• Accent Lighting: lighting that is used to accent or highlight a particular object such as a work of art. To be effective accent lighting should be approximately four or five times the level of ambient light in the room, area, or space. House plants can be accented by aiming an uplight at the wall behind the plant, creating a dramatic silhouette of the plant against the wall.
  • Ambient Lighting: general lighting that usually lights up an entire room or space
  • Area Lighting: a landscape lighting term that refers to the lighting of large landscaped areas, usually with floodlights


  • Backlighting: lighting that illuminates an object from behind. The object to be illuminated is placed between the intended viewer and the light source. If the object is opaque, backlighting can cause the edges to “glow”. With translucent objects (such as stained glass), backlighting illuminates the object by passing light through it. Backlighting is commonly used to accent artwork, photos, advertisements, or signage.
  • Baffle: in lighting this is usually a grooved surface that deflects and controls the “flow” of light to soften it and minimize reflected glare. Baffle trims (sometimes called step baffle trims or stepped baffle trims), made of plastic or metal, are often integral parts of recessed downlights. They are often colored a flat black to absorb some of the light but can have other finishes such as flat white, brushed copper, or brushed nickel.
  • 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
  • Bayonet Base: a type of lamp base with pins that serve to lock the lamp into slots in the lamp socket of a luminaire. The bayonet base got its name from the method used by soldiers to mount bayonets on their rifles.
  • Beam Pattern: synonymous with “Beam Spread” below.
  • Beam Spread: a measure of the spread of light from a reflectorized light source, a special-shaped lamp with a reflective coating inside the glass bulb to direct the light forward. The beam spread may be very narrow (narrow spot, NSP), very wide (wide flood, WFL), or something in-between (narrow flood, NFL, for example). Examples of “reflectorized light sources” are MR11, MR16, PAR20, PAR30, PAR38, R40, ER30, and BR30 lamps.
  • Binning: the process of sorting LEDs into a variety of groups based on certain performance characteristics such as color temperature and lumen output. LED manufacturers use binning to manage the slight variations that arise in LEDs during the manufacturing process.
  • Bollard: an outdoor luminaire that is a short (usually about 2-4 feet in height) but very sturdy vertical post with the light source located at or near the top. Bollards are typically used to light walkways in commercial settings.


  • CRI: see color rendering index below
  • Cable Lighting System: a low voltage lighting system where the mechanism holding the light fixtures and conducting electricity to those fixtures is a pair of taut parallel metal cables.
  • Candlepower: an obsolete term that has been replaced by the candela, a unit of measurement that refers to the luminous intensity from a light source in a specific direction.
  • Chandelier: a chandelier is often the focal point of the dining room. As such it should be hung approximately 30 inches above the tabletop and should be at least 6 inches narrower than the table on each side.
  • Circuit: a pathway for the flow of electrons, including capacitors, resistors, and/or transistors, connected by wires through which electrical current flows. If there is only one path for the current, the circuit is called a “Series Circuit”. If there are multiple paths, the circuit is called a “Parallel Circuit”.
  • Color Rendering Index: a measure of a lamp’s ability to render colors accurately. The scale ranges from 1 (low pressure sodium) to 100 (the sun). A CRI of 85 is considered to be very good.
  • Color Temperature: a measure of the color appearance or hue of a light source which helps describe the apparent “warmth” (reddish) or “coolness” (bluish) of that light source. Generally, light sources below 3200K are considered “warm;” while those above 4000K are considered “cool” light sources. The color temperature of a lamp has nothing to do with how hot the lamp will get or how much heat is given off by the lamp. The letter, K, stands for Kelvin. This term is also referred to as the Correlated Color Temperature (CCT).
  • Cove Lighting: a lighting system comprised of light sources shielded by a ledge or recess, and distributing light over the ceiling and possibly the upper part of the wall. To learn more about this topic click Cove Lighting.
  • Cross Lighting: illumination of an object from two light sources opposite of each other
  • Cut-Off Angle: the position at which a viewer can no longer see the lamp in a fixture. The cut-off angle is measured from the base of the fixture to the point at which the lamp cannot be viewed. An angle of 45 degrees or less is considered “sharp”, meaning the lamp is quickly hidden as one moves away from a fixture. The cut-off angle is important when considering glare.


