Luminance is the photometric measure of light intensity emitted or reflected from a surface per unit area in a given direction. Measured in candela per square meter (cd/m²), informally called nits. It is the physical quantity closest to perceived brightness. Formula: L = d²Φ/(dA cosθ dΩ).

What is a nit in display brightness?

1 nit = 1 candela per square meter (cd/m²). It is the standard unit for display brightness. Standard monitors: 250–350 nits. HDR phones: 1,000–2,000 nits peak. HDR TVs: 1,000–4,000 nits. Sun surface: 1.6 billion nits.

What is the difference between luminance and illuminance?

Luminance (cd/m²) = light coming FROM a surface — what eyes and cameras measure. Illuminance (lux) = light falling ON a surface — what you measure at a desk. Same units relate via: L = E × ρ / π (for a Lambertian reflector), where ρ is reflectance (0–1).

How many nits for outdoor display readability?

Minimum 1,000 nits for shaded outdoor, 2,000–3,000 for partly sunny, 5,000+ for direct sunlight. Direct sun reflects 5,000 cd/m² off a 5% reflectance screen — display must significantly exceed this.

What is a footlambert?

A footlambert (fL) = 1/π candela per square foot = 3.4263 cd/m². US-imperial luminance unit. Used primarily in US cinema (DCI: 14 fL = 48 cd/m²), military displays, and legacy documentation. Named after Lambert + foot.

What is a lambert in luminance?

1 lambert (L) = 10,000/π cd/m² ≈ 3,183 cd/m². CGS unit named after Johann Heinrich Lambert (1728–1777). The millilambert (mL ≈ 3.18 cd/m²) is more practical for modern surface measurements.

1 apostilb (asb) = 1/π cd/m² ≈ 0.3183 cd/m². Also called a blondel. Represents the luminance of a perfect diffuse reflector (Lambertian surface) under 1 lux illuminance. 1 asb = 10⁻³ millilambert.

What luminance should monitors be calibrated to?

sRGB/web: 80 cd/m² (IEC 61966-2-1). SDR video: 100 cd/m² (ITU-R BT.709). HDR mastering: 1000 cd/m² (ITU-R BT.2100 PQ). Medical DICOM: 1–600 cd/m² calibrated. Photo print proofing (ISO 12646): 160–200 cd/m². DCI cinema: 48 cd/m² (14 fL).

What is the WCAG luminance formula?

WCAG 2.2 relative luminance: L = 0.2126⋅R_lin + 0.7152⋅G_lin + 0.0722⋅B_lin, where each channel is linearized from sRGB (divide by 12.92 if ≤0.04045; else ((c+0.055)/1.055)^2.4). Contrast ratio = (L1+0.05)/(L2+0.05). AA requires ≥4.5:1 for normal text.

🔆 Luminance Conversion Calculator 2026

Convert between nits (cd/m²), footlamberts, lamberts, stilbs, apostilbs, and 10 more units instantly. Trusted reference for display engineers, cinematographers, lighting designers, and photometry professionals — with complete MathJax formulas, real-world brightness reference data, and a 2500+ word guide to luminance science.

nit ↔ cd/m² Footlambert ↔ Lambert Stilb · Apostilb 14+ Units HDR / DCI / WCAG MathJax Formulas

⇄ Luminance Unit Converter

Quick Presets (nits):

Value From Unit

Result To Unit

300 nit = 87.56 fL

Formula: value × 0.2919 (nit × π ÷ 10.7639)

📊 All Units — Simultaneous Conversion

📖 How to Use This Luminance Converter

1

Enter a Luminance Value
Type any numeric value in the "Value" field — this is the brightness level you want to convert. You can use decimals and very large numbers (e.g., 1.6e9 for the sun's surface luminance). The converter accepts both standard and scientific notation.
2

Select From and To Units
Choose your source unit from the "From Unit" dropdown (e.g., nit/cd/m² for display brightness or footlambert for cinema measurements) and your target unit from "To Unit". All 14 photometric luminance units are available, covering SI, CGS, and imperial systems.
3

Use Quick Presets for Real-World Values
Click any preset button — Cinema (48 nits), Standard Monitor (300 nits), HDR Phone (1,000 nits), HDR10 TV (4,000 nits), or White Paper (100 nits) — to instantly load that reference value. All outputs update simultaneously. This is ideal for comparing displays or checking cinema compliance against DCI standards.
4

Read All Simultaneous Conversions
The "All Units" panel below the result shows your input converted to every unit at once — perfect for professional reports. The conversion factor used is displayed in the result line so you can verify math independently. Click ⇄ to instantly swap source and target units.
5

Reference the Formulas Section
All photometry formulas (luminance definition, Lambert's cosine law, reflectance formula, WCAG luminance, and unit conversion equations) are rendered in MathJax — the same professional mathematical typesetting used in academic journals and scientific publications.

