Light Converter
Light & Photometry — From Candela to Lumen
Master photometric units across 5 categories: illuminance (lux), luminance (nit), luminous intensity (candela), luminous flux (lumen), and exposure. Understand the difference between light ON surfaces vs FROM surfaces.
Foundations of Photometry
Five Physical Quantities
Photometry measures 5 DIFFERENT things! Illuminance: light falling ON surface (lux). Luminance: light FROM surface (nit). Intensity: source strength (candela). Flux: total output (lumen). Exposure: light x time. Cannot mix!
- Illuminance: lux (light ON)
- Luminance: nit (light FROM)
- Intensity: candela (source)
- Flux: lumen (total)
- Exposure: lux-second (time)
Illuminance (Lux)
Light falling ON a surface. Units: lux (lx) = lumen per square meter. Sunlight: 100,000 lux. Office: 500 lux. Moonlight: 0.1 lux. Measures how bright a surface appears when lit.
- lux = lm/m² (lumen/area)
- Sunlight: 100,000 lx
- Office: 300-500 lx
- Cannot convert to nit!
Luminance (Nit)
Light coming FROM a surface (emitted or reflected). Units: nit = candela per square meter. Phone screen: 500 nits. Laptop: 300 nits. Different from illuminance! Measures surface brightness itself.
- nit = cd/m²
- Phone: 400-800 nits
- Laptop: 200-400 nits
- Different from illuminance!
- 5 different physical quantities - cannot mix!
- Illuminance (lux): light ON surface
- Luminance (nit): light FROM surface
- Intensity (candela): source strength in direction
- Flux (lumen): total light output
- Only convert within same category!
The Five Categories Explained
Illuminance (Light ON)
Light incident ON a surface. Measures how much light hits an area. Base unit: lux (lx). 1 lux = 1 lumen per square meter. Foot-candle (fc) = 10.76 lux. Used for lighting design.
- lux (lx): SI unit
- foot-candle (fc): imperial
- phot (ph): CGS (10,000 lx)
- Measures received light
Luminance (Light FROM)
Light emitted or reflected FROM surface. Brightness you see. Base unit: nit = candela/m². Stilb = 10,000 nits. Lambert, foot-lambert historical. Used for displays, screens.
- nit (cd/m²): modern
- stilb: 10,000 nits
- lambert: 3,183 nits
- foot-lambert: 3.43 nits
Intensity, Flux, Exposure
Intensity (candela): source strength in direction. SI base unit! Flux (lumen): total output all directions. Exposure (lux-second): illuminance over time for photography.
- candela (cd): SI base
- lumen (lm): total output
- lux-second: exposure
- All different quantities!
The Physics of Light Measurement
Inverse Square Law
Light intensity decreases with distance squared. Illuminance E = Intensity I / distance² (r²). Double distance = 1/4 brightness. 1 candela at 1 meter = 1 lux. At 2 meters = 0.25 lux.
- E = I / r²
- Double distance = 1/4 light
- 1 cd at 1m = 1 lx
- 1 cd at 2m = 0.25 lx
Flux to Illuminance
Luminous flux spread over area. E (lux) = Flux (lumen) / Area (m²). 1000 lumens over 1 m² = 1000 lux. Over 10 m² = 100 lux. Bigger area = less illuminance.
- E = Φ / A
- 1000 lm / 1 m² = 1000 lx
- 1000 lm / 10 m² = 100 lx
- Area matters!
Luminance & Reflectance
Luminance = illuminance x reflectance / π. White wall (90% reflectance): high luminance. Black surface (10% reflectance): low luminance. Same illuminance, different luminance! Depends on surface.
- L = E × ρ / π
- White: high luminance
- Black: low luminance
- Surface matters!
