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Common Lighting Terms & Definitions

AMP - Watts Divided by Volts.

ANSI - American National Standards Institute. The organization that develops voluntary guidelines and produces performance standards for the electrical and other industries.

AVERAGE RATED LIFE - An average rating, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current.

BALLAST — A device which provides the necessary starting voltage and appropriate current to a fluorescent or high intensity discharge (HID) luminaire.

BCT — Ballast Case Temperature. The measured operating temperature of a fixture’s ballast. Operating outside of a ballast’s specified operating temperature will shorten its functional lifespan.

BALLAST FACTOR — A ratio used to calculate the expected real-world performance of a lamp. Calculated as the difference between the expected performance of a lamp with a commercial ballast versus the measured performance of that lamp with a reference ballast. Rated Lamp Lumens x Ballast Factor = Net Lumens.

CRI — Color Rendering Index, sometimes CIE. The ability of a light source to accurately render an object’s color in comparison with a natural light source. Measured on a scale of 1 -100 with 100 being the ideal.

COLOR TEMPERATURE - See Kelvin Temperature.

DAYLIGHT SENSOR — A device which senses the amount of daylight in a room and controls the luminaire accordingly.

DIRECT – A direct source of light which is cast downwards from a fixture to provide lighting with uniform levels of illumination. Open, louvered, and lensed fixtures can all be “direct”. Also see Indirect and Direct/Indirect.

DID (Direct/Indirect) - A source of light in which light is cast both upwards and downwards from a fixture to provide a combination of direct and indirect illumination.

DOWNLIGHTING — Light which is cast downward from a fixture. The most common and direct form of lighting.

EFFICACY - A measure expressed in lumens per watt representing the efficiency of a lamp/ballast system or luminaire.

ECM - Energy Conservation Measures. A term commonly used by ESCOs in lighting audits/designs.

ESCO — Energy Service Company. A company dedicated to helping commercial and industrial clients reduce their energy consumption.

FC - Foot-Candle. A unit of measure for the density of light as it reaches a surface. One foot-candle is equal to 1 lumen per square foot.

HEAT SINK - A component or integral part of luminaire that conduct or convects heat away from LED components.

HI-BAY — Lighting used in industrial applications where the ceiling height is greater than 20 feet. Common in big box retail, industrial, warehouse and manufacturing spaces.

HID — High Intensity Discharge lamps. Includes HPS, PSMH and MH lamps.

HPS — High Pressure Sodium HID Lighting.

ILLUMINANCE - Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1lux.

INDIRECT - An indirect source of light which is cast upwards from a fixture and bounced down to provide lighting with minimal glare and more uniform levels of illumination.

INITIAL LUMENS - The lumens produced by a lamp after an initial burn in period (usually 100 hours).

INPUT WATTS — The total wattage required by both the ballast and the lamp in a luminaire.

INSTANT START– Ballast starting type. Applies high voltage across the lamp with no preheating of the cathode.

IT DEPENDS — The answer to most lighting questions.

JUNCTION - The p-n junction in a diode, for instance LED, where positively charged and negatively charged materials exchange electrons, emiting photons and generating heat.

JUNCTION TEMPERATURE - The temperature in the vicinity of an LED’s p-n junction. Controlling junction temperature is critical for achieving the optimal balance between lumen output and lumen maintenance.

KELVIN TEMPERATURE— A numerical scale used to describe the color of light. Light with a lower Kelvin rating will have a more yellow tint, while light with a higher kelvin rating will have a more blue tint.

KILOWATT - 1000 Watts.

KILOWATT HOUR - 1000 Watts used continuously for one hour.

LAMP — The source of light in a fixture, colloquially called a "light bulb."

LAMP DISPOSAL - Refers to the proper recycling of lamps containing mercury or other hazardous materials.

LED - Light Emitting Diode –commonly known as LED is a semiconductor devise that emits visible light of a certain color.

LED DRIVER - An electronic devise which converts input power into a constant current source despite fluctuation in voltage. It protects LED from voltage fluctuations. In simple terms an electronic devise which feed input power to LED to produce light.

LLD - Lamp Lumen Depreciation Factor. The multiplier to be used in illumination calculations to relate the initial rated output of light sources to the anticipated minimum rated output based on the relamping program to be used. (See also Lumen Depreciation and Mean Lumens).

LENS – A glass or plastic element used in luminaries to seal a fixture or control the exiting light.

LLF - Light Loss Factor. A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation

LO-BAY — Lighting used in industrial applications where the ceiling height is 20 feet or less. Common in big box retail and industrial settings.

LM-79 - The approved method by IES for making photometric measurement of LED light products. LM-79 measures total luminous flux, luminous intensity distribution, electrical power, efficacy and color characteristics (chromaticity, CCT, and CRI).

