A number of factors affect a lamp's light output over time, including lamp lumen depreciation, the lamp's interaction with the ballast, supply voltage variations, dirt or dust on the lamp, and the ambient temperature in the fixture. Even more factors can affect light output in this case, including the distribution characteristics of the fixture, fixture surface depreciation, and dirt and dust buildup. This is the amount of light measured on the workplane in the lighted space. The workplane an imaginary horizontal, tilted or vertical line where the most important tasks in the space are performed. Like lumens, footcandles can be produced as either initial or maintained quantities.
Initial footcandles indicates a light level after new lamps are installed.
Maintained footcandles indicates a light level after light loss factors are considered over a period of time. While light output may describe either the output of a light source or fixture, maintained footcandles always takes into account the efficiency of the fixture in transmitting light to the workplane. The light that we actually see, brightness can be measured as the light leaving a lamp, or the light reflecting from an object's surface. If not controlled, brightness can produce levels of glare that either impair or prevent a desired task being performed. Glare can be described as direct or reflected glare, which can then result in discomfort or disability.