Luminance Converter – Convert Between All Luminance Units
The Luminance Converter is a free online tool that instantly converts luminance values between eight standard photometric units used in display engineering, cinema, architectural lighting, photography, and scientific research. Whether you need to convert nits to foot-lamberts for projector calibration, translate lamberts to cd/m² for a lighting standard, or compare luminance values across photometric systems, this tool delivers accurate results in real time with a configurable all-units table.
All conversions are computed client-side using the candela per square meter (cd/m², also known as the nit) as the normalized base unit. Enter any value and the result updates instantly — no button press needed.
What Is Luminance?
Luminance is a photometric measure of the luminous intensity emitted or reflected from a surface per unit area in a given direction, weighted by the sensitivity of the human eye. Unlike illuminance (which measures light arriving at a surface), luminance quantifies how bright a surface appears to an observer looking at it. It is the primary quantity used to characterize the perceived brightness of displays, lamps, projection screens, and illuminated surfaces.
The SI unit of luminance is the candela per square meter (cd/m²), informally called the nit — from the Latin nitere, meaning "to shine."
Supported Luminance Units
SI / Metric Units
- Candela per square meter (cd/m², nit) — The SI unit of luminance and the international standard for screen brightness. Modern smartphone displays typically output 400–1,000 nits; HDR televisions can reach 1,000–10,000 nits; the sun at noon approaches 1.6 billion nits.
- Kilocandela per square meter (kcd/m²) — Equal to 1,000 cd/m². Useful for expressing very high luminance levels such as high-beam automotive headlights (~80–200 kcd/m²) or direct sunlight on white surfaces.
- Millinit (mnt) — Equal to 0.001 cd/m². Used for characterizing extremely dim environments such as moonlit outdoor scenes (~0.1 mnt) or the luminance of a clear night sky (~0.001 mnt).
CGS Units
- Candela per square centimeter (cd/cm², stilb) — The CGS unit of luminance, also called the stilb (sb). Equal to 10,000 cd/m². It appears in older European photometric literature and some industrial standards where CGS units remain in use.
- Lambert (L) — A CGS photometric unit defined as 1/π candela per square centimeter, equal to approximately 3,183 cd/m². The Lambert was historically significant in colorimetry and photometry but has been largely superseded by cd/m² in modern practice.
- Apostilb (asb) / Blondel (bl) — Equal to 1/π cd/m² (approximately 0.3183 cd/m²). The apostilb is exactly 10,000 times smaller than the Lambert. It is still referenced in some European lighting standards and older photometric publications.
Imperial / US Units
- Candela per square foot (cd/ft²) — The imperial equivalent of cd/m², equal to approximately 10.764 cd/m². Used in some US lighting design specifications and industrial measurement systems that operate in imperial units.
- Foot-lambert (fL) — Defined as 1/π candela per square foot, approximately 3.4263 cd/m². The foot-lambert is the standard unit for cinema and home theater projector brightness. SMPTE and THX standards specify screen luminance in foot-lamberts: theatrical projection targets 14–22 fL, while premium home theaters aim for 12–16 fL.
How Luminance Conversion Works
Every conversion normalizes through cd/m² in two steps. First, the input value is multiplied by the source unit's conversion factor to obtain the equivalent in cd/m². Then that value is divided by the target unit's conversion factor to produce the final result.
Example — converting 1,000 cd/m² to foot-lamberts:
- 1,000 cd/m² ÷ 3.4263 cd/m² per fL = 291.86 fL
Example — converting 1 Lambert to cd/m²:
- 1 L × (10,000 / π) cd/m² per L = 3,183.1 cd/m²
Common Luminance Reference Values
- Clear night sky: ~0.001 cd/m² (1 mnt)
- Full moon: ~0.01–0.1 cd/m²
- Candle flame: ~10,000 cd/m²
- Standard SDR monitor: 100–300 cd/m²
- HDR10 display peak: 1,000–4,000 cd/m²
- Cinema projection screen: ~50–75 cd/m² (14–22 fL)
- Overcast sky: ~1,000 cd/m²
- Clear blue sky: ~8,000 cd/m²
- Sun's surface as seen from Earth: ~1.6 × 10⁹ cd/m²
Practical Applications
- Display engineering: Screen brightness specifications and HDR certification (DisplayHDR, Dolby Vision) are expressed in nits (cd/m²). Converting to foot-lamberts helps compare display performance with home theater and cinema standards.
- Cinema and projection: Projector brightness specifications use foot-lamberts per SMPTE standards. Converting to cd/m² enables comparison with direct-view display technologies.
- Architectural lighting: Luminance limits for visual comfort in offices and public spaces are typically specified in cd/m². Converting to apostilbs or lamberts may be required when working with older European specifications.
- Photography and camera calibration: Exposure calculations involving luminance targets can require conversion between units depending on which photometric standard the metering system references.
- Scientific research: Vision science, colorimetry, and optical instrument calibration use various luminance units depending on the measuring equipment and the publication conventions of the field.
Tips for Using the Luminance Converter
- Enable Show all unit conversions to display your input value in all eight units simultaneously, grouped by measurement system. This is ideal for cross-referencing values across different industry standards.
- Use the Swap button to immediately reverse the conversion direction — for example, check that a cd/m² to fL conversion round-trips back to the original value.
- Increase the decimal places setting for very small conversions (such as millinits to apostilbs) where precision beyond 4 decimal places matters. The tool automatically switches to scientific notation for extreme values.
- The conversion history panel records your last 10 conversions. Click any entry to restore those values, making it easy to compare multiple luminance levels.