Glossary

accent lighting
Directional lighting to emphasize a particular object or to draw attention to a part of the field of view. See directional lighting.

adaptation
The process by which the retina of the eye becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity of the eye to light.

amperes
A measure of electrical current. In incandescent lamps, the current is relative to voltage and power as follows: Current (Amps) = Power (Watts) / Voltage (Volts).

baffle
A single opaque or translucent element to shield a source from direct view at certain angles, or to absorb unwanted light.

ballast
A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and wave form) for starting and operating; all fluorescent and HID light sources require a ballast for proper operation. Dimming ballasts are special ballasts which when used together with a dimmer will vary the light output of a lamp.

ballast factor, BF
The measured ability of a particular ballast to produce light from the lamp(s) it powers; ballast factor is derived by dividing the lumen output of a particular lamp/ballast combination by the lumen output of the same lamp(s) on a reference ballast.

beam angle
The angle between the two directions for which the intensity (candlepower) is 50% of the maximum intensity as measured in a plane through the nominal beam centerline (center beam candlepower).

beam spread
The angle (in any plane) between the two directions in the plane in which the candlepower is equal to a stated percent (usually ten percent) of the maximum candlepower in the beam.

brightness
See luminance.

candela, cd
The unit of measure indicating the luminous intensity (candlepower) of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered.

candlepower
A term used for the luminous intensity of a light source. The intensity in any one direction from the standard candle is called one candela (formerly one candlepower; cp).

candlepower distribution
A curve that represents the variation in luminous intensity (expressed in candelas) in a plane through the light center of a lamp or luminaire; each lamp or lamp/luminaire combination has a unique set of candlepower distributions that indicate how light will be spread.

candlepower distribution curve
A curve, generally polar, representing the variation of luminous intensity of a lamp or luminaire in a plane through the light center.

center beam candlepower, CBCP
The intensity of light produced at the center of a reflector lamp, expressed in candelas.

chromaticity
The aspect of color that includes consideration of its dominant wavelength and purity.

coefficient of utilization, CU
The ratio of the luminous flux (lumens) from a luminaire received on the work-plane to the lumens emitted by the luminaire's lamps alone.

color rendering index (CRI)
A measurement of the color shift an object undergoes when illuminated by the light source, as compared to a reference source at the same color temperature. Color rendering is measured on an index from 0-100, with natural daylight and incandescent lighting both equal to 100. Objects and people viewed under lamps with a high color rendering index (CRI) appear more true to life.

color temperature
The chromaticity of an ideal "black body" when it is heated to a specific temperature in Kelvin (K). It is the measure of the color of light, not actual temperature.

correlated color temperature, CCT
A specification of the color appearance of a lamp relating its color to that of a reference source heated to a particular temperature, measured in degrees Kelvin (K); CCT generally measures the "warmth" or "coolness" of light source appearance.

current, I
A measure of the flow of electricity, expressed in amperes (A).

cut-off angle
The angle (of a luminaire) from the vertical at which a reflector, louver or other shielding device cuts off direct visibility of a light source. It is the complementary angle of the shielding angle.

diffuse lighting
Light that is not predominantly incident from any particular direction.

diffuser
A device to redirect of scatter the light from a source by the process of diffuse transmission.

direct glare
Glare resulting from high luminance or insufficiently shielded light sources in the field of view, or from reflecting areas of high luminance. It is usually associated with bright areas such as luminaires, ceilings and windows that are outside the visual task or region being viewed.

direct lighting
Lighting by luminaires distributing 90 to 100 percent of the emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction. (See accent lighting.)

directional lighting
Illumination on the work-plane or on an object predominantly from a single direction.

disability glare
Glare resulting in reduced visual performance and visibility. It is often accompanied by discomfort.

discomfort glare
Glare producing discomfort. It does not necessarily interfere with visual performance or visibility.

efficacy
Efficiency of a light source expressed in lumens per watt (LPW or lm/W).

energy
A measure of work done by an electrical system over a given period of time, often expressed in kilowatt-hours (kWh).

enhancing reflections
Reflections which enhance appearance described in such terms as sparkling, glittering, etc.

ESCO
Energy Saving service Company

fenestration
Any opening or arrangement of openings or windows (normally filled with media for light control) for the admission of daylight or for the transmission of electric lighting from one room to another room.

fixture
See luminaire.

fluorescent lamp
A low pressure mercury electric discharge lamp, tubular in shape in which a fluorescent coating (phosphor) transforms ultraviolet energy into visible light.

footcandle, fc
A unit used in measuring direct illumination. It is defined as the illumination produced from a source of one candela at a point on a surface of one foot away and perpendicular to the source of light. A lumen per square foot.

frequency
The number of times per second that an alternating current system reverses from positive to negative and back to positive, expressed in cycles per second or hertz, Hz.

general lighting
Lighting designed to provide a substantially uniform illuminance throughout an area, exclusive of any provision for special local requirements.

glare
Excessive brightness that may be caused by either direct or indirect viewing of a light source; any brightness or brightness relationship that annoys, distracts or reduces visibility.

harmonic
An electrical frequency that is an integer multiple of the fundamental frequency; for example, if 60 Hz is the fundamental freqency, then 120 Hz is the second harmonic and 180 Hz is the third harmonic; some electronic devices, such as ballasts or power supplies, can cause harmonic distortion, directly affecting power quality.

