Luminance Conversion - Light Converter
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What is a Luminance - Light Converter?
A luminance converter is a tool that converts between units of luminance such as candela per square meter (nit), stilb, lambert, and foot-lambert. It is used in display technology, lighting design, photography, and vision science for measuring surface brightness.
History of Luminance - Light Measurement
The candela per square meter (nit) is the SI unit of luminance. The stilb (10,000 nits) is a CGS unit from older European literature. The lambert, named after Johann Heinrich Lambert who published foundational photometry work in 1760, is widely used in the US film and display industries.
About This Luminance - Light Converter
This luminance converter supports 16 units including candela/m², candela/cm², candela/ft², candela/in², kilocandela/m², stilb, lambert, millilambert, foot-lambert, apostilb, blondel, bril, skot, and lumen/steradian variants. It covers all major luminance measurement systems.
Understanding Luminance
Luminance measures the intensity of light emitted from or reflected by a surface in a particular direction, per unit projected area. It corresponds to what the human eye perceives as the "brightness" of a surface. The SI unit is candela per square meter (cd/m²), also called the nit. Other units include the stilb (cd/cm²), lambert (for perfectly diffusing surfaces), foot-lambert (US), and millinit. Luminance is direction-dependent — it varies with viewing angle for non-Lambertian surfaces.
Luminance conversion is essential in display technology (screen brightness specifications), lighting design (glare assessment), photography (exposure metering), and automotive engineering (dashboard and headlight design). A modern OLED display might achieve 1000 cd/m² peak luminance, while the sun's disk has luminance around 1.6 × 10⁹ cd/m². Converting between units allows comparison of display specs, compliance with visibility standards, and proper optical system design.
How to Convert Between Luminance Units
Luminance conversion involves understanding the relationships between candela-based and lambert-based units:
- Identify the source and target luminance units (cd/m², stilb, nit, lambert, foot-lambert).
- The nit IS cd/m² (same unit, alternative name): 1 nit = 1 cd/m².
- The stilb: 1 stilb = 1 cd/cm² = 10,000 cd/m².
- The lambert: 1 lambert = (1/π) cd/cm² = 3183.1 cd/m².
- The foot-lambert: 1 fL = (1/π) cd/ft² = 3.4263 cd/m².
Key Luminance Conversion Formulas
Essential relationships between luminance units:
- 1 cd/m² = 1 nit (same unit)
- 1 stilb = 10,000 cd/m² (= 1 cd/cm²)
- 1 lambert (L) = (10,000/π) cd/m² ≈ 3183.1 cd/m²
- 1 foot-lambert (fL) = 3.42626 cd/m²
- 1 cd/m² = 0.2919 fL
- 1 millilambert = 3.183 cd/m²
- 1 apostilb (asb) = 1/π cd/m² ≈ 0.3183 cd/m²
Worked Examples — Luminance Conversions
Example 1: A display specification reads 500 nit peak brightness. Express in foot-lamberts and stilb.
Solution:
500 nit = 500 cd/m² (nit ≡ cd/m²).
To fL: 500 × 0.2919 = 146.0 fL.
To stilb: 500 ÷ 10,000 = 0.05 stilb.
Answer: 500 nit = 500 cd/m² = 146 fL = 0.05 stilb.
Example 2: A cinema screen target luminance is 14 foot-lamberts (SMPTE standard). Convert to cd/m².
Solution:
Conversion: 1 fL = 3.42626 cd/m².
Multiply: 14 × 3.42626 = 47.97 cd/m².
Answer: 14 fL = 48 cd/m² (nits) — the traditional cinema screen brightness standard.
Example 3: An automotive spec calls for dashboard maximum luminance of 0.1 lambert at night. Convert to cd/m².
Solution:
Conversion: 1 lambert = 3183.1 cd/m².
Multiply: 0.1 × 3183.1 = 318.3 cd/m².
Answer: 0.1 lambert = 318 cd/m² — appropriate for backlit dashboard elements at night.
Example 4: The moon has luminance of about 2500 cd/m². Express in foot-lamberts.
Solution:
Conversion: 1 cd/m² = 0.2919 fL.
Multiply: 2500 × 0.2919 = 729.8 fL.
Answer: 2500 cd/m² ≈ 730 fL — the moon's average luminance at full phase.
