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Lighting Glossary

Sometimes lighting terms can be a little confusing. In order to help things become clearer, we have pulled together all the key lighting terms that we find are used on a day to day basis and have provided useful explanations as to what they mean and even how they may be used.

We hope these are of use to you and if there is a particular lighting term that we have missed please get in touch. We will be more than happy to help with an explanation and will then add it to this 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 |



Electric current that reverses direction at regular intervals. The rate at which alternating current changes direction is called its frequency, stated in hertz (cycles per second). Standard household current in the UK has a 50 Hz frequency.


Directional lighting used to highlight a specific object or draw attention to a display item.


The overall illumination in a room caused by any combination of daylight and general lighting, but excluding directional task or accent lighting.


A measure of electrical current. The amount of amps used by an electric light bulb can be determined by dividing watts by voltage, e.g. a UK mains-powered 50W bulb has a current of approximately 0.21 Amps (50W divided by 240V). Similarly, multiplying amps by volts gives you the wattage of the bulb, and wattage divided by amps provides you with the voltage.


In lighting terms, simply the use(s) to which a particular lamp or luminaire might be put.


The average rated life of a lamp is a statistically determined estimate of lifespan, ending when the lamp fails or loses a specific amount of its original output. In an incandescent lamp, the rated life is usually designated ‘Bp’ and is measured in hours, where ‘p’ is a percentage. For example, a B50 rated life of 1,000 hours indicates a 50% failure rate amongst tested products after that duration.

In fluorescent or LED technologies, the average rated life of a lamp refers to lumen maintenance. It is usually designated ‘Lp’ and is measured in hours, where ‘p’ is the percentage of original lumen output. For example, an L70 lifespan of 30,000 hours indicates a lumen depreciation of 30% after that duration (or 70% lumen maintenance).

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Usually a simple plate installed directly in front of a lamp to restrain its direct flow of light and reduce glare; can also refer to a specific ribbed design in the trim of a downlight, which is intended to slightly impede and diffuse light flow - also for less glare.


A device that delivers the required starting voltage and appropriate current to a fluorescent or high intensity discharge (HID) lamp.


Normally a 2-pinned lamp connector that twists into the corresponding L-shaped slots of a lighting fixture, with the bulb subsequently held in place by a retaining spring.


The angle of light produced by a lamp, measured at opposing points where the light possesses 50% of its maximum centre-line luminous intensity.


Refers to a 2-pin connector common to all G-type or GU-type lamps, with the suffixing number in any model referring to the pin spread. For example, a G5.3 lamp has a 5.3mm distance between its two pins, measured from each pin’s centre. These are a common type of connector, conforming to IEC standards (International Electrotechnical Commission).


Refers to the subjective perception of light by human vision, which, when measured, is recorded as luminance in candela per square metre (cd/m²).


Used interchangeably with the term ‘lamp’, except a bulb more correctly refers to light sources with the classic bulbous, pear-shaped housing of a traditional GLS incandescent lamp (or modern equivalent).


Many types of bulb, or lamp, incorporate size into their name, which is usually measured in eighths of an inch. For instance, an MR16 has a diameter of sixteen eighths of an inch, or two inches. Bi-pin lamps are commonly known by their base type, which is measured metrically. For instance, a G4 capsule has a 4mm measurement between the two central points of its pins.


A simple geometrically shaped luminaire usually designed and constructed with very little aesthetic consideration and for purely functional purposes.

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The SI base unit of luminous intensity, candela is the measurement of a light at source, so unlike a measurement in lumens it ignores multidirectional output. A measurement of one candela is comparable to the output of a regular candle.


More recently made with a solid block of wax and a centrally embedded wick, the candle was the primary source of light in the UK’s hotels and homes throughout the Victorian age and beyond. Electric lighting didn’t become commonplace until it was truly commercialised in the 1920s. The warm glow of a traditional candle is still a difficult light source to beat today for the restful or romantic ambience it creates.


Carbon filaments were routinely used in the manufacture of incandescent bulbs during their first years of production in late Victorian times. From the early 20th century, newly invented drawn-tungsten filaments began to be favoured for their greater durability and brightness.


