2. Illuminance
Usually we are more interested in the amount of illumination or light provided onto an
area, such as a desk, sheet of paper, etc. The amount of light on a surface area is called
illuminance (E), and is measured in lumens/m2 or lux (lx). Illuminance is used to
determine if an area is lighted well enough for reading and other activities, and therefore
used in designing buildings. Illumination and illuminance are often used interchangeably.
The illuminance on a surface varies inversely as the square of its distance from a light
source.