NetVista Thin Client ThinkPad WorkPad Illuminance Illuminance is defined below. Many readers, however, may find it helpful to understand the concept from some everyday examples.
Illumination follows a square law.
Luminance does not follow a square law, but the measurement area must be defined. The luminance of a wall, for example, is the same whether measured two meters away or four. Similarly, moving closer to or further away from a source document does not change its apparent brightness. Luminance Level and Acuity Over the years, acuity has been measured in several ways. One of the first widely accepted methods was the Snellen chart, made up of several rows of letters, with the letters on each row smaller than those on the row directly above.