Lighting Glossary

AC (Alternating Current): Electrical current in which the flow of electric charge continually reverses direction.

Acrylic: A thermoplastic material with high light transmission properties and good mechanical properties.  Acrylic is commonly used in globes and lenses used in the lighting industry.

Ambient Lighting: Lighting designed to provide uniform light levels throughout an area.

Amp (Ampere): The basic unit of electric current adopted under the System International Unites. 

ANSI: The American National Standard Institute is a private, nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel.

ASHRAE: Founded in 1894, ASHRAE was formed in 1959 as the American Society of Heating, Refrigerating and Air-Conditioning Engineers, and began doing business as ASHRAE in 2012.

Ballast: An electrical device used with fluorescent and HID lamps to supply sufficient voltage to start and operate the lamp, but then limit the current during operation

Beam Angle: Size of the cone of light produced by lighting source measured in degrees.

BUG Rating: Backlight, Uplight and Glare (BUG) ratings in commercial outdoor lighting may be used to evaluate luminaire optical performance related to light trespass, sky glow and high-angle brightness control.

Candela: The base measurement for describing luminous intensity, or how bright a light source is.

Capital: Commonly refers to the base or the fitter found on outdoor luminaires.

CCT (Correlated Color Temperature): Color temperature defines the color appearance of a white LED. CCT is defined in degrees Kelvin.  Lower CCTs are characterized by warm or yellow appearing.  High CCTs are characterized by white / blue color light.

Circadian Sleep Rhythm: A 24-hour internal clock that is running in the background of your brain and cycles between sleepiness and alertness at regular intervals. It's also known as your sleep/wake cycle.

CRI (Color Rendering Index): Based on a scale of 100, a quantitative measure of the ability of a light source to reveal the true colors of various objects in comparison with a natural light source.

Dark Sky Society: Members support educational and legislative efforts to eliminate light pollution, such as glare, light trespass and unnecessary outdoor night lighting.

DC (Direct Current): Electrical current which the flow of electric charge moves in one direction only.

Directional Light Source: A directional light source offers light that goes in a controlled direction and at a higher light intensity. Directional lamps arise from the specific shape, size of the bulb and performance parameters.

Dirt Depreciation: One of several light loss factors (see LLF), dirt can affect the brightness of a luminaire. 

DLC: DesignLights Consortium. If a lighting product has earned certification from DLC, it signals a high level of energy efficiency.  DLC listing can qualify products for utility rebates.

Efficacy: More specifically, luminous efficacy is calculated by measuring the lumen output and then dividing that number by watts. 

Finial: Decorative ornament on the top of a pole or rod.

Fitting: Commonly refers to the base or the fitter found on outdoor luminaires.

Flood Light: A lamp that provides a broad beam intended to light a general area.

Goniometer: A device used for measurement of the light emitted from an object at different angles.  Goniometers are used to measure the directional performance of a light source.  This measurement method is used as part of LM-79 testing and can produce the IES file used to model light source performance.

HID Light Sources: High-intensity discharge (HID) light sources emit light by vaporizing metallic salts inside the electric arc chamber. HID lamps. Some applications are moving from HID bulbs to LED and laser technology.

High Pressure Sodium (HPS) – High Pressure Sodium is a member of the HID light source family characterized by high efficacy, moderate life, low (yellow/orange) color temperature, and very low color rendering.  This light source is used in most outdoor applications including street, area, and parking lot lighting.

IES: Established in 1906, the Illuminating Engineering Society is the recognized technical and educational authority on illumination.  IES also authors most lighting standards used today.

IES RP-08-14: Published by the Illuminating Engineering Society (IES), the Recommended Practice of Roadway Lighting is an American National Standards Institute (ANSI)-approved standard.           

Incandescent Lighting: Light produced when a filament is heated to incandescence using electric current. Incandescent lighting is considered inefficient because most of the energy is wasted as heat rather than light.

Intelligent Street Lighting: Public street lighting that adapts to movement by pedestrians, cyclists and cars by dimming when no activity is detected, but brightens when movement is detected. Also referred to adaptive street lighting.

LCS Zonal Lumens: Luminaire Classification System (LCS) zonal lumens are computed by taking the average luminous intensity (candelas) at the center of the zone, then multiplying by the zonal constant to arrive at the zonal lumen value.

LED (Light emitting diode): Solid-state semiconductor device that converts electrical energy directly into light. LEDs are a solid state device and do not require heating of a filament to create light. Therefore, LEDs are quite energy-efficient and have very long lives.

LED Lighting Distribution Types

Type I: Great for walkways, paths and sidewalks. Used near the center of a walkway, Type I distribution provides adequate lighting for smaller pathways.

Type II: Used for wide walkways, ramps and entrance roadways. Used for smaller side streets or jogging paths.

