Glossary of lighting terminology


Lighting up the future

Glossary of Lighting Terminology*

Accent Lighting

Accent lighting is directional lighting used to emphasize a particular object, location or area.



Aluminum indium gallium phosphide is a semiconductor material used in the manufacture of HB LEDs in red, orange, green, and yellow color, to form the hetero-structure emitting light.


Ambient Light

Light diffused in the environment. When measuring a particular light source, the existing light in the environment, which is not being emitted from the source, is considered ambient light.


American National Standards Institute (ANSI)

A consensus-based organization which coordinates voluntary standards for the physical, electrical and performance characteristics of lamps, ballasts, luminaires and other lighting and electrical equipment.



Unit for measuring an electric charge per second, known as current.

Symbol: Amp, A



A three letter system that has been devised to describe lamps of different manufacture but the same application. The letters have no relationship to lamp description, but the same letters always designate the same type of lamp. Some of the application parameters they define are wattage, base type envelope size, and light center length.



The light caused by an electrical discharge between two electrodes in a gas such as xenon, argon, or air. The first usable arc as a practical light source was developed in 1809 by Sir Humphrey Davy.


Average Luminance

Luminance averaged over a specified surface.

Unit: lux (lx) = lm/m2

Symbol: Eav



Any light source used to illuminate a panel or legend from behind. LEDs are becoming the most common form of light source in backlighting, widely used in illuminating LCD panels.



Electrical device intended to limit the amount of current in an electric circuit (used for stabilising the current in the discharge). They can be as simple as a series resistor as commonly used with small neon lamps or as complex as the computerized, remote-controlled electronic ballasts used with fluorescent lamps..


Ballast Factor (BF)

This is the percentage of a lamp's rated lumen output that can be expected when operated on a specific, commercially available ballast. For example, a ballast with a ballast factor of 9.93 will result in the lamp's emitting 93% of its rated lumen output. A ballast with a lower BF results in less light output and also generally consumes less power.


Base Type

The base is the end of the lamp that fits into the receptacle connected to the electrical supply. Bases provide electrical contact to the lamp and, in most cases, also support the lamp in the fixture. There are many types of bases used in lamps, screw bases being the most common for incandescent and HID lamps, while bipin bases are common for linear fluorescent lamps.


Beam Spread (Beam Angle)

Angle in the plane through the beam axis over which the luminous intensity drops to a stated percentage (e.g. 50%) of its peak intensity.


Beam Pattern

The complete shape of the beam, as defined in the general sense. It includes any realistic or abstract patterns introduced into the beam as well as any apparatus that alters the contour of the beam.



Action of subdivision of the manufactured product into specific performance parameters such as flux, color, and forward voltage. Binning puts parts into smaller groups in order to meet specific requirements of assembly.



Attribute of visual sensation according to which an area appears to emit more or less light. Brightness according to the definition is also an attribute of colour. In British recommendations the term “brightness” is now reserved for descriptions of colour. Luminosity should be used in other instances.



is the SI base unit for luminous intensity. The operational definition of the candela describes it as the luminous intensity in a given direction of a monochromatic light source of frequency 540*10^12 Hz and a radiant intensity of 1/683 watt per steradian in that direction. When measuring the luminous intensity of light sources with different spectral content, a standard luminosity function is used as a correction factor. For reference, the luminous intensities produced by a common candle and a 100W incandescent bulb are approximately 1cd and 120cd respectively. Note that LEDs are often rated in millicandela (mcd).

Symbol: cd


Candle Power

Old definition of luminous intensity. One candle power was the intensity of a standard whale wax candle burning at 120 grains per hour. Candela is the same as candle power, 1 candela = 1 candlepower.



Cold cathode fluorescent lamps (CCFLs) refers to light sources such as neon lamps that are based on gas discharge principles, where the cathode of the lamp is not independently heated. CCFLs remain popular for LCD backlighting and computer case modification.





Center Beam Candlepower (CBCP)

Refers to the luminous intensity at the center of the beam of a blown or pressed reflector lamp (such as a PAR lamp). Measured in candelas. See also Candela.


Ceramic Metal Halide (CHM)

A Type of metal halide lamp that uses a ceramic material for the arc tube instead of glass quartz, resulting in better color rendering (greater than 80 CRI) and improved lumen maintenance.



Refers to the aspect of color quality in terms of wavelength, hue, saturation, and purity.