Light Level Benchmarks
| Condition | Illuminance (lux) | Notes |
|---|---|---|
| Starlight | 0.0001 | Darkest natural light |
| Moonlight (full) | 0.1 - 1 | Clear night |
| Street lighting | 10 - 20 | Typical urban |
| Living room | 50 - 150 | Comfortable home |
| Office workspace | 300 - 500 | Standard requirement |
| Retail store | 500 - 1000 | Bright display |
| Operating room | 10,000 - 100,000 | Surgical precision |
| Direct sunlight | 100,000 | Bright day |
| Full daylight | 10,000 - 25,000 | Overcast to sunny |
Display Brightness (Luminance)
| Device | Typical (nits) | Maximum (nits) |
|---|---|---|
| E-reader (e-ink) | 5-10 | 15 |
| Laptop screen | 200-300 | 400 |
| Desktop monitor | 250-350 | 500 |
| Smartphone | 400-600 | 800-1200 |
| HDR TV | 400-600 | 1000-2000 |
| Cinema projector | 48-80 | 150 |
| Outdoor LED display | 5000 | 10,000+ |
Real-World Applications
Lighting Design
Office: 300-500 lux. Retail: 500-1000 lux. Surgery: 10,000+ lux. Building codes specify illuminance requirements. Too low: eye strain. Too high: glare, energy waste. Proper lighting critical!
- Office: 300-500 lx
- Retail: 500-1000 lx
- Surgery: 10,000+ lx
- Building codes apply
Display Technology
Phone/tablet screens: 400-800 nits typical. Laptops: 200-400 nits. HDR TVs: 1000+ nits. Outdoor displays: 2000+ nits for visibility. Luminance determines readability in bright conditions.
- Phones: 400-800 nits
- Laptops: 200-400 nits
- HDR TV: 1000+ nits
- Outdoor: 2000+ nits
Photography
Camera exposure = illuminance x time. Lux-seconds or lux-hours. Light meters measure lux. Proper exposure critical for image quality. EV (exposure value) related to lux-seconds.
- Exposure = lux x time
- Light meters: lux
- lux-second: photo unit
- EV related to exposure
Quick Math
Inverse Square
Illuminance drops with distance². 1 cd at 1m = 1 lx. At 2m = 0.25 lx (1/4). At 3m = 0.11 lx (1/9). Quick: divide by distance squared!
- E = I / r²
- 1m: divide by 1
- 2m: divide by 4
- 3m: divide by 9
Area Spreading
Flux over area. 1000 lm bulb. 1 m away, spreads over 12.6 m² sphere surface. 1000 / 12.6 = 79 lux. Bigger sphere = lower lux.
- Sphere area = 4πr²
- 1m: 12.6 m²
- 2m: 50.3 m²
- Flux / area = illuminance
Lux to Foot-Candle
1 foot-candle = 10.764 lux. Quick: fc x 10 ≈ lux. Or: lux / 10 ≈ fc. Close enough for estimates!
- 1 fc = 10.764 lx
- fc x 10 ≈ lux
- lux / 10 ≈ fc
- Quick estimation
How Conversions Work
- Step 1: Check category
- Step 2: Only convert within category
- Illuminance: lux, fc, phot
- Luminance: nit, lambert, fL
- NEVER cross categories!
Common Conversions (Within Categories)
| From | To | Factor | Example |
|---|---|---|---|
| lux | foot-candle | 0.0929 | 100 lx = 9.29 fc |
| foot-candle | lux | 10.764 | 10 fc = 107.6 lx |
| phot | lux | 10,000 | 1 ph = 10,000 lx |
| nit (cd/m²) | foot-lambert | 0.2919 | 100 nit = 29.2 fL |
| foot-lambert | nit | 3.426 | 100 fL = 343 nit |
| stilb | nit | 10,000 | 1 sb = 10,000 nit |
| lambert | nit | 3183 | 1 L = 3183 nit |
| lumen | watt@555nm | 0.00146 | 683 lm = 1 W |
Quick Examples
Worked Problems
Office Lighting
Office needs 400 lux. LED bulbs produce 800 lumens each. Room is 5m x 4m (20 m²). How many bulbs?
Total lumens needed = 400 lx x 20 m² = 8,000 lm. Bulbs needed = 8,000 / 800 = 10 bulbs. Assumes even distribution and no losses.
Flashlight Distance
Flashlight has 1000 candela intensity. What illuminance at 5 meters?
E = I / r². E = 1000 cd / (5m)² = 1000 / 25 = 40 lux. Inverse square law: double distance = 1/4 light.
Screen Brightness
Laptop screen is 300 nits. Convert to foot-lamberts?