A ceramic metal halide lamp uses an arc tube constructed from ceramic rather than the traditional glass quartz, resulting in greater tolerance of high temperatures and less vulnerability to corrosion. This relatively new technology offers a high quality of light with exceptional colour rendering.


A highly decorative multi-armed hanging light with numerous lamps usually intended as an unabashed showpiece! For 200-300 years chandeliers made from lead crystal glass have been popular, with Venetian and Bohemian makers being particularly revered as masters of their craft.


Use of inefficient lighting requires a greater use of power, which is generated in the vast majority of cases by burning coal, oil, or gas—the three major fossil fuels. Burning fossil fuels creates carbon dioxide emissions and increases the greenhouse effect, which in turn warms the surface of the earth and causes climate change.


A small fluorescent lamp often used as a retro-fit replacement for incandescent GLS lamps, with a lifespan usually 8-15 times longer than the old technology and 70-80% more energy efficient. The lamps comprise of a ballast and a gas-filled tube; when the gas is excited it produces ultra-violet photons, which stimulate the lamp’s coating into producing visible light.


The Colour Rendering Index (CRI) is a measurement of a lamp’s ability to display colours accurately. It provides a useful though incomplete indication of light quality, using a rudimentary 8-patch test with an average score that can mask a poor performance with certain colours. Incandescent light sources routinely have a CRI 100 rating, or close to it, because they display a continuous spectrum of light. This dependable colour accuracy is nonetheless accompanied by a warm light, since the radiation of an incandescent lamp peaks towards the longer wavelengths of the visible spectrum.


The temperature, measured in Kelvin, at which a theoretical black body radiator would produce the same chromaticity as any given light under consideration. This can be applied to incandescent light sources in absolute terms, since they emulate a black body radiator by design, whereas the Correlated Colour Temperature (CCT) system is more correctly applied to light sources such as LED and fluorescent.


Can mean ‘of the time’, which needn’t be in the present, but in lighting it is commonly used to describe a modern design.


Refers to the difference in either brightness or colour between objects in the same field of view.


Other than incandescent lamps, most artificial sources of light require special control gear to start the lamp and subsequently regulate the current. These include linear and compact fluorescent lamps, metal halide lamps, cold cathode lamps and LEDs, and control gear takes the form of starters, ballast, transformers, and drivers, depending on the type of light.


Refers to the bluer white light produced by a theoretical black body radiator when it is heated to a sufficiently high Kelvin temperature. In lighting, the filament of an incandescent lamp closely mimics a black body radiator, and produces a fractionally cooler light the hotter it gets. LED lamps and CFLs are often available with a cool white output, whereas incandescent bulbs never produce a truly cool white unless they are produced with a blue-tinted glass as ‘daylight bulbs’.


An estimated temperature in Kelvin deduced by matching the light source to the colour appearance of a theoretical black body radiator as it is heated. The more a blackbody is heated, the cooler and bluer in appearance is its radiated light. Because incandescent lamps emulate a black body radiator, their given temperature accurately represents the heat of the filament, whereas other forms of light require an approximated CCT. In reality, two non-filament bulbs of the same CCT in Kelvin can appear significantly different, so bulk buys from the same manufacturer are advised.

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In an incandescent light source a daylight lamp has blue-tinted glass to counteract its natural warm output and to approximate the radiated wavelengths of daylight. A daylight lamp usually has a CCT of 5000-6000K, bearing in mind that indoors this correlates to a higher outdoor Kelvin temperature, where the less blue-sensitive cones of the eye dominate.


The single-direction flow of electrical charge produced by batteries and solar cells, often used to recharge electrical products through a car battery.


Any form of lighting that is ornate, and designed to be seen, as opposed to more functional luminaires such as recessed downlights or bulkheads. Normally refers to complete light fittings but can also refer to designer lamps such as the well-known Plumen fluorescent or old-style bulbs with decorative drawn-tungsten filaments.


A lamp surface that reflects most visible light so that it is emitted through the front of the lamp, whilst selectively absorbing UV or IR radiation and removing it from the light’s beam. In a halogen reflector this is often used to remove IR heat, which in turn causes a greater need for effective heat dissipation at the rear of the lamp.


