Luminance Converter

Luminance measures how bright a surface looks to the eye, counting the light it emits or reflects toward you per unit area. It’s the counterpart to illuminance: illuminance is light arriving at a surface, luminance is light leaving it. The SI unit is the candela per square meter (cd/m²).

Key conversions:

• 1 nit = 1 cd/m² (nit is just an older name for the same unit)
• 1 foot-lambert = 3.4263 cd/m²
• 1 lambert = 3,183.1 cd/m² (10,000/π cd/m²)

If you’ve shopped for a TV or monitor, you already know luminance by its marketing name, the nit. Every “1,000-nit HDR” claim is a luminance figure, and converting nits to cd/m² is free because they’re the same thing.

Brightness you can picture:

• A typical computer monitor: 200-500 cd/m²
• An HDR display at peak: 1,000 cd/m² and up
• An overcast sky: around 2,000 cd/m²
• Fresh snow in sunlight: over 10,000 cd/m²

The lambert family (foot-lambert, lambert) lingers in cinema and older photometry, where screen brightness was historically measured that way. The π buried in the lambert isn’t arbitrary; it falls out of assuming a perfectly diffuse surface that scatters light evenly in every direction.

A peak-brightness number on a spec sheet rarely tells the whole story. A screen that hits 1,000 nits on a small highlight but averages 400 across the full panel will look dimmer in a sunlit room than the headline promises.

Where the 3.4263 comes from, since it looks like a magic number: a foot-lambert is one lumen per square foot leaving a perfectly diffuse surface. Divide by π to turn that into candela per square foot, then multiply by the 10.7639 square feet in a square metre, and you land on 10.7639 ÷ π = 3.4263. The lambert is the same idea in centimetres: 10,000 ÷ π = 3,183.1.

That π is the giveaway that these units assume a Lambertian surface, one that looks equally bright from every angle. Matte paper and projection screens are close to it; a glossy photo or a phone screen is not, which is why an off-axis view of a laptop looks washed out while a printed page does not.

Illuminance and luminance are linked by reflectance, and this is the calculation most people actually want. For a diffuse surface, luminance (cd/m²) = illuminance (lux) × reflectance ÷ π. A sheet of white paper with 80% reflectance under 500 lux office lighting sits at 500 × 0.8 ÷ 3.1416, about 127 cd/m². That is a useful sanity check: it explains why a 400-nit monitor looks bright indoors and vanishes outdoors, where the paper next to it is being lit to 30,000 lux and reflecting nearly 8,000.

Typical reflectances: fresh snow around 0.85, white paper 0.75 to 0.85, a white painted wall 0.7 to 0.8, concrete 0.3, grass 0.15, and asphalt about 0.07. The bottom of that list is why a black car interior in summer is unbearable while the sky above it is merely bright.