light

Question

what is the relationship between lumen and candela(candle power)
please don't copy from wiki

Answer 1

Lumen and candela are different physical quantities.
Lumen is a quantity for luminous flux (a measure of the amount of light), whereas candela is a quantity for (light) source intensity (i.e. a measure of the brightness of the source).

For a source of intensity of 1 candela, the amount of light (i.e. luminous flux) that it gives out in all directions is equal to 4.pi (pi=3.14159...) lumens .

Lumen and candela can be related by the equation:

L = I.A/R^2
where L is the luminous flux in unit of lumens,
I is the source intensity in candela
A is the area of a surface (in m^2) at distance R (in m) from the source.

Example:
A surface of 1 m2 placed at a distance of 2 m from a light bulb of intensity, say, 10 candela will give a luminous flux of L given by,

L = (10x1)/2^2 lumens = 2.5 lumens