Usually in the form of a lens, a diffuser is used in lighting to scatter and soften the output of light. This has the simultaneous effect of eliminating glare, so it can be a safety element in a working environment or it can serve to create more flattering light in hospitality scenarios.


The ability of a lamp to be dimmed without impacting on its reliability. Compatibility with existing dimmer switches is often a primary concern when replacing dimmable lamps.


A device used to vary the lumen output of a source, usually by adjusting the wattage it is being operated at. Can be used to save energy and reduce running costs, since a dimmed light inherently uses less power. Also useful in adjusting ambience according to the time of day in hospitality environments.


A semiconductor component typically with two terminals, usually only allowing a unidirectional flow of current.


Frequently used to describe a movable light head, directional light is also an inherent property of an LED source, with LEDs only ever producing widespread light through luminaire design or a modifying lens.


A lamp that creates light by means of electrical discharge, usually by the ionisation of gas between two electrodes. An arc lamp is a typical example of this.


In lighting commonly refers to the scattering or spreading of light, normally by means of a diffusing lens. A synonym of ‘diffusion’.


A type of lighting that best displays items in a retail or exhibition environment, for example. Various types of luminaire and lamp lend themselves well to display lighting, with LED recently making great strides through sheer economy and concentrated efforts to improve colour rendering. Display lighting is a popular application for halogen lamps, owing to their bright, crisp output, often perceived as being very pure, and with a continuous spectrum for reliable representation of colour.


A luminaire or appliance that is double-insulated guards against electrical shock by the inclusion of two layers of insulating material, and requires no connection to earth. Such a product is known as a Class 2 appliance. An appliance that only shields live electrical parts with a single layer of insulating material (Class 1) requires a connection to electrical earth during installation.


A down-facing light, usually recessed into the ceiling for inconspicuousness and sometimes with an adjustable lighting head. Can also refer to heavily-shaded wall fittings where the design of the luminaire forces light predominantly downwards.


Used in low-profile flush and recessed fixtures and also known as a 2-D lamp, the Double-D was introduced in Europe during the mid 1980s.


Extremely powerful double-connected lamps, traditionally halogen and used generally in security lights or for floodlighting work areas and sporting venues.


Usually referring to an LED driver, which is necessary to regulate the flow of current through an LED for a more consistent, unfluctuating output of brightness.

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An alternative name for an E27 (Edison 27mm) screw-capped lamp base, which is often abbreviated to ‘ES’. The narrower E14 screw cap is known as a Small Edison Screw fitting (SES).


A measurement of how effectively a light source is in converting power into light. Efficacy always uses different units of measurement, such as lumens per watt.


Always measured using identical units of measurement, this is simply an indication of how well something works, often expressed in a percentage. For instance, you can test the optical efficiency of a luminaire by measuring its Light Output Ratio (LOR), so if 80 lumens of light is produced at lamp or LED system level and 72 lumens is delivered by the luminaire you could say it is 90% efficient.


A device used to provide the appropriate starting and electrical conditions for operating fluorescent lamps, neon lamps, or high intensity discharge (HID) lamps. An electronic ballast is advantageous over older magnetic ballasts, as it uses high frequency power that eliminates the former ill effects of humming and flickering.


An elliptically-shaped reflecting surface that focuses light directly in front of a lamp and improves efficacy in some luminaires.


Battery-powered backup lighting that either remains on (maintained) or switches on automatically (non-maintained) in the event of power loss. Can be sub-divided into two main types: escape lighting and standby lighting. The first concerns itself with providing adequate illumination to quickly evacuate a building, whereas the second aims to provide continued adequate lighting in critical, potentially life-threatening scenarios where activity cannot be interrupted, e.g. operating theatre.


A general term most commonly applied to either fluorescent or halogen lamps, which refers to the energy-saving benefits over original incandescent technology. In a fluorescent lamp this advantage might be as much as 70-80%, whereas an energy-saving halogen can save anything up to 30-50% energy, by use of premium fill gases and reflective inner surfaces that redirect IR heat back onto the filament. LED tends to exalt itself from this description, since its super-modest use of power is barely comparable to the original technology.


