Acrylic – The generic term for a family of quality light-stabilized plastics used in making fixture diffusers and lenses.
Average Maximum Candlepower – The average of the ten highest readings in a floodlight beam.
Ballast – A device which modifies incoming voltage and current to provide the circuit conditions necessary to start and operate electric discharge lamps (fluorescent and H.I.D.).
Beam Lumens – The lumens contained within the beam spread of floodlight.
Beam Spread – The vertical and horizontal displacement of the beam in degrees, bounded by the angle at which 10% of maximum candlepower occurs. (Maximum candlepower is the highest intensity beam.)
Brightness – As commonly applied, brightness (or luminance) is the intensity of the sensation which results from viewing a surface or space which directs light into the eyes.
Candela – Unit of luminous intensity.
Candlepower – Luminous intensity.
Candlepower Distribution Curve – A curve showing the variation of luminous intensity of a lamp or luminaire with angle.
Coefficient of Utilization (CU) – The percentage of light from a fixture which eaches the seeing task. It is a function of the fixture, each having its own set of CU’s for a wide range of factors.
Color Rendering – General expression for the effect of a light source on the color appearance of objects when compared with their color appearance under a reference light source.
Cutoff Angle (of luminaire) – The angle measured up from nadir between the vertical axis and the first line of sight at which the bare source is not visible.
Diffuser – See lens.
Efficacy – The ratio of light from lamp to the electrical power (watts) consumed. Usually expressed in lumens per watt.
Efficiency – See luminaire efficiency.
Fixture – The device which holds, protects, and provides an optical system and power connections for the lamp(s). Fixture usually refers to interior lighting. See Luminaire.
Flux (Luminous Flux) – See lumen.
Footcandle (fc) – A quantitative unit for measuring illuminance: the illumination on a surface one foot square on which there is a uniformly distributed flux of lumen.
Footlambert – The unit of luminance equal to 1/candle per sq. ft.
Glare – The sensation produced by luminance within the visual field that is significantly greater than the luminance to which the eyes are adapted.
Glare, Direct – Glare resulting from high luminances or insufficiently shielded light sources in the field of view.
Glare, Disability – Glare resulting in reduced visual performance and visibility and often accompanied by discomfort.
Glare, Discomfort – Glare producing discomfort. It does not necessarily interfere with visual performance or visibility.
H.I.D. – High intensity discharge lighting, including mercury vapor, metal halide and high pressure sodium light sources. Although low pressure sodium lamps are not H.I.D. sources, they often are included in the H.I.D. category.
High Intensity Discharge (H.I.D.) – The term that applies to a family of light sources consisting of mercury vapor metal halide and high pressure sodium lamps.
Illuminance – The density of luminous flux on a surface. Measured in footcandles or lux. The former term for this quantity was illumination.
Inverse Square Law – The law stating that the illuminance “E” at a point on a surface varies directly with the intensity “I” of a point source and inversely as the square of the distance “d” between the source and point. If the surface at the point is normal to the direction of the incident light, the law is expressed by E= I/d2.
Isocandela Line – A line plotted on any appropriate set of coordinates to show directions in space about a source of light in which the candlepower is the same. A series of such curves, usually for equal increments of intensity is called an isocandela diagram.
Isolux (Isofootcandle) Line – A line plotted on any appropriate set of coordinates to show all points on a surface where the illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.
Lamp– A light source. Lamps are used for outdoor lighting include H.I.D., incandescent (including tungsten halogen), and fluorescent.
Lamp Lumen Depreciation (LLD) – 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 designated period of burning time. The LLD is contingent upon relamping schedules and the specific lamp involved.
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. NOTE: Plastic lenses may be manufactured by the extrusion process or injection molded process. Injection molded lenses are more expensive.
Lighting Distribution – Luminaires are classified according to the manner in which they control or distribute the luminous flux.
Light Loss Factor – A factor used in calculating the level of illumination after a given period of time and under given conditions. It takes into account temperature, dirt accumulations on the luminaire and room surfaces, 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.
Louver – A series of baffles used to shield a source from view at certain angles or to absorb unwanted light.
Lumen – The unit of light output. Light output is also referred to as light flux.
Luminaire – A complete lighting fixture including one or more lamps and a means for connection to a power source. Many luminaries also include one or more ballasts and elements to position and protect lamps and distribute their light.
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 involved.
Luminaire Efficiency – The ratio of the light leaving a luminaire to that emitted by the lamp, or lamps used therein.
Luminance (Photometric Brightness) – The luminous intensity of any surface in a given direction per unit is of that surface as viewed from that direction. Measured in footlamberts or C/in2. All visible objects have some luminance.
Luminous Flux – The time rate of flow of light.
Lux – The metric unit of measurement of illuminance. The light on a surface of one meter square on which there is a
uniformly distributed flux of one lumen. 10.76 lux = 1 footcandle. Decalux = 10 lux.
Maintenance Factor – A multiplier which is applied to account for aging of the lamp and for dirt build-up on the luminaire during the period for which a lamp is in place. Lamp aging and luminaire dirt build-up both educe light output, the amount of reduction usually increasing with time. n common practice, maintenance factors are applied to initial footcandles o derive the minimum light level on the area being illuminated. Light loss actor includes maintenance factor.
Metercandle – The metric equivalent of a footcandle. x = lumens/square meters footcandle = 10.76 lux lux = 0.0929 footcandles.
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 affixed.
Optical System – The lamp cavity or environment (including diffusing media) designed as part of the fixture for the purpose of controlling the light output.
Overhang – In roadway lighting, the distance between a vertical line passing through the luminaire and the curb or edge of the roadway.
Parabolic – The term applied to certain low brightness louver and reflector shapes as derived from the geometric shape (curve) called a parabola where, if a light source is placed at the focal point of the parabola, the resultant emitted light will be redirected parallel to the parabola’s geometric axis.
Reflector – A device used to direct the light from a source by the processof reflection.
Refraction – The process by which the direction of a ray of light changes as it passes obliquely from one medium to another.
Refractor – A device used to redirect the luminous flux from a source, primarily by the process of refraction.
Setback – The distance that the center of the luminaire is behind the area to be lighted by that luminaire.
Shielding Angle (of a luminaire) – The angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible.
Spacing – In roadway lighting, the distance between successive light units measured along the center line of the street.
Spacing to Mounting Height Ratio – The ratio of the distance between luminaire centers to the mounting height above a reference plane, usually the work plane.
Specular Angle – The angle between the perpendicular to a surface and the reflected ray. It is numerically equal to the angle of incidence.
Specular Surface – Shiny or glossy surfaces (including mirror and polished metal) that reflect incident light, providing a relatively narrow beam pattern.
Spill Light – Lumens distributed by the luminaire which are outside the beam spread.
Styrene (Polystyrene) – The generic term for a family of plastics used in the making of fixture diffusers and lenses. Tends to yellow in time due to the effect of ultraviolet radiation from fluorescent and H.I.D. lamps.
Surface Mounted – Any fixture mounted directly on a ceiling is surface mounted.
Suspension or Pendant Mounted – Any fixture hung from a ceiling by supports (chains, hangers, stems, etc.) is suspension or pendant mounted.
Vapor-Tight Luminaire – A luminaire designed and approved for installation in damp or wet locations. Also described as enclosed and gasketed.