The Inverse Square Law assumes a point source, one so small that it has no dimension. This of course is not true for most sources, but the accuracy of the equation holds, within 1%, if the source size does not exceed 1/5th of the distance between the source and the point.
If a source exceeds the 1/5th rule, then it is arbitrarily divided until the sections meet the 1/5th rule. Then the intensity of the source in candelas at the appropriate angle is divided by the number of sections to use the Inverse Square Law. The contribution from each section must then be calculated. If an accuracy of three to five percent is acceptable then 1/3 can be substituted for 1/5.
