Light+Vocabulary

Light Vocabulary  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 area. 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. To learn more about this topic click Accent Lighting.

Alabaster: genuine alabaster is a very fine variety of crystalline gypsum (hydrous calcium sulfate) found in nature. It is translucent and its color is often white, pearly, or silky colorless but it, more often than not, has subtle reddish-brown bands of iron oxide running through it. It is often used for decorative objects such as light fixtures or figurines. Some light fixtures use faux alabaster, which is usually man-made glass. Both genuine and faux alabaster can be very attractive. The faux alabaster is generally much less expensive while real alabaster can lose some of its coloration over time. Real alabaster is expensive simply because quarrying it is costly and there is huge amount of waste when it is shaped, finished, and polished. Genuine alabaster is a minimum of 3/8in thick and weighs at least twice as much as imitations and has translucent and dark bands that are completely random in shape, size, and location. Since alabaster is a soft, porous stone it should be treated gently and never cleaned with detergents or abrasives. Instead, wipe alabaster with a slightly damp (not wet), soft cloth. Furthermore, heating alabaster for long periods of time with a high-wattage light bulb may drive out the residual water in the alabaster and turn it completely white.

A-Line Lamp: the type of incandescent light bulb that is generally used in most indoor residential lighting applications. From now until January 1, 2014 the most common standard screw-base incandescent household (A-line) light bulbs will be phased out in the U.S. Clear, frosted, soft white, and daylight light bulbs will be phased out but specialty colors and shapes will not be. The new standards for these light bulbs are technology neutral so any technology that can meet the new standards can be used - including fluorescent, halogen IR, high- efficiency incandescent, LEDs, and any technologies still to be developed.

Ambient Lighting: general lighting that usually lights up an entire space

Area Lighting: a landscape lighting term that refers to the lighting of large landscape areas, usually with floodlights

Ballast: an electrical device used with fluorescent and HID (high intensity discharge) lamps (light bulbs) to supply sufficient voltage to start and operate the lamp but then to limit the current during operation

Base: the portion of the lamp (light bulb) that provides a means of physically connecting the lamp to a socket or lampholder.

Beam Spread: a measure of the spread of light from a reflectorized light source, a special-shaped lamp with a reflective coating inside the bulb to direct the light forward. The beam spread may be very narrow (narrow spot), very wide (wide flood), or something in-between (narrow flood, for example). Examples of "reflectorized light sources" are MR11, MR16, PAR20, PAR30, PAR38, R40, ER30, and BR30 lamps.

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 cables.

Candela: the unit of measurement of luminous intensity (cd) of a light source in a given direction.

Cathode: an electrode that emits electrons. A fluorescent lamp cathode emits or discharges electrons to the cathode at the other end of the lamp (light bulb).

CCFL: a cold cathode fluorescent lamp is a type of fluorescent lamp in which the cathode (an electron-emitting electrode inside the lamp) is not independently heated although the cathode can become quite hot once the CCFL has been operating. Screw-in CCFLs come in several different wattages (present range is 3W to 18W) and several different shapes (spiral, globe, reflector, A-line, torpedo) and are generally characterized by a very thin glass envelope (2mm-4mm in diameter), a longer rated life than compact fluorescent lamps (often 25,000 hours), and a very good range of dimmability (down to 30%).

CFL: see compact fluorescent lamp

Chandelier: a chandelier is often the focal point of the dining room. As such it should be hung about 30 inches above the tabletop and should be at least 6 inches narrower than the table on each side.

Colored Glass Filter: this term means that the glass is formed with the color in the glass as opposed to the color being coated on the surface. This is accomplished by mixing various metal oxides in the glass composition. These colored glass filters are primarily used for aesthetic purposes in lighting. They are not designed for the precise control of the spectral bands, as are optical color filters (dichroic filters). To learn more about this topic click Colored Glass Filters

Color Rendering Index (CRI): 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 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.

Compact Fluorescent Lamp (CFL): a generic name used for a whole family of small, single-ended fluorescent lamps with a folded, bridged, or spiral glass tube design and with high color rendering (CRI > 80) and a long life (> 8,000 hours). To learn more about this topic click CFL Information.

Cornice Lighting: a lighting system comprised of light sources shielded by a panel parallel to the wall and attached to the ceiling and distributing light over the wall.

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.

CSA: the Canadian Standards Association, like UL and ETL in the USA, is a not-for-profit membership-based association serving business, industry, government, and consumers in Canada and the global marketplace. It is an organization that works in Canada and around the world to develop safety standards that address real needs, such as enhancing public safety and health, advancing the quality of life, and helping to preserve the environment.

