Accent Lighting: Directional lighting used to emphasize a particular object or to draw attention to a part of the field view. Adaptation: The process by which the visual system becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity of the eye to the light. Ambient Lighting: Lighting of a general, overall, uniform in nature. Ampere (amps or A): the unit of measurement of electric current flow. Baffle: An opaque or translucent element that serves to shield a light source from direct view at certain angles, or serves to absorb unwanted light. Ballast: An electrical device, used in fluorescent fixtures to supply the necessary voltage and current to start and operate the lamp(s). Electronic ballasts are gradually replacing magnetic ballast because they are more efficient, quieter, lighter and facilitate dimming. Beam Spread: The angle enclosed by two lines which intersect the candlepower distribution curve at the points where the candlepower is reduced to a stated percent of maximum (normally 50% if maximum). Channel: A group of lights of the same type, or doing the same lighting task, that operates together. All the lights in a channel dim together, i.e. they are all full-on, dimmed to 80% or 50%. etc. Circuit: The closed path over which an electric current flows. It includes the source of electric energy, (usually the electric panel box or "breaker"), the wiring and the electric load. Color Rendering Index (CRI): Measure of the degree to which the perceived colors of objects illuminated by a source conform to those of the same objects illuminated by a reference source of comparable color temperature. Color Temperature (of a Light Source): The absolute temperature of a blackbody radiator having a chromaticity (apparent color) equal to that of the light source. Contrast: The difference in the luminance of an object and its immediate background (e.g. between print and paper). Current: The flow of electricity. Current is measured in amperes (amps or A).	Diffuser: A translucent glass or plastic that shields the light source and spreads the light evenly in all directions. Diffusion: The scattering of light rays so that they are emitted or reflected in all directions. Dimming Ballast: Special fluorescent lamp ballast which when used with a dimmer control, permits varying the output. Efficacy: The ratio of output to input, measured in different units. The efficacy of a lamp is the ratio of its output (lumens) to the power (watts). Efficiency: The ratio of output to input, measured in the same units. The efficiency of a luminaire is the ratio of its output (in lumens) to the input (lumens from the lamp). Energy: The power consumed over a period of time. Electrical energy is measured in kilowatt hours (KWH). Floodlighting: Lighting desired to light a scene or object to a luminance greater than its surroundings. It may be for utility, advertising or decorative purposes. Fluorescent Lamp: A tubular electric lamp filled with mercury vapor and having a coating of fluorescent material on its inner surface. Light is emitted by the phosphors when excited by an arc discharge between cathodes at the ends of the lamp. The composition of the phosphors determines the color quality of the lamp. Footcandle (fc): A unit of illuminance, the amount of light falling on a surface. One footcandle = one lumen/sq. ft. Footlambert(fl): A unit of luminance the brightest of a surface. One footlambert = one lumen/sq. ft. Frame-In Kit: The electrified mounting frame or housing which supports the optical assembly (reflector trim) of a recessed fixture. Frame-In Kits are of various types according to the nature of the construction, and the lamp. Housing: See Frame-In Kit Halogen: (Tungsten Halogen): A type of incandescent lamp filled with a halogen gas. The advantage over regular incandescent lamps include excellent lumen maintenance and compactness. They also provide whiter light and longer life at a given light output. Illuminance: The density of luminous flux falling on a surface. The level of illumination. measured in footcandles.	Incandescent (Tungsten) Lamp: A lamp in which light is produced by a tungsten filament heated to incandescence by an electric current. Kilowatt Hour (KWH): A unit for measuring electrical energy consumption. KWH= Watts x Hours/1000. Electrical energy is sold by the "Kilowatt-Hour" (KWH). Lamp: An artificial light source. (portable luminaries, equipped with cord and plug. Lens: An glass or plastic element used in luminaries to change the direction of and control the distribution of light rays. Also called prismatic lens. Light Distribution: Luminaries: Luminaries are classified according to the distribution of light up, down, or up/down: indirect, direct or indirect/direct. Louver: A shielding element used to intercept light travelling in undesirable directions. Louvers are generally an assembly of baffle elements arranged in grid or concentric form. Low Voltage Lamps: Incandescent lamps that operates at low voltage, such as PAR 36 and MR-16 lamps that operate at 12 volts. Lumen: The unit of luminous flux. A measurement of a lamp's or luminaire's light output. Luminaire: A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and to connect the lamps to the power supply. Luminance: The luminous intensity of any light emitting surface. Often referred to as brightness. Measured in footlamberts. Matte surface: A dull surface as opposed to shiny (specular) surface. Light reflected from a matte surface is diffuse. Parabolic Reflector: A reflector shaped in a cross-section like a parabola. A small light source at the focal point of the parabola produces an essentially parallel beam of light. Power: The rate at which at which energy is developed or expended. Power is measured in watts. Power Factor: The ratio of watts to volt-amperes drawn by fluorescent ballast. It is, in effect, a measure of ballast efficiency. Commercial jobs require High Power Factor (90% or higher) because, for a given load, they draw less current, permitting more fixtures on a circuit and lower wiring costs.	Reflection: The process by which incident light is returned by a surface. It can be classified as specular or diffuse. Specular reflectors are like mirrors, returning rays of light at the same angle as they arrive, and preserving sharp images. Diffuse reflectors return light in many directions; images are blurred or absent altogether. Refraction: The process by which the direction of a ray of light changes as it passes obliquely from one medium to another. Fresnel and prismatic lenses utilize refraction to control the distribution of light. Shielding: An arrangement of light-controlling material to prevent direct view of the light source. Shielding Angle: The angle measured from the horizontal at which a light source in a luminaire first becomes visible. It is the complementary angle of the cut-off angle. Specular Reflector: A shiny, highly-polished surface that acts like a mirror. It is used to control light precisely and looks dark except when viewed from the direction in which an image of the light source is projected. Task (Visual Task): That which is to be seen. The visual function is performed. Task Lighting: Lighting directed to a specific surface or area to provide illumination for visual tasks. Thermal Protector: A device which disconnects power to a luminaire to protect against overheating due to abnormal conditions. Transformer: A device used to raise (step-up) or lower (step-down) electric voltage. Veiling Reflections: The reflections of light sources in a task which reduce the contrast between detail and background (e.g. between print and paper, or print and the computer screen), thus imposing a "veil" and decreasing task visibility. Volt (V): The unit of measurement of volt. Voltage: Electromotive force. The force or pressure which causes electricity to flow. Voltage is measured in volts. (V). Wall Wash Lighting: A lighting system that provides a smooth, even distribution of light over a vertical surface. Watt: The unit of measurement of electrical power. When power is used over a period of time, energy is consumed, measured in watt-hours or kilowatt hours.