Cold Start

Term used to describe the ignition of a cold arc lamp, i.e., a lamp that has not been electrified for a relatively long period of time.


Colour Appearance

General expression for the colour impression received when looking at a light source.


Colour Change

The facility to change the colour temperature of a lighting installation makes it possible to create a more comfortable working environment or to introduce dynamic lighting effects.


Colour Rendering

Ability of a light source to render colours naturally, without distorting the hues seen under a full spectrum radiator (like daylight lamps).

The colour rendering index (CRI) ranges from 0 to 100.

For further details see ‘Specification data lamps’.


Colour Rendering Index CRI

See Colour rendering.

Symbol: Ra


Colour Temperature (Correlated Colour Tenperature - CCT)

Refers to the numerical value given to a Colour in Kelvin (K) that describes a Colour as "warm" or "cool". High Colour temperatures (> 4000 K) are considered cool and are blue-green in hue. Lower Colour temperatures (< 3000 K) are considered warm and are yellow-red in hue. Light between 3000 K and 4000 K is considered neutral or pure. Cool light produces higher contrast and is considered better for visual tasks. Warm light is preferred for living spaces because it is considered more flattering to skin tones and clothing.

Unit: kelvin, K

Symbol: Tc


Compact Fluorescent Lamp (CFL)

The general term applied to fluorescent lamps that are single-ended and that have smaller diameter tubes that are bent to form a compact shape. Some CFLs have integral ballasts and medium or candelabra screw bases for easy replacement of incandescent lamps.


Contrast C (Between Two Parts of a Visual Field)

The relevant luminance difference between two parts in accordance with the formula:

C= (L1 - L2)/L2

Where the size of the two parts differs greatly and where:

L1 = luminance of the smallest part (object)

L2 = luminance of the largest part (background).



See Colour Rendering.



Digital Addressable Lighting Interface, a standardised communication interface to regulate lighting levels and to switch electronic HFD ballasts on and off.


Daylight Linking

Using natural light saves energy and is beneficial to individual users.

Daylight linking is a technique that regulates the light output of luminaries in accordance with daylight conditions, thus maintaining a constant level of indoor lighting and ensuring comfort at all times.



The very center or the heart of the LED.


Diffuse lens / Diffuser

Optical element used to mix light rays to improve uniformity or reduce glare (i.e. an opal cover). It makes the LED appear dimmer, but gives a wider viewing angle of the light than "transparent" lenses.



A control which varies the output of a light source by reducing the voltage or current to the lamp.



Most modern diodes are based on semiconductor p-n junctions. In a p-n junction, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but cannot flow in the opposite direction.


Direct Lighting

Lighting by means of luminaries with a light distribution such that 90 – 100% of the emitted luminous flux reaches the working plane directly, assuming that this plane is unbounded.


Disability Glare

Glare that impairs vision.


Discharge Lamp

Lamp in which the light is produced, directly or indirectly, by an electric discharge through a gas, a metal vapour, or a mixture of several gases and vapours.


Discomfort Glare

Glare that causes discomfort without necessarily impairing vision.


Dominant Wavelength

The wavelength at which the human eye perceives the light emitted from the LED to be the strongest.



Any device used to condition or control some quantity in order to provide another device with an appropriate input. In the lighting sector, an LED driver is most often a device used to control the forward current flowing through one or more LEDs..


Dust-Proof Luminaries

Luminaries constructed so that dust of a specified nature and fineness cannot enter when it is used in a dust-laden atmosphere.


Electromagnetic Spectrum

A continuum of electric and magnetic radiation that can be characterized by wavelength or frequency. Visible light encompasses a small part of the electromagnetic spectrum in the region from about 380 nanometers (violet) to 770 nanometers (red) by wavelength.


Electronic Ballast

A short name for a fluorescent high frequency electronic ballast. Electronic ballasts use solid state electronic components and typically operate fluorescent lamps at frequencies in the range of 25-35 kHz. The benefits are: increased lamp efficacy, reduced ballast losses and lighter, smaller ballasts compared to electromagnetic ballasts. Electronic ballasts may also be used with HID lamps.


Elliptical Reflector (ER) Lamp

An incandescent lamp with a built-in elliptically-shaped reflecting surface. This shape produces a focal point directly in front of the lamp which reduces light absorption in some types of luminaires. It is particularly effective at increasing the efficiency of baffled downlights.


Emergency Lighting

Lighting provided for use when the supply to the normal lighting fails.



European Ordering Code.