Dimmer: a device in an electrical circuit used for varying the brightness of lamps 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 electrical energy. (Please see the Table below.) To learn more about this topic click Dimmer FAQ and valuable Information about Dimmers.

Direct Current: electric current without alterations. DC current is usually supplied by a battery, a transformer, or photovoltaic cells.

Downlight: a small light fixture recessed into the ceiling that usually concentrates the light in a downward direction. Synonyms: recessed downlight, “can”, recessed can. To learn more about this topic click Recessed Lighting.

Efficacy: a measure used to compare light output to energy consumption. Efficacy is measured in lumens per watt. A 100-watt light source producing 1750 lumens of light has an efficacy (efficiency) of 17.5 lumens per watt (L/W).

Emergency Lighting: lighting used when the normal lighting fails. To learn more about this topic click Troubleshooting Tips for Emergency Lighting.

ETL: ETL, like Underwriters Laboratories, Inc. (UL), is an independent, not-for-profit product safety testing and certification organization. Throughout its long history its name has changed several times. Thomas A. Edison established the Lamp Testing Bureau in 1896. In 1904 Edison renamed his Lamp Testing Bureau the Electrical Testing Laboratories (ETL). In 1977 ETL officially changed its corporate name to ETL Testing Laboratories and in 1996 ETL was renamed the Intertek Testing Services, Ltd.

Filament: a tungsten wire that incandesces or lights up when an electric current runs through it.

Flexible Track Lighting System: a low voltage or a line voltage lighting system where the suspended track (sometimes called a monorail) holding the light fixtures in place and also conducting electricity to those fixtures can be bent into creative shapes.

Floodlight: a broad-beamed high intensity artificial light source

Fluorescent Lamp: a low-pressure mercury electric-discharge lamp (light bulb) in which a phosphor coating on the inside of the glass tubing transforms some of the ultraviolet energy created inside the lamp into visible light. To learn more about this topic click Fluorescent Lamps.

Foot-Candle: the amount of light reaching a subject. The commonly United States unit of measurement of lighting level (illumination) is the foot-candle (fc). 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.

Four-Way Switch: a wall switch that allows three switches like this to control one lighting system. Whenever you flip one of the four-way switches in a given circuit, the light changes its state; that is, if the light was on, it then turns off and if the light was off, it then turns on.

General Lighting: substantially uniform lighting of a space without providing for special local lighting requirements like task lighting or accent lighting.

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.

Halogen Lamp: a type of incandescent lamp (light bulb) that contains halogen gases (such as iodine, chlorine, bromine, and fluorine), which slow the evaporation of the tungsten filament. Also, sometimes called a tungsten halogen lamp or a quartz lamp. The glass envelope that surrounds the filament of a halogen lamp should not be touched with bare hands. The natural oil from human hands will only help to shorten the life of halogen lamps. If you should accidentally touch the glass bulb, you should thoroughly remove your fingerprints with methylated spirit (denatured alcohol). To learn more about this topic click Halogen Lamps.

Hardwired: technically means that the light fixture is permanently connected to an electrical source. The light fixture is not hardwired if it gets power via a cord & plug.

HID Lamp: high intensity discharge (HID) lamps (light bulbs) have a longer life and provide more light (lumens) per watt than most other light sources. Available in mercury vapor, metal halide, high pressure sodium, and low pressure sodium types.

High Pressure Sodium Lamp: a high intensity discharge (HID) light bulb that illuminates by radiation from sodium vapor producing a yellowish color rendering, when supplied with electricity from a ballast. This type of light bulb is often used in street lights and is available with clear or phosphor coated glass envelopes. Often abbreviated as HPS lamps they are considered to be quite efficient producing about 100 lumens/watt.

Household Wire: "Household wire" is the name that we use on this web site to refer to the "non-metallic sheathed electrical cable" that is used behind all of the walls and ceilings in your home to connect the electrical panel to switches, junction boxes, receptacles, exhaust fans, and light fixtures. The term, "Romex" is often used to refer to this type of wire but, strictly speaking, this is not correct. 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. 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.

Incandescence: light emission by a heated filament.

Incandescent Lamp: lamp (light bulb) in which light is produced by means of an element heated to the point of incandescence by the passage of an electric current.

Infrared Radiation: a type of invisible radiation for which the wavelengths are longer and frequency lower than those for visible radiation.

