Point-source illuminance calculator

A small source illuminates a normal surface in inverse proportion to the square of the unobstructed distance.

Units

E_{\text{fc}} = \frac{I}{d^{2}}

Calculator

Enter the values you know and leave one box empty. Calculate finds the empty one, the result or any input.

Given

E_{\text{fc}} = \frac{1800}{{12}^{2}} = 12.5

Symbol	Meaning	Unit
$E_{\text{fc}}$	normal-plane illuminance	fc
$I$	luminous intensity toward the point	cd
$d$	distance from the source	ft

Symbol	Meaning	Unit
$E_{\text{lux}}$	normal-plane illuminance	lux
$I$	luminous intensity toward the point	cd
$d$	distance from the source	m

Results are estimates for study and screening. Check the inputs, units and assumptions before you use them for design, billing or contract decisions.