Since early experiments, it has been confirmed that illuminances produced by the source of light decrease inversely to the square of the distance from the plane to illuminate the source. This ratio is expressed by the following formula:
where Ε is the illuminance level in lux (lx), Ι is the intensity of the source in candelas (cd), and d is the distance from the source of light to the perpendicular receptor plane.
In this way, an illuminance ratio Ε1 and Ε2 may be established, between two planes separated by a distance d and D from the source of light, respectively:
This law is fulfilled when we are dealing with a punctual source of perpendicular surfaces to the direction of the luminous flux. However, the law is supposed to be accurate enough when the distance undergoing measurement is, at least, five times the maximum dimension of the luminaire (the distance is big in relation to the size of the area of the source of light).