Escape Lighting

That part of emergency lighting provided to ensure that an escape route can be effectively identified and used in the case of failure of the normal lighting system.


Field Angle

The angular dimension of the cone of light from reflectorized lamps (such as R and PAR types) encompassing the central part of the beam out to the angle where the intensity is 10% of maximum.



The wire inside an incandescent lamp envelope that glows and emits light when heated, i.e., when electricity passes through it. Filament configurations are identified by letters and a suffix number (or number and letter) indicating the shape of the filament and the number of any supports.


Flashed Area

Of a luminaries in a given direction. The area of the orthogonal projection of the luminous surface on a plane perpendicular to the specified direction.


Fluorescent Lamp

A lamp utilizing an electric discharge through low pressure mercury vapor to produce ultra-violet (UV) energy. The UV excites phosphor materials applied as a thin layer on the inside of a glass tube which makes up the structure of the lamp. The phosphors transform the UV to visible light.



Unit of measurement for illuminance, or the intensity of light that is incident upon a surface. It can be defined as the intensity of light resulting from 1 lumen of luminous flux being distributed uniformly across a surface with an area of 1 square foot.

Symbol: fc


Fresnel Lens or Reflector

A Fresnel lens or reflector is an optical element of reduced height.

By means of discrete steps, the basic shape is flattened yet the optical characteristics are retained.


Full Spectrum Lighting

A marketing term, typically associated with light sources that are similar to some forms of natural daylight (5000K and above, 90+ CRI), but sometimes more broadly used for lamps that have a smooth and continuous color spectrum.


Gas Filled

Gas filled lamps use an inert gas to protect the filament during operation. In most automotive lamps, the gas most used is a mixture of 88% argon and 12% nitrogen. The pressure of the fill gas at room temperature is about 3/4 of an atmosphere.

The use of a fill gas reduces the net rate of tungsten evaporation by several orders of magnitude, so that little blackening of the bulb occurs during the life of the lamp. Since the net evaporation rate in gas is less than in a vacuum, the filament can be operated at a higher temperature for the same life. For this reason , gas filled lamps are more efficient than vacuum lamps.

The blackening that does occur is usually confined to the top of the bulb because of convection currents produced during operation. To avoid "water cycle" and premature blackening, "getters" are usually used to chemically or physically bind contaminants and assure a "clean" fill gas. Purity of the gas fill initially and during operation is critical in gas filled lamps.



Condition of vision in which there is discomfort or a reduction in the ability to see significant objects, or both, due to an unsuitable distribution or range of luminance or to extreme contrast in space or time.


Halogen Lamp

A halogen lamp is an incandescent lamp with a filament that is surrounded by halogen gases, such as iodine or bromine. Halogen gases allow the filaments to be operated at higher temperatures and higher efficacies. The halogen participates in a tungsten transport cycle, returning tungsten to the filament and prolonging lamp life.

Halogen lamps, in addition to being gas filled, have a gaseous halogen compound added to the gas fill. The halogen compound constitutes a small portion of the fill gas and must be accurately controlled. The quantity used is on the order of 100 parts per million of fill gas and is usually a bromine compound. Its purpose is to prevent any blackening of the bulb by returning evaporated tungsten back to the filament through a "halogen cycle". Too little halogen permits blackening of the bulb to occur. Too much halogen destroys the tungsten filament.

Since with proper operation of the "halogen cycle" no blackening occurs, the bulb can be smaller than what is normally used in regular gas filled lamps. With smaller envelopes, higher molecular weight inert gasses and higher fill pressures are practical. Krypton is generally used in automotive halogen lamps and the fill pressure at room temperature is approximately 5 atmospheres.

The higher molecular weight gas and higher fill pressure permits operation of the filament at even higher temperatures than regular gas filled lamps, so that for the same life, halogen lamps have the highest efficiency of all miniature lamps.


Halogen HV

“High voltage” 230 V halogen lamp, which does not require a ballast or a transformer.


Halogen LV

“Low voltage” 6 V, 12 V or 24 V halogen lamps. An electronic or electromagnetic transformer is required to operate low-voltage halogen lamps and is often integrated into the luminaries.



High Brightness Light Emitting Diodes (HB LEDs) is a loosely defined term generally referring to LEDs capable of producing large amounts of luminous flux for illumination applications, and which are generally operated at much higher power levels than LEDs used for indication purposes. Do not confuse with High Bay LED Luminaries.