📐 Luminance Formulas — MathJax Rendered

Luminance — Formal Photometric Definition (SI)

\[ L_v = \frac{d^2\Phi_v}{dA \cos\theta \, d\Omega} \quad \text{(cd/m²)} \]

\( L_v = \) Luminance (cd/m²) · \( \Phi_v = \) Luminous flux (lumens) · \( A = \) Surface area (m²) · \( \theta = \) Angle to surface normal · \( \Omega = \) Solid angle (steradians)

\( \text{For a Lambertian (perfectly diffuse) emitter: } L_v = \frac{M_v}{\pi} \quad \text{where } M_v = \text{luminous exitance (lm/m²)} \)

Luminance is defined as the luminous intensity per unit projected area per unit solid angle. The cosθ factor accounts for the projected area — a surface tilted at angle θ from the observer's line of sight appears smaller by factor cosθ, making it look less bright despite emitting the same total light. For display technology: a display with luminous exitance M_v lm/m² has luminance L_v = M_v/π cd/m² if it follows Lambert's law (equal brightness from all viewing angles). Most LCD displays are approximately Lambertian, while OLEDs have narrower emission profiles.

Unit Conversion Formulas — Nit, Footlambert, Lambert, Stilb, Apostilb

\( 1 \text{ fL (footlambert)} = \frac{\pi}{10.7639} \text{ cd/m}^2 = \frac{1}{\pi} \text{ cd/ft}^2 \approx 3.4263 \text{ cd/m}^2 \)

\( 1 \text{ L (lambert)} = \frac{10000}{\pi} \text{ cd/m}^2 \approx 3183.1 \text{ cd/m}^2 \quad \Rightarrow \quad 1 \text{ mL} = \frac{10}{\pi} \approx 3.1831 \text{ cd/m}^2 \)

\( 1 \text{ sb (stilb)} = 1 \text{ cd/cm}^2 = 10{,}000 \text{ cd/m}^2 \)

\( 1 \text{ asb (apostilb / blondel)} = \frac{1}{\pi} \text{ cd/m}^2 \approx 0.31831 \text{ cd/m}^2 \)

\( \text{General: } L_B = L_A \times \frac{F_A}{F_B} \quad \text{where } F = \text{cd/m}^2 \text{ equivalent of each unit} \)

All luminance unit conversions route through cd/m² (nit) as the pivot. The footlambert is defined as 1/π cd/ft² — the π factor arises from Lambert's cosine law for a perfectly diffuse (Lambertian) surface. Converting: 1 ft² = 0.092903 m², so 1 cd/ft² = 10.7639 cd/m², and 1 fL = (1/π) × 10.7639 = 3.4263 cd/m². The lambert (L) similarly derives from 1/π cd/cm². The stilb is simply 1 cd/cm² without the π factor — used for very high luminances like arc lamps. The apostilb (asb), equal to 1/π cd/m², is the luminance of a Lambertian surface with an illuminance of 1 lux; it is also called the blondel after physicist André-Eugène Blondel.

Lambert's Cosine Law and Lambertian Reflectance

\( L_v = \frac{E_v \cdot \rho}{\pi} \quad \text{(cd/m², for a perfect Lambertian reflector)} \)

\( L_v = \) Luminance (nits) · \( E_v = \) Illuminance (lux) · \( \rho = \) Reflectance (0–1)

\( \text{Example: White paper } (\rho = 0.9) \text{ under } 500 \text{ lux: } L_v = \frac{500 \times 0.9}{\pi} \approx 143 \text{ cd/m}^2 \)

\( \text{Black surface } (\rho = 0.05): \; L_v = \frac{500 \times 0.05}{\pi} \approx 7.96 \text{ cd/m}^2 \)

Lambert's cosine law (introduced by Johann Heinrich Lambert in "Photometria", 1760) states that an ideal diffuse (Lambertian) surface reflects or emits light with luminance that is constant in all directions, even though the luminous intensity falls off as cosθ. The apparent brightness (luminance) of a Lambertian surface is the same from any viewing angle, which is why a uniformly lit wall appears equally bright whether viewed straight-on or at an angle. Real surfaces deviate from ideal Lambertian behaviour: glossy surfaces show specular highlights (Phong model), and retroreflective materials (road signs, bicycle reflectors) return most light toward the source. In display calibration, the ANSI/IEC definition of screen luminance assumes a purely diffuse (Lambertian) output, which is why the π factor appears in specifications.

