(Opt; Symb: E) The luminous flux, in footcandles, falling on a surface, expressed as E = F/A, where F is the luminous flux in lumens and A is the area. Note: The typical unit is the foot candle, the illumination falling on a surface 1 foot from a 1-candlepower source, which is equal to 1 lumen per square foot. Ancther expression is E = I/d2, where I is the source intensity in candlepower and d is the distance to the incident surface