Usually refers to the inner glass enclosure—the bulb within the bulb—that is inherent in some lamp types, such as the quartz envelope of a halogen.


A subset of escape lighting that specifically refers to the direct evacuation route of a building. The other form of escape lighting is ‘open area lighting’.

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A string of electric lamps—often multi-coloured—wired to a flexible cable. Used as decoration in indoor or outdoor events such as wedding receptions or birthday parties.


Filaments are specified with a lettered code in which C is a coiled wire filament, CC is a coiled wire that is looped into a bigger coil, and SR is a straight ribbon filament. Numbers represent the type of filament-support arrangement.


The surface texture, colour, or appearance of a luminaire or any crafted product, or simply the material with which it is constructed.


The ability of a ceiling light to maintain structural integrity during a fire, which is tested and expressed in terms of minutes, e.g. fire-rating of 30 minutes. Such fittings are usually constructed with a heat-expandable material that disallows the possibility of flames immediately taking hold of a ceiling cavity.


A chiefly British term, very commonly used with reference to a light fixture and other types of furnishing or décor.


An undesirable aspect of lighting that is most often related to old fluorescent lamps with low-frequency magnetic ballasts. If a flickering light is used in a common work area it can cause nausea, headaches, and might trigger an epileptic fit in sufferers of photosensitive epilepsy. Eliminated in most modern lighting solutions.


An artificial light source delivering light across a wide area, and often the domain of powerful multidirectional lighting technologies such as halogen or HID lamps. An LED floodlight design has to negotiate the inherent directional property of LED illumination, which is why in some quarters it is criticised—perhaps irrationally—as a form of street lighting.


An extremely efficient lamp that uses an electric discharge through low pressure mercury vapour to create ultraviolet (UV) energy. The UV excites phosphor materials, which are applied as a thin interior coating in a glass tube, and the phosphors transform the UV to visible light.


An installation that is fitted in extremely close proximity to either a wall or ceiling for either low-key effect or to avoid obstruction in confined spaces.


Literally refers to the complete electromagnetic spectrum from infrared to near-ultraviolet, though it is often used erroneously to describe a continuous spectrum of light, in which all wavelengths of visible light are represented by the source. In some instances it is used merely to describe lamps with a high CRI rating, which is misleading at best.


A circuit-breaking safety device, usually constructed with a thin strip of metal that fractures when overloaded with current.

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Artificial lighting used for overall, general-purpose illumination, and categorised with daylight as an ambient light source.


A symptom of strong, unmodified bright light, in which human vision is temporarily and significantly impeded. Glare is a mandatory safety consideration in many areas of light installation, and is accounted for in various regulations and requirements.


Usually distinguished from a golfball by its larger size, a globe is a spherically shaped bulb often with an opal finish, and usually used openly for a wide spread of diffused light.


A ‘General Lighting Service’ bulb, so named because of its compatibility with a wide range of fittings. The standard pear-shaped GLS bulb dates back to Victorian times, and its shape is still used in the retro-fitting replacements of today. Industrial ‘rough service’ bulbs, which are similar but more robustly constructed, momentarily escaped the EU incandescent GLS phase-out and can still be bought from specialist dealers in a variety of wattages.


A small spherical bulb, typically used less openly than the larger globes and ideal for compact light fittings.


An effect in which greenhouse gases, which includes carbon dioxide, traps infrared heat and holds it in the earth’s atmosphere, resulting in a heating of the earth’s surface that is commonly termed as global warming. LED lighting is seen as a significant means of slowing the Greenhouse Effect, since less use of electricity equates to less burning of fossil fuels and production of CO2.


Earthed (commonly used term in America); the condition of being connected to electrical earth.


A bi-pin, mains-powered derivative of an MR16 reflector bulb, so named because the measurement between its two pins is 10mm from centre to centre. A very commonly used lamp in many types of spotlighting luminaire.

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A type of incandescent lamp with an interior quartz envelope containing a tungsten filament and one of several types of halogen gas. The gas enables the filament to reach higher temperatures than its traditional incandescent counterpart, thus it burns at a greater temperature for a bluer, brighter output. Though original halogen technology was only marginally more efficient than incandescent, recent energy-saving halogen lamps use premium gases and IR heat reflectors to achieve greater efficiency.