WCAG Relative Luminance — Accessibility Contrast Formula

\( c_{\text{lin}} = \begin{cases} \dfrac{c_{\text{sRGB}}}{12.92} & \text{if } c_{\text{sRGB}} \leq 0.04045 \\ \left(\dfrac{c_{\text{sRGB}} + 0.055}{1.055}\right)^{2.4} & \text{otherwise} \end{cases} \quad (c \in \{R,G,B\}) \)

\( L = 0.2126 \cdot R_{\text{lin}} + 0.7152 \cdot G_{\text{lin}} + 0.0722 \cdot B_{\text{lin}} \quad (0 \leq L \leq 1) \)

\( \text{Contrast Ratio} = \frac{L_1 + 0.05}{L_2 + 0.05} \quad (L_1 \geq L_2) \quad \text{WCAG AA: CR} \geq 4.5:1 \)

WCAG (Web Content Accessibility Guidelines) 2.2 defines "relative luminance" L as a weighted sum of linearized sRGB channels. Green receives the highest weight (0.7152) because the human eye's peak photopic sensitivity is around 555 nm — the green region of the spectrum. This relative luminance L ranges from 0 (absolute black) to 1 (reference white), making it directly comparable to physical luminance ratios. The contrast ratio formula adds 0.05 to both numerator and denominator to avoid division-by-zero for pure black (L=0) and to reflect the fact that no real display produces absolute zero luminance. Black-on-white: L1=1, L2=0 → CR = (1+0.05)/(0+0.05) = 21:1. WCAG AA Normal Text requires CR ≥ 4.5:1; AAA requires ≥ 7:1.

📊 Complete Luminance Conversion Reference

From Unit	To cd/m² (nit)	Exact Factor	Notes
1 nit (cd/m²)	1 cd/m²	1 (exact)	SI base unit, most common for displays
1 kcd/m²	1,000 cd/m²	1,000	Kilocandela — outdoor signage, HB LEDs
1 cd/cm² (stilb)	10,000 cd/m²	10,000	CGS unit, very high luminance arc lamps
1 cd/ft²	10.7639 cd/m²	1/0.092903	Imperial candela per sq. foot
1 cd/in²	1,550.003 cd/m²	144 × 10.7639	Very bright — laser spots, concentrated beams
1 footlambert (fL)	3.42626 cd/m²	π/10.7639	US cinema standard; 1 fL = 1/π cd/ft²
1 lambert (L)	3,183.099 cd/m²	10,000/π	CGS luminance unit — rarely used today
1 millilambert (mL)	3.18310 cd/m²	10/π	More practical than lambert for dim surfaces
1 apostilb (asb)	0.31831 cd/m²	1/π	Also "blondel". Diffuse reflector of 1 lux
1 skot	0.001 asb ≈ 3.18×10⁻⁴	0.001/π	Historical scotopic unit for very dim surfaces

🌍 Real-World Luminance Reference Values

Source / Surface	cd/m² (nits)	Footlamberts (fL)	Lamberts (L)
☀️ Sun's surface (photosphere)	1.6 × 10⁹	4.67 × 10⁸	502,655
💡 White LED bulb surface	≈ 100,000–300,000	29,000–87,570	31–94
🔆 HB display (peak HDR)	10,000	2,919	3.14
📺 HDR10+ / Dolby Vision TV	1,000–4,000	292–1,168	0.31–1.26
📱 iPhone 15 Pro (outdoor peak)	2,000	584	0.63
🖥️ Calibrated sRGB monitor	80–160	23–47	0.025–0.050
🖥️ Standard office monitor	250–350	73–102	0.079–0.110
📽️ DCI cinema screen (peak white)	48	14.0	0.015
🎬 Dolby Cinema screen	108	31.5	0.034
📄 White paper (500 lux office)	80–143	23–42	0.025–0.045
☁️ Overcast sky	1,000–10,000	292–2,919	0.31–3.14
🌙 Full moonlit snow	0.01–0.1	0.003–0.029	3.1×10⁻⁶
⭐ Minimum scotopic (dark adapted)	≈ 0.000001	≈ 2.9×10⁻⁷	3.1×10⁻¹⁰

