Illumination

Illumination
Law of Inverse Square:

The amount of Light which falls on one unit of the area of these surfaces is inversely proportional to the square of the distance from the source. The brightness of the surface as it appears to the eye when viewed from a fixed distance depends on the amount of light that falls on it.

A stronger light will be placed further away from the surface than a weaker one to produce the same effect.

Candela
Candela: Unit of Light source measurrement.

Luminous Intensity (Symbol I)
This has to do with the strength of a source of Light and is measured in Candelas. This is a measure of the rate at which a source of Light is sending out Luminous energy in a given direction.

Luminous Flux (O) measured in Lumen

A divergent beam (cone or Pyramid)

Consider a surface, a distance d from the source, half way the beam length.

The light incident on a unit surface area of the surface is inversely proportional to the square of the distance.

Hence:

AI/d2 = O

Given an area 'A' and a luminous intensity I, the amount of light incident on the area 'A' is proportional to AI/d2

If A and d are measured in  SI unit meter. this amount of light, referred to as luminous flux is measured in lumens.

When unity = 1 lumen.

This is the amount of light falling on a unit area 1m2 illuminated by a source with luminous intensity of 1 Candela placed a unit distance 1m away.