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Lighting Glossary
An electrical control device that initiates the light arc in fluorescent and high intensity discharge (HID) lights with high starting voltage.  
The unit of measure for the intensity of light at the source roughly equal to the amount of light in any direction from the flame of a candle.
Color Rendering Index (CRI)
Color rendering is the ability of a light source to produce color in objects. The CRI is expressed on a scale from 0-100, where 100 is the best in producing vibrant color in objects. Relatively speaking, a source with a CRI of 80 will produce more vibrant color in the same object than a source with a CRI of 60.
Color Temperature
A measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (°K). Incandescent lights have a low color temperature (approximately 2800°K) and have a red-yellowish tone; daylight has a high color temperature (approximately 6000°K) and appears bluish (the most popular fluorescent light Cool White is rated at 4100°K). Today, the phosphors used in fluorescent lights can be blended to provide any desired color temperature in the range from 2800°K to 6000°K.
Efficacy is an industry term for the amount of light produced per watt of electricity (comparable to efficiency). It is the rate at which a light bulb is able to convert electrical power (watts) into light (lumens), expressed in terms of lumens per watt (LPW). For example, a 100 watt lamp producing 1750 lumens gives 17.5 lumens per watt.
The term used to describe the total light which comes off a surface. Exitance is dependent upon the illuminance on and the reflectance of the surface.
The unit of measure for the density of light as it reaches a surface. One footcandle is equal to one lumen per square foot. Measured footcandles are sensitive to the distance from the source to the surface of measure (inverse square law) and the angle at which the light reaches the surface (cosine law).
The density of luminous flux on a surface, is measured in footcandles (one lumen per square foot) or lux (one lumen per square meter).
The result of the use of light.
The energy that allows us to see. Light can be expressed in terms of intensity (candela), flux (lumen), luminance (candela/square foot), and exitance (lumen/square foot).
Lighting Power Budget
Lighting power budget is the upper limit of power to be available to provide the lighting needs for a building, including all permanently connected lighting.
Lighting Power Density
Lighting power density is the maximum allowable lighting density permitted by the code. It is expressed in watts per square foot for a given occupancy/space type.
The unit of measure for the light energy which flows in air. The total light output from electric sources is expressed in lumens. A uniform source of one candlepower placed in a sphere emits 12.57 lumens or mean spherical candela equals to 12.57 lumens.
Luminaires are a complete lighting unit consisting of a light or lights together with the parts designed to distribute the light, to position and protect lights and to connect the lights to their power supply. Many luminaires include one or more ballasts.
This term is used to describe the specific light which comes off the surface whether off a filament, light bulb, lens, louver, tabletop, etc. Luminance varies with both the direction at which you view the surface and its gloss characteristics. Luminance is measured in candela per square foot.