1 nit = 0.2919 foot-lambert. 300 nit x 0.2919 = 87.6 fL. Historical cinema standard was 16 fL, so laptop is 5.5x brighter!
Common Mistakes
- **Mixing categories**: Cannot convert lux to nit! Different physical quantities. Lux = light ON surface. Nit = light FROM surface. Need reflectance to relate them.
- **Forgetting inverse square**: Light decreases with distance SQUARED, not linearly. 2x distance = 1/4 brightness, not 1/2!
- **Confusing lumen and lux**: Lumen = total output (all directions). Lux = output per area (one direction). 1000 lm bulb does NOT produce 1000 lux!
- **Ignoring reflectance**: White wall vs black wall under same illuminance have vastly different luminance. Surface matters!
- **Candela vs candle power**: 1 candela ≠ 1 candle power. Pentane candle = 10 candela. Historical units varied!
- **Display brightness units**: Manufacturers mix nits, cd/m², and % brightness. Always check actual nits for comparison.
Fun Facts
Candela is SI Base Unit
Candela is one of 7 SI base units (with meter, kilogram, second, ampere, kelvin, mole). Defined as luminous intensity of source emitting 540 THz light with radiant intensity 1/683 watt per steradian. Only unit based on human perception!
Lumen Defined from Candela
1 lumen = light from 1 candela source over 1 steradian solid angle. Since sphere has 4π steradians, 1 candela isotropic source emits 4π ≈ 12.57 lumens total. Lumen derived, candela fundamental!
555 nm is Peak Sensitivity
Human eye most sensitive to 555 nm (green-yellow). 1 watt of 555 nm light = 683 lumens (maximum possible). Red or blue light: fewer lumens per watt. That's why night vision is green!
HDR Displays = 1000+ Nits
Standard displays: 200-400 nits. HDR (High Dynamic Range): 1000+ nits. Some reach 2000-4000 nits! Sun reflection: 5000+ nits. HDR mimics real-world brightness range for stunning images.
Foot-Candle from Actual Candles
1 foot-candle = illuminance 1 foot from 1 candela source. Originally from actual candle at 1 foot distance! = 10.764 lux. Still used in US lighting codes.
Cinema Brightness Standard
Cinema projectors calibrated to 14-16 foot-lamberts (48-55 nits). Seems dim compared to TV/phone! But in dark theater, creates proper contrast. Home projectors often 100+ nits for ambient light.
The Evolution of Light Measurement: From Candles to Quantum Standards
Ancient Light Sources (Pre-1800)
Before scientific photometry, humans relied on natural light cycles and crude artificial sources. Oil lamps, candles, and torches provided inconsistent illumination measured only by comparison.
- Candles as standards: Tallow, beeswax, and spermaceti candles used as rough references
- No quantitative measurements: Light described qualitatively ('bright as daylight', 'dim as moonlight')
- Regional variations: Each culture developed its own candle standards with no international agreement
- Discovery limitation: No understanding of light as electromagnetic radiation or photons
Birth of Scientific Photometry (1800-1900)
The 19th century brought systematic attempts to standardize light measurement, driven by gas lighting adoption and early electrical illumination.
- 1799 - Rumford's photometer: Benjamin Thompson (Count Rumford) invented shadow photometer for comparing light sources
- 1860s - Candle standards emerge: Spermaceti candle (whale oil), carcel lamp (vegetable oil), Hefner lamp (amyl acetate) compete as references
- 1881 - Violle standard: Jules Violle proposed platinum at freezing point (1769°C) as light standard - 1 square cm emits 1 Violle
- 1896 - Hefner candle: German standard using controlled amyl acetate flame, still used into 1940s (0.903 modern candela)
International Standardization (1900-1948)
Early 20th century efforts unified competing national standards into the International Candle, precursor to the modern candela.
- 1909 - International Candle: Agreement between France, UK, and USA defines standard as 1/20th of platinum blackbody radiator at freezing point
- 1921 - Bouger unit proposed: Based on platinum standard, approximately equal to modern candela
- 1930s - Pentane standard: Some countries used standardized pentane lamp instead of platinum
- 1940s - War disrupts standards: WWII highlights need for universal, reproducible measurement independent of artifacts
The Candela Becomes an SI Base Unit (1948-1979)
Post-war international cooperation established the candela as the seventh SI base unit, initially defined by platinum blackbody radiation.