  • Dichroic Filter: a material that splits visible light into different wavelengths, allowing for precise control of the spectral band. Dichroic filters are coated on the glass and only allow certain wavelengths or colors to pass through. Dichroic filters are different from colored glass filters, which are used mostly for aesthetic purposes.
  • Diffuser: a transparent or translucent piece of glass, silicone, or plastic designed to control light by scattering or diffusing it in order to create softer light without much glare
  • Dimmable: any lighting product (light fixture or light bulb) that is designated as dimmable can be dimmed if the correct dimming device (such as a dimmer) is used to decrease or increase the amount of light that light fixture gives off.
  • Dimmer: a device in an electrical circuit used for varying the brightness of light bulbs in a lighting installation. Dimming controls are ideal for almost any type of room because they can change the amount of lighting to suit each mood or activity and they can help you look good. The use of dimmers with incandescent, xenon, and halogen light sources also increases the life of the lamps and decreases the use of electricity.
  • Driver, LED: an electronic device that acts as the power supply for LEDs. A driver regulates the current in order to maintain steady lumen output and prevent variation.


  • Electronic Ballast: a type of ballast with electronic components that increases the standard operating frequency of electricity from 60 cycles per second (the U.S. standard) to 20 kHz (20,000 cycles/second) or higher. This increase in operating frequency is important for greatly reducing the stroboscopic effect or flickering that is associated with fluorescent lamps. Electronic ballasts are an improvement over magnetic ballasts because they are quieter, lighter in weight, and more efficient in converting electrical energy into light energy while producing less heat.
  • Electronic Transformer: a type of transformer that includes an inverter, which allows for a substantially smaller size compared to a magnetic transformer with comparable wattage. The inverter causes the current to alternate at a frequency of 20-50 kHz. The higher the frequency, the smaller the transformer can be. The transformer “transforms” line voltage (usually 120-277 volts) into low voltage (usually 12 or 24 volts). Light fixture manufacturers often incorporate built-in electronic transformers in the fixture design because of their small size. Note: due to its very high frequency the voltage of electronic transformers can not be measured with standard voltmeters; instead, a “true RMS” voltmeter with sufficient range should be used.


  • fc: see foot-candle below
  • Filter: a piece of plastic or glass designed to transmit a certain narrow range of light (wavelength) while reflecting or absorbing the wavelengths of light that are not transmitted. See colored glass filter and dichroic filter.
  • Foot-Candle: the USA unit of measurement of lighting level (illumination or the amount of light reaching a subject) and sometimes spelled footcandle. The international unit of measurement of lighting level (Illumination) is the lux (lx). The relationship between the lux and the foot-candle is 1 fc = 10.76 lux.
  • Framing Projector: a light fixture with a lens and adjustable shutters, which allow the fixture to “project” a beam of accent lighting with a variable size in order to light a well-defined area such as a hanging frame of artwork
  • Fresnel Lens: (pronounced fray-nell) a special type of lens developed by the French physicist Augustin-Jean Fresnel for lighthouses. Compared to a conventional bulky lens, a Fresnel lens is much thinner, lighter in weight, larger, and flatter.
  • Frosted Lens: a white translucent lens that creates a soft (diffused) light


  • Glare: direct glare is caused by light coming directly to the eye from a light source. Indirect glare is light reflected from a surface in the direction of the eye. Both can harm vision and cause visual discomfort or disability.
  • Glare Bomb: a light fixture that obviously produces way too much glare
  • Gobo: a thin metal stencil with a cut-out pattern that produces an image projected onto a surface when a light beam is directed through it


  • Halogen Cycle: a regenerative cycle of tungsten and halogen atoms, which helps minimize the evaporation of tungsten atoms from the filament of a halogen lamp and the blackening of the glass envelope during the life of the lamp
  • Heat Sink: a component found in well-designed LED light fixtures that lowers the temperature of the LEDs by dissipating their heat. Heat sinks are also found in other electronic devices such as computers and lasers. They are often made of aluminum and have grooves, fins, and sometimes a fan.


  • IESNA: founded in 1906, the Illuminating Engineering Society of North America is a non-profit organization made up of lighting professional members that seeks to “improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public.” (see
  • Illuminance: the total luminous flux incident on a surface per unit area; a measure of how much of the incident light illuminates the surface; measured in lux or foot-candles
  • Illumination: see illuminance above
  • Incandescent Lamp: a lamp in which light is produced by the passage of an electric current through a tungsten filament which is heated to the point of incandescence
  • Indirect Glare: glare or excessive brightness reflected off another surface separate from the light source. Indirect glare, sometimes referred to as reflected glare, can be a reflection off a computer or television screen or even a magazine. Similar to direct glare, the light source should be considered when trying to prevent indirect glare.
  • Indirect Lighting: lighting that uses luminaires to direct most, if not all, of the light toward the ceiling or wall, providing soft, glare-free illumination without seeing the luminaire directly
  • Induction Lamp: a special type of fluorescent lamp that uses electricity to generate an electromagnetic field that causes the gaseous mercury atoms inside the glass envelope to emit ultraviolet radiation, which in turn, is converted to visible light by the phosphor coating on the inside of the glass envelope. Induction lamps have no electrodes and, therefore, have longer rated lamp lives than standard fluorescent lamps because the deterioration of the tungsten filaments in a standard fluorescent lamp is usually the main cause of a fluorescent lamp to stop working.
  • Inverse Square Law: a law that states that the illuminance (E) at a point on a plane perpendicular to the line joining the point and a source is inversely proportional to the square of the distance (d) between the source and the plane, E = I/d². This means, for example, that if the distance between a light source and the object being lit is doubled or tripled, that the object being lit receives 1/4 or 1/9 illumination (respectively) as it did originally.


