Glossary Of Lighting Terms

Learn about the lingo and what the specs in LED lighting mean

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0-10 Volt

0–10 V is one of the earliest and simplest electronic lighting control signaling systems; simply put, the control signal is a DC voltage that varies between zero and ten volts.

Ambient Lighting

Lighting throughout an area that provides general illuminations.


A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and wave form) for starting and operating.  See electronic ballast, magnetic ballast.

Batwing Distribution

A symmetrical light distribution producing light angles to the right and left of the observer with comparatively little direct downward illumination.  The shape is thus similar to that of a batwing.

Batwing Lens (linear prismatic)

A lens, usually of molded or extruded acrylic, consisting of a series of prisms that cause a batwing distribution of light.


The unit of measure indicating the luminous intensity (candlepower) of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered.

Canopy Light

Light Canopies are lamp parts used to cover ceiling boxes. They can also be used as back plates for wall sconces. On occasion, by drilling side holes they can be used to hang glass with bead chain.

Color Rendering Index (CRI)

A method of describing the effect of a light source om the color appearance of objects being illuminated, with a CRI of 100 representing the reference condition (and thus the maximum CRI possible).  In general, a lower CRI indicates that some colors may appear unnatural when illuminated by the lamp.

Color Temperature (chromaticity)

The degree of warmth or coolness of a light source, measured in degrees Kelvin (K).  The higher the degree K, the more blue, or cooler the lamp appears.  The lower the degree K, the more red or warmer the lamp appears.

Compact Fluorescent Lamps

Small diameter tubes that are bent so they begin and end in a sing base.  This allows them to be produced in a wide variety of configurations, greatly extending the applications for fluorescent lighting.


The difference between the luminance (brightness) of the detail in a visual task and the luminance of it’s immediate background (e.g., between the print and the paper).

Correlated Color Temperature (CCT)

A description of the color appearance of a light source in terms of its warmth and coolness.  The CCT relates the color appearance of the light emitted by the lamp to the color appearance of the reference material heated to a high temperature (measured on the Kelvin scale, abbreviated K).  As the temperature rises, the color appearance shifts form yellow to blue.  Thus, lamps with a low CCT (3000 degrees K or less) have a yellow-white color appearance and are described as “warm”.  Lamps with a high CCT (4000 K or higher) have a blue-white color appearance and are described as “cool”.


A device to redirect or scatter the light from a source, primarily by the process or diffuse transmission.

Direct Glare

Glare resulting from the excessive brightness or insufficiently shielded light sources in the field of view.


The DesignLights Consortium (DLC) promotes quality and energy efficient lighting for the commercial sector. When you buy a product that’s on DLC’s Qualified Products List (QPL), you’ve got yourself a superior lighting product.


A measurement of how effective the light source is in converting electrical energy to LUMENS of visible light. Expressed in LUMENS-PER-WATT (LPW) this measure gives more weight to the yellow region of the spectrum and less weight to the blue and red region where the eye is not as sensitive.

Electric-Discharge Lamp

A lamp in which light is produced by passing an electric current through a gas.  These lamps may be named after the gas they contain, as in mercury lamps, sodium lamps, neon lamps; or they may be named for their operating parameters or dimensions, as in short arc lamps, high pressure lamps, etc.

Electronic Ballast

A ballast that uses solid state electronic components and operates lamps (typically fluorescent) at frequencies in the range of 25-35 kHz.  See ballast, magnetic ballast.

Emergency Driver

An LED driver is the power supply for an LED system, much like a ballast is to a fluorescent or HID lighting system.  LED drivers are usually compact enough to fit inside a junction box, include isolated Class 2 output for safe handling of the load, operate at high system efficiency, and offer remote operation of the power supply.

Emergency Fluorescent Ballast

A fixture that operates one or two lamps for a set period of time in the event of a power failure.  Emergency ballasts are also used for normal, day to day operation.

Emergency Light

An emergency light is a battery-backed lighting device that switches on automatically when a building experiences a power outage. Emergency lights are standard in new commercial and high occupancy residential buildings, such as college dormitories.

Energy Star

ENERGY STAR is the trusted, government-backed symbol for energy efficiency helping us all save money and protect the environment through energy-efficient products and practices.
The ENERGY STAR label was established to:
-Reduce greenhouse gas emissions and other pollutants caused by the inefficient use of energy; and
-Make it easy for consumers to identify and purchase energy-efficient products that offer savings on energy bills without sacrificing performance, features, and comfort.

