irradiance always follows the Inverse Square Law. Suppose from a point source the radiant flux is being received by two surfaces of A

_{1}and A

_{2}where they are equal surface area. They are placed at r

_{1}and r

_{2}distance. Now the flux received by the surface

And the flux received by the surface

Where, I

_{e,λ}radiant intensity and ω solid angle. Again the radiant flux received per unit area for A

_{1}and A

_{2}are

Here A

_{1}and A

_{2}are equal. Putting the φ

_{e,λ}= I

_{e,λ}ω in the equation we get

This is Inverse Square Law of irradiance. If we convert this irradiance into Illuminance then we should follow the conversion equation i.e.

Where, K

_{m}is the constant which is called maximum spectral luminous efficacy and its value is 683 lm/W. By definition the luminous flux received by unit area of the detector is called Illuminance. Its unit is Lux or Lumen per sq. meter (lm/sq. m). It also follows the same inverse square law, i.e.

E

_{v}is related to the surface dA where luminous flux is falling on this surface perpendicularly. E'

_{v}is related to the surface dA' where this surface creates an angle Ɵ to the base plane. As per figure above,

This above equation can be written making it generalized,