Glossary of Terms

Common Lighting Terms & Definitions

ACCENT LIGHTING — Any Source from almost any direction which is used in addition to more basic lights to call attention to an object or area – not the Lighting.

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

ASHREA — American Society of Heating, Refrigerating and Air-Conditioning Engineers

AVERAGE RATED LIFE — The useful life of LED lighting products is defined differently than that of other light sources, such as incandescent or CFL. This is because LEDs typically do not “burn out” or fail. Instead, they experience lumen depreciation, where the amount of light produced decreases and light color appearance can shift over time. Instead of basing the useful life of an LED product on the time it takes for 50% of a large group of lamps to burn out (as is the case with traditional sources), LED product “lifetime” is set based on a prediction of when the light output decreases 30 percent.

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

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.

BEAM — A cone of light emitted by a Luminaire.

BEAM ANGLE — Beam angle is the point at which the Intensity of a source drops to 50% of maximum (center reading) measured in degrees of the full angle.

CANDELA — Unit of luminous intensity, describing the intensity of a light source in a specific direction.

CANDELA DISTRIBUTION — A curve, often on polar coordinates, illustrating the variation of luminous intensity of a lamp or luminaire in a plane through the light center.

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

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 (or no color shift). A low CRI rating suggests that the colors of objects will appear unnatural under that particular light source.

COEFFICIENT OF UTILIZATION — The ratio of lumens from a luminaire received on the work plane to the lumens produced by the lamps alone. (Also called “CU”).

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.

DIFFUSE — Term describing dispersed light distribution. Refers to the scattering or softening of light.

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.

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.

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.

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.

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.

INVERSE-SQUARE LAW — Light from a point source falls-off inversely to the square of the distance. Move the light from 10-feet-away to 20-feet-away and you have only 1/4 of the intensity; 40-feet, 1/16th.

KELVIN TEMPERATURE — A numerical scale used to describe the color of light. 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 3200K are considered “warm;” while those above 4000K are considered “cool” sources. Light with a lower Kelvin rating will have a more yellow tint, while light with a higher kelvin rating will have a 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

LED DRIVER — are current control devices that replace the need for resistors. LED Drivers respond to the changing input voltage while maintaining a constant amount of current (output power) to the LED as its electrical properties change with temperature

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

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).

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

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 — The deterioration in the amount of light that is emitted from a lamp over time. A lamp with a good lumen maintenance will emit a consistent amount of light over its lifetime, emitting as much as 90% of its original capability at the end of its lifespan. A lamp with poor lumen maintenance will lose as much as 50% of its ability to emit light over time.

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.

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.

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

PAR — Parabolic Aluminized Reflector lamps

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.

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.

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.

SPECTRUM, VISIBLE & ELCTROMAGNETIC — The full range of electromagnetic wavelengths extends from the shortest gamma rays of 1 millionth mm to radio waves of 6 miles. Buried in the middle is the visible spectrum, the tiny portion to which the human eye is sensitive. The spectrum can be “seen” when White Light is intercepted by a prism or rain-drops.

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.

VOLTS — A measure of electrical “pressure.” Formula: volts = watts ÷ amps.

WATTS (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 AC circuits, PF must be included.)