Glossary of lighting terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Ampere (Amp)

The unit for measuring rate of flow of electrical current.

Array

LEDs can be single-chip based, but an LED light source that populates many independent chips built as one package is called an array.

B

Ballast

A device that maintains the current through a lamp at the desired constant value, sometimes also providing the necessary starting voltage and current.

C

Candela (cd)

The luminous intensity as defined by the international metric standard (SI). The term, retained from the early days of lighting, defines a standard candle of a fixed size and composition as a basis for evaluating the intensity of other light sources.

Chip

Small square of semi-conducting material, also known as a die. It is the active light-emitting component of an LED.

Chroma

Describes the degree of departure from gray of the same lightness and increasing color (e.g. red, redder, pure red).

Cold Cathode Fluorescent Lighting (CCFL)

CCFL is often used as a backlight for LCD displays and usually contains mercury or other regulated heavy metals.

Color Definition

The color of uniformly illuminated objects described using three terms: Hue, Lightness and Chroma.

Color Rendition Index (CRI)

A measure of a lamp’s ability to render colors accurately. The higher the CRI (based upon a 0-100 scale), the more natural the colors appear. Natural outdoor light has a CRI of 100. Common lighting sources have a large range of CRI.

Color Spectrum

All wavelengths perceived by human sight, usually measured in nanometers (nm).

Color Temperature

A measure of the color appearance of a light source, helping to describe the apparent ‘warmth’ (red) or ‘coolness’ (blue) of that light source.

Compact Fluorescent Lamp (CFL)

Also known as a compact fluorescent light, a CFL is a type of fluorescent lamp designed to replace an incandescent lamp and can fit into an existing fixture formally used for incandescents. CFLs give off the same amount of visible light as incandescents, but generally use less power and have a longer life. Like all fluorescents, CFLs contain mercury, which complicates their disposal.

Conformal Coating

Conformal coating material is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes that if uncoated (non-protected) could result in a complete failure of the electronic system.

Cool White

A description of light with a correlated color temperature between 5000K and 7500K, usually perceived as slightly blue.

Correlated Color Temperature (CCT)

CCT is the measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow / gold / orange or more blue, in terms of the range of available shades of "white." CCT is given in kelvins (unit of absolute temperature).

D

Diffuser

An optical element used to mix light rays to improve uniformity.

Diode

A device, such as a two-element electron tube or a semiconductor, through which current can pass freely in only one direction.

E

Efficacy

A measure to compare light output to energy consumption. Efficacy is expressed in lumens per watt (lm/W).

F

Flux

The sum of all the lumens (lm) emitted by a source.

Foot-candle (FC)

A unit of illuminance or light falling onto a surface. It stands for the light level on a surface, one foot from a standard candle. One foot-candle is equal to one lumen per square foot.

G

Graphical User Interface (GUI)

A type of user interface that allows people to interact with electronic devices such as computers. A GUI utilizes graphical icons and visual indicators as opposed to text-based interfaces.

H

Heat Sink

An environment or object that absorbs and dissipates heat from another object using thermal contact (either direct or radiant).

High Intensity Discharge (HID) Lamp

A type of electrical lamp that consists of a sealed arc tube inside a glass envelope, or outer jacket. The inner arc tube is filled with elements that emit light when ionized by electrical current. HID lamps are available in mercury vapor, metal halide, high-pressure sodium and low-pressure sodium types.

High Pressure Sodium (HPS) Lamp

A type of HID lamp that uses sodium under high pressure as the primary light-producing element.

Hue

Describes the situation when the appearance of different colors is similar (e.g. matching blues and pinks).

I

Illuminance

The density of light incident on a surface or light level on a surface. Measured in foot-candles or lux.

Incandescent Lamp

A lamp (light bulb) in which light is produced by means of an element heated to the point of incandescence by the passage of electric current. Incandescent lamps are among the least expensive to buy, but due to their relative inefficiency and short life spans, they are more expensive to operate.

Infrared Radiation (IR)

A type of invisible radiation for which the wavelengths are longer and frequency lower than those for visible radiation.

J

Junction Temperature

The temperature at the point where an individual diode connects to the base. Most significantly, junction temperature affects the lifetime of the LED – the lower the temperature, the longer the life.

K

Kelvin Temperature

Term and symbol (K) used to indicate the comparative color appearance of a light source when compared to a theoretical blackbody. Yellowish incandescent lamps are 3000K. Fluorescent light sources range from 3000K to 7500K and higher.