💡 Understanding Luminance Units

🔆

Candela/m² — The Nit (SI)

The International System (SI) unit of luminance. 1 nit = 1 cd/m². "Nit" is an informal but universally accepted name derived from the Latin nitere (to shine). Every modern display spec uses nits: TV/monitor brightness ratings, HDR mastering levels, outdoor signage NIT levels, and phone brightness comparisons all use cd/m².

🇺🇸

Footlambert (fL) — Cinema Standard

The US imperial luminance unit, defined as 1/π candela per square foot. The DCI (Digital Cinema Initiatives) specification mandates 14 fL (≈48 cd/m²) as peak white for theatrical projection. The name combines "foot" (area) and "lambert" (the originating CGS unit). Dolby Cinema runs at 31.5 fL (108 nits). Used throughout US cinema, military display specs, and some aviation contracts.

🔬

Lambert (L) and Millilambert (mL)

CGS photometric unit. 1 L = 10,000/π cd/m² ≈ 3,183 cd/m². Named after Johann Heinrich Lambert, the Swiss mathematician who published Photometria in 1760 — the first systematic treatment of light measurement. Because 1 lambert equals a very high luminance (approximately 31× brighter than a standard monitor), the millilambert (mL ≈ 3.18 cd/m²) or microlambert is more commonly used for dim surfaces.

⚡

Stilb (sb) — Extreme Luminance

CGS unit defined as 1 cd/cm² = 10,000 cd/m². The name comes from Greek stilbein (to glitter). Used historically for arc lamps, carbon arc projectors, and flashlamps — sources so bright that nits require nine-figure numbers. Modern usage is largely confined to older photometry literature and some specialised arc lamp specifications. A standard professional cinema projector bulb has a luminance of roughly 10–30 stilbs at its arc.

🌫️

Apostilb (asb) / Blondel

1 apostilb = 1/π cd/m² ≈ 0.3183 cd/m². The apostilb represents the luminance of a perfectly diffuse (Lambertian) surface that receives an illuminance of 1 lux. Also known as the blondel (after André-Eugène Blondel). Used in European photometry standards, particularly in older literature and some medical imaging specifications. 1 asb = 0.001 millilambert = 0.1 millinit/π.

📐

Luminance vs Illuminance vs Luminous Intensity

Luminance (cd/m²) = brightness of a surface as perceived by your eye — direction-dependent. Illuminance (lux) = light falling on a surface — depends on the light source, not the surface. Luminous Intensity (candela) = light emitted in a direction — independent of distance. Luminous Flux (lumen) = total light power — omnidirectional. Photographers care about illuminance (exposure); display engineers care about luminance (brightness); light bulb marketers care about luminous flux (lumens).

💡 Quick Unit Memory Aid: 1 fL ≈ 3.426 nits · 1 nit ≈ 0.2919 fL · 1 lambert ≈ 3,183 nits · 1 stilb = 10,000 nits · 1 apostilb ≈ 0.318 nits. When in doubt, convert everything to cd/m² (nits) first — it is the universal photometric pivot unit used in all modern display, cinema, and lighting standards.

📚 Complete Guide to Luminance — Science, Display Technology, and Photometry

Luminance is the single most important concept in visual science, display engineering, and photometric measurement. It is the physical quantity that most closely corresponds to what the human visual system experiences as "brightness" — and yet it is frequently confused with related concepts like illuminance, luminous flux, and radiance. Understanding luminance precisely is essential for display calibration engineers, cinematographers setting cinema projector levels, web designers verifying WCAG accessibility compliance, lighting architects designing human-centric workplaces, and medical imaging specialists ensuring diagnostic displays meet DICOM GSDF standards.

The formal definition is elegant but dense: luminance is the luminous intensity per unit projected area of a surface, measured in a specified direction, per unit solid angle. The SI unit is the candela per square meter (cd/m²), informally called the "nit" — from the Latin nitere (to shine). The term "nit" was popularized in the display industry, particularly after Apple began using it in marketing materials for MacBook Retina displays around 2012. Before that, cd/m² was the preferred term in technical literature. Today both are used interchangeably, with "nit" dominant in consumer electronics and "cd/m²" preferred in scientific and standards documents.

