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Glossary of Definitions

AMBIENT LIGHT: The general illumination that comes from all directions in an area.

AmPERE: A unit of measure of electrical current. It is defined as the current required to produce a certain force between two parallel and infinitely long wires separated by one meter.

BALLAST: A device used to operate fluorescent and HID lamps. The ballast provides the necessary starting voltage, while limiting and regulating the lamp current during operation.

BEAM SPREAD: The angle between the two directions in the plane in which the intensity is equal to a given percentage (usually 10%) of the maximum beam intensity.

BUG RATING: The Backlight, Uplight, and Glare Rating System, developed by the IESNA, is a way to evaluate optical performance related to light trespass, sky glow, and high angle brightness control in terms of the quantity of light (lumens) exiting a luminaire at defined angles.

CANDELA (cd): Unit of luminous intensity, describing the intensity of a light source in a specific direction.

CANDLEPOWER: A measure of luminous intensity of a light source in a specific direction, measured in candelas (see above).

COLOR RENDERING INDEX (CRI): A scale of the effect of a light source on the color appearance of an object compared to its color appearance under a reference light source. Expressed on a scale of 1 to 100, where 100 indicates no color shift. A low CRI rating suggests that the colors of objects will appear unnatural under that particular light source.

COLOR TEMPERATURE (CCT): The color temperature is a specification of the color appearance of a light source, relating the color to a reference source heated to a particular temperature, measured by the thermal unit Kelvin. The measurement can also be described as the "warmth" or "coolness" of a light source. Generally, sources below 3,200?K are considered "warm;" while those above 4,000?K are considered "cool" sources.

CONES AND RODS: Retinal receptors. Cones dominate the response when the luminance level is high, and provide color perception. Rods dominate at low luminance levels. No rods are found in the central part of the fovea. Rods have no color perception ability.

CUT-OFF ANGLE: The angle from a fixture's vertical axis at which a reflector, louver, or other shielding device cuts off direct visibility of a lamp.

CUTOFF FIXTURE: A fixture that provides a shielding of the emitted light where the candela per 1,000 lumens does not exceed 2.5% at 90? above nadir, and the candelas per 1,000 lumens is less than 10% at 80? above nadir.

DARK SKY: The night sky free from light pollution that interferes with the view of the stars. Dark sky compliant fixtures cast light only downward.

DIFFUSER: A translucent piece of glass or plastic sheet that shields the light source in a fixture. The light transmitted throughout the diffuser will be redirected and scattered.

DOWNLIGHT: A type of luminaire where most of the light is directed downward. May feature an open reflector and/or shielding device.

EFFICACY: A measure of the energy efficiency of a light source in terms of Lumens (light output) per rated wattage of the source.

EFFICIENCY: A measure of the effective or useful output of a system compared to the input of the system.

ELECTRONIC BALLAST: A ballast that uses semi-conductor components to increase the frequency of fluorescent lamp operation (typically in the 20-40 kHz range). Smaller inductive components provide the lamp current control. Fluorescent system efficiency is increased due to high frequency lamp operation.

FIXTURE: The assembly that holds the lamp in a lighting system. It includes the elements designed to give light output control, such as a reflector (mirror) or refractor (lens), the ballast, housing, and the attachment parts. The term “fixture” does not include the lamp.

FOOTCANDLE (FC): The English unit of measurement of the illuminance (or light level) on a surface. One footcandle is equal to one lumen per square foot.

FULL-CUTOFF FIXTURE: A fixture that allows no emission above a horizontal plane through the fixture.

GLARE: The effect of brightness or differences in brightness within the visual field sufficiently high to cause annoyance, discomfort or loss of visual performance.
HID: Abbreviation for high intensity discharge. Generic term describing mercury vapor, metal halide, high pressure sodium, and (informally) low pressure sodium light sources and luminaires.

HIGH PRESSURE SODIUM LAMP: A high intensity discharge (HID) lamp whose light is produced by radiation from sodium vapor (and mercury).

INTERNATIONAL DARK-SKY ASSOCIATION (IDA, INC.): A non-profit organization whose goals are to build awareness of the value of dark skies, and of the need for quality lighting.

KELVIN: Kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units (SI) and assigned the unit symbol K.

LAMP: The source of electric light. To be distinguished from the whole assembly (see luminaire). Lamp often is used to denote the bulb and its housing

LED: A light-emitting diode. It is a semiconductor light source.

LENS: Transparent or translucent medium that alters the directional characteristics of light passing through it. Usually made of glass or acrylic.

LIGHT POLLUTION: Any adverse effect of manmade light. Often used to denote urban sky glow.

LIGHT TRESPASS: Light falling where it is not wanted or needed. Spill light. Obtrusive light.

LUMEN (L): A unit of light flow, or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp.

LUMENS PER WATT (L/W): A measure of luminous efficacy. It is the ratio of luminous flux to power (watts) consumed by the light source.

LUMINAIRE: A complete lighting unit consisting of a lamp or lamps, along with the parts designed to distribute the light, hold the lamps, and connect the lamps to a power source.

MESOPIC: Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations. Most night-time outdoor and traffic lighting scenarios are in the mesopic range.

METAL HALIDE: A type of HID lamp in which most of the light is produced by radiation of metal halide and mercury vapors in the arc tube. Available in clear and phosphor-coated lamps.

OPTICS: A term referring to the components of a light fixture (such as reflectors, refractors, lenses, louvers) or to the light emitting or light-controlling performance of a fixture.

PHOTOMETRIC REPORT: A photometric report is a set of printed data describing the light distribution, efficiency, and zonal lumen output of a luminaire. This report is generated from laboratory testing.

PHOTOPIC: Photopic vision is the vision of the eye under well-lit conditions. In humans and many animals, photopic vision allows color perception, mediated by cone cells.

POWER FACTOR (PF): The power factor of an electric power system is defined as the ratio of the real power flowing to the load to the apparent power (product of current x voltage) in the circuit and is a dimensionless number between 0 and 1.

REFLECTANCE: The ratio of light reflected from a surface to the light incident on the surface. Reflectances are often used for lighting calculations. The reflectance of a dark carpet is around 20%, and a clean white wall is roughly 50% to 60%.

REFLECTOR: The part of a light fixture that shrouds the lamps and redirects some light emitted from the lamp.

SEMI-CUTOFF FIXTURE: A fixture that provides a shielding of the emitted light where the candela per 1,000 lumens does not exceed 5% at 90? above nadir, and the candelas per 1,000 lumens is less than 20% at 80? above nadir.

VOLTAGE: The difference in electrical potential between two points of an electrical circuit.

WATT (W): The unit for measuring electrical power. It defines the rate of energy consumption by an electrical device when it is in operation. The energy cost of operating an electrical device is calculated as its wattage times the hours of use. In single phase circuits, it is related to volts and amps by the formula: Volts x Amps x PF = Watts. (Note: For alternating current circuits, PF must be included.



Last updated: 3/6/2015 2:48:15 PM