Heat Sink

Metal device that uses thermal contact to absorb and dissipate thermal energy from a high temperature object to a lower temperature object that has a much greater heat capacity. This process acts as a cooling mechanism because of thermal equilibrium. Heat sinks are typically designed with many fins in order to maximize surface area for a rapid transfer of thermal energy to the surrounding cooler air.



Electronic regulating ballast which enables the light to be dimmed.

Ballast functions according to the DALI protocol.



Electronic regulating DALI ballast for various fluorescent lamps. With this new-style electronic ballast standard push-buttons can be used to dim the light output down to 3%. The lamp can also be switched on or off using the push-button control.

Ballast functions according to the DALI protocol.



Compactly built electronic ballast for various compact fluorescent lamp types. See also ‘Specification data luminaries’.



Electronic ballast for various fluorescent lamp types with warm-start principle. See also ‘Specification data luminaries’.



Electronic regulating ballast for various fluorescent lamp types by which light dimming is possible. The main ranges of HFR ballasts allow step-less dimming down to 3%. See also ‘Specification data luminaries’.


High Intensity Discharge Lamp (HID)

Under this name you can find lamps of mercury vapor, metal halide, high-pressure sodium and less common, xenon short-arc lights. The light-producing element of these light types is a well-stabilized arc discharge contained within a refractory envelope, or arc tube.


High-Pressure Sodium Lamp (HPS)

HPS lamps are high intensity discharge light sources that produce light by an electrical discharge through sodium vapor operating at relatively high pressures and temperatures.


Horizontal Luminance (Ehor)

Luminance incident on the horizontal surface.

Unit: lux (lx) = lm/m2

Symbol: Ehor


HPI Plus Lamp

Metal-halide high-intensity discharge lamp combining high luminous efficacy with white light of moderate colour rendering. Main indoor applications are big industrial halls and lighting of discount stores, hyper-/supermarkets and DIY shops. HPI Plus lamps are usually applied in high-bay luminaries and floodlights.



The luminous flux density at the surface being lit. The luminance in the full summer sun is approx. 100 000 lux. Recommended luminance for work places range from 200 lux for rough work to 2000 lux for detailed or critical work.

Unit: lux (lx) = lm/m2

Symbol: E


Incandescent Lamp

Lamp in which light is produced by means of an element heated to incandescence by the passage of an electric current until it radiates electromagnetic energy in the visible spectrum.


Indirect Lighting

Lighting by means of luminaries with a light distribution such that not more than 10 per cent of the emitted luminous flux reaches the working plane directly, assuming that this plane is unbounded.


Induction Lighting QL

Electrode-less induction lighting system characterised by good light quality, high luminous efficacy and a phenomenal life (60 000 hours).


Infrared (IR)

The invisible portion of the electromagnetic spectrum which the wavelength is longer than visible light and extends from the range of 750 nm to 1 mm.


Infrared Remote Control

In offices and meeting rooms where the lighting requirement changes frequently, wireless infrared control offers the flexibility to set the lighting and change it at any time.


Instant Start

A type of ballast designed to start fluorescent lamps as soon as the power is applied. Most T8 fluorescent lamps are being operated on electronic instant-start ballasts. These ballasts should be bypassed when relamping with LED tubes.


Isocandela Curve (Diagram)

Curve traced on an imaginary sphere with the source at its centre and joining all adjacent points corresponding to those directions in which the luminous intensity is the same, or a plane projection of this curve.


Isolux Curve (Diagram)

Locus of points on a surface where the luminance has the same value.


Jet-Proof Luminaries

Luminaries constructed to withstand a direct jet of water from any direction.


Kilowatt (kW)

A measure of electrical power equal to 1000 watts.


Kilowatt Hour (kWh)

The standard measure of electrical energy and the typical billing unit used by electrical utilities for electricity use. In example: a 100-watt lamp operated for 10 hours consumes 1000 watt-hours (100 x 10) or one kilowatt-hour.



The term used to refer to the complete light source package, including the inner parts as well as the outer bulb or tube. "Lamp" is also commonly used to refer to a type of small light fixture such as a table lamp.



The electrical device that supports a lamp in a luminaire, and generally contains the contacts that make the electrical connection to the contacts of the lamp base.


LED (Light Emitting Diode)

Light Emitting Diode used as a light source. Solid-state semiconductor device that converts electrical energy directly into light of a specific colour or white light. The Colour of the emitted light depends on the composition and condition of the semi conducting material used, and can be infrared, visible, or ultraviolet.


