Multiply the candlepower rating measured in candela of spherical light sources, such as incandescent or regular household light bulbs with a wire filament, by a conversion factor of 1 lumen per candela to obtain the mean spherical candlepower (MSCP) for the light source, measured in lumen. The intensity or candlepower of a spherical light source is considered equivalent in value to the MSCP for that source.
Convert the beam angle in degrees to a solid angle in steradians for non-spherical light sources that only specify the beam angle in degrees, as is typical with narrow beam light sources such as the LED. Non-spherical light sources do not emit light uniformally in all directions, but typically produce directional beams. Use the following formula to convert the beam angle in degrees (Q) to a solid angle (P) in steradians: P = 6.28 x (1-cosine(Q/2)). For example, a 90 degree beam angle is equivalent to a solid angle of 1.8 steradians.
Multiply the candlepower rating in candela of a non-spherical light source by the solid angle through which the light beam is distributed (measured in steradians) to calculate the total light output. For example, a non-spherical light source rated as 2 candela with a solid beam angle of 1.8 steradians has a total light output of 3.6 lumen.
Divide the total light output in lumens by 12.57 to obtain the mean spherical candlepower, which is the average light brightness of the source radiating equally in all directions.