Exit Sign

An exit sign is a device in a public facility (such as a building, aircraft or boat) denoting the location of the closest emergency exit in case of fire or other emergency. Most relevant codes (fire, building, health or safety) require exit signs to be permanently lit.


Used to refer to the beam pattern of a reflector lamp, which disperses the light over a wide beam angle, typically 20 degrees or more. (“Flood” as opposed to “spot”).

Fluorescent Lamp

A low pressure mercury, electric-discharge lamp in which phosphor coating transforms ultraviolet energy (created by electric discharge) into visible light.


A complete lighting unit consisting of a lamp or lamps together with parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.  Also called Luminaire.

Footcandle (fc)

A unit of measurement indicating how much illumination is reaching a surface.  It is equal to one lumen striking an area of one square foot.

General Lighting

Lighting designed to provide a substantially uniform level of illumination throughout an area, exclusive of any provision for special local requirements.


A sensation caused by light within the visual field that is brighter than the level of light to which the eyes are adapted, causing annoyance, discomfort or loss in visual performance.

Halogen Lamps

High pressure tungsten filament lamps containing halogen gases.  The halogen gases allow the filaments to operate at higher efficacies than incandescent lamps.

High Bay Lighting, High Bay Fixtures

Lighting designed for locations (typically industrial) with a ceiling height of 25 feet and above.

High –Intensity Discharge (HID) Lamps

Lamps in which an arc passing between two electrodes in a pressurized tube causes various metallic additives to vaporize and release large amounts of light.

High-Pressure Sodium (HPS) Lamps

High-intensity discharge light sources that produce light by an electrical discharge through sodium vapor operating at relatively high pressures and temperatures.


The amount of light that reaches a surface.  Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter).  One footcandle equals 10.76 lux, although for convenience the IESNA uses 10 lux as the equivalent.

Indirect lighting

Lighting by luminaires distributing 90 to 100 percent of the emitted light upward.

Instant Start

A method of starting fluorescent lamps by applying high voltage, without heating the electrodes (as in rapid start sytems).  Higher starting voltage places greater stress on the electrodes, slightly shortening lamp life.  However, the elimination of electrode heating reduces system energy consumption, which more than offsets the slightly shorter lamp life.  See rapid start.

Intelligent Lighting

Fluorescent high-bay lighting that is forward-thinking in its design, accommodating occupancy an daylight harvesting systems, security applications, and programmable control systems.


A generic term for a man made source of light.

Lamp Flicker

Cyclic variation in output of a light source.  High frequency electronic ballasts provide a minimal level of lamp flicker.

Lamp Lumen Depreciation Factor (LLDF)

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.


A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. The light is not particularly bright, but in most LEDs it is monochromatic, occurring at a single wavelength.


A glass or plastic shield that covers the bottom of a luminaire to control the direction and brightness of the light it emits.


Radiant energy that is capable of exiting the retina and producing a visual sensation.


A series of baffles used to shield a light source from view at certain angles or to absorb unwanted light.  The baffles are usually arranged in a geometric pattern.

Low Bay Lighting, Low Bay Fixtures

Lighting designed for use when the bottom of the luminaire is less than 20 feet above the floor.  Fixtures are usually 22-28” in diameter in order to spread the light evenly.


The unit of measurement defining the output of a lamp.  Could be said to measure a volume of light.

Lumen Depreciation

The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve.  (Sometimes called Lumen maintenance curve).

Lumens Per Watt (LPW)

Lumens per watt (lm/W) refers to the energy efficiency of lighting: how much visible light you get for a given amount of electricity.


A complete lighting unit consisting of a lamp or lamps together with parts designed to distribute the light, position and protect the lamps, and connect the lamps to the power supply.

Luminaire Efficiency

The ratio of the light emitted by a luminaire to the light emitted by the lamp within it.  Components of a luminaire such as reflectors and diffusers absorb some of the light from the lamp(s).  A highly efficient luminaire emits most of the light that the lamp(s) emits.


Light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or square meters).

Luminance Contrast

The relationship between the luminances of an object and its immediate background.

Luminance Ratio

The ratio between the luminances of any two area in the visual field.

