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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

1 lux = 0.0929 fc = 1 lm/m²

How It Works

Illuminance measures the amount of light falling on a surface per unit area. The SI unit is lux (lx), equal to one lumen per square metre (lm/m²). The foot-candle (fc) is the imperial unit: 1 fc = 10.764 lux. Phot (ph) is a CGS unit equal to 10,000 lux and is rarely used in modern practice. Illuminance drives ambient light sensor (ALS) design and helps set LED driver current for consistent brightness across varying environments.

Worked Example

Office lighting standard of 500 lux: 500 lux = 46.45 fc = 500,000 mlux = 0.5 klux = 0.05 phot. Direct sunlight of 100,000 lux: 100,000 lux = 9290 fc = 100 klux = 10 phot = 100,000,000 mlux.

Practical Tips

  • ✓Typical illuminance levels: 1 lux (moonlight), 100 lux (corridor), 300–500 lux (office), 1000 lux (bright workshop), 10,000–100,000 lux (outdoor sunlight).
  • ✓ALS ICs like VEML7700 and BH1750 have ranges of 0–65,535 lux; use auto-gain or range switching for applications spanning moonlight to sunlight.
  • ✓Display brightness (nits/cd/m²) and scene illuminance (lux) are linked by reflectance: a 50% reflective surface at 1000 lux appears as ~160 nits in diffuse reflection.

Common Mistakes

  • ✗Confusing lux (illuminance, light hitting a surface) with lumens (luminous flux, total light emitted by a source) — lux = lumens / area.
  • ✗Foot-candle to lux conversion factor is 10.764, not 10 — use the precise value for accurate light level compliance checks.
  • ✗ALS sensors often output counts or voltage, not direct lux; always apply the datasheet sensitivity factor (lux/count or lux/V) to get calibrated readings.

Frequently Asked Questions

Lumens (lm) measure total light output from a source. Candela (cd) measures light intensity in a specific direction. Lux measures how much of that light actually arrives at a surface: 1 lux = 1 lm/m². A 1000 lm bulb illuminating 1 m² produces 1000 lux on that surface.
Standards recommend 300–500 lux for office and reading tasks, 750–1000 lux for detailed work like PCB inspection, and 100–200 lux for corridors and general areas. Below 50 lux is considered dim for sustained work.
The BH1750 returns a 16-bit value from an I2C register. In H-resolution mode, divide the raw count by 1.2 to get lux. Range is 1–65,535 lux; use low-resolution mode for bright environments.
The sun illuminates a huge solid angle hemisphere. 100,000 lux over a 1 m² surface = 100,000 lumens hitting that square metre. A 1000 lm LED lamp directed into a 10 m² room produces ~100 lux average — the same lumen count but spread over a larger area.

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