Glossary - Z

Z 

zonal-cavity inter reflectance method a procedure for calculating coefficients of utilization, wall exitance coefficients, and ceiling cavity exitance coefficients, taking into consideration the luminaire intensity distribution, room size and shape (cavity ratio concepts), and room reflectances. It is based on flux transfer theory. 

zonal constant a factor by which the mean intensity emitted by a source of light in a given angular zone is multiplied to obtain the lumens in the zone. See Chapter 2, Measurement of Light and Other Radiant Energy. 

zonal factor interreflection method a procedure used for calculating coefficients of utilization, based on integral equations, that takes into consideration the ultimate disposition of luminous flux from every 10° zone from luminaires. 

zonal factor method a procedure for predetermining, from typical luminaire photometric data in discrete angular zones, the proportion of luminaire output that would be incident initially (without interreflections) on the workplane, ceiling, walls, and floor of a room. 

zonal multipliers multipliers for the flux in each 10-degree conical zone from 0° (nadir) to 90° (horizontal) from a luminaire, expressing the fraction of that zonal flux that is directly incident on the floor of a room cavity. These multipliers are a function of the room cavity ratio and are used to determine the direct ratio.

Glossary - W

W 

watt† the unit of power (rate of doing work). In electrical calculations, one watt is the power produced by a current of one ampere across a potential difference of one volt. The symbol often used in equations is "P," although "W" is also acceptable. 

wavelength† the distance between two successive points of a periodic wave, in the direction of propagation, at which the oscillation has the same phase.

wide-angle diffusion that in which flux is scattered at angles far from the direction that it would take by regular reflection or transmission. See narrow-angle diffusion. 

wide-angle luminaire a luminaire that concentrates the light within the cone of a comparatively large solid angle. See narrow-angle luminaire. 

workplane the plane on which a visual task is usually done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 m (30 in.) above the floor.

Glossary - V

V 

vacuum lamp an incandescent lamp in which the filament operates in an evacuated bulb. 

valance a longitudinal shielding member mounted across the top of a window or along a wall (and is usually parallel to the wall) to conceal light sources, giving both upward and downward distributions. 

valance lighting comprising light sources shielded by a panel parallel to the wall at the top of a window. 

vapor-tight luminaire a luminaire designed and approved for installation in damp or wet locations. It also is described as enclosed and gasketed. 

veiling brightness a brightness superimposed on the retinal image that reduces its contrast. It is this veiling effect produced by bright sources or areas in the visual field that results in decreased visual performance and visibility. 

veiling reflection regular reflections that are superimposed upon diffuse reflections from an object that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called reflected glare. Another kind of veiling reflection occurs when one looks through a plate of glass. A reflected image of a bright element or surface can be seen superimposed on what is viewed through the glass plate. 

visibility the quality or state of being perceivable by the eye. In many outdoor applications, visibility is defined in terms of the distance at which an object can be just perceived by the eye. In indoor applications it usually is defined in terms of the contrast or size of a standard test object, observed under standardized viewing conditions, having the same threshold as the given object. See visibility (meteorological). 

vision† See central (foveal) vision, mesopic vision, peripheral vision, photopic vision, and scotopic vision. 

visual acuity a measure of the ability to distinguish fine details, measured with a set of optotypes (test types for determining visual acuity) of different sizes. Quantitatively, it is the reciprocal of the minimum angular size in minutes of the critical detail of an object that can just be seen. 

visual angle the angle that an object or detail subtends at the point of observation. It usually is measured in minutes of arc. 

visual comfort probability (VCP) the rating of a lighting system expressed as a percent of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual comfort probability is related to the discomfort glare rating (DGR). 

visual field the locus of objects or points in space that can be perceived when the head and eyes are kept fixed. Separate monocular fields for the two eyes can be specified or the combination of the two. See binocular portion of the visual field, central visual field, monocular visual field, and peripheral visual field. 

visual perception the interpretation of impressions transmitted from the retina to the brain in terms of information about a physical world displayed before the eye.  Note Visual perception involves any one or more of the following recognizing the presence of something (object, aperture, or medium); identifying it; locating it in space; noting its relation to other things; and identifying its movement, color, brightness, or form. 

visual performance the quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy. 

visual range (of a light or object) the maximum distance at which that particular light (or object) can be seen and identified. 

visual task conventionally designates those details and objects that must be seen for the performance of a given activity, and includes the immediate background of the details or objects.  Note The term visual task as used is a misnomer because it refers to the visual display itself and not the task of extracting information from it. The task of extracting information also has to be differentiated from the overall task performed by the observer. 

volt† the difference in electrical potential between two points in a circuit. It is also called the electromotive force. The symbol often used in equations is "E" (from the latter term), although "V" is also acceptable.