  • Junction Box: a plastic or metal container inside which all standard electrical wiring connections must be made. A junction box protects and conceals these electrical connections.


  • Kelvin Temperature Scale: a temperature scale that references to absolute zero (0 degrees Kelvin), which, in theory, is the absence of all thermal energy. In lighting, the Kelvin temperature scale is useful when describing the color temperature of a light source.
  • Kilowatt Hour: the standard measurement of electrical energy equal to one kilowatt of electricity used over the period of one hour


  • LED: light emitting diode. A small electronic device that lights up when electricity is passed through it. LEDs are quite energy-efficient and have very long lives. They can be red, green, blue or white in color.
  • LM-79: a lighting measurement standard published by the IES in 2008 that provides specific practices for testing LED performance. LM-79 covers testing procedures for determining how the color and light emitted from LEDs is perceived by people and how power is measured for LEDs.
  • Layers of Light: layers of light in a given space are created by introducing task lighting (lighting by which people perform tasks), accent lighting (lighting used to highlight specific objects such as works of art), decorative lighting (lighting created by very attractive light sources such as chandeliers or mini pendants), and general lighting (lighting that fills the space). This technique (long favored by cameramen and cinematographers) can eliminate ugly shadows in the room and on your face.
  • Leadership in Energy and Environmental Design (LEED): an internationally recognized green building certification system, providing third-party verification that a building or community was designed and built using strategies aimed at improving performance across all the metrics that matter most: energy savings (including lighting energy efficiency), water efficiency, CO2 emissions reduction, improved indoor environmental quality, and stewardship of resources and sensitivity to their impacts. Developed by the U.S. Green Building Council (USGBC), LEED provides building owners and operators a concise framework for identifying and implementing practical and measurable green building design, construction, operations and maintenance solutions. LEED is flexible enough to apply to all building types – commercial as well as residential. It works throughout the building lifecycle – design and construction, operations and maintenance, tenant fitout, and significant retrofit. (see
  • Lens: in lighting (or optics), a lens is a transparent object that transmits and reshapes the direction of light. Made of clear glass or transparent plastic, a convex-type of lens can converge (focus) light while a concave-type of lens can diverge (spread) light. See spread lens, linear spread lens, and solite lens.
  • Light Trespass: outdoor light that is emitted into an unintended area; also sometimes known as spill light
  • Linear Spread Lens: a lens designed to produce the asymmetrical distribution of light in one direction as opposed to a spread lens, which produces symmetrical distribution in all directions. This lens diverges light in one axis and leaves the other axis unchanged. A linear spread lens is sometimes called an elongated lens, elliptical lens, or a Skytex lens.
  • Louver: a type of “screen” made of translucent or opaque material and geometrically designed to prevent lamps from being viewed directly within a given angle. Louvers are intended to minimize direct or indirect glare. To learn more about this topic click Louvers.
  • Low Voltage: although “low voltage” is generally defined as anything below 30 volts, low voltage lighting systems usually operate on 12 volts and sometimes 24 volts. It is important to note that a low voltage lighting system uses a transformer (electronic or magnetic) to transform the “incoming” voltage (usually 120 volts) to 12 or 24 volts because that is the voltage needed by the light bulbs in that lighting system. That is, the transformer of a low voltage lighting system uses the line voltage supplied in the home/building/facility but the light bulbs in that lighting system use the low voltage supplied by the transformer.
  • Lumen: an international unit (SI) of measurement used to describe the amount of light that a light source produces or emits.
  • Luminaire: a light fixture. This should not be confused with the term, luminary, which is a small open paper bag, sometimes with a design on it, containing a lit votive candle set in sand. Luminaries are often used to line outdoor walkways to provide a festive mood to a setting and a certain level of safety. In Mexico and the southwest USA, the terms, luminary and luminaries, become luminaria and luminarias. The term, luminary, can also refer to a person who has attained eminence in his/her field or is an inspiration to others.


