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Glossary

A

Absorption

Conversion of incident radiant flux (light) to another form of energy, usually heat.

Accent Lighting

Directional lighting designed to emphasise a particular object or to draw attention to a part of the field of view.

Adaptation

The process whereby the eye becomes accustomed to varying quantities of light or to light of a different colour.

Alternating Current (AC)

Flow of electricity which cycles in alternate direction many times per second.  The number of cycles per second is referred to as frequency.

Amalgam

A mixture of mercury and other metals used in Compact Fluorescent lamps to allow the lamps to have a stable light output over a wide range of temperatures. The amalgam causes the lamp to have a 60 second warm-up time.

Ambient Light

The general lighting present in an area - excluding task lighting and accent lighting but includes electric and/or natural lighting producing uniform general illumination.

Amperes (or amps)

The unit of measurement of electric current. This is current related to voltage and power:
Current (amps) =Power (watts) / Voltage (volts)

Arc

Intense luminous discharge formed by the passage of electric current in a gaseous medium across a space between electrodes.

Arc Lamp

A light source containing an arc (see above).

Average Rated Life

The number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life.

B

Ballast

A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation.

Barn Doors

Typically, it consists of four adjustable shields that are attached to the face of the luminaire to reduce glare.

Base

The end of the lamp which inserts into a lamp holder.

Base Temperature (Maximum)

The maximum operating temperature permitted for the base in Celsius. Fixture manufacturers need to ensure that these conditions are satisfied in their fixture.

Bayonet

A lamp cap type, which uses keyways instead of threads to connect the bulb to the fixture base. The bulb is locked in place by pushing it down and turning it clockwise.

Beam Angle

The angle at which luminous intensity is 50 percent of the maximum intensity.

Brightness

A visual sensation that describes how much light an area appears to emit. There are no units of measure for brightness; its measurement is relative.

Bulb

The glass outer or envelope of a lamp. A lamp is commonly referred to as a bulb.

Burning Position

The position in which a lamp is designed to be operated.

C

Candela

The unit of measure for intensity in a given direction. The term has been retained from the early days of lighting when a standard wax candle of a fixed size and composition was used as a basis for evaluating the intensity of other light sources.

Capacitor

A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field.

Cathode

Metal filaments that emit electrons in a fluorescent lamp. Negatively charged free electrons emitted by the cathode are attracted to the positive electrode (anode), creating an electric current between the electrodes.

Chromaticity

Refers to the dominant or complementary wavelength and purity aspects of the colour taken together, or of the aspects specified by the chromaticity coordinates of the colour taken together.

Colour Appearance

The colour impression when looking directly at a light source.

Colour Consistency

The measure of how close in colour appearance random samples of a lamp or source tends to be.

Colour Rendering

Expression for the effect that the light has on the colour appearance of objects.

Colour Temperature

A term used to describe the "whiteness" of light. It is the temperature of a piece of metal (or black body) that emits the same colour light as the compared light source.

Compact Fluorescent Lamp (CFL)

The general term applied to fluorescent lamps that are single-ended and have smaller diameter tubes that are bent to form a compact shape.

Cool White

A term loosely used to denote a colour temperature of around 4,100 K. The Cool White (CW) designation is used specifically for T12 and other fluorescent lamps using halo phosphors and having a CRI of 62.

Cut-off Angle

The critical viewing angle beyond which a source can no longer be seen because of an obstruction.

D

Daylight Harvesting

Lighting design for building interiors that makes use of daylight as a way of reducing energy consumption.

Daylight Lamp

A lamp resembling the colour of daylight, typically with a colour temperature of 5500K to 6500K.

Dichroic Reflector

A reflector (or filter) that reflects one region of the spectrum while allowing the other region(s) to pass through. A reflector lamp with a dichroic reflector will have a "cool beam" i.e. most of the heat has been removed from the beam by allowing it to pass through the reflector while the light has been reflected.

Diffuser

Device used to alter the distribution of light and depending essentially on the phenomenon of diffusion.

Diffusing

Those surfaces and glazing that redistribute some of the incident flux by scattering in all directions.

Diffused Lighting

Lighting in which the light is not coming mainly from one particular direction.

Dimmer, Dimming Control

A device used to lower the light output of a source, usually by reducing the wattage it is being operated at. Dimming controls are increasing in popularity as energy saving devices.

Direct Light

Light emitted by a luminaire in the general direction of the task to be illuminated. The term usually refers to light emitted in a downward direction.

Directional Lighting

Lighting in which the light on the working plane or on an object is coming predominantly from one particular direction.

Disability Glare

Glare resulting in reduced visual performance and visibility.

Dispersion

To split light into the colours of the rainbow.

Downlight

Small luminaire, which distributes the light downward, usually recessed in the ceiling.

Driver

For Light Emitting Diodes, a device that regulates the voltage and current powering the source.

E

Efficacy

Efficacy is the rate at which a lamp is able to convert electrical power (watts) into light (lumens), expressed as lumens per watt. Lumens/Watts=LPW

Electronic Ballast

A shortened 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 25-35 kHz. The benefits are: increased lamp efficacy, reduced ballast losses, and lighter, smaller ballasts.

