Lux to Candela Calculator
Enter a measured lux value, the illuminated area, distance, and incidence angle. The calculator exposes the geometric assumptions behind the candela estimate.
Estimated candela:
Implied lumens:
A QUICK WALKTHROUGH
How to use this tool
- Enter illuminance in lux and the illuminated area in square metres.
- Enter source-to-surface distance and the angle from the source axis.
- Calculate candela using I = E × d² ÷ cos(θ), with lumens shown as E × area.
Geometry is required
Lux measures illuminance on a surface, while candela measures directional intensity. This estimate uses a small, uniformly illuminated planar area at a known distance and incidence angle. Area is also used to show the implied lumens.
Formula
For the stated point-source approximation, I = E × d² ÷ cos(θ), where E is lux, d is metres, and θ is the angle between the source axis and the surface normal. Implied flux is Φ = E × A.
Estimate only
The model does not account for beam non-uniformity, reflections, source size, occlusion, or a large-area solid-angle correction. At 90° or beyond, the forward-facing estimate is undefined and is rejected.
GOOD TO KNOW
Common questions
Can lux be converted to candela directly?
No. Distance and surface geometry are required. Enter the illuminated area too when you want the implied lumens.
Why does the area not appear in the candela formula?
For a small planar area, area appears in both flux and solid angle and cancels; it remains useful as a check and for the implied-lumens output.
What does 0° mean?
The surface normal points toward the source axis, so cos(θ) equals 1.