LED Circuit

The basic LED circuit is an electrical circuit used to power an LED. It consists of up to four components connected in series: a voltage source, a current limiting resistor, a LED, and optionally a switch to open and close the circuit. Two diodes may be placed in parallel in the circuit, but connected anode to cathode; the second diode may be used to protect the LED against reverse bias, which can damage the LED, or it may be another LED which is illuminated when the polarity of the voltage source is reversed. The LEDs used will have a forward voltage specified at the intended operating current. Ohm's law, V=IR, is used to calculate the resistor that is used to attain the correct current. The resistor value is computed by subtracting the forward bias voltage from the supply voltage, and then dividing by the desired operating current.



An optical lens is an optical device with axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens is a lens consisting of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Manufactured lenses are typically made of glass or transparent plastic.



Radiant energy that can be sensed or seen by the human eye. Visible light is measured in lumens.


Light Engine

A light engine is generally one or more high-brightness LEDs in a pre-fabricated array that is used for illumination applications.


Light Output Ratio (LOR)

Ratio of the total light emitted by a luminarie to the total light output of the lamp(s) it contains measured at standard operating conditions.


Light Pipe

A light pipe is an optical conduit that is used to pass light from one location to another. These devices come in many arrangements including single, multiple, and panel mount. Light pipes protect circuits from electrostatic discharge (ESD) by isolating the device’s front panel from the circuit board.



Testing methodology for creating a level field for product evaluation. It looks at 25ºC ambient, power supply, stabilization, orientation, electrical instruments, and testing equipment; also defines what information is required (total light output, voltage, current, power, calculates efficacy, lumen distribution, CCT, CRI, spectral distribution, testing lab, and equipment used). LM-79 requires that solid state lighting products be tested to “Absolute Photometry” (not "Relative Photometry”)



Industry standard that helps users evaluate LED products providing a method for measuring the lumen maintenance of LED packages, arrays, and modules (i.e., the LED light source) at various temperatures. It is not a measure of LED system performance or reliability. It only describes how to measure how one part of an LED luminaire—the LED light source—performs over a period of time and under certain set conditions.


Low-Pressure Sodium Lamp

Sodium vapour lamp in which the partial pressure of the vapour during operation does not exceed 5 Pa – for example: a SOX lamp.



Is the SI unit of luminous flux and is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.

Symbol: lm


Lumen Depreciation

The decline in the light output of a light source during its lifetime.


Lumen Maintenance

A measure of how well a lamp maintains its light output over time. It may be expressed numerically or as a graph of light output vs time.



Appliance that distributes, filters or transforms the light given by a light source (LED or lamp), and which contains all the necessary elements for fixing and protecting the light source(s) and facilitates their power supply and operating conditions.



The light intensity per square metre of apparent area of the light source, luminarie or illuminated surface. Where surfaces are lit, the luminance depends upon both lighting level and reflection characteristics of the luminous surface.

Unit: cd/m2

Symbol: L


Luminare Efficiency

The ratio of total lumens emitted by a luminaire to those emitted by the lamp or lamps used in that luminaire.


Luminous Efficacy

The quantity of light that a light source emits per unit of electrical energy consumed. Notice that the luminous efficacy can be specified for a light source or for a lighting system (lamp and ballast). The luminous efficacy of a system is lower than that of the light source.

Unit: lm/W


Luminous Flux

The total light output emitted by a light source. Also the total light falling on a surface. The light output of a light source is measured in lumen.

Unit: lumen

Symbol: φ


Luminous Intensity

The luminous flux in a given direction, e.g. from a floodlight or projector.

Unit: candela (cd) = one lumen per steradian

Symbol: l


Luminous Intensity Distribution Diagram (Table)

Luminous intensity shown in the form of a polar diagram, Cartesian diagram or table, in terms of candelas per 1 000 lumens of lamp flux.

The diagram (table) for non-symmetrical light distributions gives the light distribution of a luminarie in at least two planes:

  1. In a vertical plane through the longitudinal axis of the luminarie,
  2. In a plane at right angles to that axis.

The luminous intensity diagram (table) can be used:

  • To provide a rough idea of the light distribution of the luminarie,
  • For the calculation of luminance values at a point,
  • For the calculation of the luminance distribution of the luminarie.



The standard unit of luminance of a surface being lit. One lux equals one lumen per square metre.