Glossary - U

U 

ultraviolet lamp a lamp that emits a significant portion of its radiative power in the ultraviolet (UV) part of the spectrum; the visible radiation is not of principal interest. 

upward component that portion of the luminous flux from a luminaire emitted at angles above the horizontal. See downward component. 

Glossary - T

T 

table lamp a portable luminaire with a short stand, suitable for standing on furniture. 

tail lamp a lighting device used to designate the rear of a vehicle by a warning light. 

task ambient lighting a combination of task lighting and ambient lighting within an area such that the general level of ambient lighting is lower than and complementary to the task lighting. 

task lighting directed to a specific surface or area that provides illumination for visual tasks. 

thermopile† a thermal radiation detector consisting of a number of thermocouples interconnected in order to increase the sensitivity to incident radiant flux. 

torchère an indirect floor lamp that sends all or nearly all of its light upward. 

total internal reflectance (TIR)† total reflection of a light ray at a surface of a transmitting medium occurs when the angle of incidence exceeds a certain value whose sine equals n2/n1, the ratio of indices of refraction, or when sin r = 1, where r equals

transition lighting in roadway lighting, lighting gauged to compensate for visual adaptation between regions of high and low light level, as when entering tunnels. 

translucent† transmitting light diffusely or imperfectly. 

transparent† having the property of transmitting rays of light through its substance so that bodies situated beyond or behind can be distinctly seen (opposed to opaque and usually distinguished from translucent). 

troffer a long recessed lighting unit usually installed with the opening flush with the ceiling. The term is derived from "trough" and "coffer"

Glossary - S

S 

scoop a floodlight consisting of a lamp in an ellipsoidal or paraboloidal matte reflector, usually in a fixed relationship, though some types permit adjustment of the beam shape. 

scotopic vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m2. See photopic vision. 

self-ballasted lamps any arc discharge lamp of which the current-limiting device is an integral part. 

semi-direct lighting involving luminaires that distribute 60 to 90% of the emitted light downward and the balance upward. 

semi-indirect lighting involving luminaires that distribute 60 to 90% of the emitted light upward and the balance downward. 

service period the number of hours per day for which daylighting provides a specified illuminance level. It often is stated as a monthly average. 

set light in theatrical lighting, the separate illumination of background or scenic elements. 

shade a screen made of opaque or diffusing material that is designed to prevent a light source from being directly visible at normal angles of view. 

shielding angle (of a luminaire) the angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible. See cutoff angle (of a luminaire). 

side light lighting from the side to enhance subject modeling and place the subject in depth, apparently separated from the background. 

signal shutter a device that modulates a beam of light by mechanical means for the purpose of transmitting intelligence. 

sky factor the ratio of the illuminance on a horizontal plane at a given point inside a building due to the light received directly from the sky, to the illuminance due to an unobstructed hemisphere of sky of uniform luminance equal to that of the visible sky. 

sky light† visible radiation from the sun redirected by the atmosphere. 

soft light (1) diffuse illumination that produces soft-edged, poorly defined shadows on the background when an object is placed in its path; (2) a luminaire designed to produce such illumination. 

solar efficacy the ratio of the solar illuminance constant to the solar irradiance constant. The current accepted value is 94.2 lm/W. 

spacing-to-mounting-height ratio, S/MH_wp† the ratio of the actual distance between luminaire centers to the mounting height above the workplane. Also, an obsolete term that described a characteristic of interior luminaires. See luminaire spacing criterion. 

special color rendering index, Ri measure of color shift of various standardized special colors, including saturated colors, typical foliage, and Caucasian skin. It also can be defined for other color samples when the spectral reflectance distributions are known. 

specular surface one from which the reflection is predominantly regular. See regular (specular) reflection. 