The necessary process in light sources of becoming cooler through heat loss, with built-in heat sinks being a primary means of achieving this whether in the lamp or the luminaire.


A device or system designed to dissipate heat, which is described as active if it is accompanied by an electric fan, or passive if the process is entirely unassisted.


Lighting systems designed for industrial purposes usually with a 25ft height or above in mind.


See Electronic Ballast.


A lamp that produces a discharge caused by electrical breakdown of gas between two tungsten electrodes. Can be sub-divided into various types: Mercury-vapour lamps, Metal-halide (MH) lamps, Ceramic MH lamps, Sodium-vapour lamps, Xenon short-arc lamps. Very powerful yet energy-efficient lamps often used for lighting large outside areas.


An energy-efficient form of sodium vapour lamp used in many outdoor applications including street lighting. Offers better colour rendition than low-pressure sodium lamps.


The ‘Halo’ standard of track lighting usually signified by its use of three contacts.


A colour, or shade.

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The total luminous flux incident on a surface.


Lighting or light.


The original form of electric lighting patented by Thomas Edison in America and the lesser-known Joseph Swan in Victorian Britain. A bulb-shaped glass enclosure imaginatively dubbed a bulb contains a filament, through which electric current is passed to create a glowing light. The filament was originally constructed from carbon, which was superseded within a couple of decades by a drawn-tungsten replacement for greater longevity and less blackening of the glass. This classic bulb is very commonly known as a GLS lamp, as are any same-shaped modern replacements.


A form of electromagnetic radiation that lies between the visible and microwave portions of the electromagnetic spectrum, with longer wavelengths than those of visible light.


A 2-digit rating used internationally for luminaires signifying the level of protection against ingress of solid particles or liquids.


The lumen output of a lamp at the beginning of its life.


A form of ballast designed to start fluorescent lamps immediately the power is applied.


A set of electronic circuits on one small semiconductor chip.

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The ‘Juno’ standard of track lighting, usually signified by the 1-inch space between the track tips of the fitting. The L-type track fitting has a marginally narrower gap.

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An SI measurement unit for temperature in physics using an absolute thermometric scale, in which the null point is absolute zero, equating to −273.15 °C. The Kelvin Scale is used in lighting to describe the colour temperature of a lamp’s output, using the Plankian locus of a theoretical black body radiator as the reference. The blackbody does not exist in real life, though in lighting the thermal radiation of a burning filament closely simulates it. This means incandescent light sources are accurately represented by the Kelvin scale. For other light sources a Correlated Colour Temperature (CCT) should be correctly applied, and because the chromacities can vary in any given Correlated Colour Temperature it is advisable to buy bulbs in batches from the same manufacturer.


A 1,000 watt measurement of electrical power.


A unit of energy equal to the work done by a power of 1000 watts operating for one hour, e.g. ten 100W lamps burning for an hour. The Kilowatt hour (kWh) is commonly used in electricity billing by energy companies.


A noble gas used in many types of gas discharge or fluorescent lamp.

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A somewhat ambiguous term often used interchangeably with ‘bulb’ in the lighting industry, though a bulb is traditionally bulb-shaped. A lamp can also be the fitting that holds the bulb.


A semiconductor light-emitting diode that glows when a voltage is applied to it. The astounding energy efficiency of an LED allows it to equal the output of other light sources whilst using a small fraction of the energy!


An assembly of one or more LED dies including wire bonding or other types of electrical connection, perhaps with an optical element and thermal, mechanical, and electrical interfaces.


Sometimes used to collectively refer to the LED(s) and its/their driver in an integral LED luminaire.


Electromagnetic radiation that is visible to the human eye, ranging from about 390 to 740 nm in wavelength.


A brightening of the sky caused by poorly managed and unnecessary man-made light, which inhibits our view of the stars and planets in all urban areas.


Exposure to daylight or to specific wavelengths of light using various controllable light sources, often useful in treating skin disorders and some psychiatric disorders such as SAD.


