Illuminance (in lux, i.e., lux or lx) is the amount of energy received per unit area of visible light. It is a physical quantity used to indicate the intensity of light and the degree to which an object’s surface is illuminated. The SI derived unit of illuminance is lux or lx, meaning lumen per square meter.
The two concepts, illuminance and luminance, are often confused. Unlike luminance, illuminance is an objective parameter that is determined by using a device to measure the amount of light that actually reaches a given surface. In lighting design, illuminance is referred to represent the brightness of a space or place.
To help visualize the preceding information, Fig. 1.2.1 lists the illuminance values for several scenarios that we often come across in both human society and nature.