The the luminous intensity of a point source is known, then the illuminance it will produce at a certain distance can be described with the inverse square law. This hold true only if the distance to the light source is at least five times the largest dimension of the source.
The inverse square law
Because the illuminance depends on the area onto which the light is falling, it is proportional to the inverse squared distance to the source:
E = I / d²
This formula can only be applied when the illuminance on a plane perpendicular to the light source is required.