Lighting Glossary
Lighting Glossary 
absorption
A process by which incident radiant flux is converted to another form of energy, usually (and ultimately) heat.
accent light
Directional lighting designed to emphasize a particular object or to draw attention to a part of the field of view.
accommodation
The process by which the eye changes focusfrom one distance to another.
adaptation
The process by which the eye becomes accustomed to varying quantities of light or to light of a different color.
altitude
The vertical angular distance of a point in the sky above the horizon. Altitude is measured positively from the horizon to the zenith, from 0 to 90 degrees.
ambient light
Electric and/or natural lighting throughout a space that produces uniform general illumination.
artificial sky
An enclosure that simulates the luminance distribution of a real sky for the purpose of testing physical daylighting models. See hemispherical dome artificial sky and mirror-box artificial sky.
azimuth
The horizontal angular distance between the vertical plane containing a point in the sky and true south.
baffle
A single opaque or translucent element to shield a source from direct view at certain angles, or to absorb unwanted light.
ballast
A magnetic or electronic device used to control the starting and operation of discharge lamps.
ballast factor (BF)
The ratio of lamp lumen output on a particular ballast as compared to that lamp's (lamps') rated lumen output on a reference ballast under ANSI test conditions (free, unmoving air at 25° C)
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 component
That component of flux received directly (or by specular reflection or transmission) from a point source (such as the sun or small lamp). It is a direct component.
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.
blinding glare
Glare that is so intense that, for an appreciable length of time after it has been removed, no object can be seen.
bulb
The glass outer envelope component of a lamp.
brightness
See luminance.
candela (cd)
The SI unit of luminous intensity (formerly called the candle). One candela equals one lumen per steradian-the luminous intensity, in a give direction, of a source that emits monochromatic radiation at a frequency of 540E12 hertz and of which the radiant intensity in that direction is 1/683 watts per steradian.
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
Refers to the dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together.
clerestory
That part of a building rising clear of the roofs or other parts, whose walls contain windows for lighting the interior.
coefficient of utilization (CU)
The ratio of lumens from a luminaire received on the work plane to the total quantity of lumens emitted by the lamps of that luminaire.
color rendering index (CRI)
A measurement of the amount of color shift that objects undergo when lighted by a light source as compared with the color of those same objects when seen under a reference light source of comparable color temperature. CRI values generally range from 0 to 100.
color temperature
The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source (see correlated color 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.
cone
A retinal receptor that dominates the retinal response when the luminance level is high and provides the basis for the perception of color.
contrast
The ratio of the luminance of an object to that of its immediate background.
cosine law
The law that the illuminance on any surface varies as the cosine of the angle of incidence. (The angle of incidence is the angle between the normal to the surface and the direction of the incident light.)
current (I)
A measure of the flow of electricity, expressed in amperes (A).
cut-off angle
The critical viewing angle beyond which a source can no longer be seen because of an obstruction (such as a baffle or overhang).
dark adaptation
The process by which the retina becomes adapted to a luminance of less than 0.01 footlamberts.
daylight factor (DF)
The ratio of daylight illumination at a given point on a given plane due to the light received directly or indirectly from a sky of assumed or known luminance distribution, to the illumination on a horizontal plane due to an unobstructed hemisphere of this sky, expressed as a percentage. Direct sunlight is excluded for both values of illumination. The daylight factor is the sum of the sky component, the external reflected component, and the internal reflected component. The interior plane is usually horizontal. If the sky condition is the C.I.E. standard overcast condition, then the DF will remain constant, regardless of absolute exterior illuminance. If used in conjunction with other than standard overcast conditions, the sky conditions should be specified. The term is also informally applied to the ratio of horizontal interior to exterior illuminance in the fenestration plane; under clear sky conditions, the DF remains constant only if the fenestration is completely diffusing (such as an ideal opalescent glass).
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.
diffusing (surface)
Those surfaces and glazing that redistribute some of the incident flux by scattering in all directions.
