Luminance Converter

Luminance is how bright a surface looks from where you are standing — light per unit area per unit solid angle — and it has picked up more competing units than almost any other photometric quantity. This converter holds six: candela per square metre (the SI unit, also called the nit), stilb, apostilb (also called the blondel), lambert, foot-lambert and skot.

Two of those are plain decimal shifts. The stilb is candela per square centimetre, so 10,000 cd/m²; the nit is not a separate unit at all, just another name for cd/m², which is why it does not get its own row. The other three are the reason this conversion trips people up: the lambert family carries a factor of 1/π baked into its definition. They measure the luminance of a perfectly diffusing surface that emits a given amount of light per unit area, and integrating a Lambertian emitter over the hemisphere brings in that π. So an apostilb is 1/π cd/m², a lambert is 10⁴/π, and a foot-lambert — one lumen per square foot from a perfect diffuser — is 1/(π × 0.09290304) = 3.4262591 cd/m². Nothing about the surface in front of you is guaranteed to diffuse perfectly, so a reading converted from foot-lamberts describes an idealised surface rather than the one you measured.

The foot-lambert is still the working unit of cinema, which makes it the number most people arrive here with. The SMPTE reference for 2D projection is 14 fL on screen, and this tool puts that at 47.97 cd/m² — the rounded 48 nits quoted throughout the industry. For comparison, a reference SDR display is mastered at 100 cd/m² and the PQ curve used for HDR runs to 10,000.

Readings show eight significant digits, cap the decimals at eight, and drop to exponential notation below 0.0001.