Light intensity measured on a plane at a specific location is called illuminance. You can measure illuminance using a light meter located on the work surface where tasks are performed. Therefore, the amount of light delivered into the space and the reflectance of the surfaces in the space affects your ability to see. Any particular lamp or group of lamps may vary from published rated life. For discharge light sources such as fluorescent and HID lamps, the average rated lamp life is affected by the burn cycle, or the number of starts and the duration of the operating cycle each time the lamp is started.
All light sources used for lighting lose their ability to produce light as they age.
For example, a 32-watt T8 lamp has an initial lumen rating of 2,900 lumens and a design lumen rating of 2,610. Another characteristic of a light source is the color temperature. It is measured on a Kelvin scale ranging from 1,500K, which appears red-orange, to 9,000K which appear blue. Color rendition describes the effect a light source has on the appearance of colored objects. The higher the CRI, the less distortion of the object s color by the lamp s light output.