Hertz, Hz
A unit of freqency equal to one cycle per second; see frequency.

high intensity discharge (HID) lamps
A general group of lamps consisting of mercury, metal halide and high pressure sodium lamps.

hot restrike
The lamp does not have to cool down before being turned back on.

illumination
The result of the use of light, illuminance, the density ofluminous flux on a surface, is usually measured in footcandles.

incandescent filament lamp
A lamp in which light is produced by a filament heated to incandescence by an electric current.

indirect lighting
Lighting by luminaires distributing 90 to 100 percent of the emitted light upward.

instant start
A circuit used to start specially designed fluorescent lamps without the aid of a starter. The circuit utilizes higher open circuit voltage than is required for the same length preheat lamps, to strike the arc instantly.
This circuit is used today in slimline and cold cathode lamps. Instant start 40-watt bipin lamps are made with a short-circuiting device built into the base.

ionization smoke detector
Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. Ionization smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.
Ionization smoke detectors feature a harmless radioactive source within a dual detection chamber. Ionizaiton smoke detectors repsond to invisible by-products of combustion. They operate by sensing for a change in the electrical conductivitiy across the detection chamber. The advantage of the ionization detector is that the smoke can be invisible to the human eye, while remaining very much visible to the ionization detector.
Ionization smoke detectors respond first to fast flaming fires. A flaming fire devours combustibles extremely fast, spreads rapidly, and generates considerable heat with little smoke.

kilowatt, kW
A larger unit of power, a thousand watts (watts x 1000 kilowatts).

kilowatt hour (kWh)
The measure of electrical usage from which electricity billing is determined. For example, a 100-watt bulb operated for 1000 hours would consume 100 kilowatt hours (100 watt x 1000 hours = 100 kWh). At a billing rate of $0.10/kWH, this bulb would cost $10.00 (100 kWh x $0.10/kWh) to operate.

lamp
Manufactured light source; the 3 broad categores of electric lamps are incandescent, fluorescent, and high intensity discharge (HID). Also a generic term for a man-made source of light.

lamp life
An average rating, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current; manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID lamps when performing lamp life testing procedures; every lamp type has a unique mortality curve that depicts its average rated life.

lamp lumen depreciation factor, LLD
The multiplier to be used in illumination calculations to relate the initial rated output of light sources to the anticipated minimum rated output based on the relamping program to be used.

LEED (Leadership in Energy and Environmental Design)
LEED is a third party certification program and the nationally accepted benchmark for the design, construction and operation of high performance green buildings. LEED promotes a whole-building approach to sustainability by recognizing performance in five key areas of human and environmental health: sustainable site development, water savings, energy efficiency, materials selection and indoor environmental quality.

lens
A glass or plastic element used in luminaires to change the direction and control the distribution of light rays.

level of illumination
See luminance.

light
The term generally applied to the visible energy from a source. Light is usually measured in lumens or candlepower. When light strikes a surface it is either absorbed, reflected or transmitted. Light is said to travel in straight lines.

light loss factor, LLF
A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions. Formerly called maintenance factor.

light source
See lamp.

louver
A series of baffles used to shield a source from view at certain angles or to absorb unwanted light. The baffles are usually arranged in a geometric pattern.

lumen, lm
The unit that expresses the total quantity of light given off by a source, regardless of direction. A lumen is defined as the amount of light falling on a surface of one square foot, every point of which is one foot away from a source of one candlepower. A uniform source of one candlepower placed in a sphere emits 12.57 lumens, or mean spherical candela equals 12.57 lumens.

lumen depreciation
The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve (sometimes called lumen maintenance curve) depicting the pattern of decreasing light output.

lumen maintenance
See lumen depreciation.

lumens per watt (LPW)
A measure of the efficacy of a light source in terms of the light produced for the power consumed. For example, a 100-watt lamp producing 1750 lumens gives 17.5 lumens per watt.

luminaire
A light fixture; the complete lighting unit, including lamp, reflector, ballast, socket, wiring, diffuser, and housing.

luminaire efficiency
The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

luminance (L) or brightness
The light emitted, transmitted or reflected from a unit area of the source of surface is its brightness. It is usually expressed in candles per square inch or Lamberts or foot Lamberts.

luminance contrast
The relationship between the luminances of an object and its immediate background.

luminance ratio
The ratio between the luminances of any two areas in the visual field.

luminous flux
The time rate of flow of light.

lux, lx
A unit of illuminance equal to 1 lumen per square meter.

matte surface
A non-glossy dull surface as opposed to a shiny (specular) surface. Light reflected from a matte surface is diffuse.

modeling
The effect of using highly directional light to create form through shadows and highlights.