The history of luminance measurement stretches back to Johann Heinrich Lambert (1728–1777), a Swiss-German polymath who published Photometria in 1760 — the first rigorous, quantitative treatment of light measurement. Lambert introduced what we now call Lambert's cosine law: the brightness of a perfectly diffuse (Lambertian) surface is the same from all viewing angles, even though the luminous intensity decreases as cosθ. This is because although a tilted surface emits less light per steradian at angle θ (intensity × cosθ), it also appears smaller (area × cosθ), and these two effects cancel each other. Lambert's law is fundamental to display technology, rendering algorithms, and photographic exposure calculation. The now-deprecated CGS unit "lambert" (L) and the related footlambert (fL) both carry Lambert's name.

The International System of Units (SI) formally adopted the candela as one of its seven base units in 1948, defined at that time as 1/60 of the luminous intensity of 1 cm² of a blackbody at the temperature of freezing platinum (2,042 K). In 1979, CGPM (the General Conference on Weights and Measures) redefined the candela in terms of radiant intensity and the luminous efficacy of the 540 THz frequency: 1 candela = the luminous intensity of a source emitting monochromatic radiation at 540 × 10¹² Hz (green light, λ ≈ 555 nm) with radiant intensity of 1/683 watts per steradian. The 683 lm/W figure represents the maximum possible luminous efficacy at the peak wavelength of human photopic vision. Luminance in cd/m² is then derived from this base unit: L = d²Φ_v / (dA cosθ dΩ).

Display brightness standards have evolved enormously over the past three decades. Standard Definition television (SDTV) was calibrated to approximately 100 cd/m² peak white. The ITU-R BT.709 standard (1990, the basis for HDTV and Blu-ray) specifies a reference display with 100 cd/m² peak luminance and 0.1 cd/m² black level — a contrast ratio of 1,000:1. High Dynamic Range (HDR) video formats dramatically raised these targets: HDR10 (from the Consumer Electronics Association, 2015) supports content mastered at up to 1,000 cd/m² (though real consumer TVs rarely hit this). HDR10+ (Samsung, 2017) extends mastering to 4,000 nits. Dolby Vision (2014) supports mastering at up to 10,000 nits, with some reference monitors capable of 4,000 nits in practice. Apple's XDR (Extreme Dynamic Range) displays offer 1,600 nits of peak brightness in HDR mode. The ITU-R BT.2100 standard formalizes HDR display requirements and specifies Perceptual Quantizer (PQ) and Hybrid Look-Ahead Gain (HLG) electro-optical transfer functions (EOTFs).

Cinema luminance standards are defined by the Digital Cinema Initiatives (DCI), a joint venture of major Hollywood studios established in 2002. The DCI specification mandates a peak white luminance of 48 cd/m² (14 footlamberts) for theatrical digital cinema. The footlambert remains entrenched in US cinema because all legacy projector specifications used imperial units. Converting: 14 fL × 3.4263 = 47.97 ≈ 48 cd/m². Premium cinema formats push higher: Dolby Cinema (using Dolby's own two-head projector system) achieves 108 cd/m² (31.5 fL). IMAX with Laser reaches up to 22 fL (75 nits) for 2D and 14 fL (48 nits) for 3D (the polarization filters cut approximately half the light). Samsung Onyx (direct-view LED cinema screens) achieves 48–146 cd/m² (14–43 fL) without any polarization loss, making them inherently brighter for 3D content.

Monitor calibration standards differ by profession. The ICC (International Color Consortium) and IEC 61966-2-1 sRGB standard specifies a reference display white point at D65 (6,500 K) with 80 cd/m² luminance — this is the target for web content and print-match workflows. Medical imaging uses DICOM GSDF (Grayscale Standard Display Function), which specifies display luminance characteristics for diagnostic monitors — typically 1–600 cd/m² range with ≤3.5% luminance deviation between calibration and any point in the GSDF curve. ACES (Academy Color Encoding System, used in film VFX) specifies reference displays at 48, 100, or 1,000 cd/m² depending on the viewing environment. ISO 12646 specifies softcopy proofing displays for print colour matching: minimum 80 cd/m², white point D50 (5,000 K), uniformity ≤15%.

