How this calculator works
Luminous flux counts light weighted by human visual sensitivity. Candela is flux per solid angle, while lux is flux per receiving area. The conical model assumes constant intensity within the cone and returns on-axis illuminance on a perpendicular plane. Its lux result is not the average over a finite planar beam footprint. Area modes instead assume the entered flux is distributed uniformly over the entered surface.
Formula
Ω = 2π[1-cos(θ/2)]; Φ = IΩ; Eaxis = I/r². For uniform received flux, E = Φ/A.
Worked example
A uniform hemispherical source with full cone angle 180 degrees and intensity 100 cd emits 628.319 lm. At 2 m on axis it gives 25 lx. Distributing 1000 lm evenly over 10 m2 gives 100 lx.
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Frequently asked questions
Can lumens be converted to lux without geometry?
You need a receiving area or a directional model and distance.
Is the cone angle a half-angle?
Enter the full opening angle. A hemisphere is 180 degrees and a full sphere is 360 degrees.
Does this include losses?
No. Absorption, reflectors and non-uniform beam shape need measured factors or a photometric model.