  • Magnetic Ballast: a magnetic ballast uses magnetic inductance to regulate the voltage of a fluorescent lamp. Magnetic ballasts are noisier, heavier, and less efficient than electronic ballasts. Since magnetic ballasts do not alter the frequency of the electricity supplied to the lamp(s), a flicker or stroboscopic effect can be expected. Some people are more affected by this flickering of the light source and can develop headaches as a result
  • Magnetic Transformer: a magnetic transformer includes an iron core wrapped with two sets of wires. The transformer “transforms” line voltage (usually 120 volts) into low voltage (usually 12 or 24 volts). One set of the wires connects to the line voltage side, which is called the primary side of the transformer while the second set of wires connects to the low voltage side, which is called the secondary side. Magnetic transformers are often larger, heavier, noisier, and less efficient than electronic transformers.
  • Moonlighting: an outdoor lighting technique that simulates the filtering of natural light from the moon through an object such as a tree. A luminaire can be placed directly high on a tree to achieve this landscaping effect.
  • Motion Sensor: synonymous with motion detector
  • MR11: a halogen multi-faceted reflector lamp that measures 11/8 inches in diameter and which directs a sharp, well-defined beam of light. To learn more about this topic click MR11 Low Voltage Halogen Lamps.
  • MR16: a halogen multi-faceted reflector lamp that measures 16/8 inches in diameter and which directs a sharp, well-defined beam of light. To learn more about this topic click MR16 Low Voltage Halogen Lamps.


  • National Electric Code: a set of standards in the U.S. for electrical installation of wires and devices. The code is updated every three years and is published by the National Fire Protection Association.
  • Nominal Length: the nominal length of any object generally refers to its approximate length; the nominal length of an object should never be construed as its exact length. For example, a light bulb can be referred to as 1/2 inch long (the nominal length) but actually measure 0.53 in length.


  • Optics: a branch of physics that studies infrared, ultraviolet, and visible light and how they interact with matter. Optics also focuses on the construction of instruments/accessories/devices that detect and manipulate light, such as mirrors, lenses, filters, and louvers.
  • Outlet: a connection to power supply for an electric plug. Outlet design varies from country to country, but in the United States an outlet generally has 2 sockets that are designed for a plug with 2 or 3 prongs.


  • Path Light: a luminaire that illuminates a walkway. Path lights can be solar powered or wired and be programmed to turn on and off at certain brightness levels throughout the day.
  • Pendant: pendant lights can provide both task and general lighting. Equipped with shades or globes to avoid glare, they are suspended from the ceiling over dinette tables, game tables, kitchen counters, or other work areas. When used over end tables or night tables, they free up the space occupied by table lamps. In general, pendants should be hung about 30 inches above the tabletop and be about 12 inches narrower than the table on all sides. Not to be confused with a jewelry pendant, which is usually a small to medium-sized ornamental piece of jewelry attached to a necklace or bracelet.
  • Photocell: a device that detects levels of daylight and adjusts luminaires accordingly. It is often used with a street light to turn the light on at dusk and off at dawn.
  • Preheat: the process of heating a filament prior to striking the arc in a fluorescent lamp. Also known as switch start fluorescent lamps, preheat fluorescent lamps require a starter unlike rapid start fluorescent lamps.
  • Prismatic Lens: see spread lens below


  • Receptacle: the receiving end on an outlet that connects the power supply to a plug. A receptacle can have 2 to 4 holes, including a connection for grounding.
  •  a light fixture (usually circular but sometimes square) recessed into the ceiling that usually concentrates the light in a downward direction. Recessed downlights are usually composed of 3 key components: the housing, usually hidden above the ceiling, the trim, usually very visible, and the light source, which could be an incandescent, halogen, fluorescent, LED, or HID lamp. Synonyms: downlight, can, recessed can, high hat, pot light.
  • Retrofit: replacing old and/or inefficient lighting technology with new lamps, ballasts, luminaires, or equipment that improves the efficiency or safety of the lighting system
  • ROMEX® Wire: ROMEX® is a trademark of the Southwire Company which refers to their specific brand of “non-metalic sheathed electrical cable”. The term, “Romex”, is often incorrectly used as a generic term to refer to any “non-metalic sheathed electrical cable” just like the brand name Kleenex is often incorrectly used to refer to any tissue. Non-metalic sheathed electrical cable is the type of wiring that is used throughout your home to connect virtually everything that is electrical. The wiring behind all of the walls and ceilings in your home connecting the electrical panel to switches, junction boxes, receptacles, exhaust fans, and light fixtures is non-metallic sheathed electrical cable. (See the image at right.) The non-metallic sheathing is the outside rubber insulation around the entire cable. The cable inside the sheathing is usually made up of 3 wires: one wire with white insulation (neutral wire), one wire with black insulation (power wire), and one copper wire with no insulation (ground wire). The size of the non-metallic sheathed electrical cable that is used to connect lighting fixtures in a home is usually described as “14/2 with ground” (although this may vary with geographical location). The “14/2” refers to the two insulated wires that are 14 gauge in size and the “ground” refers to the uninsulated copper wire. On our web site we often refer to this non-metallic sheathed electrical cable as “household wire”.