Instant Start: refers to fluorescent lamps that start instantly without pre-heating the cathodes and without the need for starters.

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.

IP Ratings: Ingress Protection (IP) ratings, developed by the European Committee for Electro Technical Standardization, are used to specify the environmental protection an enclosure provides to the electrical equipment inside that enclosure. An IP rating normally has two numbers associated with it: (1) protection from solid objects or materials and (2) protection from liquids (water). Example: With an IP rating of IP54, the "5" describes the level of protection from solid objects (protected against dust limited ingress with no harmful deposit) and the "4" describes the level of protection from liquids (protection against water sprayed from all directions with limited ingress permitted). An “X” can used for one of the digits if there is only one class of protection, e.g., IPX1 describes only the level of protection from liquids (protection against vertically falling drops of water, e.g., condensation).

Lamp: a light source such as an incandescent, halogen, or fluorescent lamp. A lamp is sometimes called a “light bulb” or simply a “bulb”. To learn more about this topic click Halogen Lamps.

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), decorative lighting (lighting created by very attractive light sources), 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.

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. To learn more about this topic click LED FAQ.

LEED: LEED (Leadership in Energy and Environmental Design) is 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.

Light Fixture: a complete lighting unit consisting of a lamp (light bulb) or lamps, a housing, and a connection to the source of electrical power. Examples of light fixtures are wall sconces, recessed downlights, mini pendants, table lamps, floor lamps, track heads, step lights, picture lights, desk lamps, and recessed troffers.

Light Source: a source of light (visible portion of the electromagnetic spectrum). There are many light sources such as candle light, gaslight, oil lamps, kerosene lamps, stars including our own sun, lightning, aurorae, fluorescent light bulbs including CFLs, incandescent light bulbs (including halogen and xenon), high pressure sodium light bulbs, metal halide light bulbs, low pressure sodium light bulbs, neon lamps, cold cathode fluorescent lamps, chairladies sources like lightning bugs and "light sticks", LEDs of various colors, and organic light emitting diodes.

Light Trespass: also known as "spill light". Light that is emitted into an unintended area.

Line Voltage: in the USA and Canada the standard residential line voltage is 120 volts. (Before 1960 residential line voltage was 110 volts, a standard established by Thomas Edison.) Residential electric clothes dryers, however, operate on 240 volts. In the USA commercial line voltage may be 120 volts or 277 volts. In the USA both the residential and the commercial line voltage may vary by ±10%. Thus, residential line voltage may be as low as 108 volts and as high as 132 volts. The standard line voltage is different in other countries. For example, in Japan it is 100V; in Mexico it is 127V; in China and Russia it is 220V; in France, Germany, Italy, England, New Zealand, and India it is 230V, and in Australia it is 240V.

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's 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. To learn more about this topic click Low Voltage Lighting: 15 Things You Need to Know.

Lumen: a unit of measure used to describe the amount of light that a light source produces or emits. For reference please consult the table below.

Mercury Vapor Lamp: a high intensity discharge (HID) light bulb that produces light by radiation from mercury vapor, when supplied with electricity from a ballast. Mercury vapor light bulbs usually have very long lifetimes and are available with clear or phosphor-coated glass envelopes. The Energy Policy Act (EPAct) of 2005 included a provision that no new ballasts for mercury vapor light bulbs may be imported or manufactured for "general illumination use" in the U.S., effective January 1, 2008.

Metal Halide Lamp: a high intensity discharge (HID) light bulb that produces light by radiation from certain metallic vapors (such as scandium, sodium, thallium, and indium), when supplied with electricity from a ballast. Known for producing accurate color rendition with a range of 65-90 and are, therefore, often used to light large gymnasiums and athletic stadiums; can be produced with almost any color temperature from 2700K to 20,000K; relatively unaffected by ambient temperatures and can, therefore, be used indoors and outdoors; has high efficacy of between 65-115 lumens/watt, which makes it approximately 5 times as efficient as a typical incandescent light bulb; has a long life of 15,000-20,000+ hours

M.O.L.: maximum overall length of a lamp (light bulb) — from tip to tip.

MR11 Lamp: a halogen metalized reflector lamp (light bulb) 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 Lamp: a halogen mirrored 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.