LM-80 - A measurement standard developed by IES which allows user to evaluate and compare the lumen maintenance of LED components from different manufacturer at standard operating condition. LED packages, arrays or LED modules can be tested at three junction temperatures typically at 55°C, 85°C & manufacturer specified temperature for 6000 hours. The approved method of measuring lumen maintenance is only for LED light source not complete luminaire.

LPW - Lumens Per Watt. The number of lumens produced by a light source for each watt of electrical power supplied to the light source. See Efficacy.

LUMEN DEPRECIATION - The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve (sometimes called a lumen maintenance curve) depicting the pattern of decreasing light output.

LUMEN MAINTENANCE — Lumen maintenance is the standard lighting term for the percentage of initial lumens that a light source maintains over time. For instance, P-2’s QHC maintains 70% of initial lumens over 60,000 hours and specified as L70 over 60,000 hours (or 30% of deterioration in the amount of light produce by QHC over a period of 60,000 hours). Lumen maintenance is often specified as L50, L70, L80, or L90. In each case, L stands for lumen maintenance and the number is the percentage of light output remaining.

LUMENS - A unit of luminous flux; overall light output; quantity of light, expressed in lumens.

LUMINAIRE — A complete lighting unit which contains a lamp, housing, ballast, sockets and any other necessary components.

LUMINAIRE EFFICIENCY - The ratio of lumens emitted by a luminaire to the total lumens emitted from the light source within the luminaire.

LUX - A unit of illuminance equal to 1 lumen per square meter.

MEAN LUMENS - The average lumen output of a lamp over its rated life. Mean lumen values for fluorescent and HID lamps are typically measured at 40% of their rated lives.

MH — Metal Halide HID lighting.

— A highly effective reflecting material manufactured by Alanod which appears white but reflects nearly as much light at traditional specular surfaces.

OCCUPANCY SENSOR — A device which activates a fixture upon sensing the presence of a person.

PHOSPHORS — Substances which emit light after being bombarded by electrons. Phosphors are used to coat the inside of fluorescent lamps.

PHOTOPIC LUMENS — A type of light measured in lumens that is generally detected by common light meters and accounts for part of the human eye’s perception of brightness.

POWER FACTOR - A measure of the effectiveness with which an electrical device converts volt-amperes to watts;devices with power factors (< 0.90) are “high power factor” devices.

PS (Programmed Rapid Start) - A method of starting fluorescent lamps, associated with electronic ballasts, where low voltage is applied to the cathode prior to lamp ignition. Recommended for use with occupancy sensors.

PSMH — Pulse Start Metal Halide HID Lighting.

RAPID START - A method of starting typically associated with magnetic ballasts; where a low filament voltage is applied to preheat the cathodes.

RE-STRIKE - Refers to the restarting of a previously operating lamp shortly after turnoff. Metal halide lamps typically require a minimum of 4-15 minutes to restart after turn-off.

RLO — Relative Light Output. The ratio of light output between a fixture's potential light output at optimum ambient temperatures and actual light output at actual ambient temperatures. For example, if a fixture at its optimal temperature of 75°F produces 10,000 Lumens and 8,000 Lumens 50°F, that fixture's RLO at 50°F is 8,000 Lumens ÷ 10,000 Lumens, or 80%.

SCOTOPIC LUMENS — A type of light that is not generally detected by common light meters but which accounts for part of the human eye’s perception of brightness.
SEMI-SPECULAR — A reflective but somewhat diffuse surface.
SOLID-STATE LIGHTING - A description of the devices that do not contain moving parts or parts that can break, rupture, shatter, leak or contaminate the environment.

S/P RATIO— The ratio of scotopic to photopic lumens produced by a light source. An appropriate S/P ratio will provide for a more comfortable atmosphere and better perceived brightness.

SPECULAR — A highly polished or mirrored surface.

T5 — 5/8" diameter fluorescent lamps. “T” stands for tubular, while the number “5” stands for the 5 in 5/8”. Therefore a T8 lamp would be a Tubular 8/8”, or 1” diameter lamp.

T8 — 1" diameter fluorescent lamps.

T12 — 1 1/2" diameter fluorescent lamps.

THERMAL CHARACTERISTICS — The manner in which a luminaire manages heat, either dissipating heat or retaining it.

THD - Total Harmonic Distortion. A measure of the distortion of an electrical wave form. Excessive THD (defined by ANSI as greater than 32%) may cause adverse effects to the electrical system.

TROFFER — A recessed luminaire shaped like an inverted trough used to enclose and reflect fluorescent lamps.

UPLIGHTING — A source of light which is cast upwards to illuminate a ceiling cavity for aesthetic reasons. When combined with reflective ceiling materials, uplighting can function as a source of indirect lighting.


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