| Color Rendering | General expression for the effect of a light source on color appearance of objects when compared with their color appearance under a reference light source. The Color Rendering Index (CRI) scale ranges from 0 to 100, with higher numbers designating better color rendering. |
| Correlated Color Temperature (CCT) | The color appearance of light produced by a light source measured with the Kelvin Scale (K). Low CCTs have a warm appearance (red, orange, yellow). High CCTs have a cool appearance (blue or blue-white). |
| Efficacy | The ratio of light from a lamp, fixture or luminaire to the electrical power (watts) consumed. Usually expressed in lumens per watt. |
| Efficiency | The absolute percentage of light produced within a fixture that is not absorbed by the fixture, but actually leaves the fixture. |
| Footcandle (fc) | A quantitative unit for measuring luminance; the illumination on a surface one foot square on which there is a uniformly distributed flux of one lumen. |
| HID | High intensity discharge lighting, including mercury vapor, high pressure sodium and metal halide light sources. |
| Illuminance (Illumination) | The total amount of visible light illuminating a point on a surface from all directions above the surface. |
| Lamp | A light source. Lamps used for outdoor lighting include high intensity discharge, incandescent and fluorescent. |
| Lamp Lumen Depreciation | A factor used in lighting calculations to account for the light loss that takes place in a lamp due to the gradual decay in lumen output over a specified period of burning time. |
| Lens | The shielding or diffuser portion of a fixture made of plastic or glass, through which the light passes on its way to the seeing task. |
| Lighting Distribution | Luminaires are classified according to the manner in which they control or distribute the luminous flux. |
| Light Emitting Diode (LED) | An LED is a small light source that becomes illuminated by the movement of electrons through a semiconductor material. LEDs have the ability to produce large numbers of lumens from far less energy (watts) than conventional light sources. |
| Light Loss Factor | A factor used in calculating the level of illumination after a given period of time and under given conditions that takes into account dirt accumulations on the luminaire, lamp depreciation, maintenance procedures and atmosphere conditions. |
| Light Trespass | A situation which occurs when, due to lack of adequate beam control, light from a source is distributed onto areas where the illumination is not wanted. |
| Lumen | The unit of light output. Light output is also referred to as light flux. |
| Luminaire (outdoor lighting fixture) | A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, position and protect the lamps, and to connect the lamps to the power supply. |
| Luminaire Dirt Depreciation (LDD) | A factor used in lighting calculations to account for the light loss due to the accumulation of dirt on the luminaire. The LDD is contingent upon environment, cleaning schedules and the type of luminaire used. |
| Mounting Height | The vertical distance between the luminaire and the surface to be lighted. It includes both the pole length and the base above grade to which the pole is mounted. |
| Optical System | The lamp cavity or environment designed as part of the luminaire for the purpose of controlling light output. |
| Reflector | A device used to direct the light from a source by the process of reflection. |
| Refractor | A device used to direct the luminous flux from a source primarily by the process of refraction. |
| Uniformity | A measure of how evenly or smoothly the lighting level is spread over an area. It is expressed as a uniformity ratio of average foot-candles divided by the minimum allowable foot-candles. The lower the ratio, the better. Visually, observing an area with bright spots beneath outdoor lighting fixtures surrounded by dark spots is an indication of poor lighting uniformity. |
| Watt | A measure of the amount of electrical power required by a product to operate at any given instant. |
