Candela to lumen conversion

From LED LInear.   A good sourc fro basic inofrmation on LED's

This calculator allows you to do an approximate conversion between millicandelas (or candelas) and lumens for an LED where you know the beam angle. The reason it's approximate is because the specs don't usually include information about how the luminous intensity (in candelas) was measured. This calculator is perhaps most useful in comparing the light output of LEDs with different beam angles.

To briefly explain the math the wizard does, it simply applies the conversion 1 candela = 1 lumen/steradian. To do this, it converts the beam angle you supply into a solid angle in units of steradians.

The likely thing for a manufacturer to do is to list their luminous intensity spec in the most favorable way possible. So it can be the case that this wizard provides inaccurate values--this happens because the assumption in the wizard is that the luminous intensity value you supply represents an average value across the beam angle you supply. It's too tempting for the suppliers to list a maximum value instead, since there is not a standard for measuring this. Beware especially of wide-angle LEDs, which will probably be distorted the most by this calculation.