Illuminance

Illuminance $E$ (in lux, lx) describes how strongly a point light source lights a surface at a distance $r$:

$E=\frac{I}{r^2}$

where $I$ is the luminous intensity of the source (in candelas, cd). This is another example of the inverse-square law — illuminance falls off with the square of the distance, so moving twice as far from the lamp gives four times weaker lighting.

The calculator finds any of the three quantities ($E$, $I$, $r$). The same $1/r^2$ rule governs gravity and the interaction of charges — light, like those forces, "spreads out" over an ever larger spherical surface.