  • Sconce: see wall sconce below. Not to be confused with a scone, which is a flat, round cake of wheat flour.
  • Shield: an opaque or semi-opaque element that serves to shield a light source from direct view at certain angles
  • Spread Lens: a lens used in the front of a luminaire designed to diverge light evenly in all directions. Also called a prismatic lens, it is designed with perpendicular ribs on one side of the lens.
  • Step Light: a luminaire that is specifically designed to illuminate stairs


  • Task Lighting: lighting that is specifically installed to light an area where a task is performed. Under cabinet lighting used in a kitchen is a good example of task lighting.
  • Terminal Block: a special electrical connector that uses insulating plastic on the outside and metal screws and clamps on the inside to create good splices for electrical wires, especially low voltage electrical wires. The screws inside the terminal block allow the installer to be certain that the electrical connections are very secure and tight, which, in turn, will minimize the possibility of arcing and overheating with low voltage connections.
  • Track Lighting: lighting that uses several luminaires attached to a track mounted on a ceiling or wall to illuminate a space. The track provides the current for the various luminaires, which allows them to be manipulated into different positions.
  • Transformer: an electrical device that transforms the line voltage of a facility (usually 120 volts for residential settings and 277 volts for commercial settings) into the voltage that a low voltage lighting system requires (12 volts or 24 volts).


  • Uplight: for a suspended light fixture (such as a single pendant light, an entire row of linear luminaires, or a semi-flush mount ceiling fixture) the amount of light lumens directed upward at or above 90 degrees. Also, refers to a single light fixture that is generally placed on the floor or recessed in the floor that projects light upward.
  • Uplighting: a lighting technique that directs light on an object from a light fixture aimed upward. Uplighting can create an intimate environment such as using a candle in a restaurant, or it can create harsh shadows resulting in an eerie effect like that created by uplighting a statue. This technique is also used in landscape lighting to accent trees or bushes.


  • Volt: the standard unit of electrical force or pressure between two points in an electric circuit. The greater the voltage, the faster electrons will travel through a circuit, meaning the greater the current running through a circuit. The standard household line voltage in the U.S. is approximately 120 volts. The unit is named after Alessandro Volta, an Italian physicist who made the first electric cell.
  • Voltage: the difference in electrical charge between two points in a circuit expressed in volts; the electric pressure that exists between two points and is capable of producing a flow of current when a closed circuit is connected between the two points; the rate at which energy is drawn from a source that produces a flow of electricity in a circuit; synonyms: electrical potential, electromotive force, EMF; Formulae: volts = amps x ohms, V = I x R, voltage = electrical current x electrical resistance.
  • Voltage Drop: the loss of voltage caused by the electrical resistance of the wire and the light fixtures in the circuit. It can become especially noticeable in low voltage circuits (where the operating voltage is 12 or 24 volts). Voltage drop may be minimized by using a thicker wire with a lower gauge, shortening the distance between the low voltage transformer and the light fixtures, and/or using a DC transformer.


  • Wall Sconce: a luminaire (light fixture) affixed to the wall and usually decorative in nature.
  • Watt: a standard unit of power (the rate at which work is performed). The power created by 1 ampere passing across a potential difference of 1 volt is equal to 1 watt (W=AV). One watt also equal 1 joule per second (W=J/s). The unit is named after James Watt because of his work on steam engine technology. In lighting, watts indicate the amount of power a light bulb consumes not the light output of that light bulb.


  • Xenon Lamp: a type of incandescent light bulb that contains xenon gas in the glass envelope. The primary reason that this is done is to lengthen the average rated life of the lamp. Depending on the lamp a typical xenon lamp may have a rated life of 10,000 hours whereas a similar halogen lamp may have a rated life of 2,000 hours. Unlike halogen lamps, xenon lamps may be touched with bare hands without affecting the rated life of the lamp. Xenon lamps also operate at cooler temperatures than comparable halogen lamps. Like halogen lamps, xenon lamps may be dimmed (using the right dimmer) whether the lamp is rated for low voltage (12 volts or 24 volts) or line voltage (120 volts).