speed of light† the speed of all radiant energy, including light, is 2.9979258 × 10⁸ m/s in vacuum (approximately 186,000 mi/s). In all material media the speed is less and varies with the material's index of refraction, which itself varies with wavelength. 

sphere illumination illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center. 

spotlight any of several different types of luminaires with narrow beam angle designed to illuminate a well-defined area. In motion pictures, generic for Fresnel lens luminaires. Also, a form of floodlight, usually equipped with lenses and reflectors to give a fixed or adjustable narrow beam. 

standard source an electric light source having the same spectral power distribution as a specified standard illuminant. 

starter a device used in conjunction with a ballast for the purpose of starting an electric-discharge lamp. 

state of chromatic adaptation the condition of the chromatic properties of the visual system at a specified moment as a result of exposure to the totality of colors of the visual field currently and in the past. 

stray light (in the eye) light from a source that is scattered onto parts of the retina lying outside the retinal image of the source. 

street lighting luminaire a complete lighting device consisting of a light source and ballast, where appropriate, together with its direct appurtenances such as globe, reflector, refractor, housing, and such support as is integral with the housing. The pole, post, or bracket is not considered part of the luminaire.  Note Modern street lighting luminaires contain the ballasts for high-intensity discharge lamps where such lamps are used; a photocontrol can be mounted on the luminaire. 

street lighting unit the assembly of a pole or lamp post with a bracket and a luminaire. 

striplight (theatrical) once an open trough reflector containing a series of lamps; now usually a compartmentalized luminaire with each compartment containing a lamp, reflector, and color frame holder, wired in rotation in three or four circuits and used as borderlights, footlights, or cyclorama lighting from above or below. Often in short 0.9- to 2.4-m [3- to 8-ft] portable sections. 

stroboscopic lamp (strobe light) a flash tube designed for repetitive flashing. 

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

surface-mounted luminaire a luminaire that is mounted directly on a ceiling. 

suspended (pendant) luminaire a luminaire that is hung from a ceiling by supports. 

Systeme Internationale (SI)† a measurement system used throughout the world, commonly referred to as the metric system. Public Law 100-418 designated the metric system as the preferred system of weights and measures for the United States. 

Glossary - R

R 

radiant energy, Q energy traveling in the form of electromagnetic waves. It is measured in units of energy such as joules or kilowatt hours. See spectral radiant energy. 

radiometry the measurement of quantities associated with radiant energy and power. 

rated lamp life the life value assigned to a particular type lamp. This is commonly a statistically determined estimate of average or of median operational life. For certain lamp types other criteria than failure to light can be used; for example, the life can be based on the average time until the lamp type produces a given fraction of initial luminous flux. 

reflectance of a surface or medium, ρ = Φr/Φi the ratio of the reflected flux to the incident flux. Reflectance is a function of: 

1. Geometry 

a. of the incident flux  b. of collection for the reflected flux 

2. Spectral distribution

a. characteristic of the incident flux  b. weighting function for the collected flux 

3. Polarization 

a. of the incident flux  b. component defined for the collected flux. 

reflectance factor, R the ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal (glossless), perfectly diffuse (lambertian), completely reflecting standard surface irradiated in exactly the same way as the sample. Note the analogies to reflectance in the fact that nine canonical forms are possible that "spectral" can be applied as a modifier, that it can be luminous or radiant reflectance factor, and so on. Note that reflectance cannot exceed unity, but reflectance factor can have any value from zero to values approaching infinity. 

reflected glare glare resulting from reflections of high luminances in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. See veiling reflection. 

reflection a general term for the process by which the incident flux leaves a (stationary) surface or medium from the incident side without change in frequency.  Note Reflection is usually a combination of regular and diffuse reflection. See regular (specular) reflection, diffuse reflection, and veiling reflection. 

reflectivity† reflectance of a layer of a material of such a thickness that there is no change of reflectance with increase in thickness. 

reflectometer a photometer for measuring reflectance.  Note Reflectometers can be visual or physical instruments. 

reflector a device used to redirect the flux from a source by the process of reflection. See retro-reflector. 

refraction† the process by which the direction of a ray 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 flux from a source, primarily by the process of refraction. 