Maintained Emergency Lighting

Emergency lighting where the lamps are operated from the normal supply during standard conditions. In an emergency situation the emergency lamp (usually one lamp in luminaries of two or more lamps) remains in operation.


Maintained Luminance (Luminance)

Value below which the average luminance on the specified surface is not allowed falling. The maintained luminance is specified at the end of the maintenance cycle, taking into consideration the maintenance factor.

It is one of the main specification elements for the lighting designer.

In the various standards the maintained luminance is specified for various activities.

Unit: lux

Symbol: Em, Lm


Maintenance Factor

Correction factor used in lighting design to compensate for the rate of lumen depreciation, caused by lamp ageing (lumen depreciation and lamp failure) and dirt accumulation (luminaire and environment).

It determines the maintenance cycle needed to ensure that luminance does not fall below the maintained value.


Maximum Overall Length (MOL)

The end-to-end measurement of a lamp, expressed in inches or millimeters. This dimension represents the overall length of the lamp from the top of the glass to the bottom of the base.



A Metal Core Printed Circuit Board (MCPCB) incorporates a base metal material as a means for heat dissipation. The metal core usually consists of an aluminum alloy.


Mercury Lamp

High-intensity discharge lamp for use in industry and large public spaces. Higher quality light characteristics are obtained by applying metal-halide lamps.


Metal-Halide Lamp

Single- or double-ended discharge lamp for use in industry, public spaces and shops. Metal-halide lamps combine a natural white colour with a pleasant light and a high luminous intensity.


Minatures / Miniature Lamp

The term miniature lamp applies to units ranging in size from the so-called "grain of wheat" lamps to automotive headlights. It is not as much indicative of physical size as of low operating voltage - rarely more than 28 volts and often as little as 1.5 or 2.5 volts.


Movement Detection

Sensing of occupancy by movement detection ensures lights are activated only when needed and enables the lighting in a specific area to be controlled.



Unit of measurement. 1 billion nm is equal to 1 meter.

Symbol: nm


Non-Maintained Emergency Lighting

Emergency lighting where the emergency lighting lamps come into operation only when the power supply to normal lighting fails.


Obtrusive Light

Spill light which because of quantitative, directional and spectral attributes in a given context, gives rise to annoyance, discomfort, distraction or a reduction in the ability to see essential information.



Omnidirectional Lighting Control, a Philips-patented series of optics for TL5 and TL-D lamps offering optimal lighting efficiency in combination with excellent glare and luminance control all around the luminarie.


OLED (Organic Light-Emitting Diode)

LED whose emissive electroluminescent layer is composed of a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different Colours. Such systems can be used in television screens, computer displays, and portable system screens such as PDA's, advertising, information, indication and, most recently, lighting. OLEDs typically emit less light per area than inorganic solid-state based LEDs and can degrade over time, but they do not require a backlight to function and can therefore be very thin. They also draw far less power and can operate longer on the same battery charge.



An optical lens transmits or refracts light, converging or diverging the beam. The viewing angle of a lens controls how narrow or wide the beam of light is from an LED. Lenses have either a clear or diffused package which refers to how quickly the light dissipates at the edge of the viewing angle. The optic may include louvers for reduction of glare.


Package Size

The package size of a surface mount component (SMD) is typically written as a four digit number in units of hundredths of an inch or tenths of a millimeter. For example, "0402 (1005 metric)" would be 0.04"x 0.02"(1.0 mm x 0.5 mm).


PAR Lamp

PAR is an acronym for parabolic aluminized reflector. A PAR lamp, which may utilize either an incandescent filament, a halogen filament tube or a HID arc tube, is a precision pressed-glass reflector lamp. PAR lamps rely on both the internal reflector and prisms in the lens for the control of the light beam.



The Peltier (or thermoelectric) effect refers to a current that creates a temperature difference between an upper and lower conductor. These conductors are attempting to recreate the electron equilibrium that existed before current was applied by absorbing energy at one conductor and releasing it at the other. A Peltier device can be used for both heating and cooling but is much more economical to be used for only cooling.



An inorganic chemical compound processed into a powder and deposited on the inner glass surface of fluorescent tubes and some mercury and metal-halide lamp bulbs. Phosphors are designed to absorb short wavelength ultraviolet radiation and to transform and emit it as visible light.



A photometer is a device used for measuring luminous intensity or luminance.


