Both units measure illuminance — the amount of light falling on a surface — but they use different area references. One lux (lx) equals one lumen per square metre (lm/m²). One foot-candle (fc) equals one lumen per square foot. Because one square foot equals approximately 0.0929 m², there are about 10.764 lux per foot-candle.

Lux is the SI standard used in most countries; foot-candles remain common in North American lighting specifications and photography.

Lux (lx) is the SI unit of illuminance, defined as one lumen per square metre (lm/m²). It measures the density of light flux incident on a surface.

A lumen (lm) measures total light output from a source, while lux measures how that light spreads over an area — the same 1000 lm focused onto 1 m² produces 1000 lx, but spread over 10 m² produces only 100 lx. Lux depends on both the luminous flux and the distance from the source (following an inverse-square relationship for point sources).

Typical illuminance targets in photography and video:

• Portrait studio: 500–1000 lx (about 46–93 fc)
• Broadcast television studio: 1000–2000 lx (about 93–186 fc)
• Product photography: 1000–5000 lx (about 93–465 fc)
• Outdoor overcast sky: 1000–10 000 lx
• Direct sunlight: 32 000–100 000 lx

Light meters in photography often display foot-candles (FC) or EV (exposure value). To convert a meter reading in lx to the equivalent in fc, divide by 10.764.

Phot (ph) and nox are older CGS-era units that predate the SI system.

A phot equals 10 000 lux — one lumen per square centimetre. It was used in early photometric research and occasionally appears in older scientific literature, but has been superseded by lux.

A nox equals 0.001 lux. It was proposed in the 1940s to quantify very low illuminance levels such as moonlight (roughly 100 nox, equivalent to about 0.1 lx) or starlight (around 1 nox, equivalent to about 0.001 lx). The unit never gained official standardisation and is primarily a historical reference in modern photometry.