Luminous Flux

The time rate of flow of light.

Lux (lx)

A unit of illuminance.  One lux is one lumen per square meter.

Magnetic Ballast

A ballast that uses “core & coil” assembly to transform electrical current to start an arc between electrodes.  See ballast, electronic ballast.

Mean Lumens

Lumen output of a light source after the source has been used.

Modular Reflector

A device used to redirect the luminous flux from a sourced by the process of reflection.

Prismatic Lens

An optical component of a luminaire that is used to distribute the emitted light.  It is usually a sheet of plastic with a pattern of pyramid-shaped refracting prisms on one side.  Most ceiling-mounted luminaires in commercial buildings use prismatic lenses.

Programmed Rapid Start

A new “smart” method of starting fluorescent lamps by heating electrodes on their optimum operating temperature before voltage is applied.  Programmed rapid start greatly reduces stress on electrodes and increases the number of lamp switching cycles without adversely effecting lamp life – a critical issue in occupancy sensor applications. The level of maintained electrode heat can also be optimized for a balance between lamp life and energy efficiency.  Initial system costs, maintenance considerations, and the increasing use of dimming and occupancy sensor controls all factor into the selection of lamp starting and operating methods.  See rapid start.


The branch of physics dealing with the nature and properties of electromagnetic energy in the light spectrum and the phenomena of vision. In the broadest sense,optics deals with infrared light, visible light, and ultraviolet light.

Quality of Lighting

Pertains to the distribution of luminance in a visual environment.  The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease, of seeing, safety, and aesthetic for the specific visual tasks involved.

Rapid Start

A method of starting fluorescent lamps in which the ballast simultaneously applies starting voltage across the lamp while heating the electrodes.  The electrode heating reduces the voltage needed to start the lamp but remains constant after starting, consuming additional power.  While rapid start (RS) systems consume more energy that instant (IS) systems, they also place less stress on electrodes and deliver longer lamp life in typical operating cycles.  See programmed rapid start, instant start.

Recessed Can

A recessed can light is a light fixture that is installed into a hollow opening in a ceiling.

Recessed High Bay

A recessed high bay is designed to deliver maximum efficiency and lighting control. Precise photometric distribution provides the high performance general lighting required in offices, schools, hospitals and retail interiors.

Recessed Panel

A recessed light panel is a panel light fixture that is installed into a hollow opening in a ceiling.

Recessed Troffer

A long recessed light fixture, especially on in the ceiling or for a fluorescent light. An example of a troffer is a fluorescent light fixture recessed in the ceiling.


A general term for the process by which the incident flux leaves a surface or medium from the incident side, without changing in frequency.


A device used to redirect the luminous flux from a source by the process of reflection.


A connecting device for a luminaire that slips over the end of a bracket arm and is secured by a clamp and/or set screws.

Specular Reflector

A mirror-like surface for which the angle of reflection equals the angle of incidence; contrast with a diffuser.

T8 and T12

The “T” designation in fluorescent lamp nomenclature stands for tubular – the shape of the lamp. The number immediately following the T gives the diameter of the lamp in eighths of an inch. A T12 lamp is therefore twelve-eighths of an inch, or one-and-one half inches, in diameter. A T8 lamp is eight-eighths of an inch, or one inch, in diameter. Both T12 and T8 lamps use the medium bi-pin base, which allows T8 lamps to fit into the same luminaires as T12 lamps of the same length.


T5 lamps are fluorescent lamps that are 5/8″ of an inch in diameter. This report discusses only linear T5 lamps. Differences in length and socket pin design versus conventional fluorescent lamps prevent any problems with electric circuits or human factors. This section focuses on the physical characteristics of T5 systems compared with T8 systems.
“HO” stands for high output. T5 HO lamps deliver more light than standard T5 lamps and are available in higher wattages. HO lamps have the same diameter and length as standard T5 lamps. Table 1-2 compares characteristics of T5 and T5 HO lamps.

Wall Pack

Wall packs are the most widely used commercial outdoor lighting fixtures used today. Wall packs are powerful light fixtures that are installed in outdoor locations of commercial buildings. They use high intensity lamp heads that are energy efficient, durable, and feature low levels of light pollution.


A unit of electrical power equal to 1 joule per second.  Lamps are rated in watts to indicate power consumption.
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