the chip and must be capable of withstanding constant optical radiation and elevated operating temperatures without loss of transparency.

energy gap: See band gap.

epitaxy: The growth of one thin film layer using the crystalline structure of the preceding layer as a template.

exciton: An excitation of a molecule when an excited electron and hole are loosely bound together. This excitation is mobile and eventually decays by the recombination of the electron and the hole.

fluorescence: The emission of light as a result of, and only during, the absorption of radiation of shorter wavelengths. (ANSI and IES, 2010)

fluorescent lamp: A tubular electric lamp that is coated on its inner surface with a phosphor and that contains mercury vapor whose bombardment by electrons from the cathode provides ultraviolet light, which causes the phosphor to emit visible light either of a selected color or closely approximating daylight. (California Energy Commission, 2002)

footcandle (ftc): A lumen per square foot; a unit of illuminance. While commonly used, it is not an SI unit. The corresponding SI unit is lux.

glare: The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted; it may cause annoyance, discomfort, or loss in visual performance or visibility. (ANSI and IES, 2010)

halogen lamp: A type of incandescent lamps in which the tungsten filament has been enclosed in a capsule containing a halogen gas, typically bromine.

high-intensity discharge (HID) lamps: Electric lamps with tubes filled with gas and metal salts. The gas initiates an arc, which evaporates the metal salts, forming a plasma. These lamps are generally used to light large spaces or roadways. Mercury, metal halide, and high-pressure sodium lamps are examples of specific types of HID lamps.

illuminance: The total luminous flux incident on a surface per unit area. Commonly referred to as brightness, it indicates how bright an illuminated space is. Illuminance depends on the luminous flux of the light sources, their distances from the illuminated surface, and some of the reflectance properties of nearby surfaces.

incandescence: The self-emission of radiant energy in the visible spectrum, due to the thermal excitation of atoms or molecules. (ANSI and IES, 2010)

incandescent lamp: A light source that generates light by passing an electric current through a thin filament wire (usually tungsten) to a temperature of approximately 2,500-3,000 kelvin (K) where the filament glows or incandesces.

internal quantum efficiency (IQE): The percentage of photons generated relative to the current (of electrons or holes) injected into a device.

lamp: A replaceable component that produces light. The term lamp can refer to an incandescent bulb, a CFL bulb, or an LED replacement bulb.

light-emitting diode (LED): A p-n junction semiconductor device that emits optical radiation under an applied voltage. The optical emission may be in the ultraviolet or infrared wavelength regions as well as visible light. (Industrial Fiber Optics, 2004)

LED array: An assembly of LED packages or dies that are intended to connect to an LED driver, created by mounting and interconnecting individual LED devices on a printed circuit board, which is then connected thermally to the heat sink.

LED package: The LED package is the structure in which the LED chip is mounted and through which access to the LED terminals is provided. The assembly typically includes one or more LED dies with electrical connections and may include an optical element as well as thermal, mechanical, and electrical interfaces. (ANSI and IES, 2010)

LED lamp, integrated: An integrated assembly composed of LED packages or LED arrays, LED driver, ANSI standard base and other optical, thermal, mechanical, and electrical components. The device is intended to connect directly to the branch circuit through a corresponding ANSI standard socket. (ANSI and IES, 2010)

LED lamp, non-integrated: An assembly comprised of an LED array or LED packages and ANSI standard base. The device is intended to connect to the LED driver of an LED luminaire through an ANSI standard socket and not to the branch circuit directly. (ANSI and IES, 2010)

LED light engine: An integrated assembly comprised of LED packages or LED arrays, LED driver, and other optical, thermal, mechanical, and electrical components. The device is intended to connect directly to the branch circuit through a custom connector compatible with the LED luminaire for which it was designed and does not use an ANSI standard base. (ANSI and IES, 2010)

lighting power density (LPD): The spatial average power consumption of the installed luminaires in a building or in a space. It is expressed in units of Watts per square feet of floor area (W/ft2).

lumen (lm): A measure of the amount of light, or luminous flux, emitted by a source per unit time.

lumen maintenance: The relationship between temperature, operating time, and light output.

luminance: A measure of the amount of light per unit area of a surface. The luminance of an illuminated object is dependent on both the incident illuminance and the reflectance of the object. Luminance is the common measure of the intensity of displays. Lighting products available in variable-sized flat forms, such as sheets or tapes, often report luminance because luminous flux depends on the surface area of the product.

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