The "Point by Point" method makes use of the inverse-square law, which states that the illuminance at a point on a surface perpendicular to the light ray is equal to the luminous intensity of the source at that point divided by the square of the distance between the source and the point of calculation.

E | Illuminance in footcandles | |

I | Luminous intensity in candles | |

D | Distance in feet between the source and the point of calculation |