retina a membrane lining the posterior part of the inside of the eye. It comprises photoreceptors (cones and rods) that are sensitive to light and nerve cells that transmit to the optic nerve the responses of the receptor elements. 

rhythmic light a light that when observed from a fixed point has a luminous intensity that changes periodically. See equal interval (isophase) light, flashing light, group flashing light, interrupted quickflashing light, and occulting light. 

room cavity the cavity formed by the plane of the luminaires, the work plane, and the wall surfaces between these two planes. 

room cavity ratio (RCR) a number indicating room cavity proportions, calculated from the length, width, and height. See zonal-cavity interreflectance method. 

room utilization factor (utilance) the ratio of the luminous flux (lumens) received on the workplane to that emitted by the luminaire. Note This ratio sometimes is called interflectance. Room utilization factor is based on the flux emitted by a complete luminaire, whereas coefficient of utilization is based on the total flux generated by the lamps in a luminaire. 

room surface dirt depreciation (RSDD) the fractional loss of task illuminance due to dirt on the room surfaces.

Glossary - Q

Q 

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 aesthetics for the specific visual tasks involved.  

Glossary - P

P 

panel (open) face exit sign† a transilluminated sign where both the exit legend and background are translucent. 

parking lamp a lighting device placed on a vehicle to indicate its presence when parked. 

partial diffusion† See incomplete diffusion. 

partly cloudy sky a sky that has 30 to 70% cloud cover. 

passing beams† See lower (passing) beams. 

pendant luminaire† See suspended (pendant) luminaire. 

perceived light source color the color perceived to belong to a light source. 

perceived object color† the color perceived to belong to an object resulting from characteristics of the object, of the incident light, and of the surround, the viewing direction, and observer adaptation. See object color. 

percent flicker a relative measure of the cyclic variation in output of a light source (percent modulation). It is given by the expression where A is the maximum and B is the minimum output during a single cycle. See Chapter 6, Light Sources. 

perfect diffusion that in which flux is uniformly scattered in accord with Lambert's cosine law. 

peripheral vision the seeing of objects displaced from the primary line of sight and outside the central visual field. 

peripheral visual field that portion of the visual field that falls outside the region corresponding to the foveal portion of the retina. 

phosphor mercury lamp† see mercury-fluorescent lamp. 

phosphorescence the emission of light as the result of the absorption of radiation, and continuing for a noticeable length of time after excitation (longer than approximately 10−8 s). 

phot, ph a unit of illuminance equal to one lumen per square centimeter. The use of this unit is deprecated. 

photobiology a branch of biology that deals with the effects of optical radiation on living systems. 

photometer an instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux, or illuminance. See densitomer, goniophotometer, illuminance (lux or footcandle) meter, integrating photometer, reflectometer, spectrophotometer, and transmissometer. 

photometry the measurement of quantities associated with light.  Note Photometry can be either visual, in which the eye is used to make a comparison, or physical, in which measurements are made by means of physical receptors. 

phototherapy the treatment of disease involving the use of optical radiation. 

physical photometer an instrument containing a physical receptor and associated filters that is calibrated so as to read photometric quantities directly. See visual photometer. 

pilot house control a mechanical means for controlling the elevation and train of a searchlight from a position on the other side of the bulkhead or deck on which it is mounted. 

point-by-point method† a method of lighting calculation, now called the point method. 

point method a lighting design procedure for predetermining the illuminance at various locations in

point source a source of radiation whose dimensions are sufficiently small, compared with the distance between the source and the irradiated surface, that these dimensions can be neglected in calculations and measurements. 

point vision the mode of vision of a small source of light such that the sensation is determined by its intensity rather than by its size. Point vision occurs with sources so small that their form or shape is not perceived and that they appear as points of light; this generally means less than 1-minute angular subtense.

pole (roadway lighting) a standard support generally used where overhead lighting distribution circuits are employed. 

portable lighting lighting involving equipment designed for manual portability. 

portable luminaire a lighting unit that is not permanently fixed in place. See table lamp and floor lamp. 

preheat (switch start) fluorescent lamp a fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrodes in order to start the arc. 

projector a lighting unit that, by means of mirrors and lenses, concentrates the light to a limited solid angle so as to obtain a high value of luminous intensity. See floodlight, searchlight, and signaling light. 

