Luminance & Illuminance Converter

Convert lighting measurements between every common photometric unit. Enter one value, choose whether you are measuring illuminance or luminance, pick the unit you have, and the tool shows the equivalent in every other unit of that same family at once — plus a table of typical real-world values for reference.

Illuminance vs. luminance — why they are different

These two quantities are constantly confused, but they measure different things and are not directly interconvertible.

• Illuminance is the amount of light falling onto a surface — luminous flux per unit area. Its SI unit is the lux (1 lux = 1 lumen per square metre). This is what a light meter reads when you point it at a wall or lay it on a desk, and what building codes specify ("maintain 300–500 lux on office desks").
• Luminance is the amount of light emitted or reflected from a surface toward your eye — luminous intensity per unit projected area. Its SI unit is the candela per square metre (cd/m²), also called the nit. This is what describes how bright a screen, a lamp, or a patch of sky actually looks.

Going from one to the other requires knowing the surface's reflectance and the viewing geometry (for a perfectly diffuse "Lambertian" surface, luminance ≈ illuminance × reflectance ÷ π). Because that assumption is not universal, this tool deliberately keeps the two families separate and never silently converts lux into cd/m². Choose the Quantity first, and the unit list and results switch to match.

How to use it

1. Select the Quantity — Illuminance (light on a surface) or Luminance (light from a surface).
2. Type the Amount you want to convert.
3. Choose the Unit that amount is in.
4. Optionally set the Decimal precision (0–10 places).

The results table updates instantly with the value expressed in every unit of that family. Use the copy or download buttons to export the full table as CSV.

Illuminance units supported

Lux (lx), foot-candle (fc, 1 fc = 10.7639 lx), phot (ph, 1 ph = 10,000 lx), millilux (mlx), and nox (nx).

Luminance units supported

Candela per m² / nit (cd/m²), foot-lambert (fL, 1 fL = 10.7639/π ≈ 3.4263 cd/m²), stilb (sb, 1 sb = 10,000 cd/m²), lambert (L, 1 L = 10,000/π ≈ 3183.1 cd/m²), and candela per ft² (1 cd/ft² = 10.7639 cd/m²).

Is a nit the same as a cd/m²?

Yes. "Nit" is simply another name for candela per square metre; display manufacturers quote peak brightness in nits, which you can read directly as cd/m².

Why is a foot-lambert not exactly 10.7639 cd/m²?

The foot-lambert builds a factor of 1/π into its definition (it measures the luminance of a perfectly diffuse surface), so 1 fL = (1/π) × 10.7639 cd/m² ≈ 3.4263 cd/m² — unlike cd/ft², which is a plain area conversion of 10.7639.

All calculations run entirely in your browser; nothing you enter is sent to a server.