Luminance Conversion Quick Reference
Common luminance conversions for display and lighting engineering:
| From | To |
|---|---|
| 1 cd/m² (nit) | 0.2919 fL |
| 1 foot-lambert | 3.4263 cd/m² |
| 1 stilb | 10,000 cd/m² |
| 1 lambert | 3183.1 cd/m² |
| 1 millilambert | 3.183 cd/m² |
| 1 apostilb | 0.3183 cd/m² |
| 1 cd/m² | 10⁻⁴ stilb |
| 1 cd/m² | π apostilb |
| 1 cd/ft² | 10.764 cd/m² |
| 100 cd/m² | 29.2 fL |
| 1000 cd/m² | 292 fL |
| 10,000 cd/m² | 2919 fL |
Understanding Luminance Measurement Systems
The SI unit cd/m² (candela per square meter) directly expresses luminous intensity per unit area in the direction of observation. The alternative name "nit" (from Latin nitere, to shine) is widely used in the display industry. The candela itself is an SI base unit defined by the luminous efficacy of 540 THz radiation. The cd/m² is intuitive: it tells you how many candelas of intensity a surface emits per square meter of projected area.
The lambert (L) and foot-lambert (fL) incorporate π because they assume Lambertian (perfectly diffuse) emission, where luminance is the emitted luminous flux per unit projected area per unit solid angle. For a Lambertian emitter: luminance (cd/m²) = illuminance (lux) × reflectance / π. The 1/π factor converts between lumens/area (exitance) and cd/m² (luminance). The stilb (1 cd/cm²) is simply the CGS version of cd/m² with centimeters. The foot-lambert = (1/π) cd/ft² is the US standard for cinema and display industry, still specified in SMPTE standards.
Real-World Applications of Luminance Conversion
Display Technology
Display brightness is specified in nits (cd/m²) internationally or foot-lamberts (US cinema). HDR10 targets 1000 nit peak; Dolby Vision targets 4000 nit. Converting to fL: 1000 nit = 292 fL, far exceeding cinema's 14 fL standard.
Lighting Design & Glare
Glare assessment uses luminance ratios. A window at 5000 cd/m² next to a desk at 50 cd/m² creates a 100:1 ratio causing discomfort. Standards specify maximum luminance in cd/m², requiring conversion from historic fL-based guidelines.
Automotive Displays
Dashboard brightness must be visible in 10,000 cd/m² ambient sunlight yet comfortable at night (<50 cd/m²). Specifications may mix nit and fL depending on manufacturer country, requiring consistent conversion.
Photography & Exposure
Scene luminance determines exposure settings. A "Zone V" (middle gray) corresponds to about 12.7 cd/m² (3.7 fL). Light meters may report in either unit, and conversion ensures proper exposure calculations.
Road Safety & Signage
Retroreflective sign luminance must meet minimum standards in cd/m² (international) or fL (US DOT). Sign materials are specified by coefficient of retroreflection, and converting final luminance values ensures regulatory compliance.
Common Pitfalls in Luminance Conversion
The most common confusion is between luminance (cd/m², light emitted/reflected BY a surface toward the observer) and illuminance (lux, light arriving ON a surface). They have completely different units and physical meanings. A white paper in bright sunlight (100,000 lux illuminance) has luminance of about 25,000 cd/m², while black paper at the same illuminance has only 1000 cd/m². Another pitfall is the π factor in lambert and foot-lambert: this applies only to perfectly diffuse (Lambertian) surfaces. Real surfaces may be specular (mirror-like) or anisotropic, where the lambert-based units become ambiguous. Also, "brightness" is technically a subjective perceptual term, while "luminance" is the objective physical measurement — but the display industry uses them interchangeably, which confuses purists. Finally, don't confuse cd/m² with cd/cm² (= 1 stilb = 10,000 cd/m²) — a 10,000× error from misreading the area unit.
Key Takeaways
- 1 nit = 1 cd/m² — "nit" is just an alternate name for the SI luminance unit.
- 1 foot-lambert = 3.426 cd/m² — the key US-to-SI luminance conversion.
- The π factor in lambert/fL conversions assumes Lambertian (perfectly diffuse) surfaces.
- Display brightness: typical laptop 300 nit, HDR 1000+ nit, direct sunlight readable >3000 nit.