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 . Often accompanied by discomfort glare.
discomfort glare
Glare producing discomfort. Does not necessarily interfere with visual performance or visibility.
efficacy
A measure of the luminous efficiency of a radiant flux, expressed in lumens per watt as the quotient of the total luminous flux by the total radiant flux. For daylighting, this is the quotient of visible flux incident on a surface to radiant flux on that surface. For electric sources, this is the quotient of the total luminous flux emitted by the total lamp power input.
emissivity/emittance
The ratio of radiance (for directional emissivity) or radiant exitance (for hemispherical emissivity) of an element of surface on a temperature radiator to that of a blackbody at the same temperature. By Kirchoff's Law, for a given wavelength of the electromagnetic spectrum, emissivity of a surface equals its absorptivity (and is the reciprocal of its reflectivity).
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
exitance
The density of light reflecting from a surface at a point, measured in lumens per square foot (formerly "footlamberts"). It is determined by multiplying the footcandles striking a diffuse reflecting surface times the reflectance of that surface.
fenestration
Any opening or arrangement of openings (normally filled with glazing media) for the admission of daylight, including any devices in the immediate proximity of the opening that affect distribution (such as baffles, louvers, draperies, overhangs, light-shelves, jambs, sills, and other light-diffusing materials).
fixture
Informal substitute term for luminaire.
fluorescent
A discharge lamp in which a phosphor coating transforms ultraviolet energy into visible light.
flux
The time rate of flow. For example, volume per hour is the flux of a fluid.
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.
footlambert (fl)
A unit of luminance equal to 0.3183010 candela per square foot, or to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at a rate of one lumen per square foot, or to the average luminance of a surface emitting or reflecting light at that rate. An unobstructed sky of one footlambert uniform luminance contributes one footcandle of illuminance on a horizontal plane.
fovea
A small region at the center of the retina, subtending about two degrees and forming the site of the most distinct vision and greatest color discrimination.
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
See direct glare, disability glare, discomfort glare, reflected glare.
glare index
A method of predicting the presence of discomfort glare due to daylighting. Factors affecting the glare index include the size and relative position of fenestration, sky luminance, and interior luminance. Most widely used in Europe, the glare index is similar to the index of sensation and the discomfort glare rating, which are used in North America for electric lighting applications.
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 lamps (HID)
A general group of lamps consisting of mercury, metal halide and high pressure sodium lamps. Also refered to as HID lamps.
hue
The attribute of a color that allows it to be classified as red, yellow, blue, and so on.
IALD
International Association of Lighting Designers
IESNA
Illuminating Engineering Society of North America
illuminance
The density of incident luminous flux on a surface; illuminance is the standard metric for lighting levels, and is measured in lux (lx) or footcandles (fc).
incandescence
The emission of visible electromagnetic radiation due to the thermal excitation of atoms or molecules.
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.
indirect sources
Surfaces which, after being illuminated by other sources (direct sources such as the sun, sky, or electric light, or other indirect sources), have measurable luminance and, in turn, become sources themselves.
infrared radiation (IR)
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.
inverse-square law
The law stating that the illuminance at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between that source and that surface.
irradiance (E)
The density of radiant flux incident on a surface.
isolux (isofootcandle) line
A line plotted on any appropriate set of coordinates to show all the points on a surface where the daylight illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.
lamp
An electrically energized source of light, commonly called a bulb or tube.
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 (LLD)
The decrease over time of lamp lumen output, caused by bulb wall blackening, phosphor exhaustion, filament depreciation and other factors.
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 exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum (light) extends from about 0.38 to 0.77 microns.
light loss factor (LLF)
A factor used in calculating the illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, lamp depreciation (of electric luminaries), dirt accumulation on luminaire and room surfaces, maintenance procedures and atmosphere conditions. Formerly called maintenance factor.
light shelf
A horizontal shelf positioned (usually above eye level) to reflect daylight onto the ceiling and to shield direct flare from the sky.