Neodymium: a rare earth element discovered in 1885 with an atomic number of 60 in the Periodic Table of Elements. It is a fairly common silvery metal that is used to make slightly purple glass envelopes for incandescent light bulbs, eye protection goggles for welding and glass blowing, laser rods, filters to color correct light for art displays, and lenses used by astronomers to calibrate spectrometers, optical instruments used for analyzing light. Using this substance in an incandescent light bulb makes it less "yellowish" and more like natural outdoor light; however, the use of these "daylight" light bulbs can "expose" the flaws or inconsistencies in things.

Opaque: a term that describes a material that does not transmit any visible light. A wooden door, aluminum foil, and bricks are all examples of opaque materials.

Open Circuit: an electric circuit in which the flow of electricity is interrupted due to an open circuit breaker or a blown fuse or a burned out electrical component or an on/off switch that has been turned off

Parallel Circuit: an electric circuit that has two or more paths for the electricity to flow. The light fixtures in a parallel electric circuit are "parallel" to each other like rungs on a ladder. If one of the light fixtures in a parallel circuit burns out, the other light fixtures will remain on because the electric current would still be able to flow through them.

PAR Lamp: PAR is an acronym for a parabolic aluminized reflector. A PAR lamp, which may use an incandescent filament, a halogen filament tube, or an HID arc tube is a precision pressed-glass reflector lamp that reflects light coming from the filament much like a parabola. PAR lamps (such as PAR20, PAR30, and PAR38) rely on both the internal reflector and prisms in the lens for the control of the light beam. To learn more about this topic click PAR Halogen Lamps.

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.

PL Lamp: a nickname for a twin tube fluorescent lamp coined by Philips Lighting, a large manufacturer of lamps.

Portable Lamp: a table lamp, floor lamp, pharmacy lamp, or reading lamp which can be easily moved and plugged into a receptacle; officially called a portable luminaire (light fixture).

Quick Connector: an electrical connector made up of plastic on the outside and metal on the inside that allows electrical connections to be made by simply pushing wires into the holes of the quick connector. A quick connector is sometimes called a push-in wire connector or a poke-in wire connector, which is different from screw-on wire connectors (wire nuts).

Radio Frequency Interference (RFI): interference to the radio frequency band cased by other high frequency equipment or devices in the immediate area. Fluorescent lighting systems and AC electronic transformers can generate RFI.

Rapid Start: a fluorescent system that does not require starters and takes 1 to 2 seconds to emit light. The ballast preheats the electrodes within the fluorescent lamp and initiates the arc without a starter or the application of high voltage.

Rated Life: The rated life of a lamp signifies the time at which 50% of a large quantity of these lamps will have burned out. That means that 50% of these lamps will burn out before the rated life and 50% will burn out after the rated life. The rated life does not mean that every one of the lamps will last at least that long. Also, please note that the Actual Life of a Lamp = the Rated Life of that Lamp x (Rated Voltage/Operating Voltage) raised to the 12th power. For example, the Actual Life of a certain lamp that is designed to be used with 130 volts is equal to the Rated Life of that Lamp (let's say 1000 hours) x (130 volts/120 volts) raised to the 12th power. The Actual Life of this lamp is, therefore, equal to (1000 hours) x (1.083) raised to the twelfth power = (1000 hours) x (2.61) = 2610 hours. This means that an incandescent lamp that has a rated life of 1000 hours and is designed to be used with 130 volts but is used with 120 volts instead will have an "Actual Life" of 2610 hours.

Recessed Downlight: a small light fixture recessed into the ceiling that usually concentrates the light in a downward direction. Synonyms: downlight, can, recessed can. To learn more about this topic click Recessed Lighting FAQ.

R Lamp: an incandescent, cone-shaped, light bulb that has a reflecting surface on the inside rear of the glass envelope. Variations of this lamp type are the bulged reflector lamp (BR), the ellipsoidal reflector lamp (ER), and the small reflector lamp (R). Since mid-2008 BR and ER light bulbs greater than 65 watts, used mostly in commercial retail applications, have been outlawed from being manufactured. These banned light bulbs can be replaced with more-efficient halogen PAR light bulbs. Also since mid-2008, R20 light bulbs can be no more than 45 watts. If a higher wattage reflector-type lamp is needed, a more efficient halogen PAR20 light bulb can be used. The 65-watt BR30 light bulb, commonly used in homes and restaurants, may still be manufactured and sold.

RLM: originally this term stood for reflector light microscopy; however, today it means reflector luminaire manufacturer. It is a light fixture that is designed to project light downward, a characteristic that the International Dark Sky Association strongly approves of. These retro-style fixtures were originally commonly used in industrial areas; however, today the RLM-style light fixtures are used in non-industrial installations such as multi-family sites, restaurants, retail stores, and hotels in both indoor and outdoor applications.