The human visual system has an extraordinary luminance dynamic range — approximately 10 orders of magnitude from the absolute threshold of detection (~10⁻⁶ cd/m²) to the pain threshold (~10⁸ cd/m² for direct solar viewing). This range is handled through: (1) photoreceptor switching — rod cells operate from 10⁻⁶ to ~0.01 cd/m² (scotopic vision), cone cells from ~0.01 to 10⁷ cd/m² (photopic vision), with overlap in the mesopic range (~0.01–10 cd/m²); (2) pupil adaptation — the pupil diameter varies from ~2 mm (bright light) to ~8 mm (dark), a factor of 16× in area and therefore 16× in retinal luminance; (3) neural gain control — retinal and cortical circuits adjust sensitivity over minutes (dark adaptation takes up to 30 minutes for full rod sensitivity). The Weber-Fechner law approximates perceived brightness as proportional to the logarithm of luminance, which is why the WCAG contrast ratio L1/L2 and the Perceptual Quantizer (PQ) curve for HDR are defined on roughly logarithmic scales.

⚠️ Display Brightness vs. Perceived Brightness: Two displays with identical luminance (nits) can appear to have very different brightness depending on ambient light level, room illuminance, and the display's black level. A 300-nit display in a dark room appears blazingly bright, while the same display becomes difficult to read under direct sunlight (50,000–100,000 lux of ambient illuminance). This is quantified by the Ambient Light Contrast (ALC) metric. For outdoor readability, industry thumb-rule: display luminance should exceed ambient luminance divided by the display reflectance (typically 4–7%). A 5% reflectance display in 10,000 lux direct sun needs: L > 10,000 × 0.05 = 500 cd/m² just to break even, and 3–5× that for comfortable reading.

✅ Luminance in Photography and Exposure: Camera exposure relates directly to luminance. The reflected-light exposure equation is: Ev (EV at ISO 100) = log₂(L × S / K) where L is luminance (cd/m²), S is ISO sensitivity, and K is the reflected-light meter calibration constant (typically 12.5). A scene with 1000 cd/m² at ISO 100 yields EV = log₂(1000 × 100/12.5) = log₂(8000) ≈ 13 EV (roughly f/8 at 1/125s). Incident light meters instead measure illuminance (lux), not luminance — they use a white dome integrator to sum light from all directions. For HDR photography, luminance mapping to display values uses tone-mapping operators (Reinhard, ACES RRT, etc.) that compress the full scene luminance range into the display's gamut while preserving perceived contrast.

☀

Written & Reviewed by Num8ers Editorial Team — Photometry, Display Calibration & Visual Science Researchers Last updated: April 2026 · Sources: CIE 017.3:2011 — International Lighting Vocabulary (6th edition) — all luminance unit definitions · ISO 11664-1:2007 — Colorimetry, Standard CIE illuminants · IEC 61966-2-1:1999 (sRGB standard) — reference display white luminance 80 cd/m² · ITU-R BT.709-6 (2015) — HDTV system standard · ITU-R BT.2100-2 (2018) — HDR television, PQ/HLG EOTF specification · DCI Specification Version 1.4.4 (2021) — Digital Cinema Initiatives, 48 cd/m² / 14 fL peak white standard · DICOM Supplement 58 — Grayscale Standard Display Function (GSDF), 2003 · SMPTE ST 2094 series — Dynamic metadata for HDR10+/Dolby Vision · ACES Project Committee (2014) — Academy Color Encoding System reference display specs · Johann Heinrich Lambert, "Photometria" (1760) — original photometric science treatise · BIPM — Bureau International des Poids et Mesures, candela definition (CGPM 1979, 2019 SI revision: 683 lm/W at 540 THz) · ISO 12646:2015 — Softcopy proofing display requirements · W3C WCAG 2.2:2023 — Relative luminance formula (SC 1.4.3, SC 1.4.6) — www.w3.org/TR/WCAG22/ · Apple Inc. ProMotion XDR display specs (1,600 nit HDR peak) — apple.com/macbook-pro · Samsung Electronics — Onyx Cinema LED specification (LED digital cinema systems) · André-Eugène Blondel — apostilb (blondel) photometric unit, named by IEC Committee. All conversion factors use exact mathematical expressions with π=3.14159265358979 (IEEE 754 double precision).

❓ Frequently Asked Questions — Luminance Units & Conversion

What is luminance and how is it different from brightness?

