About the Illuminance Converter
Light levels are measured in lux almost everywhere in the world, except the US, where foot-candles remain common in photography, film, and lighting design. Our Illuminance Converter translates between the two instantly.
How It Works
The calculator applies the precise conversion factor between lux and foot-candles (1 foot-candle ≈ 10.7639 lux), since both units measure the same physical quantity, the amount of light falling on a surface, just scaled to different reference distances.
Formula & Methodology
Illuminance measures how much light lands on a given area, and both units define that concept identically, just using different length units for the reference distance. A foot-candle is the illuminance produced by one candela of light intensity at a distance of one foot, while lux uses that same definition at one meter. Because area scales as the square of distance, and a meter is roughly 3.2808 feet, the conversion factor between the two comes out to that ratio squared, about 10.7639, which is why the constant looks less tidy than a simple length conversion despite both units sharing an identical underlying definition.
Step-by-Step: Calculating It By Hand
- 1To convert foot-candles to lux: multiply the foot-candle value by 10.7639.
- 2To convert lux to foot-candles: divide the lux value by 10.7639.
- 3Double-check which unit your light meter or lighting specification is actually using before comparing figures.
- 4For a rough mental estimate, treating 1 foot-candle as 'about 10 lux' is close enough for casual comparisons, though not precise enough for photography or lighting design work.
Examples
Foot-candles to lux
50 foot-candles, a typical well-lit retail or office reading, converts to approximately 538.2 lux.
Everyday light level reference
Direct sunlight measures around 100,000 lux, a well-lit office around 300 to 500 lux, and a dim living room around 50 to 100 lux - useful reference points when interpreting either unit.
Advantages
- Converts precisely between lux and foot-candles in either direction
- Uses the exact standard conversion constant rather than a rough approximation
- Useful for photography, film lighting, and general lighting design
- Instant results as you change the input value
Common Mistakes
- Rounding the conversion factor too casually, treating it as exactly 10, for precision lighting work
- Confusing illuminance (light falling on a surface) with luminance (light emitted or reflected as perceived)
- Not checking which unit a light meter or lighting spec is actually using before comparing readings
- Assuming a lux or foot-candle reading is independent of distance and angle from the light source
Edge Cases to Watch For
- Photography light meters and film industry lighting specs often use foot-candles even outside the US, a holdover from historical industry convention rather than the country of use.
- Because the conversion factor is a squared length ratio rather than a round number, mental approximations (treating it as roughly 10) introduce a small but real error for precise lighting design work.
- Illuminance measures light falling on a surface, which differs from luminance (brightness as perceived from a light-emitting or reflecting surface) - the two are easy to conflate but aren't interchangeable.
- Measured lux or foot-candle readings vary considerably with distance from the light source and the angle of the measuring surface, so the reading itself, not just the unit, depends heavily on setup.
Common Use Cases
- Photography and film lighting setup and equipment specifications
- Comparing lighting design standards across countries
- Understanding light meter readings given in an unfamiliar unit
- General reference for interpreting illuminance specifications