**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,