The expected lifespan of a lamp, given in hours. Can also refer to the lifespan of an LED luminaire, where a high-quality LED is an integral part of the luminaire design. In that case the whole luminaire has a definitive lifespan, which is marketable because of its extremity.


Low voltage lamps require a 12V transformer for conversion from 240V, and offer several advantages over mains-powered lights, including an extremely wide choice in designs and beam angles. There are many types of lamp available in low-voltage form, including MR11, MR16, AR111, and PAR 36. Low-voltage lighting is popular outdoors for its safety and sheer ease of installation, which avoids the need to dig a deep trench system for cables!


The Lightolier standard of track lighting, distinguishable from J-type by a slightly narrower 7/8-inch gap between the contact tips of the fitting.


The SI derived unit of luminous flux measuring the total ‘amount’ of visible light emitted by a source, independent of its distribution.


The decline of light output in a lamp, measured in lumens, over a period of time.


A measurement of how a lamp maintains its lumen output over a period of time. An LED lamp is considered to have reached the end of its useful life when lumen maintenance reaches 70%.


The classic measurement of luminous efficacy, which describes how well a light source converts electric energy into light.


Primarily describes a complete light fitting that includes all necessary parts and wiring for operation, though it can also refer to a fitting minus its lamps.


The photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per square metre.


See efficacy.


A measurement in lumens of the radiant power of light emitted from a source without regard for the direction in which it is emitted.


A unit of illuminance equal to one lumen per square meter.

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A maintained emergency light is designed to function as normal amongst the regular luminaires of a lighting scheme, but using battery back-up it remains switched on during a loss of power.


The average light output of a lamp over its rated life.


A high intensity discharge (HID) lamp with a mercury-filled arc tube producing UV light that excites the phosphor layer of the outer bulb, creating light.


Electric light source that produces light by an electric arc through a gaseous blend of vaporised mercury and metal halides.


Emits microwave pulses and registers any changes due to reflection off a moving object. Because microwave radiation can penetrate materials used in luminaire construction such as plastic and glass, the motion sensor can be discreetly installed within the fitting itself.


Any light that radiates in a very narrow range of wavelengths—theoretically only one wavelength—with extremely poor colour rendering as a result. A SOX street light is an example of this.


Low voltage multi-faceted reflector lamps with their diameter provided in their name using 1/8-inch increments, e.g. an MR16 has a 16/8-inch diameter, or 2-inches.

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Gas discharge lamp containing neon gas at a low pressure in a glass capsule. Produces a pink or red glow when a voltage is applied.


An emergency light that spends most of its life switched off, but which activates automatically in the event of a power failure. Uses battery back-up power to typically provide up to 3 hours of emergency lighting.

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Translucent or opaque glass, usually white, which significantly softens and spreads the output of light from a luminaire. An opal finish may be applied to a lamp or the lens of a luminaire—often called a diffuser.


The optical system of a lamp, or luminaire, often used to modify the particular properties of a light to suit a specific application.

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Parabolic aluminized reflector lamp, commonly used in retail, entertainment, and transport environments.


A light fitting usually suspended from the ceiling by a cord, chain, or rod. Often a pendant hangs quite low, and can be used as such to define space over each table of a restaurant. A pendant might equally be fixed close to the ceiling, in which case it is known as a ‘semi-flush pendant’.


A luminescent substance that produces light when excited by radiation.


A light sensor often used to automatically switch on security lights when ambient light levels are sufficiently low.


A passive infrared sensor that triggers illumination in security lights or work lights whenever it detects body warmth through IR radiation.


Strong thermoplastic that is highly impact-resistant, yet lightweight and flexible. Often used to construct light fittings—the casing and/or diffuser—and frequently used in exterior commercial lighting because of its resistance to vandalisation and accidental damage. Also suitable for use in constructing outdoor coastal lights.


Prismatic diffusers are often favoured for reducing glare in industrial areas, since they maintain a good transmission of light.


A means of starting a discharge lamp using a high-voltage ignitor to create a series of pulses. This starting method is said to have many benefits, including longer lamp life and greater lumen maintenance.

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The emission of energy in the form of subatomic particles or electromagnetic waves.