1948 Definition: 1948 (9th CGPM): Candela defined as luminous intensity of 1/600,000 m² of platinum at freezing point. First time 'candela' officially replaced 'candle'. Established photometry within SI framework alongside meter, kilogram, second, ampere, kelvin, and mole.
Challenges:
- Platinum dependence: Required precise control of platinum purity and temperature (1769°C)
- Difficult realization: Few laboratories could maintain platinum freezing point apparatus
- Spectral sensitivity: Definition based on photopic vision (human eye sensitivity curve)
- Terminology evolution: 'Nit' informally adopted for cd/m² in 1967, though not official SI term
Quantum Revolution: Linking Light to Fundamental Constants (1979-Present)
The 1979 redefinition freed the candela from material artifacts, instead linking it to the watt through human eye sensitivity at a specific wavelength.
1979 Breakthrough: 16th CGPM redefined candela based on monochromatic radiation: 'The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² Hz (555 nm, peak human eye sensitivity) and has a radiant intensity of 1/683 watt per steradian.' This makes 683 lumens exactly equal to 1 watt at 555 nm.
Advantages:
- Fundamental constant: Linked to the watt (SI power unit) and human photopic luminosity function
- Reproducibility: Any lab can realize the candela using laser and calibrated detector
- No artifacts: No platinum, no freezing points, no physical standards required
- Wavelength precision: 555 nm chosen as peak of photopic vision (where eye is most sensitive)
- 683 number: Chosen to maintain continuity with previous candela definition
Modern Impact:
- LED calibration: Critical for energy efficiency standards (lumens per watt ratings)
- Display technology: HDR standards (nits) based on precise candela definition
- Lighting codes: Building requirements (lux levels) traceable to quantum standard
- Astronomy: Stellar luminosity measurements connected to fundamental physics
Technological Revolutions in Lighting (1980s-Present)
Modern lighting technology has transformed how we create, measure, and use light, making photometric precision more important than ever.
LED Era (2000s-2010s)
LEDs revolutionized lighting with 100+ lumens/watt (vs. 15 lm/W for incandescent). Energy labels now require precise lumen ratings. Color rendering index (CRI) and color temperature (Kelvin) become consumer specifications.
Display Technology (2010s-Present)
HDR displays reaching 1000-2000 nits. OLED pixel-level control. Standards like HDR10, Dolby Vision require precise luminance specifications. Smartphone outdoor visibility drives 1200+ nit peak brightness. Cinema maintains 48 nits for proper contrast.
Smart Lighting & Human-Centric Design (2020s)
Circadian rhythm research drives tunable lighting (CCT adjustment). Lux meters in smartphones. Building codes specify illuminance for health/productivity. Photometry central to wellness design.
- Only SI unit based on human perception: Candela uniquely incorporates biology (eye sensitivity) into physics definition
- From candles to quantum: Journey from crude wax sticks to laser-defined standards in 200 years
- Still evolving: LED and display technology continue to drive photometric innovation
- Practical impact: Your phone screen brightness, office lighting, and car headlights all trace back to 683 lumens = 1 watt at 555 nm
- Future: Potential further refinement as we understand vision science better, but current definition remarkably stable since 1979
Pro Tips
- **Check category first**: Always confirm you're converting within same category. Lux to fc: OK. Lux to nit: WRONG!
- **Inverse square fast**: Distance x2 = brightness /4. Distance x3 = brightness /9. Quick mental math!
- **Lumen ≠ Lux**: 1000 lumen bulb spread over 1 m² = 1000 lux. Over 10 m² = 100 lux. Area matters!
- **Foot-candle quick**: fc x 10 ≈ lux. Close enough for rough estimates. Exact: fc x 10.764 = lux.
- **Display comparison**: Always use nits (cd/m²). Ignore % brightness specs. Only nits are objective.
- **Room lighting estimate**: 300-500 lux typical office. Total lumens needed = lux x area (m²). Then divide by lumens per bulb.