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 quantity of luminous flux emitted within a unit solid angle (one steradian) by a point source with one candella intensity in all directions.
lumen method
A method of estimating the interior illuminance due to window daylighting at three locations within a room. Based on empirical studies, the use of this method is primarily limited to North America.
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 complete lighting unit, consisting of a lamp or lamps together with the components required to distribute the light, position the lamps, and connect the lamps to a power supply. Often referred to as a "fixture."
luminaire dirt depreciation
A multiplier used in lighting calculations to account for the reduction in illuminance produced by the accumulation of dirt on a luminaire.
luminaire efficiency
The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.
luminance
The luminous intensity of a surface in a given direction per unit area of that surface as viewed from that direction; often incorrectly referred to as "brightness."
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)
The SI unit of illuminance equal to one lumen per square meter.
matte surface
Surface from which the reflection is predominantly diffuse, with or without a negligible specular component.
modeling
The effect of using highly directional light to create form through shadows and highlights.
monitor
A raised section of roof that includes a vertically (or near-vertically) glazed aperture for the purpose of daylighting illumination.
Nanometer (nm)
A unit of length equal to 10 -9 meters; commonly used as a unit of wavelength.
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 nonmetallic building materials.
overcast sky
A sky luminance distribution three times brighter near (C.I.E. Standard Condition) the zenith than at the horizon, as defined by a formula proposed by Moon and Spencer in 1942 and adopted by the Commission International de l'Eclairge in 1955
overhang
A horizontal building projection, usually above a window, for the purpose of shading.
photocell
A device that measures the amount of incident light present in a space.
peripheral vision
The seeing of objects displaced from the primary line of site and outside of the central visual field.
point method
A method of estimating the illuminance at various locations in a building using photometric data.
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.
pupil
The opening in the iris of the eye that admits light.
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.
radiant energy (radiation)
Energy traveling in the form of electromagnetic waves. Measured in units of energy such as joules, ergs, or kilowatt-hours.
reference ballast
A ballast specially constructed to have certain prescribed characteristics for use in testing electric-discharge lamps and other ballasts.
reflectance(rho)
The ratio of reflected flux to incident flux.
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 light changes as it passes obliquely from one medium to another in which its speed is different.
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.
retina
A light-sensitive membrane lining the posterior part of the inside of the eye.
rods
Retinal receptors that respond to low levels of luminance but cannot distinguish hues. Not present in the center of the fovea region.
room cavity ratio
In lighting calculations, a measure of room proportion as determined by dimensions of length, width, and height.
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.
skylight
A relatively horizontal glazed roof aperture for the admission of daylight.
spectral power distribution (SPD)
A curve illustrating the distribution of power produced by the lamp, at each wavelength across the spectrum.
specular angle
The angle of mirror reflection (angle of incidence equals angle of reflectance).
specular reflection
The process by which incident light is redirected at the specular (mirror) angle.
specular surface
A shiny, highly polished surface which reflects light at an angle equal to that of the incident light.
specular transmission
The process by which incident flux passes through a surface or medium without scattering.
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.
task light
Light that is directed to a specific surface or area to provide illumination for visual tasks.
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.
ultraviolet radiation (uv)
Any radiant energy within the wavelength range of 0.001 to 0.38 microns.
veiling reflection
Specular reflection superimposed upon diffuse reflection from an object that partially or totally obscures the details to be seen by reducing the contrast. Controlled by distributing the source over a larger area, relocating the source out of the reflected field of view, changing the task surface specular reflectance or tilt, or relocating the observer.
visual acuity
A measure of the ability to distinguish fine details.
visual comfort probability (VCP)
The rating of a lighting system expressed as a percentage of the people who, when viewing from a specified location and in a specified direction, will be expected to find it comfortable in terms of discomfort glare.
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 (E)
A measure of electrical potential, expressed in volts (V).
watt (W)
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.