PL Lamp

Single-ended compact fluorescent lamp in which the discharge tube is folded into two, four or six limbs. PL lamps are characterised by unusually high light output for their length. They are applied in compact luminaries for professional and domestic use. Also available with integrated ballast.


Power Factor (PF)

Ratio of the circuit power in watts over the product of the root-meansquare values of voltage and current. For sinusoidal waveforms, it is equal to the cosine of the angle of phase difference between voltage and current. For electronic ballasts the power factor is 0.95 so that no additional phase compensation is required.


Preheat Circuit

A type of fluorescent lamp-ballast circuit used with the first commercial fluorescent lamp products. A push button or automatic switch is used to preheat the lamp cathodes to a glow state. Starting the lamp can then be accomplished using simple "choke" or reactor ballasts.



Pulse-Width Modulation (PWM) of a signal or power source involves the modulation of its duty cycle to either convey information over a communications channel or control the amount of power sent to a load. Pulse-width modulation uses a square wave whose pulse width is modulated resulting in the variation of the average value of the waveform.


QL Lamp

See Induction lighting.


Rapid Start Circuit

A fluorescent lamp-ballast circuit that utilizes continuous cathode heating, while the system is energized, to start and maintain lamp light output at efficient levels. Rapid start ballasts may be either electromagnetic, electronic or of hybrid design. Full-range fluorescent lamp dimming is only possible with rapid start systems.


Rated Life

For most lamp types, rated lamp life is the length of time of a statistically large sample between first use and the point when 50% of the lamps have died. It is possible to define "useful life" of a lamp based on practical considerations involving lumen depreciation and color shift.

Rated life is not guaranteed life and does not necessarily represent service life expectancy. Rated life, along with rated amperes at design volts, is a factor establishing the laboratory efficiency of a lamp. It is useful only as an index of comparative longevity of various lamps.

The life of an incandescent filament is voltage sensitive. A 10% increase in applied voltage will reduce the filament life to only 30% of its design value, while a 10% decrease in applied voltage will multiply the design life by a factor of approximately three. It is often feasible to "derate" an incandescent lamp for the sake of increased life.


Reflector Lamp

A light source with a built-in reflecting surface. Sometimes, the term is used to refer specifically to blown bulbs like the "R" and "ER" lamps; at other times, it includes all reflectorized lamps like PAR and MR.



Optical element used to concentrate light rays to focus or improve the direction of light. A prismatic cover is an example of a refractor.



The term RGB stands for red, green, and blue, the three primary Colours of light. When the three Colours are mixed the result of the Colour of light appears white to the human eye. By adjusting intensities of each Colour can produce all of the other Colours in the spectrum.



White SON or SDW-T lamps offer a high luminous efficacy in combination with warm-white light. The colour rendering is excellent.

SDW-T lamps are applied in shops and public spaces where the atmosphere should be warm and cosy.


Scotopic/Photopic (S/P) Ratio

This measurement accounts for the fact that of the two light sensors in the retina, rods are more sensitive to blue light (Scotopic vision) and cones to yellow light (Photopic vision). The Scotopic/Photopic (S/P) Ratio is an attempt to capture the relative strengths of these two responses. S/P is calculated as the ratio of scotopic lumens to photopic lumens for the light source on an ANSI reference ballast. Cooler sources (higher color temperatures lamps) tend to have higher values of the S/P Ratio compared to warm sources.



The International System of Units


Sky Glow

The brightening of the night sky that results from the reflection of radiation (visible and non-visible), scattered from the constituents of the atmosphere (gas molecules, aerosols and particulate matter), in the direction of observation. It comprises two separate components as follows:

a. Natural sky glow: That part of the sky glow which is attributable to radiation from celestial sources and luminescent processes in the earth’s upper atmosphere.

b. Man-made sky glow: That part of the sky glow which is attributable to man-made sources of radiation (e.g. outdoor electric lighting), including radiation that is emitted directly upwards and radiation that is reflected from the surface of the earth.


Sodium Lamp, SON Lamp

High-pressure discharge lamp with a yellowish colour appearance and an extremely high efficiency. SON and SON Comfort lamps, with improved colour characteristics, are mainly applied in high-bay industrial applications.


Solid-State Lighting

Description of a lighting device that does not contain moving parts or parts that can break, rupture, shatter, leak or contaminate the environment (such as a lamp).


Spectral Power Distribution (SPD)

A graph of the radiant power emitted by a light source as a function of wavelength. SPD's provide a visual profile of "finger print" of the color characteristics of the source throughout the visible part of the spectrum.


