Average Rated Life
Lamps are rated in terms of average life, a statistical average at which point half of the lamps are still operating and the other half are not.

Color Rendering Index (CRI)
Measured on a scale of 0-100, where a light source with 100 CRI is best at producing vibrant color in objects. A higher CRI rating typically denotes a higher quality lamp.

Color Temperature
Measured in Kelvin (K). Relates to how interior space feels. For example, a color temperature of 3000K appears to be warm while 4100K appears to be cool. Color temperature can play an integral part in creating a given atmosphere for a space, whether it be soft and comfortable for relaxing at home or sharp and precise for work environments where visual clarity is a must.

Footcandle
A footcandle (ftcndl or fc) represents the amount of lumens falling on an area expressed in square feet. One lumen falling on a surface of one square foot produces one footcandle.

Glare
Glare is any unwanted light that causes discomfort, interference with vision, or eye fatigue. Direct glare is caused by light coming directly to the eye from a source or bright surface. Discomfort or reflected glare is light reflected from a surface in the direction of the eye.

Illuminance
Illuminance is the density of light falling on a surface. It is expressed in footcandles or lux. The reference plane can be horizontal, e.g. a counter or desk, or it can be vertical, e.g., a sign on the wall.

Lamp Lumen Depreciation (LLD)
Lumen depreciation is a term that describes the normal attrition of light. Essentially, the longer a lamp operates, the less light it delivers. Different light sources have different rates of depreciation.

Louver
A framed opening, as in a wall, door, or window, fitted with fixed or movable horizontal slats for admitting air and light and shedding rain. May also refer to one of the slats used in such an opening.

Luminaire Dirt Depreciation (LDD)
Dirt or dust that accumulates on luminaires decreasing the total output of light, lowering the overall efficiency of the system.

Luminance
Luminance is a measure of light as it emanates from a surface. A mirror is perceived as brighter than a white surface under the same illuminance. Luminance is expressed in candelas per square inch or centimeter and may vary greatly due to surface reflectance and glossiness.

Luminous Efficacy
The luminous efficacy is a light engineering term for the measure of the rate at which a lamp is able to convert electrical power (watt) to light (lumen). The luminous efficacy is therefore expressed in lumens per watt (LPW).

Luminous Flux
The luminous flux is the quantity of light flow produced by a lamp. Flux is often used to describe the total light output. It is expressed in the unit lumen.

Luminous Intensity
Luminous intensity is a measure of light at a source emitted in a certain direction. It is expressed in candelas.

Lux
The metric equivalent of the footcandle. The amount of lumens falling on an area, with the are expressed in square meters. Approximately 10 Lux is equivalent to one footcandle.

Uniformity Ratio (U)
The uniformity ratio, U, refers to the uniformity of a lighting installation, expressed as Emin/Eave, where E is the illuminance in footcandles or lux.

Visual Comfort Probability (VCP)
The visual comfort probability (VCP) is a term that refers to the number of people (expressed as a percentage) that feel comfortable in an environment illuminated by a specific luminaire.

Watt
The watt (W) is the unit of energy used to measure power used or consumed by a lamp. A kilowatt (KW) equals 1000 watts.