As a surface that is illuminated by a light source moves away from the light source, the surface appears dimmer. In fact, it becomes dimmer much faster than it moves away from the source. The inverse square law, which quantifies this effect, relates illuminance and luminous intensity as follows:

E illuminance [lx] I luminous intensity [cd] d distance between light source and surface [m] α angle between plummet and strike of the beam of light

For example, if the illuminance on a surface is 40lx at a distance of 0.5m from the light source, the illuminance decreases to 10lx at a distance of 1 meter, as shown in the figure Figure 2 9: inverse square law below. [2]