Lux and Foot-Candles — What Illuminance Actually Measures

Illuminance is the measure of light falling on a surface. One lux equals one lumen per square metre. It is the number produced by a lighting calculation software, verified by a lux meter during commissioning, and referenced in every lighting standard from schools to operating theatres.

The scale is worth understanding intuitively, not just technically.

A moonlit night sits around 0.1 to 0.3 lux — enough to perceive shapes but not to read. A well-lit residential living room operates at around 150 to 300 lux. A standard office environment requires 500 lux at desk level, per EN 12464-1. A detailed technical workshop may need 750 lux or more. A surgical theatre demands upward of 10,000 lux on the operating field.

The difference between 200 and 500 lux in a workspace is not merely a number. It is the difference between eye strain and effortless focus — measurable in productivity, error rates, and how long an occupant can work before feeling fatigued. These are not subjective impressions. They are documented outcomes.

Foot-candles are the imperial equivalent, still widely used in the United States and in some international project documentation. One foot-candle equals approximately 10.76 lux. If you are working across markets, knowing both units prevents specification errors and miscommunication on site.

One important distinction: illuminance tells you how much light arrives at a surface. It does not tell you how much of that light reaches the eye — that is luminance, which we will cover separately. It does not tell you whether the light is pleasant, well directed, or spectrally appropriate. Lux is a necessary measure. It is not a sufficient one.

Lux tells you how much light arrives. Everything before that is about making sure it gets there.