English unit of measurement for the illumination (E) on a surface. (See "lux" for metric.) One footcandle is the illumination on a surface that is one foot from and perpendicular to a uniform point source of one candela. Combining the inverse square law and the cosine law, the formula for footcandles (FC) is:

where CD is the candlepower, A is the angle of incidence of the light beam (see diagram under "lux,") and D is the distance of the surface from the light source.