Illuminance

Illuminance is the density of luminous flux arriving at a surface, measured in lux (or foot-candles). It answers the practical question “how much light lands on the task?” and is the quantity most indoor and area lighting standards specify — EN 12464-1 for workplaces, EN 12193 for sport. Designs distinguish average illuminance from uniformity (the ratio of minimum to average), because a space that averages 200 lx with black corners fails its purpose. Maintained illuminance — the level after lamp depreciation and dirt accumulation, controlled by a maintenance factor — is the figure a specification legally means unless stated otherwise.