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Illuminance Unit Converter

Convert illuminance between lux, foot-candles, millilux, kilolux, and phot for ambient light sensor and LED design.

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Formula

1lux=0.0929fc=1lm/m21 lux = 0.0929 fc = 1 lm/m²

How It Works

This calculator converts between lux, foot-candles, phot, and kilolux for lighting designers, sensor engineers, and display professionals. Per SI Brochure (BIPM), lux is the SI unit of illuminance: 1 lx = 1 lm/m^2 (lumen per square meter), measuring light incident on a surface. The foot-candle (fc) is the imperial unit: 1 fc = 1 lm/ft^2 = 10.764 lx exactly (since 1 ft = 0.3048 m exactly). Illuminance spans 6 orders of magnitude: 0.001 lx (starlight), 1 lx (full moon), 500 lx (office per ISO 8995), 10,000-100,000 lx (outdoor sunlight). Ambient light sensors (ALS) used in smartphones and displays typically cover 0.01-100,000 lx range with 16-bit resolution (0.0036 lx/count).

Worked Example

Problem

An office lighting retrofit targets 500 lx at desk level per ISO 8995-1. Calculate required LED fixture output for a 4m × 5m room with 0.6 utilization factor and 0.8 maintenance factor.

Solution
  1. Room area: 4 × 5 = 20 m^2
  2. Required lumens: Phi = E × A / (UF × MF) = 500 × 20 / (0.6 × 0.8) = 20,833 lm
  3. Convert illuminance to fc: 500 lx / 10.764 = 46.5 fc (US spec often uses fc)
  4. LED fixture selection: 4 × 5000 lm panels = 20,000 lm (slight under-spec)
  5. Actual illuminance: 20,000 × 0.6 × 0.8 / 20 = 480 lx (4% below target)
  6. Power budget: 5000 lm at 120 lm/W = 42 W per fixture, total 168 W
  7. Energy: 168 W × 8 hr × 250 days = 336 kWh/year (compared to 1200 kWh for fluorescent)

Practical Tips

  • ✓Standard illuminance per ISO 8995-1 / IESNA: offices 300-500 lx, detailed work 500-1000 lx, corridors 50-100 lx, warehouses 100-200 lx, outdoor walkways 10-50 lx. Night vision adaptation requires < 1 lx for 30+ minutes
  • ✓ALS IC selection per ams/Vishay datasheets: VEML7700 range 0-120,000 lx with 16-bit resolution; BH1750 range 1-65,535 lx. For smartphones, use auto-gain for 0.01 lx (night) to 100,000 lx (direct sun) coverage
  • ✓Display brightness vs ambient: per ISO 9241-303, display luminance should be 3-5x ambient reflectance. At 500 lx office with 50% desk reflectance, reflected luminance = 80 cd/m^2, so display should be 250-400 cd/m^2

Common Mistakes

  • ✗Confusing lux (illuminance, light hitting surface) with lumens (luminous flux, total light emitted) - 1000 lm LED illuminating 2 m^2 produces 500 lx average, but intensity varies with distance and angle
  • ✗Using approximate fc-to-lux conversion 10 instead of 10.764 - causes 7.6% error, potentially failing compliance verification. For regulatory compliance (OSHA, ISO), use exact factor
  • ✗Not calibrating ALS sensors for specific spectral response - silicon photodiodes peak at 900 nm (IR), while human eye peaks at 555 nm (green per CIE). Sensors need optical filtering or multi-spectrum compensation

Frequently Asked Questions

Per SI Brochure: candela (cd) is SI base unit for luminous intensity (light per solid angle). Lumen (lm) = cd·sr, total luminous flux from source. Lux (lx) = lm/m^2, illuminance on surface. A 1000 lm bulb distributing light over 2 m^2 produces 500 lx. Inverse-square law: E = I/d^2 where I in candela, d in meters.
Per ISO 8995-1 and IESNA: reading/office work = 300-500 lx, detailed drafting = 750-1000 lx, precision assembly = 1000-2000 lx. Below 50 lx, visual acuity drops significantly. For people over 60, increase by 50-100% due to reduced pupil size and lens clarity.
Per ROHM datasheet: BH1750 returns 16-bit I2C value. In H-resolution mode (1 lx resolution): lux = raw_count / 1.2. In H-resolution2 mode (0.5 lx): lux = raw_count / 2.4. Max value 65535 = 54,612 lx typical. Integration time 120 ms default; use 24 ms for fast response at reduced resolution.
Per inverse-square law: E = Phi / (4*pi*d^2) for point source, or E = Phi / A for uniform distribution. The sun delivers 1361 W/m^2 (solar constant) × 683 lm/W photopic conversion × atmospheric losses = ~100,000 lx at ground. A 1000 lm LED in 10 m^2 room = 100 lx average (plus utilization factor). Same lumens, different area.

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