- Cinema standard: 14 fL = 48 cd/m² (SDR); HDR cinema targets 106 fL = 363 cd/m².
- Luminance ≠ illuminance: luminance is light FROM a surface; illuminance is light ON a surface.
Metric Conversion Factor Tables for Luminance - Light Converter
| Units to convert | Multiply By The Number | Convert as Unit |
|---|---|---|
| Candela/sq meter | 0.0001 | Candela/sq centimeter |
| Candela/sq centimeter | 10000 | Candela/sq meter |
| Candela/sq meter | 0.09290304 | Candela/sq foot |
| Candela/sq foot | 10.763910417 | Candela/sq meter |
| Candela/sq meter | 0.00064516 | Candela/sq inch |
| Candela/sq inch | 1550.0031 | Candela/sq meter |
| Candela/sq meter | 0.001 | Kilocandela/sq meter |
| Stilb (sb) | 10000 | Nit (nt) |
| Lambert (L) | 0.3183098862 | Stilb (sb) |
| Lambert (L) | 3183.0988618 | Nit (nt) |
| Lambert (L) | 1000 | Millilambert (mL) |
| Foot-lambert (fL) | 1.0763910417 | Millilambert (mL) |
| Skot | 10000 | Bril |
Luminance - Lightconverters & it's abbreviations
| Unit | Abbreviation | Unit | Abbreviation | Unit | Abbreviation |
|---|---|---|---|---|---|
| candela/square meter | cd/m² | candela/square centimeter | cd/cm² | candela/square foot | cd/ft² |
| candela/square inch | cd/in² | kilocandela/square meter | kcd/m² | stilb | sb |
| lambert | L | millilambert | mL | foot-lambert | fL |
| apostilb | - | blondel | - | bril | - |
| skot | - |
Frequently Asked Questions
What is a nit in display brightness?
A nit is another name for candela per square meter (cd/m²), the SI unit of luminance. Modern smartphones have 500-2000 nits peak brightness, while HDR TVs can reach 1000-4000 nits.
How do I convert foot-lamberts to nits?
Multiply the foot-lambert value by 3.4263 to get nits (cd/m²). For example, 14 fL × 3.4263 = 48 nits (typical cinema screen brightness).
What is the difference between luminance and illuminance?
Illuminance (lux) measures light falling on a surface. Luminance (cd/m²) measures light emitted or reflected from a surface toward the viewer. A bright screen has high luminance; a well-lit desk has high illuminance.
What is a lambert?
A lambert is a CGS unit of luminance equal to approximately 3,183 cd/m². It assumes perfectly diffuse (Lambertian) reflection or emission. The millilambert (1/1000 lambert) is more commonly used.
How bright is 1000 nits?
A 1000-nit display is very bright, suitable for HDR content and outdoor visibility. For comparison, a typical indoor monitor is 250-350 nits, and direct sunlight on a white surface is about 30,000 nits.
Complete list of Luminance - Light conversion units and its conversion.
- 1 candela/square meter = 0.0001 candela/square centimeter
candela per sq m to candela per sq cm → - 1 candela/square centimeter = 10000 candela/square meter
candela per sq cm to candela per sq m → - 1 candela/square meter = 0.09290304 candela/square foot
candela per sq m to candela per sq ft →
- 1 candela/square meter = 0.001 kilocandela/square meter
candela per sq m to kilocandela per sq m → - 1 kilocandela/square meter = 1000 candela/square meter
kilocandela per sq m to candela per sq m → - 1 stilb [sb] = 10000 nit [nt]
stilb to nit →
- 1 lambert = 1000 millilambert
lambert to millilambert → - 1 foot-lambert = 1.0763910417 millilambert
ft lambert to lambert → - 1 skot = 0.000092903 foot-lambert
skot to nit →
- 1 candela/square foot = 10.763910417 candela/square meter
candela per sq ft to candela per sq m → - 1 candela/square meter = 0.00064516 candela/square inch
candela per sq m to candela per sq in → - 1 candela/square inch = 1550.0031 candela/square meter
candela per sq in to candela per sq m →
- 1 nit [nt] = 0.0001 stilb [sb]
nit to stilb → - 1 lambert = 0.3183098862 stilb
lambert to stilb → - 1 lambert = 3183.0988618 nit
lambert to nit →
- 1 skot = 10000 bril
skot to bril →