A fluorescent lamp-ballast circuit that applies voltage to simultaneously energise the system and provide continuous cathode heating. This has the advantage of increasing lamp and cycle life, but consumes marginally more energy because the cathodes continually draw heating power while the lamp is in operation.


The average lifespan of any given type of lamp, tested under laboratory conditions using a large quantity of samples and usually given in hours. In the case of LEDs, lamps are considered to be at the end of their useful life when they reach 70% lumen maintenance.


A type of installation where the light is fitted into a ceiling cavity or wall cut-out for purposes of inconspicuousness and reduced glare. Recessed lights are also useful in confined spaces, and their seamless look is well suited to contemporary premises. Downlights are the classic example of a recessed light, which can emit downward light from a ceiling in whichever way is required, from floodlighting to spotlighting.


A classic type of incandescent lamp with an interior coating of aluminium designed to project light to the front of the bulb, thus providing a focused beam for spotlighting purposes. Such lamps are named with an ‘R’ followed by their diameter in millimetres, e.g. R80 (80mm incandescent reflector bulb). Other types of reflector lamp are generically known merely as ‘reflectors’, which might include a multi-faceted reflector (MR), or parabolic aluminized reflector (PAR).


Not specifically a lighting term but often used to describe luminaires or any product reminiscent in style of a particular period in time. Usually refers to the relatively recent past.


The process of installing new technology in place of old, usually implying compatibility with existing connectors or fastenings for a seamless transition. Often refers to the direct replacement of incandescent lamps with CFL or LED alternatives.


An EU directive known as the Restriction of Hazardous Substances Directive, which was effected in 2006 and is required to be enforced in all member states. The directive restricts the use of six materials in the manufacture of electrical products: Lead (Pb), Mercury (Hg), Cadmium (Cd), Hexavalent chromium (Cr6+), Polybrominated biphenyls (PBB), Polybrominated diphenyl ether (PBDE).


An industrial-strength General Lighting Service lamp (GLS) that is still permitted to be sold by lighting retailers despite the EU incandescent phase-out, but which cannot legally be sold for purposes of household use.

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Seasonal Affective Disorder is a mood disorder where sufferers display symptoms of depression during winter months. Associated with the lack of seasonal daylight, SAD can be effectively treated with bright light therapy, which usually involves light sources with an illuminance of at least 2500 lux.


Correctly refers to an upward facing wall-mounted light fitting, holding either traditional candles or electric lamps. Often used incorrectly to describe any wall light.


A solid substance such as silicon or germanium that is a poor conductor at low temperatures, but whose conductivity improves with the application of heat, light, or voltage. In LED technology light is produced when a voltage is applied to a semiconductor material.


Used to describe any light fitting that fits close to the wall, or ceiling, but still requires a small gap for its fixing. A ‘flush’ light-fitting leaves no such gap, and is installed so it hugs the surface it is mounted on.


The E14 Small Edison Screw cap fitting—abbreviated as SES—has a threaded metal bulb base with a 14mm diameter.


A gas discharge lamp in which an electric current passes through a tube of sodium vapour to produce a yellow light. Commonly used in street lighting.


Describes the power of the light at each wavelength in the visible spectrum. By viewing an SPD graph it is possible to evaluate the quality of colour rendering in any given light source.


The reflection of light, absent of any diffusion, whereby the light reflects off of an object at precisely the same angle as it strikes. An easy analogy is ‘mirror-like reflection’.


Light rays from a focused light source that are surplus to requirements, illuminating an area that is not meant to be lit.


A lamp designed to illuminate a very defined area, usually a reflector. Correctly used this term refers to a light whose beam angle is narrow—between 10 and 30°, but it is also used generically when referring to any multi-faceted reflectors or PAR lights.


An electronic device that helps in starting a discharge lamp, normally by providing a high-voltage surge.


A light fitting that has an independent switch, rather than requiring control from a wall-mounted mains switch. Very useful in fittings such as bedside reading lamps, where it eliminates the need for users to haul themselves out of bed after reading!

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Supplementary lighting deliberately designed with a particular task in mind, e.g. a reading light of any description, or kitchen under-cabinet lights for food preparation.