Emergency Lighting

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

Escape Lighting

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

Eye Sensitivity

A curve depicting the sensitivity of the human eye as a function of wavelength (or colour). The peak of human eye sensitivity is in the yellow-green region of the spectrum.

F

Field Angle

The angular dimension of the cone of light from reflectorised 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.

Filament

Metal tungsten wire heated by the passage of electrical current, used to emit light in incandescent lamps. In fluorescent lamps the filament is coated with emission mix and emits electrons when heated.

Fixture

Informal substitute term for luminaire.

Flood

Used to refer to the beam pattern of a reflector lamp, which disperses the light over a wide beam angle, typically 20 degrees or more.

Floodlight

A luminaire used to light a scene or object to a level much brighter than its surroundings. Usually floodlights can be aimed at the object or area of interest.

Fluorescence

A physical phenomenon whereby an atom of a material absorbs a photon of light an immediately emits a photon of longer wavelength. If there is a significant delay the phenomenon is called phosphorescence rather than fluorescence. It is interesting that "phosphors" used in lamps exhibit "fluorescence," not "phosphorescence."

Fluorescent Lamp

A high efficiency lamp utilising an electric discharge through inert gas and low pressure mercury vapour to produce ultraviolet (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.

Footcandle

A standard measurement of illuminance, representing the amount of illuminance on a surface one foot square on which there is a uniformly distributed flux of one lumen.

Fovea

A small region at the centre of the retina, subtending about two degrees and forming the site of the most distinct vision and greatest colour discrimination.

Frequency

Rate of alternation in an AC current. Expressed in cycles per second or Hertz (Hz).

G

General Lighting

Substantially uniform lighting of an area, excludes task lighting.

Glare

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 amount of light.

H

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.

Hertz (Hz)

Unit used to measure frequency of alteration of current or voltage

High Intensity Discharge Lamps (HID)

HID lamps include groups of lamps known as Mercury Vapour, Metal Halide and High Pressure Sodium.

I

Illuminance

The "density" of light (lumens/area) incident on a surface; i.e. the light level on a surface. Illuminance is measured in footcandles or lux.

Illumination

Application of light to a scene, objects or their surroundings so that they may be seen.

Incandescent Lamp

A light source that generates light utilising a thin filament wire (usually of tungsten) heated to white heat by an electric current passing through it.

Indirect Lighting

The method of lighting a space by directing the light from luminaires upwards towards the ceiling. The light scattered off the ceiling produces a soft, diffuse illumination for the entire area.

Infrared Radiation

Radiation with wavelengths too long to be perceived by the human eye (that is, longer than 0.77 microns) and less than 1,000 microns. Room IR is infrared radiation in the 7.7-8.0 micron region and typical of that radiated from surfaces near room temperature.

Intensity

Intensity of a source in a given direction. Used mainly in directional light sources, the strength of the beam. Unit candela, cd.

Inverse Square Law

Formula stating that if you double the distance from the light source, the light level goes down by a factor of 4, if you triple the distance, it goes down by a factor of 9, and so on.

K

Kelvin

A unit of temperature starting from absolute zero, parallel to the Celsius (or Centigrade) scale. 0C is 273K.

Kilowatt (Kw)

A measure of electrical power equal to 1000 watts.

Kilowatt-Hour (KwH)

Unit of electrical power consumed over a period of time. One thousand watts used for one hour equals one kilowatt-hour.

L

Lamp

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

Lamp Rated Life

The number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp, or group of lamps, may vary from the published rated life. Rated life is based on standard test conditions.

Light

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

Light Emitting Diode (LED)

A semicondurtor that when positively biased emits visible light. Some LEDs today incorporate fluorescent materials to change the colour characteristics of the emitted light.

Light Pollution

Light that is directed to areas where it is not needed, and thereby interferes with some visual act.

Light Trespass (Spill Light)

Light that is not aimed properly or shielded effectively can spill out at into areas that don't want it: it can be directed towards drivers, pedestrians or neighbours.

Louvre

A fixed shield, usually divided into small cells, which is attached to the face of a luminaire to reduce direct glare.

Lumens

A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time.

Lumens/Watts

A ratio expressing the luminous efficacy of a light source.

Luminaire

A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply.

Lux (lx)

A unit of illuminance or light falling onto a surface. One lux is equal to one lumen per square meter. Ten lux approximately equals one footcandle.

M

Maximum Beam Candlepower (MBCP)

The maximum intensity of the beam in a given direction.

Mercury Vapour Lamp

A high-pressure mercury lamp in which the light is produced by the mercury vapour and sometimes by a layer of fluorescent material on the inner surface of the outer bulb excited by the ultraviolet radiation of the discharge.

Metal Halide Lamp

Discharge lamp which produces a white light with good colour rendering and high efficiency.

Monochromatic Light

Light with only one wavelength (i.e. colour) present..

Mortality Rate

The number of operating hours elapsed before a certain percentage of the lamps fail.

