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Glossary

Accent Lighting

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

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.

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

(in any plane) the angle 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 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)

The measure of a light source's ability to render the color of objects

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)

Current (I)

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

Cut-Off Angle

(of a luminaire) the angle 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

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.)

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 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 of illuminance equal to 1 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

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.

HID

See High Intensity Discharge Lamps.

High Intensity Discharge Lamps

A general group of lamps consisting of mercury, metal halide and high pressure sodium lamps. Also refered to as HID lamps.

Illuminance

Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux.

Incandescent Filament Lamp

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

Indirect Lighting

By luminaires distributing 90 to 100 percent of the emitted light upward.

Lamp

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

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.

LED (Light Emitting Diode)

A semiconductor device that emits optical radiation in the visible, UV, or IR region(s) when forward-biased.

LED Driver

Power source and control circuitry designed to operate an LED package, an LED module, or an LED lamp.

LED Package

An assembly of one or more LEDs, possibly with a lens or reflector and thermal, mechanical and electrical interfaces.

Lens

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

Level of Illumination

See illuminance.

Light

Radiant energy that is capable of producing a visual sensation.

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)

A unit of luminous flux; the overall light output of a luminous source is measured in 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.

Luminaire

A light fixture; the complete 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)

Light emitted, transmitted, or reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or square meters).

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 to the -9th meters; commonly used as a unit of wavelength.

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 greater than 0.90 are 'high power factor' devices.

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.

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.

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

Used to provide an additional quantity and quality of illumination that cannot be readily obtained by a general system and that supplements the general 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.

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 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 environment.