nanometer, nm
A unit of length equal to 10-9 meters; commonly used as a unit of wavelength.

non hot restrike
The lamp must cool down before being turned back on.

photoelectric smoke detector
Photoelectric smoke detectors look for the presence of visible by-products of combustion in the detection chamber. When a sufficient density of visible combustibles fill the detection chamber, the photoelectric smoke detector sounds an alarm condition.
Photoelectric smoke detectors respond first to slow smoldering fires. A smoldering fire generates large amounts of thick, black smoke with little heat and may smolder for hours before bursting into flames.

power
The rate at which energy is taken from an electrical system or dissipated by a load, expressed in watts (W); power that is generated by a utility is typically expressed in volt-amperes (V-A).

power factor
A measure of the effectiveness with which an electrical device converts volt-amperes to watts; devices with power factors >0.90 are "high power factor" devices.

preheat
A circuit used in fluorescent lamps where the electrodes are heated or warmed to a glow stage, by an auxiliary switch or starter (can be a glow switch, thermal type or mechanical device like a push button) before the lamps are lighted. This system was used on the original fluorescent lamps and is still in use today.

quality of lighting
Pertains to the distribution of luminance in a visual environment. The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety and esthetics for the specific visual tasks involved.

quantity of light
The product of the luminous flux by the time it is maintained. It is the time integral of luminous flux.

rapid start
A circuit designed to start lamps by continuously heating or preheating the electrodes. This circuit is a modern version of the trigger start system and requires lamps designed for this circuit. In the rapid start two-lamp circuit, one end of each lamp is connected to a separate starting winding. The other end of each lamp is connected to a common winding. Except for slimline lamps, all modern fixtures using 40-watt and higher lamps are equipped with Rapid Start ballasts.

rated average life
The operating life (hours) at which 50% of the lamps are still operating. Where a plus (+) is used in starting the life, survival rate is 67% at the started time.

reference ballast
A ballast specially constructed to have certain prescribed characteristics for use in testing electric-discharge lamps and other ballasts.

reflectance, rho
The percentage of light reflected back from a surface, the difference having been absorbed or transmitted by the surface.

reflected glare
Glare resulting form specular reflections of high luminance in polished or glossy surfaces in the field of view.

reflection
The process by which flux leaves a surface or medium from the incident side.

reflector
A device used to redirect the light by the process of reflection.

refraction
The process by which the direction of a ray of light changes as it passes obliquely from one medium to another.

refractor
A device used to redirect the luminous flux from a source, primarily by the process of refraction.

resistance, R
A measure of resistance to flow of current, expressed in ohms.

shielding
A general term to include all devices used to block, diffuse or redirect light rays, including baffles, louvers, shades, diffusers and lenses.

shielding angle
The complementary angle of the cut-off angle of a luminaire.

smoke detector
Smoke detectors consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up. Smoke detectors can run off of a 9-volt battery or 120-volt house current. The two most common types of smoke detectors used today are photoelectric smoke detectors and ionization smoke detectors.

spectral power distribution, SPD
A curve illustrating the distribution of power produced by the lamp, at each wavelength across the spectrum.

specular surface
A shiny, highly polished surface which reflects light at an angle equal to that of the incident light.

supplementary lighting
Lighting used to provide an additional quantity and quality of illumination that cannot be readily obtained by a general lighting system and that supplements the general lighting level usually for specific task requirements.

transmission
The process by which incident flux leaves a surface or medium on a side other than the incident side, the characteristics of many materials such as glass, plastics and textiles.

transmittance
The ratio of the flux transmitted by a medium to the incident flux.

trigger start
A circuit used to eliminate the starter and start the preheat lamp almost instantly. In this circuit, each electrode is connected to a separate winding in the ballasts so that the electrode is continuously heated. This circuit is primarily used on 20-watt and lower wattage fluorescent lamps today.

veiling reflections
Reflections which partially or totally obscure the details to be seen by reducing the contrast.

visual comfort probability, VCP
A discomfort glare calculation that predicts the percent of observers positioned in the least favorable part of the room who would be expected to judge a lighting condition to be comfortable. VCP rates the luminaire in its environment, taking into account such factors as illuminance level, room dimensions and reflectances, luminaire type, size and light distribution, number and location of luminaires, and observer location and line of sight. The higher the VCP the more comfortable the lighting environment.

visual field
The location of objects or points in a space where the head and eyes are kept fixed.

visual surround
All portions of the visual field except the visual task.

visual task
Those details and objects which must be seen for the performance of a given activity, including the immediate background of the details or objects.

voltage (V or E)
A measurement of electromotive force or the pressure of electricity. This is analogous to the pressure in a water line, i.e., pounds per square inch. The voltage of a circuit is the electrical pressure it gives. In an incandescent lamp "voltage" designates the supply voltage to which the lamps should be connected. In other lamp types, it may refer to "operating voltage" of a lighted arc discharge lamp.

watt, W
Unit used to measure power consumption of a lamp. A unit of electrical power equal to 1 joule per second.

work plane
The plane at which work usually is done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meters (30 inches) above the floor.