Luminance is the physical, measurable quantity of light intensity emitted or reflected from a surface per unit area in a given direction — measured in cd/m² (nits). Brightness is the subjective perception of luminance by the human visual system. The same luminance can appear brighter or dimmer depending on adaptation level, surrounding luminance (simultaneous contrast), and viewing angle. Photometry quantifies luminance; psychophysics quantifies brightness (using units like the "bril" or logarithmic scales). In common use, "brightness" is often used loosely to mean luminance — for example, "display brightness 400 nits."

What is a nit and why are displays rated in nits?

1 nit = 1 candela per square meter (cd/m²), the SI unit of luminance. The informal term "nit" (from Latin nitere) was popularized by the display industry because "cd/m²" is cumbersome in marketing materials. Standard SDR HD monitors peak at 200–350 nits. HDR10 content targets 1,000 nits. Dolby Vision masters content at up to 10,000 nits. Apple's iPhone 15 Pro reaches 2,000 nits of peak outdoor brightness. Samsung's QD-OLED displays achieve up to 3,000 nits on small highlights. All of these are physically measured in cd/m² using a tristimulus colorimeter or telespectroradiometer.

How do I convert nits to footlamberts?

Multiply nits by 0.29186 (or exactly π/10.7639).
Formula: fL = cd/m² × (π/10.7639) ≈ cd/m² × 0.2919
Examples: 48 nits = 14 fL (DCI cinema white) · 300 nits = 87.56 fL · 1,000 nits = 291.9 fL · 4,000 nits = 1,167 fL.
Reverse (fL to nits): multiply by 10.7639/π ≈ 3.4263. So 14 fL × 3.4263 = 47.97 ≈ 48 nits.

What is the DCI cinema brightness standard?

The DCI (Digital Cinema Initiatives) specification mandates 48 cd/m² (14.0 footlamberts) as peak white luminance for theatrical digital cinema projection. This standard was set to match the legacy THX cinema certification standard and the practical limits of xenon arc lamp projectors circa 2005. Premium cinema formats exceed this: Dolby Cinema achieves 108 cd/m² (31.5 fL) using dual Christie laser projectors. IMAX with Laser: 75 cd/m² 2D / 37 cd/m² 3D. HDR cinema (HFR/SDR hybrid): up to 300 cd/m². Samsung Onyx LED: 48–146 cd/m².

What is the difference between luminance, illuminance, and luminous intensity?

Luminance (cd/m²): Light coming FROM a surface in a specific direction. Direction-dependent. What your camera and eye measure. Examples: display brightness, sky brightness, paper whiteness.
Illuminance (lux): Light falling ON a surface. Direction-independent (integrates all directions). Measured by putting a meter ON the surface. Examples: office lighting level, sunlight on desk (100,000 lux), moonlight (0.1 lux).
Luminous Intensity (candela): Light emitted in a specific direction from a point source. Independent of distance (unlike illuminance, which follows inverse-square law). Flashlight beam specification.

How many nits do I need for outdoor display readability?

General rule: display luminance ≥ 3× ambient light reflected back. Direct tropical sun: ~100,000 lux. A display with 5% reflectance reflects 5,000 cd/m². To overcome: need ≥ 15,000 nits (impractical). Typical outdoor display solutions: (1) Shade the display (reduce ambient by 90%), (2) Use anti-reflective coating (reduce to 1–2% reflectance), (3) Use 1,000–2,000 nit displays in shaded outdoor areas, (4) Use 3,000–5,000 nit displays for partial shade/overcast. High-brightness outdoor monitors for bus stops, ATMs: typically 1,500–3,000 nits. Smartphone "outdoor mode": 1,000–2,000 nits peak on small screen area.

What is Lambert's cosine law and why does it matter?

Lambert's cosine law states that an ideal diffuse (Lambertian) surface emits light with luminance independent of viewing angle — because although intensity falls as cosθ, the apparent area also shrinks as cosθ, cancelling exactly. This means: a Lambertian surface looks equally bright from any angle. Real-world implications: (1) Matte paper is approximately Lambertian — it looks the same brightness from any reading angle. (2) LCD displays deviate from Lambertian — luminance falls by 20–40% at ±60° viewing angle. (3) 3D rendering engines use Lambert's law for diffuse shading calculations. (4) The apostilb and footlambert units are defined specifically for Lambertian surfaces (the 1/π factor comes from integrating intensity over a hemisphere: ∫₀^(π/2) cosθ sinθ dθ dφ = π).

