A footcandle (fc) is a unit of illuminance equal to one lumen per square foot. One footcandle is approximately 10.764 lux. It is used mainly in countries that use imperial units; Nordic Lights technical lighting data generally uses SI units such as lux. Footcandles and lux describe the same physical quantity using different area units, so measurements can be converted between them. Illuminance at a work surface changes with distance, aiming, light pattern, and overlap from multiple lamps. When comparing values, the measurement plane and conditions must be the same; a lamp’s lumen output cannot be converted directly to footcandles without distribution and geometry.