Spill Light (Stray Light)

Light emitted by a lighting installation which falls outside the boundaries of the site for which the lighting installation is designed.



Device for starting a discharge lamp, in particular a fluorescent lamp, that preheats the electrodes and/or causes a voltage surge in combination with the series ballast.



SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane. The name is derived from the Greek stereos for "solid" and the Latin radius for "ray beam".

Symbol: sr


Switching and Dimming Control

As more and more light sources can be dimmed economically, lighting controls need to provide both switching and light regulating capabilities.


Thermal Grease

Thermal grease is a thermally conductive compound that aids in the bonding of a heat sink to a device where heat dissipation is required, such as a microprocessor.


Thermal Pad

Thermal pads are used as an interface for heat dissipation between a component to be cooled and a heat sink. Many factors influence the decision to use thermal pad including thermal and electrical conductivity of the material, long-term stability, reliability, and ease of application. The fundamental characteristic of a thermal pad is how it can soften at higher temperatures. This allows it to conform to the surfaces that it is bonded to which increases surface area for heat transfer.


Thermal Tape

Thermal tape is a thermally conductive adhesive used to mount a heat sink to a device where heat dissipation is required, such as a microprocessor.


Threshold Increment (TI)

The measure of disability glare expressed as the percentage increase in the contrast required between an object and its background for it to be seen equally well with a source of glare present. Higher values of TI correspond to greater disability glare.



A method for lumen-depreciation projection based on LM-80 data. Provides guidelines for using LM-80 data to estimate the light source lumen maintenance beyond the LM-80 test period. LM-80 and TM-21 are designed to work hand-in-hand, with TM-21 using the LM-80 data, along with in-situ temperature performance data, to project the lumen maintenance of an LED light source.


Uniformity Ratio

Ratio of the minimum over the average luminance for a specified area (Emin/Eave). When defined as such, the uniformity ratio is also the ratio of the minimum over the maximum luminance for a specified surface area (Emin/Emax).


Ultra-Violet (UV)

The invisible portion of the electromagnetic spectrum with the wavelength between 1nm and 400nm.


Utilisation Factor

Utilisation factor (UF) of a lighting installation represents the percentage of the luminous flux of the lamp(s) that reaches the defined working plane. UF can be seen as the efficiency of the lighting installation. UF is used to calculate the number of luminaries required.



The source of light in all miniature lamps is an incandescent tungsten filament which must be protected from exposure to the atmosphere. In vacuum type lamps, the filament is protected by processing the lamp so that a vacuum exists inside the glass envelope. The level of vacuum prior to operation of the lamp is on the order of 10-2 mm Hg or better, and during operation lamps will generally improve to about 10-4 mm Hg because of gettering of residual gasses by tungsten evaporation.

Tungsten evaporation rates in vacuum lamps are generally much higher than in gas filled or halogen lamp types. As a consequence, normal bulb blackening causes an almost constant decrease in light output during the life of the lamp.


Vertical Illuminance

Luminance incident on the vertical surface.

Unit: lux (lx) = lm/m2

Symbol: Evert


Viewing Angle

The viewing angle refers to the cone shaped pattern in front of the light source. The viewing angle is determined to front and sides of the LED at which the light output falls off 50 percent.


Visible Spectrum

The light spectrum between 400 nm and 700 nm which is detectable by the human eye.


Visual Guidance

The sum of the measures taken to give the user of a space an unambiguous and immediately recognisable picture of the course of the path ahead. Visual guidance is important in shops and other public buildings. The lighting designer takes the demands for visual guidance into consideration.



A measurement of the electromotive force in an electrical circuit or device expressed in volts.

Symbol: V



The unit for measuring electrical power. Watt does not relate to the light output level, instead it defines the rate of energy consumed by the device.

Symbol: W



Distance between repeating units of a propagating wave of a given frequency. It is measured in meters (m) and is commonly designated by the Greek letter lambda (λ). The wavelength is related to the frequency by the formula: wavelength = wave speed / frequency. Wavelength is therefore inversely proportional to frequency. Higher frequencies have shorter wavelengths while lower frequencies have longer wavelengths, assuming the speed of the wave is the same. Visible light ranges from deep red, roughly 700 nm (10-9 m), to violet, roughly 400 nm.

Symbol: λ

* These definitions are provided for your convenience but 3elamp accepts no responsibility or liability for any mistake or defective definition.

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