Glossary - O

O 

object color† the color of the light reflected or transmitted by an object when illuminated by a standard light source, such as CIE source A, B, C, or D65. See standard source and perceived object color.  

occulting light a rhythmic light in which the periods of light are clearly longer than the periods of darkness. 

opaque† impenetrable to light; not able to transmit, or not transmitting light. 

orientation the relation of a building with respect to compass directions. 

overcast sky one that has 100% cloud cover; the sun is not visible. 

overhang the distance between a vertical line passing through a specified point (often the photometric center) of a luminaire and the curb or edge of a roadway.

Glossary - N

N 

narrow-angle diffusion that in which flux is scattered at angles near the direction that the flux would take by regular reflection or transmission. See wide-angle diffusion. 

narrow-angle luminaire a luminaire that concentrates the light within a cone of a comparatively small solid angle. See wide-angle luminaire. 

normal lighting† permanently installed task and corridor electric lighting normally for use when the premises are occupied. 

Glossary - M

M 

maintenance factor (MF)† a factor formerly used to denote the ratio of the illuminance on a given area after a period of time to the initial illuminance on the same area. This term is obsolete and is no longer valid. See light loss factor. 

matte surface a surface from which the reflection is predominantly diffuse, with or without a negligible specular component. See diffuse reflection. 

mean spherical luminous intensity the average value of the luminous intensity in all directions for a

mercury lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury operating at a partial pressure in excess of 105 Pa (approximately 1 atm). Includes clear, phosphor-coated (mercury-fluorescent), and self-ballasted lamps. 

mercury-fluorescent lamp (phosphor mercury lamp) an electric-discharge lamp having a highpressure mercury arc in an arc tube and an outer envelope coated with a fluorescing substance (phosphor) that transforms some of the ultraviolet energy generated by the arc into light. 

mesopic vision vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, that is, between about 3.4 and 0.034 cd/m2. 

metal halide lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation--possibly in combination with metallic vapors such as mercury. Includes clear and phosphor-coated lamps. 

metamers lights of the same color but of different spectral power distribution.  Note The term "metamers" is also used to denote objects that, when illuminated by a given source and viewed by a given observer, produce metameric lights. 

modeling light illumination that reveals the depth, shape, and texture of a subject; key light, cross lighting, counter-key light, side light, back light, and eye light are types of modeling light. 

monocular visual field the field for a single eye. See binocular portion of the visual field. 

mounting height (roadway)† the vertical distance between the roadway surface and the center of the apparent light source of a luminaire. 

mounting height above the floor (MHf)† the distance from the floor to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. 

mounting height above the workplane (MHwp)† the distance from the work plane to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. 

multiple-beam headlamp a headlamp so designed as to permit the driver of a vehicle to use any one of two or more distributions of light on the road.

Glossaray - L

L 

lamp a generic term for a source created to produce optical radiation. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible.  Note Through popular usage, a portable luminaire consisting of a lamp with shade, reflector, enclosing globe, housing, or other accessories is also sometimes called a lamp. In such cases, in order to distinguish between the assembled unit and the light source within it, the latter is often called a bulb or tube, if it is electrically powered. See also luminaire. 

lamp burnout factor the fractional loss of task illuminance due to burned-out lamps left in place for long periods. 

lamp lumen depreciation (LLD) factor the fractional loss of lamp lumens at rated operating conditions that progressively occurs during lamp operation. 

lamp position factor The ratio of the luminous flux of a lamp at a given operating position to the luminous flux when the lamp is operated in the position at which the lamp lumens are rated. 

lamp post a standard support provided with the necessary internal attachments for wiring and the external attachments for the bracket and luminaire. 

lamp shielding angle, φ the angle between the plane of the baffles or louver grid and the plane most

laser an acronym for light amplification by stimulated emission of radiation. The laser produces a highly monochromatic and coherent (spatially and temporally) beam of radiation. A steady oscillation of nearly a single electromagnetic mode is maintained in a volume of an active material bounded by highly reflecting surfaces, called a resonator. The frequency of oscillation varies according to the material used and the methods of initially exciting or pumping the material. 

lateral width of a light distribution in roadway lighting, the lateral angle between the reference line and the width line, measured in the cone of maximum candlepower. This angular width includes the line of maximum candlepower. See reference line and width line. 