Mounting Height

Distance from the bottom of the fixture to either the floor or work plane, depending on usage

N

Near Infrared (solar infrared)

The region of the electromagnetic spectrum between 0.77 to 1.4 microns. Most of the infrared solar radiation falls into this region. This near infrared (or solar IR) region is transmitted, absorbed, and reflected in a similar manner to visible light by most glazing and non-metallic building materials.

O

Operating Current

Current in amps consumed by a lamp at rated watts.

Operating Position 

The manufacturer-recommended operating position for a lamp.

Operating Voltage

The measured voltage across the  lamp during operation.

P

PAR Lamp

An incandescent or tungsten-halogen incandescent lamp with a hard glass bulb and an interior reflecting surface, a precisely placed filament, and a lens to control beam spread. Can be used outdoors unprotected as they are made of "hard" glass that can withstand adverse weather.

Peak Intensity

The luminous intensity of a luminaire or lamp in the direction of the beam axis.

Pendant Luminaire

A luminaire which is suspended from a ceiling or other support by using a cord, chain etc.

Photocell

A device that measures the amount of incident light present in a space. Photocells are usually used as a switch to turn on or off.

Point Method

A method of estimating the illuminance at various locations in a building using photometric data.

Point Source

A source of light in which the dimensions are small, compared with the distance between the source and the working surface. Produces sharp shadows.

Power factor (PF)

A general term for the release of energy in a "wave" or "ray" form. All light is radiant energy or radiation, as is heat, UV, microwaves, radio waves, etc.

R

Radiation

A general term for the release of energy in a "wave" or "ray" form. All light is radiant energy or radiation, as is heat, UV, microwaves, radio waves, etc.

Rapid Start Fluorescent Lamp

A fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starter or the application of high voltage. Takes about one second to start.

Rated Average Lamp Life

Also referred to as Lamp Rated Life. Lamps are tested in controlled settings and the point at which 50% of a given sample burns out is listed as the lamps' rated average lamp life.

Recessed Luminaire

Luminaire mounted above the ceiling or behind a wall or other surface so that any visible projection is insignificant.

Reflector Lamp

Lamp in which part of the bulb is coated with a reflecting material, either diffuser or specular, so as to control the light.

Room Cavity Ratio

In lighting calculations, a measure of room proportion as determined by dimensions of length, width, and height.

S

Shielding

Blocking an electric or magnetic field with a metallic substance in order to prevent direct view of the light source. The incident field induces currents in the metallic substance, and these currents induce a field that opposes the incident field. Shielding reduces radiated electromagnetic waves. Electronic components, wires, lamps, and devices can all be shielded.

Spacing

The distance between the centres of two successive luminaires in an installation.

Spacing to Mounting Height Ratio (S/M)

The ratio of the distance between luminaire centres to the height above the work plane. Gives the maximum spacing of luminaires at which even illumination will be provided.

Speed of Light

Approximately 186,000 miles per second.

Specular Angle

The angle of mirror reflection (angle of incidence equals angle of reflectance).

Specular Reflection

Reflection from a smooth, shiny surface, as opposed to diffuse reflection.

Spill Light

Light that falls outside of the area intended to be lighted.

Spot

A colloquial term referring to a reflector lamp with a narrow beam of light, typically around 10 degrees or less. It comes from the fact that such a lamp produces a narrow spot of light as opposed to a wide flood of light.

System

A term referring to the lamp and ballast combination, and sometimes to the entire lighting delivery system including the fixture, the optics, the particular layout and the lighting controls.

T

Track Mounting

For track luminaires, the method by which the track is attached to the ceiling.

Transformer

Device used to raise or lower the voltage to a lamp.

Transmission

Passage of light through a material.

Tungsten-Halogen Lamp

A gas filled incandescent lamp with a tungsten filament containing a certain proportion of halogens.

U

Ultraviolet Radiation

Invisible radiation in which the wavelengths are shorter then those for visible radiation.

Uplight

Light directed upward at greater than 90° above nadir. The source of uplight can be from a combination of direct uplight and reflected light.

V

Valance Lighting

Lighting from light sources on a wall typically above eye level, shielded by horizontal panels. The light may be upward or downward directed.

Veiling Reflection

Effective reduction in contrast between task and its background caused by the reflection of light rays; sometimes called "reflected glare."

Visual Acuity

A measure of the ability to distinguish fine details.

Visual System

The group of structures comprising the eye, the optic nerve, and certain parts of the brain, which is responsible for seeing.

Volt

A measure of electrical pressure between two points. The higher the voltage, the more current will be pushed through a resistor connected across the points.

Voltage

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

W

Warm-up Time

The amount of time from turn-on to 90% light output.

Warm White

Refers to a colour temperature around 3000K, providing a yellowish-white light.

Watt

A unit of electrical power. Lamps are rated in watts to indicate the rate at which they consume energy.

Wavelength

The distance between two neighbouring crests of a travelling wave. The wavelength of light is between 400 and 700 nanometres.

Work Plane

This is the plane at which work is usually done. For instance, the table top is the working plane. 

Zenith

The direction directly above the luminaire. Zenith is opposite nadir. In astronomical usage, zenith is the highest point in the sky, directly above the observation point.