- **Scientific notation auto**: Values ≥ 1 million or < 0.000001 automatically display in scientific notation (e.g., 1.0e+6) for readability!
Complete Photometric Reference
Illuminance
Light falling ON a surface - lux, foot-candle, phot. Units: lm/m². Cannot convert to other categories!
| Unit | Symbol | Notes & Applications |
|---|---|---|
| lux | lx | SI unit of illuminance. 1 lx = 1 lm/m². Office: 300-500 lux. Sunlight: 100,000 lux. |
| kilolux | klx | 1000 lux. Bright outdoor conditions. Direct sunlight ranges. |
| millilux | mlx | 0.001 lux. Low-light conditions. Twilight levels. |
| microlux | µlx | 0.000001 lux. Very dark conditions. Starlight levels. |
| foot-candle | fc | Imperial illuminance. 1 fc = 10.764 lux. Still used in US codes. |
| phot | ph | CGS unit. 1 ph = 10,000 lux = 1 lm/cm². Rarely used now. |
| nox | nx | 0.001 lux. Night-time illumination. From Latin 'night'. |
| lumen per square meter | lm/m² | Same as lux. Direct definition: 1 lm/m² = 1 lux. |
| lumen per square centimeter | lm/cm² | Same as phot. 1 lm/cm² = 10,000 lux. |
| lumen per square foot | lm/ft² | Same as foot-candle. 1 lm/ft² = 1 fc = 10.764 lux. |
Luminance
Light emitted/reflected FROM a surface - nit, cd/m², foot-lambert. Different from illuminance!
| Unit | Symbol | Notes & Applications |
|---|---|---|
| candela per square meter (nit) | cd/m² | Modern luminance unit = nit. Displays rated in nits. Phone: 500 nits. |
| nit | nt | Common name for cd/m². Display brightness standard. HDR: 1000+ nits. |
| stilb | sb | 1 cd/cm² = 10,000 nits. Very bright. Rarely used now. |
| candela per square centimeter | cd/cm² | Same as stilb. 1 cd/cm² = 10,000 cd/m². |
| candela per square foot | cd/ft² | Imperial luminance. 1 cd/ft² = 10.764 cd/m². |
| candela per square inch | cd/in² | 1 cd/in² = 1550 cd/m². Small area, high brightness. |
| lambert | L | 1/π cd/cm² = 3,183 cd/m². Perfectly diffuse surface. |
| millilambert | mL | 0.001 lambert = 3.183 cd/m². |
| foot-lambert | fL | 1/π cd/ft² = 3.426 cd/m². US cinema standard: 14-16 fL. |
| apostilb | asb | 1/π cd/m² = 0.318 cd/m². CGS unit. |
| blondel | blondel | Same as apostilb. 1/π cd/m². Named after André Blondel. |
| bril | bril | 10^-7 lambert = 3.183 x 10^-6 cd/m². Dark-adapted vision. |
| skot | sk | 10^-4 lambert = 3.183 x 10^-4 cd/m². Scotopic vision unit. |
Luminous Intensity
Light source strength in a direction - candela (SI base unit), candle power. Different physical quantity!
| Unit | Symbol | Notes & Applications |
|---|---|---|
| candela | cd | SI base unit! Light intensity in a direction. LED: 1-10 cd typical. |
| kilocandela | kcd | 1000 candela. Very bright sources. Searchlights. |
| millicandela | mcd | 0.001 candela. Small LEDs. Indicator lights: 1-100 mcd. |
| hefnerkerze (hefner candle) | HK | 0.903 cd. German candle standard. Amyl acetate flame. |
| international candle | ICP | 1.02 cd. Early standard. Platinum at freezing point. |
| decimal candle | dc | Same as candela. Early French term. |
| pentane candle (10 candle power) | cp | 10 cd. Pentane lamp standard. 10 candle power. |
| carcel unit | carcel | 9.74 cd. French lamp standard. Carcel oil lamp. |
| bougie decimal | bougie | Same as candela. French decimal candle. |
Luminous Flux
Total light output in all directions - lumen. Cannot convert to intensity/illuminance without geometry!
