From AMS Glossary
This is the illuminance provided by a light source of luminous intensity one candela (formerly candle) at a distance of 1 ft, hence the name. Full sunlight with zenith sun produces an illuminance of about 10 000 foot-candles on a horizontal surface. Full moonlight provides an illuminance of about 0.02 foot-candles. Adequate illumination for reading is taken to be about 10 foot-candles; that for machine shop work is about 40 foot-candles.