lens† a glass or plastic element used in luminaires to change the direction and control the distribution of light rays; also, the part of the eye that allows objects at different distances to be focused onto the retina. 

life performance curve a curve that presents the variation of a particular characteristic of a light source (such as luminous flux, intensity, etc.) throughout the life of the source.  Note Life performance curves sometimes are called maintenance curves, for example, lumen maintenance

life test of lamps a test in which lamps are operated under specified conditions for a specified length of time for the purpose of obtaining information on lamp life. Measurements of photometric and electrical characteristics can be made at specified intervals of time during this test. 

light radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nm.  Note The subjective impression produced by stimulating the retina is sometimes designated as light. Visual sensations are sometimes arbitrarily defined as sensations of light, and in line with this concept, it is sometimes said that light cannot exist until an eye has been stimulated. Electrical stimulation of the retina or the visual cortex is described as producing flashes of light. In illuminating engineering, however, light is a physical entity--radiant energy weighted by the luminous efficiency function. It is a physical stimulus that can be applied to the retina. See spectral luminous efficacy of radiant flux and values of spectral luminous efficiency for photopic vision. 

light adaptation the process by which the retina becomes adapted to a luminance greater than about 3.4 cd/m2. 

light-emitting diode (LED) a p-n junction solid-state diode whose radiated output is a function of its physical construction, material used, and exciting current. The output can be in the IR or in the visible region. 

light loss factor (LLF) Formerly called maintenance factor. The ratio of illuminance (or exitance or luminance) for a given area to the value that would occur if lamps operated at their (initial) rated lumens and if no system variation or depreciation had occurred. Components of this factor can be either initial or maintained.  Note The light loss factor is used in lighting calculations as an allowance for lamp(s) or luminaire(s) operating at other than rated conditions (initial) and for the depreciation of lamps, light control elements, and room surfaces to values below the initial or design conditions, so that a minimum desired level of illuminance can be maintained in service. The light loss factor had formerly been widely interpreted as the ratio of average illuminance in service to initial illuminance. 

light meter A common name for an illuminance meter. See illuminance (lux or footcandle) meter. 

light source color the color of the light emitted by a source.  Note The color of a point source can be defined by its luminous intensity and chromaticity coordinates; the color of an extended source can be defined by its luminance and chromaticity coordinates. See color temperature, correlated color temperature, and perceived light source color. 

lighting effectiveness factor (LEF) the ratio of equivalent sphere illumination to measured or calculated task illuminance. 

lightness (of a perceived patch of surface color) the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting. 

louver (or louver grid) a series of baffles used to shield a source from view at certain angles, to absorb or block unwanted light, or to reflect or redirect light. The baffles are usually arranged in a geometric pattern.

louver shielding angle, θ the angle between the horizontal plane of the baffles or louver grid and the plane at which the louver conceals all objects above. See lamp shielding angle.  Note The planes usually are so chosen that their intersection is parallel with the louvered blade. 

louvered ceiling a ceiling-area lighting system composed of a wall-to-wall installation of multicell louvers shielding the light sources mounted above it. See luminous ceiling. 

low-bay lighting interior lighting where the roof trusses or ceiling height is approximately 7.6 m (25 ft) or less above the floor. 

low-key lighting a type of lighting that, applied to a scene, results in a picture having gradations from middle gray to black with comparatively limited areas of light grays and whites. See high-key lighting. 

low-pressure mercury lamp a discharge lamp (with or without a phosphor coating) in which the partial pressure of the mercury vapor does not exceed 100 Pa during operation. 

low-pressure sodium (LPS) lamp a discharge lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of 0.1 to 1.5 Pa (approximately 10−3 to 10−2 Torr). 

lower (passing) beams one or more beams directed low enough on the left to avoid glare in the eyes of oncoming drivers and intended for use in congested areas and on highways when meeting other vehicles within a distance of 300 m (1000 ft). Formerly traffic beam. 

lumen, lm SI unit of luminous flux. Radiometrically, it is determined from the radiant power as in luminous flux. Photometrically, it is the luminous flux emitted within a unit solid angle (1 sr) by a point source having a uniform luminous intensity of 1 cd. 

lumen depreciation† the decrease in lumen output that occurs as a lamp is operated, until failure. 

lumen (or flux) method a lighting design procedure used for predetermining the relation between the number and types of lamps or luminaires, the room characteristics, and the average illuminance on the workplane. It takes into account both direct and reflected flux. 

luminaire (light fixture) a complete lighting unit consisting of a lamp or lamps and ballast(s) (when applicable) together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply. 

luminaire dirt depreciation (LDD) the fractional loss of task illuminance due to luminaire dirt accumulation. 

