Luminance Converter
Luminance measures the brightness of a light source or surface as perceived from a specific direction, expressed as luminous intensity per unit area. It is fundamental to lighting design, photography, display technology, and vision science. The SI unit is candela per square metre (cd/m²), also called a nit. A typical computer monitor displays at 150-300 nits, while direct sunlight reflects at tens of thousands of nits. Older and alternative units include the stilb (10,000 cd/m²), the lambert (π/1 cd/cm² or about 3,183 cd/m²), the millilambert for dimmer surfaces, the footlambert used in some photography contexts, and the apostilb for very dim light. Different disciplines and countries developed these units historically, making conversion essential when comparing brightness specifications across photography, lighting, displays, and research. A typical room-temperature tungsten lamp has a luminance around 30,000 cd/m² or 3 stilbs. This calculator displays nine luminance units: candela per square metre variants, stilbs, lamberts, millilamberts, footlamberts, apostilbs, and nits, with NIST-defined precision.
NIST conversion factors
All conversions below use standard NIST-defined factors, with all units expressed relative to cd/m² (nit).
| Unit | Symbol | cd/m² |
|---|---|---|
| Candela per square metre | cd/m² | 1 |
| Candela per square foot | cd/ft² | 10.76391 |
| Candela per square centimetre | cd/cm² | 10,000 |
| Stilb | sb | 10,000 |
| Lambert | L | 3,183.0989 |
| Millilambert | mL | 3.18310 |
| Footlambert | fL | 3.42626 |
| Apostilb | asb | 0.31831 |
| Nit | nit | 1 |
Luminance converter: frequently asked questions
What is luminance?
Luminance is the luminous intensity (light) per unit area, measured as the brightness of a light source from a specific direction. The SI unit is candela per square metre (cd/m²), also called a nit. It measures perceived brightness, not just raw light output.
What is the difference between luminance and illuminance?
Luminance (cd/m²) is the brightness of a light source or reflecting surface per unit area, measured from a viewing direction. Illuminance (lux) is the total amount of light falling on a surface per unit area. They are related but different quantities.
What is a stilb?
A stilb is an older CGS unit of luminance equal to 10,000 candelas per square metre (10,000 nits). It is still used in some specialized contexts. One stilb equals a very bright surface, like a traditional light source.
What is a lambert?
A lambert is a non-SI unit of luminance equal to 1/pi candelas per square centimetre, or about 3,183 candelas per square metre. The millilambert is 1/1,000 of a lambert, used for dimmer surfaces.
Why are there so many luminance units?
Different fields and countries developed luminance units independently. SI uses cd/m² (nits). Photography uses footlamberts. Older contexts use stilbs, lamberts, and apostilbs. Converting ensures clarity when comparing brightness specifications across disciplines.
Official sources
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.