What is WCAG luminance and how is it calculated?

WCAG relative luminance is a dimensionless value (0–1) calculated from sRGB colour values using a gamma linearization and weighted sum: L = 0.2126R_lin + 0.7152G_lin + 0.0722B_lin. The linearization converts display values to linear light: values ≤0.04045 divide by 12.92; others use the sRGB gamma curve ((c+0.055)/1.055)^2.4. The weights reflect human cone sensitivity (green=71.52%, red=21.26%, blue=7.22%). WCAG 2.2 contrast ratio: CR = (L1+0.05)/(L2+0.05) where L1≥L2. AA normal text requires CR≥4.5:1, AA large text ≥3:1, AAA normal ≥7:1. Pure black on white: CR=21:1. Check any colour pair at our HTML Color Picker.

How is luminance measured in practice?

Luminance is measured with a luminance meter (also called a spot photometer or telespectroradiometer). The instrument collects light from a small solid angle (typically 1°, 1/3°, or 1/10° acceptance angle) aimed at a specific surface area. Common instruments: Konica Minolta CS-100A (point luminance meter, 1° angle), Photo Research PR-670 (telespectroradiometer), Admesy Rhea (high-speed display measurement), Klein K-10A (colorimeter). For flat-panel display calibration: measurements are taken at screen centre and multiple off-axis points to check viewing angle uniformity. DICOM GSDF calibration requires measuring luminance at multiple grey levels from black to white and fitting a correction curve. Photography light meters measure reflected light (related to luminance) or incident light (related to illuminance).

What is the footlambert used for today?

Footlamberts are used primarily in: (1) US cinema industry — DCI spec mandates 14 fL peak white; THX certification requires ≥12 fL. (2) US military display specifications — MIL-SPEC-461, avionic display contracts. (3) Legacy projector documentation — many older training materials and specifications use fL. (4) US sports video boards — LED scoreboard specifications sometimes still use fL. Outside the US, fL is rarely encountered in modern documents — cd/m² has replaced it globally. The ANSI standards body (ANSI/NAPM IT7.228) formally deprecated fL in display calibration specifications, but the unit persists in cinema contracts due to the global dominance of Hollywood studio distribution agreements.

What is the difference between HDR10, HDR10+, and Dolby Vision brightness levels?

HDR10: Static metadata, max content light level (MaxCLL) up to 10,000 nits mastered; most content targets 1,000 nits. Consumer TVs: 600–1,000 nits actual peak.
HDR10+: Dynamic metadata (scene-by-scene), same 10,000 nit mastering capability. Samsung, Panasonic, Amazon. Consumer TVs: 1,000–2,000 nits.
Dolby Vision: Dynamic metadata, mastered at up to 10,000 nits. Dolby Vision IQ adapts to room light. Reference monitors: 1,000–4,000 nits. Consumer TVs: 1,000–3,000 nits.
HLG (Hybrid Log-Gamma): Broadcast-focused HDR (BBC, NHK). No metadata, backward-compatible with SDR. Reference display: 1,000 nits at 75% signal level.

What luminance should I use for display calibration?

sRGB / web design: 80 cd/m² white point (IEC 61966-2-1) — for dark rooms, print-matching. In practice, most users work at 120–200 cd/m² for bright office environments.
video editing (SDR): 100 cd/m² (ITU-R BT.709 reference). Many editors use 120 cd/m² for practical ambient conditions.
Photo/print proofing (ISO 12646): Minimum 80 cd/m², typically 160–200 cd/m².
HDR mastering (PQ): 1,000 cd/m² reference (ITU-R BT.2100).
Medical imaging (DICOM GSDF): 1–600 cd/m² calibrated range.
Cinema (DCI): 48 cd/m² (14 fL) at screen.

How accurate is this luminance converter?

The converter uses mathematically exact conversion factors, including the precise value of π (IEEE 754 double-precision: 3.141592653589793). All factors involving π (footlambert, lambert, millilambert, apostilb) use JavaScript's Math.PI constant, not the 3.14159 approximation. For footlambert: the exact factor is π/10.7639104167 = 3.426259099… cd/m². The tool is accurate to the precision of IEEE 754 double-precision floating-point arithmetic (approximately 15–17 significant decimal digits). For practical display calibration purposes, differences beyond the 4th significant figure are below the measurement uncertainty of even professional luminance meters (typically ±2%).

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