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

luminaire spacing criterion (SC)† a classification parameter for indoor luminaires relating to the distribution of the direct illuminance component produced on the work plane. The SC of a luminaire is an estimated maximum ratio of spacing to mounting height above the work plane for a regular array of that luminaire such that the work plane illuminance will be acceptably uniform.  Note The SC is not a recommendation for the spacing-to-mounting-height ratio for an installation. It is a characteristic that assists in identifying appropriate luminaires when illuminance uniformity is a design goal. The SC evolved but is distinctly different from an obsolete luminaire parameter called the spacingto-mounting-height ratio. See Chapter 9, Lighting Calculations, for the SC algorithm. 

luminaire surface depreciation factor the fractional loss of task illuminance due to permanent deterioration of luminaire surfaces. 

luminance, L = d2φ/(dω dA cos θ) (in a direction and at a point of a real or imaginary surface) the quotient of the luminous flux at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction. The luminous flux can be leaving, passing through, and/or arriving at the surface. Formerly, photometric brightness.  By introducing the concept of luminous intensity, luminance can be expressed as L = dI/(dA cos θ). Here, luminance at a point on a surface in a direction is interpreted as the quotient of luminous intensity in the given direction, produced by an element of the surface surrounding the point, by the area of the orthogonal projection of the element of surface on a plane, perpendicular to the given direction

luminance contrast the relationship between the luminances of an object and its immediate background. It is equal to (L1 − L2)/L1 or (L2 − L1)/L1 = |∆L/L1|, where L1 and L2 are the luminances of the background and object, respectively. The form of the equation must be specified. The ratio ∆L/L1 is known as Weber's fraction.  Note See note under luminance. Because of the relationship among luminance, illuminance, and reflectance, contrast often is expressed in terms of reflectance when only reflecting surfaces are involved. Thus, contrast is equal to (ρ1 − ρ2)/ρ1, or (ρ2 − ρ1)/ρ1, where ρ1 and ρ2 are the reflectances of the background and object, respectively. This method of computing contrast holds only for perfectly diffusing surfaces; for other surfaces it is only an approximation unless the angles of incidence and view are taken into consideration. See reflectance. 

luminance difference the difference in luminance between two areas, such as the detail of a visual task and its immediate background, in which case it is quantitatively equal to the numerator in the formula for

luminance factor, β the ratio of the luminance of a surface or medium under specified conditions of incidence, observation, and light source, to the luminance of a completely reflecting or transmitting, perfectly diffusing surface or medium under the same conditions.  Note The reflectance or transmittance cannot exceed 1, but luminance factor can have any value from 0 to values approaching infinity. 

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

luminance threshold the minimum perceptible difference in luminance for a given state of adaptation of the eye. 

luminescence any emission of light not ascribable directly to incandescence. See electroluminescence, fluorescence, and phosphorescence. 

luminous ceiling a ceiling area lighting system comprising a continuous surface of transmitting material of a diffusing or light-controlling character with light sources mounted above it. See louvered ceiling. 

luminous density, w = dQ/dV quantity of light (luminous energy) per unit volume. 

luminous efficacy of radiant flux the quotient of the total luminous flux by the total radiant flux. It is expressed in lumens per watt. 

luminous efficacy of a source of light the quotient of the total luminous flux emitted the total lamp power input. It is expressed in lumens per watt.  Note The term luminous efficiency has in the past been extensively used for this concept. 

luminous efficiency† See spectral luminous efficiency of radiant flux. 

luminous energy† See quantity of light. 

luminous flux density at a surface, dΦ/dA the luminous flux per unit area at a point on a surface.  Note This need not be a physical surface; it can also be a mathematical plane. See also illuminance and luminous exitance. 

lux, lx the SI unit of illuminance. One lux is one lumen per square meter (lm/m2). See the Appendix for conversion values.