Type III: Used for roadway lighting, general parking areas and anywhere where a larger area of lighting is needed.

Type IV: Produces a semicircular light used for the perimeters of parking areas and businesses.

Type V: Produces the same intensity from all angles. Used for luminaire mounting at the center of roadways, center islands of parkways and intersections. Also used for large, commercial parking lot lighting.

Type VS: Produces a square distribution with the same intensity at all angles for a more defined edge. Like Type V, used at the center of roadways, center islands of parkways and intersections. Used for large, commercial parking lot lighting.

Light Trespass: The poor control of outdoor lighting that crosses property lines and detracts from property values and quality of life.

LLF (Light Loss Factors): Factors that contribute to the loss of light include lamp burnouts, lamp lumen depreciation (LLD) and fixture (luminaire) dirt depreciation. These factors are called recoverable because preventive maintenance can reduce the extent of light loss. 

LM-79: An approved method for taking electrical and photometric measurements of solid-state lighting (SSL) products. It covers total flux (light output), electrical power, efficacy, chromaticity, and intensity distribution.

LM-80: Refers to a method for measuring the lumen depreciation of solid-state lighting (SSL) sources, such as LED packages, modules and arrays.

Lumen: An international unit (SI) of measurement used to describe the amount of light that a light source produces or emits. The higher the lumen, the brighter the light. Most people used the term “watts” to describe incandescent lighting.

Lumen Depreciation: The amount of light produced from a light source or from a luminaire when it’s brand new compared to the amount of light output at a specific time in the future.

Luminaire: Lighting fixture, complete with lamp, housing, power supply and optical components, often used for outdoor walkways to provide safety and pleasant illumination.

Luminous Flux: The quantity of the energy of the light emitted in all directions.

Metal Halide (MH): High-intensity light source produced by the radiation from mercury, plus halides of metals such as sodium, scandium, indium, and symposium.

NEMA: The National Electrical Manufacturers Association, the largest trade association of electrical equipment manufacturers in the U.S.

Non-directional Light Source:  A non-directional light source offers light that spreads in more than one direction and from all sides of the bulb (360 degrees). 

Optical Focal Point: Because of the curvature of the lens surfaces, different rays of an incident light beam are refracted through different angles, so that an entire beam of parallel rays can be caused to converge on, or to appear to diverge from, a single point, or focal point.

Peak Candela: The maximum intensity generated by a flashing device during its light pulse.

Photometric Curve: The photometric distribution curve is one of the lighting designer's most valuable tools. It is a cross-sectional ”map” of intensity (candelas) measured at many different vertical angles.

Photometric Sphere: Integrating spheres are used for a variety of optical, photometric or radiometric measurements. The sphere is used to determine bulk lumen output. A detector connected to an integrating sphere can accurately measure the sum of all the ambient light incident on a small circular aperture.  The sphere is used to determine bulk lumen output of the tested light source.

Polycarbonate: A type of plastic resin, polycarbonate and various polycarbonate blends are used by manufactures in a variety of LED applications due to their properties to meet demanding and variable requirements.

Angle of Refraction: The angle of refraction is the angle between a refracted ray and a line drawn normal to the interface between two media at the point of refraction.

Reflection: Reflection occurs when light bounces off of an object

Refraction: The bending of light as it passes from one transparent substance to another. Light refracts whenever it travels at an angle into a substance with a different refractive index.

Refractive Globes: A type of globe post top used in outdoor luminaires that spreads the light being distributed from the light sources to reduce glare and to maximize the light distribution performance of the system.

Remote Phosphor Technology: The use of phosphor in LED technology to deliver a more uniform and attractive light output.  This method produces a non-directional light source in most applications.

Smart City: Using information and communication technology, a smart city can improve its core systems. Data is collected through sensors and other devices and communicated via wireless networks. Data can sense what is currently happening and predict what will take place in the future, all contributing to improved safety and efficiency in lighting, traffic and energy use.

Solid State Lighting: Lighting technology which gives illumination when electricity is passed through a semiconductor as opposed to a gas or vacuum tube. A description of the devices that do not contain moving parts or parts that can break, rupture, shatter, leak or contaminate the environment.

Surge Suppression: Using an appliance or device to protect electrical devices from voltage spikes.

Tenon Mounting: Durable brackets engineered to provide versatile mounting options for optimal positioning of the light fixture to maximize lighting effectiveness.

Ultraviolet (UV): Invisible radiation that is shorter in wavelength and higher in frequency than visible violet light. UV rays are light waves that are similar to the light from the sun.

Uniformity: The evenness of light distribution.

Uniformity Ratio: Either the ratio of the illuminance in the brightest-lit spots to that in the dimmest areas, or of the average illuminance of the whole area to that of the dimmest sports.

Watt: The unit of electrical power as used by an electrical device during its operation. For example, a light source with a higher lumen per watt value is more efficient.