This is a simple calculator that accounts for the distance error associated with the inverse square law when calculating candlepower from relatively close distance lux measurements of high etendue lights, more accurately calculating the true candlepower without having to take extremely long distance measurements. The inverse square law alone does not account for the distance behind the light at which the beam divergence meets to a point, which must be included in the distance measurement for accurate candlepower calculations.|
This calculator will work for only for lights having a uniform beam, such as lasers, lens-focused searchlights, and long focal length parabolic searchlights. Even most lights having a uniform beam still have a degree of variation of intensity within the beam. This can be accounted for by measuring the diameter of the beam just inside the feathered edge of the beam, at the distance the lux reading is measured. Accuracy increases with longer distance measurements, however this will achieve accuracy to within a few percent within just a few hundred feet.
More information available at Candlepowerforums.com
Comprehensive Calculation & Beam Modeling for Parabolic Search Lights Here