An LED (Light Emitting Diode) is an electronic semiconductor component. When current flows is applied in the forward direction of the device it emits light. More details... (Wikipedia)

The first problem that someone new to lighting design has is defining the requirement in specific terms that can be used to calculate how much light is required.

See below some Key Words,  simple explanations

A traditional candle emits light with a luminous intensity of roughly one candela (cd).

A steradian is defined as conical in shape; where point P represents the center of the sphere. The solid (conical) angle q, representing one steradian, is such that the area A of the subtended portion of the sphere is equal to r2, where r is the radius of the sphere.

If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen (lm).

In some LED specification the Luminous intensity is given in candela or milli candela (mcd), click on to convert millicandela to lumen (LED Center)

To specify what illumination level is needed on a surface you need to calculate how much light is needed per square meter of surface.

This unit of measurement is called Lux (lx= lm/m2) and is the amount of total light in lumens divided by the area being illuminated.

Converting from Candela to Lux
To convert from Candela values to Lux: Divide the Candela values by the square of the distance in meters from the light source to your illuminated surface.

Converting from Lux to Candela.
To convert from Lux to Candela: Multiply the Lux values by the square of the distance in meters from the light source to your illuminated surface.

Radiometric Power (mW)
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