A lighting scheme where lamps can be positioned anywhere along a continuous track containing electrical conductors. Very useful in commercial display situations where the position of displayed items is apt to change regularly.


A style of decoration focused on creating comfort and a relaxing, casual ambience. Light fittings in this style tend to be decorative, slightly old-fashioned and homely, as opposed to contemporary designs that often display clean lines, geometric shapes, and simple finishes.


Lighting transformers are used to convert a mains 240V input voltage down to 12V output for low-voltage lighting installations.


A triple-phosphor coating used inside fluorescent lamps for enhanced light levels, increased lifespan, and improved colour rendering.


A chemical element often used to manufacture filaments in electric light bulbs because of its extremely high melting point. The typical 2700K temperature in a tungsten filament is a literal measurement of its heat, since a filament emulates a black body radiator to produce light. This physical temperature in an incandescent bulb equates to something in the region of 2500°C.

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Part of the electromagnetic spectrum of light that radiates at wavelengths shorter than those of visible light, but longer than X-rays. UV rays are most hazardous at their shortest wavelength, where they can be rapidly mutagenic or carcinogenic, but such rays do not occur naturally on earth and are only present in strictly regulated man-made UV sources. Less harmful long-wave UV rays account for the vast majority of UV radiation reaching the earth’s surface, with prolonged exposure still posing health risks.


A dual effect in which a wall luminaire—often cylindrical in shape—emits light from both the top and bottom of its shade, simultaneously producing uplighting and downlighting.


A wall luminaire that is designed and installed in such a way that it permits light only from the top of its shade. Some wall lights have a parallel design that allows them to be installed as either uplight or downlight.

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A measure of potential energy that is relative between two points.

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Thin slices of substrate material such as pure silicon, or virgin silicon, upon which semi-conductor materials such as Gallium nitride (GaN) can be deposited to make LED chips. It is the potential large-scale industrialisation of this process that is expected to bring the price of LED lighting down, replacing the more expensive method of making LEDs using sapphire or silicon carbide substrates.


An SI derived unit of power that is defined as one joule per second, used in measuring the rate of energy conversion or transfer. Corresponds to the rate of energy consumption in an electric circuit where the potential difference is one volt and the current one ampere.


A lighting effect where an area of a wall or a whole wall is bathed in light, usually at an acute angle with the luminaire(s) close to the plane of the wall, or on it.


Simply a lamp or luminaire that delivers warm-looking yellow or orange illumination. Colour temperature is derived using the Kelvin scale and the corresponding colours of a theoretical blackbody radiator as it is being substantially heated. The plotted path of this transition in colour is known in physics as the Plankian locus. Any lamp or luminaire with a colour temperature (or CCT) of between 2700K and 3200K is commonly described as a warm light. In incandescent lamps the given Kelvin temperature is more than nominal—it reflects their actual temperature.


Fluorescent lights usually take a brief but noticeable time to warm up whilst the applied electrical current heats the cathodes. Typically the light will reach 80% of its maximum output within a few seconds, but reaching maximum output can take up to two minutes or more.


The colours of the visible spectrum—namely red, orange, yellow, green, blue, indigo, and violet—appear as white when we view them all combined. However, in electric lighting there are ways of duping human eyesight into perceiving a white light. For instance, the primary red, green , and blue colours of the additive RGB model produce white when blended. In LED lighting, bright white light is produced from adding a yellow phosphor to a blue light source. The thickness of the phosphor dictates colour temperature in an LED, and affects efficacy.


A horizontal working plane is usually deemed to be 76cm (30”) from the floor, and luminance is assessed at this level.

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A premium fill gas used in some electric light bulbs. In a halogen lamp use of Xenon creates greater efficiency through being a poorer conductor of heat than Argon. The filament of the lamp burns brighter through better retention of heat, and yet its lifespan is maintained through the greater ability of Xenon to redeposit evaporated tungsten onto the filament.

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Yellow phosphor is used to convert blue light to white light in an LED, usually impregnated into a plastic cover above a printed circuit board where the LED is mounted.

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The direction directly behind or above a luminaire, at a 180° angle to the main direction of light.

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