Sconce: an ornamental light fixture attached to a wall. Synonym: wall sconce. Not to be confused with a scone, which is a flat, round cake of wheat flour.

Security Lighting: lighting that is used to protect people and property from criminal activity. When properly employed, security lighting eliminates shadows near buildings and provides even landscape illumination for pedestrian safety.

Series Circuit: an electric circuit that has only one path for the electricity to flow. The light fixtures in a series electric circuit are connected to each other in a single line like links in a chain. If one of the light fixtures in a series circuit burns out, the other light fixtures would not remain on because the electric circuit would become an open circuit and the electric current would not be able to get to the other light fixtures. Another term for a series circuit is a "daisy chain".

Shield: an opaque or semi-opaque element that serves to shield a light source from direct view at certain angles

Silhouetting: a landscape lighting technique used to create a distinct outline or silhouette of plants or other objects

Single-Pole Switch: a standard on/off wall switch that is the only switch that controls one or more light fixtures in a single circuit

Sky Glow: also, known as "light pollution". The "haze" or "glow" that surrounds highly populated areas and reduces the ability to view the nighttime sky. Specifically, light that enters the sky from an outdoor lighting system by indirect light reflected from atmospheric particles such as fog, dust, or smog.

Specular Reflection: reflection, without diffusion, in accordance with the laws of geometrical optics, as in a mirror.

Starter: an electrical device used in conjunction with a ballast for the purpose of starting an electric discharge lamp like a fluorescent lamp or an HID lamp.

Task Lighting: lighting that is specifically installed to light an area where a task is performed.

Three-Way Switch: a wall switch that allows two switches like this to control one lighting system. Whenever you flip one of the three-way switches in a given circuit, the light changes its state; that is, if the light was on, it then turns off and if the light was off, it then turns on.

Transformer: an electrical device that transforms the line voltage of a facility (usually 120 volts for residential settings or 277 volts for commercial settings) into the voltage that a low voltage lighting system requires (12 volts or 24 volts). To learn more about this topic click Low Voltage Transformer FAQ, Electronic Transformer FAQ, and Advantages of Toroidal Magnetic Low Voltage Transformers.

Translucent: a term that describes a material that transmits some visible light usually with some distortion. A frosted piece of glass, a stained glass window, and paper are all examples of translucent materials.

Transparent: a term that describes a material that transmits most, if not all, of the visible light incident upon it with very little, if any, distortion. A typical glass window pane, the air, and clear plexiglas are all examples of transparent materials

Troffer: a large recessed ceiling luminaire (light fixture) that often uses 1-4 T5, T8, or T12 fluorescent lamps (light bulbs), usually measures 24in by 48in or 24in by 24in, and is usually installed with the opening flush with the ceiling

UL: Underwriters Laboratories, Inc., like Electrical Testing Laboratory (ETL), is an independent, not-for-profit product safety testing and certification organization. The UL symbol with the letter, "C", and the letters, "US", indicate that the lighting product is UL listed in both Canada and the United States.

Uplight: the percentage of lamp lumens directed from a luminaire (light fixture) at or above 90 degrees.

UV Radiation: is not visible to the naked human eye and is generally considered to be that electromagnetic radiation which has a wavelength of less than 400 nanometers (nm) and greater than 100 nm. To learn more about this topic click UV Filters.

Vapor-Tight Luminaire: a luminaire (light fixture) constructed so that a specified vapor or gas cannot enter its enclosure — usually water vapor

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 Grazing: a lighting term that refers to the dramatic highlight and shadow effects on surfaces such as draperies, stone, and brick. Grazing creates easily distinguishable high and low levels of light on the surface.

Wall Sconce: a luminaire (light fixture) affixed to the wall and usually decorative in nature. To learn more about this topic click Valuable Information About Wall Sconces.

Wall Washing: a lighting technique that produces a relatively smooth, even level of illumination on the wall and reduces the texture of the surface.

Wattage: the amount of electrical power consumed by a lamp or light fixture measured in “watts”. One watt is equal to the power dissipated by 1 ampere of electrical current flowing across a resistance of 1 ohm or the power produced by 1 ampere of electrical current under an electromotive force of 1 volt. One horsepower is equal to 746 watts.

Xenon Lamp: a type of incandescent lamp 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 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). To learn more about this topic click Xenon Light Bulbs FAQ and Advantages of Xenon Light Bulbs.