Kilowatt (KW)

The measure of electrical power equal to 1000 watts.

Kilowatt-Hour (kWh)

A unit of electricity supply or consumption of one thousand watts acting over a period of one hour. The typical billing unit used by electrical utilities for use.

L

LED

A light-emitting diode (LED) is a solid-state semiconductor device that converts electrical energy directly into light. The semiconductor is comprised of two regions. The p-region contains positive electrical charges while the n-region contains negative electrical charges. When voltage is applied and current begins to flow, the electrons move across the n region into the p region. The process of an electron moving through the p-n junction releases energy. The dispersion of this energy produces photons with visible wavelengths.

Leadership in Energy and Environmental Design (LEED)

A list of standards and certification scheme for environmentally sustainable construction developed by the US Green Building Council (USGBC).

Lightness

Describes a range of grayness between black and white.

Low-Pressure Sodium Lamp

A type of HID lamp that produces light from sodium gas contained in a bulb operating at a partial pressure (.013 to 1.3 Pascal). Low-pressure sodium lamps are more energy efficient than HID lamps but have a very low color rendition index.

Lumen (lm)

The international unit of luminous flux or quantity of light and equals the amount of light that is spread over a square foot of surface by one candle power when all parts of the surface are exactly one foot from the light source.

Lumen Maintenance

The remaining flux percentage at the rated life of a light source. A measure of how well a lamp maintains its light output over time.

Luminaire

A lighting fixture complete with installed lamps and all other accessories.

Lux (lx)

The international unit of illuminance, or luminous flux incident on a unit area, frequently defined as one lumen per square meter.

M

Metal Halide Lamp

A member of the (HID) family of lamps, metal halides produce high light output for their size. Like most HID lamps, metal halide lamps operate under high pressure and temperature, and require special fixtures to operate safely. They are also considered a "point" light source; so reflective luminaries are often required to concentrate the light for purposes of the lighting application.

O

Optic

A device that changes the direction of a ray of visible light, usually by reflection, such as a mirror, or refraction by a lens.

Organic Light-Emitting Diodes (OLEDs)

OLEDs are based on organic (carbon based) materials. In contrast to LEDs, which are small point sources, OLEDs are made in sheets, which provide a diffuse area light source. OLED technology is developing rapidly and is increasingly used in display applications such as cell phones and PDA screens. However, OLEDs are still some years away from becoming a practical general illumination source.

P

Photometry

The science of measurement of visible light in terms of its perceived brightness to human vision.

P-N Junction

The area on an LED chip where the positively and negatively charged regions meet. When voltage is applied and current begins to flow, the electrons move across the n region into the p region. The process of an electron moving through the p-n junction releases energy. The dispersion of this energy produces photons with visible wavelengths. In short, the area on a chip where light is produced.

R

RGB

RGB stands for red, green, and blue, the three primary colors of light. When the primaries are mixed, the resulting light appears white to the human eye. Mixing the light from red, green, and blue LEDs is one way to produce white light. The other approach is known as phosphor conversion.

S

Solid-State Lighting

A type of lighting device, such as an LED, that does not contain moving parts or parts that can break, rupture, shatter, leak or contaminate the environment.

T

Thermal Management

Controlling the operating temperature of the product, such as an LED, through design. Traditional examples include heat sinks and improved airflow.

Thermal Resistance (C/W)

A measure of the heat transfer capacity of the LED with lower resistance preferred.

Thermocouple

A device consisting of two dissimilar conductors with their ends connected together. When the two junctions are at different temperatures, a small voltage is generated.

U

Underwriters Laboratories (UL)

A privately owned, independent U.S. organization which tests and lists electrical (and other) equipment for electrical and fire safety according to recognized UL standards.

Ultraviolet Light (UV)

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light. UV light is found in sunlight and is emitted by electric arcs and specialized lights such as black lights. As an ionizing radiation it can cause chemical reactions, and causes many substances to glow or fluoresce.

V

Volt

A measure of electrical pressure between two points or the electrical potential difference between oppositely charged conductors.

W

Warm White

A description of light with a correlated color temperature between 3000K and 3500K, usually perceived a slightly yellow..

Watt

The unit of electrical power as used by an electrical device during its operation. Many lamps come with rating in watts to indicate their power consumption. A light source with a higher lumen per watt value is more efficient.