| Unit | Symbol | Notes & Applications |
|---|---|---|
| lumen | lm | SI unit of luminous flux. Total light output. LED bulb: 800 lm typical. |
| kilolumen | klm | 1000 lumens. Bright bulbs. Commercial lighting. |
| millilumen | mlm | 0.001 lumen. Very dim sources. |
| watt (at 555 nm, peak luminous efficacy) | W@555nm | 1 W at 555 nm = 683 lm. Peak luminous efficacy. Green light maximum. |
Photometric Exposure
Light exposure over time - lux-second, lux-hour. Illuminance integrated over time.
| Unit | Symbol | Notes & Applications |
|---|---|---|
| lux-second | lx⋅s | Illuminance over time. Photography exposure. 1 lx for 1 second. |
| lux-hour | lx⋅h | 3600 lux-seconds. 1 lx for 1 hour. Longer exposures. |
| phot-second | ph⋅s | 10,000 lux-seconds. Bright exposure. |
| foot-candle-second | fc⋅s | 10.764 lux-seconds. Foot-candle for 1 second. |
| foot-candle-hour | fc⋅h | 38,750 lux-seconds. Foot-candle for 1 hour. |
Photometry Conversion Best Practices
Best Practices
- Know the quantity: Lux (ON surface), nit (FROM surface), candela (source), lumen (total) - NEVER mix!
- Only convert within same category: lux↔foot-candle OK, lux↔nit IMPOSSIBLE without surface data
- For lumen to lux: need area and light distribution pattern (not simple division!)
- Display brightness in nits: 200-300 indoor, 600+ outdoor, 1000+ HDR content
- Lighting codes use lux: office 300-500 lx, retail 500-1000 lx, verify local requirements
- Photography: lux-seconds for exposure, but modern cameras use EV (exposure value) scale
Common Mistakes to Avoid
- Trying to convert lux to nit directly: Impossible! Different quantities (ON vs FROM surface)
- Converting lumens to lux without area: Must know lit area and distribution pattern
- Ignoring inverse square law: Light intensity decreases with distance² (double distance = 1/4 light)
- Mixing categories: Like converting meters to kilograms - physically meaningless!
- Using wrong unit for application: Displays need nits, rooms need lux, bulbs rated in lumens
- Confusing candela with candlepower: Old imperial unit, not the same as modern candela (cd)
FAQ
What's the difference between lux and nit?
Completely different! Lux = illuminance = light falling ON a surface (lm/m²). Nit = luminance = light coming FROM a surface (cd/m²). Example: desk has 500 lux illuminance from overhead lights. Computer screen has 300 nits luminance that you see. Cannot convert between them without knowing surface reflectance! Different physical quantities.
Can I convert lumens to lux?
Yes, but need area! Lux = lumens / area (m²). 1000 lumen bulb lighting 1 m² surface = 1000 lux. Same bulb lighting 10 m² = 100 lux. Also affected by distance (inverse square law) and light distribution pattern. Not a direct conversion!
Why is candela an SI base unit?
Historical and practical reasons. Luminous intensity is fundamental - can be directly measured from a source. Lumen, lux derived from candela using geometry. Also, candela only SI unit based on human perception! Defined using human eye spectral sensitivity at 555 nm. Special among SI units.
What's a good screen brightness?
Depends on environment! Indoor: 200-300 nits sufficient. Outdoor: need 600+ nits for visibility. HDR content: 400-1000 nits. Too bright in dark = eye strain. Too dim in sunlight = can't see. Many devices auto-adjust. Phones typically 400-800 nits, some reach 1200+ for bright sunlight.
How many lumens do I need?
Depends on room and purpose! General rule: 300-500 lux for offices. Bedroom: 100-200 lux. Kitchen: 300-400 lux. Multiply lux x room area (m²) = total lumens. Example: 4m x 5m office (20 m²) at 400 lux = 8,000 lumens needed. Then divide by lumens per bulb.
Why can't I mix these categories?
They're fundamentally different physical quantities with different dimensions! Like trying to convert kilograms to meters - impossible! Illuminance is flux/area. Luminance is intensity/area. Intensity is candela. Flux is lumens. All related by physics/geometry but NOT directly convertible. Need additional information (distance, area, reflectance) to relate them.
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