Convert luminance units instantly — candela per square meter (cd/m²), nit, stilb, foot-lambert, lambert and more. Essential for display engineers, photographers, cinema professionals and anyone working with screen brightness or lighting standards.
Luminance is the photometric quantity that describes how much light is emitted, reflected or transmitted from a surface per unit area in a given direction. It is the physical correlate of perceived brightness — what your eyes actually detect when looking at a screen or a lit surface. The SI unit is the candela per square meter (cd/m²), informally called the nit, and conversions between cd/m², stilb, foot-lambert and lambert appear constantly in display engineering, cinema projector calibration, and architectural lighting design.
💡 SI & Metric Units (cd/m², stilb, lambert)
The candela per square meter (cd/m²) was formally adopted as the SI unit of luminance by the International System of Units and is now the universal standard in display specifications, HDR standards (HDR10, Dolby Vision) and smartphone brightness ratings. The stilb (sb) is the CGS equivalent: 1 sb = 10,000 cd/m², originally defined by the French photometrist André Blondel around 1900. The lambert (L) — also attributed to Blondel — equals 1/π cd/cm², or approximately 3,183.1 cd/m², and was widely used in European photometric literature through the mid-20th century before SI standardisation replaced it.
🎬 Imperial & Cinema Units (foot-lambert, cd/ft²)
The foot-lambert (fL) is the primary luminance unit in the North American cinema industry and is defined as 1/π candela per square foot, equal to approximately 3.4263 cd/m². SMPTE 196M specifies 16 fL as the standard screen brightness for commercial cinema. The candela per square foot (cd/ft²) is the direct imperial equivalent of cd/m²: 1 cd/ft² equals 10.7639 cd/m². The candela per square inch (cd/in²), also written as cd/in², is used in backlight engineering for small panels and equals 1,550 cd/m². Despite metrication, foot-lamberts remain ubiquitous on projector data sheets and cinema compliance reports worldwide.
Luminance vs Illuminance — the key distinction: Luminance (cd/m²) measures how bright a surface appears — light leaving toward the viewer. Illuminance (lux / foot-candle) measures how much light falls onto a surface. A dim room can have high illuminance from a powerful lamp while the walls show low luminance. Display engineers care about luminance; lighting architects care about both. Use our Illuminance Converter for lux and foot-candle conversions.
A commercial cinema projector calibrated to the SMPTE 196M standard of 16 foot-lamberts delivers approximately 54.8 nits at the screen surface. Dolby Cinema laser projectors target ≈ 31 fL (106 cd/m²) for HDR content.
Smartphone HDR
1,000 cd/m² × 0.29186 = 291.86 fL
A flagship smartphone displaying HDR content at 1,000 nits is emitting 291.9 foot-lamberts — more than 18× the brightness of a cinema screen. This is why phones feel blinding in a dark theatre.
Legacy Photometry
0.5 sb × 10,000 = 5,000 cd/m²
A 1960s European photometry paper reports surface luminance of 0.5 stilb. Converting to SI: 5,000 cd/m² — roughly equal to a modern premium outdoor display panel running at full brightness.
Outdoor Digital Sign
5,000 cd/m² ÷ 10.7639 = 464.5 cd/ft²
A highway LED billboard running at 5,000 nits for daylight visibility equals 464.5 cd/ft². North American sign codes sometimes specify brightness limits in cd/ft², making this conversion essential for compliance reports.
Calibrate digital cinema projectors to the SMPTE 16 foot-lambert standard by converting between fL and cd/m² for compliance reports and white-point measurements.
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Display Engineers
Specify peak brightness for HDR10, Dolby Vision, and HLG mastering in nits (cd/m²) and convert to foot-lamberts for North American client deliverables.
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Photographers & Videographers
Match monitor brightness to target luminance for colour-grading suites and ensure accurate soft-proofing between cd/m² calibration values and display specs.
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Architectural Lighting Designers
Convert between cd/m² and foot-lambert for outdoor digital signage, ensuring legibility at calculated luminance levels in roadway and façade lighting projects.
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Photometry Researchers
Translate luminance data reported in historical CGS units (stilb, lambert) into SI values for meta-analyses and cross-era comparison of lighting experiments.
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Medical & Dental Equipment Techs
Calibrate X-ray viewing lightboxes and dental curing light meters where luminance specifications may appear in lambert or foot-lambert from legacy equipment manuals.
Mental Math Tips — Luminance
nits → fL
Divide nits by 3.43 to get foot-lamberts. A 300-nit monitor ≈ 87.5 fL. Or memorise: 100 nits ≈ 29 fL. For quick rough work, "divide by 3" is accurate to within 10%.
sb → nits
Move the decimal 4 places right: 0.1 stilb = 1,000 cd/m². Stilbs are so large that most practical display values are tiny fractions — a 500-nit display is only 0.05 sb.
fL → nits
Multiply foot-lamberts by 3.43 to get nits. Cinema standard 16 fL × 3.43 ≈ 54.9 nits. Dolby Cinema can hit ~108 nits (≈31.5 fL). A handy check: 1 fL × π ≈ 3.14 cd/m².
Frequently Asked Questions — Luminance Converter
1 foot-lambert equals approximately 3.4263 cd/m² (nits). Conversely, 1 nit equals about 0.2919 foot-lamberts. This conversion is critical for cinema projectionists matching SMPTE brightness standards, where the target is 16 foot-lamberts (≈ 54.8 nits) for standard 2D projection.
A nit and a candela per square meter (cd/m²) are exactly the same unit. "Nit" is the informal trade name for cd/m² used primarily in the display and TV industry. Both measure luminance — the intensity of light per unit area emitted from a surface in a specific direction.
1 stilb (sb) = 10,000 cd/m² (nits). To convert stilb to cd/m², multiply by 10,000. To convert cd/m² to stilb, divide by 10,000. The stilb is the CGS unit of luminance, defined as one candela per square centimeter, and is mostly found in older European photometric literature.
Outdoor displays in direct sunlight require at least 1,000 nits (cd/m²) for comfortable viewing. High-brightness digital signage typically runs at 2,500–5,000 nits, while direct-view LED billboards can exceed 10,000 nits. Standard indoor monitors at 200–400 nits are completely washed out in full sun.
1 lambert (L) equals approximately 3,183.1 cd/m². The lambert is a CGS photometric unit defined as the luminance of a perfectly diffuse surface emitting or reflecting 1 lumen per square centimeter. It is largely obsolete in modern standards but still appears in older photometry textbooks and cinema documentation.
Typical smartphone screens range from 400 to 800 nits for standard use. High-end flagship phones can peak at 1,500–2,000 nits in HDR mode for outdoor legibility. Budget phones usually offer 400–500 nits. Outdoor readability generally requires at least 600 nits.
SMPTE standard 196M specifies a screen luminance of 16 foot-lamberts (approximately 54.8 cd/m²) for commercial cinema projection. Dolby Cinema and IMAX laser systems can achieve 31 foot-lamberts (≈ 106 cd/m²) or higher for HDR presentations, dramatically exceeding the traditional standard.