Glossary - L

laboratory reference standard† the highest-ranking order of standards at each laboratory.

Lambert a lambertian unit of luminance equal to 1/n candela per square centimeter. This term is obsolete, and its use is deprecated.

lambertian surface a surface that emits or reflects light in accordance with Lambert's cosine law. A lambertian surface has the same luminance regardless of viewing angle.

Lambert's cosine law I9 = I0 cos 9 the law stating that the luminous intensity in any direction from an element of a perfectly diffusing surface varies as the cosine of the angle between that direction and the perpendicular to the surface element.

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, 。 the angle between the plane of the baffles or louver grid and the plane most nearly horizontal that is tangent to both the lamps and the louver blades. Notes (i) The lamp shielding angle is formed by a sight line tangent to the lowest part of the brightness area to be shielded. *H* is the vertical distance from the brightness source to the bottom of the shielding element. *D* is the horizontal distance from the brightness source to the shielding element. Lamp shielding angle *。* = arctan (*H*/*D*). (ii) The lamp shielding angle frequently is larger than the louver shielding angle, but never smaller. See *louver shielding angle.*

landing direction indicator a device to indicate visually the direction currently designated for landing and takeoff.

landing light an aircraft aeronautical light designed to illuminate a ground area from the aircraft.

landmark beacon an aeronautical beacon used to indicate the location of a landmark used by pilots as an aid to en route navigation.

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.

level of illumination† See illuminance.

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 curves.

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. See also *dark adaptation.*

light center (of a lamp) the center of the smallest sphere that would completely contain the light- emitting element of the lamp.

light center length the distance from the light center to a specified reference point on the lamp.

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.

linear light a luminous signal having a perceptible physical length.

linear polarization the process by which the transverse vibrations of light waves are oriented in or parallel to a specific plane. Polarization can be obtained by using either transmitting or reflecting media. Linnebach projector a lensless scenic projector using a concentrated source in a black box and a slide or cutout between the source and the projection surface.

liquid crystal display (LCD) a display made of material whose reflectance or transmittance changes when an electric field is applied.

local lighting lighting providing illuminance over a relatively small area or confined space without providing any significant general surrounding lighting.

localized general lighting lighting utilizing luminaires above the visual task and contributing also to the illuminance of the surround.

long-arc lamp an arc lamp in which the distance between the electrodes is large. Note This type of lamp (such as xenon) generally has an arc tube containing gas at high pressure. The arc fills the discharge tube and is therefore wall stabilized.

longitudinal roadway line (LRL) any line along a roadway that is parallel to the curb line.

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 。 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.

lumen-second (lm × s) a unit of quantity of light, the SI unit of luminous energy (also called a *talbot*). It is the quantity of light delivered in one second by a luminous flux of 1 lumen.

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 ambient temperature factor the fractional lumen change of a fluorescent luminaire due to internal luminaire temperatures differing from the temperatures at which photometry was performed. This factor takes into consideration a variation in ambient temperature surrounding the luminaire, the means and conditions of mounting the luminaire, and the use of any insulation in conjunction with the application of the luminaire.

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 *spacing- to-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 = d2。/(dm dA cos 。) (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 *。*). 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 can be measured at a receiving surface by using

This value can be less than the luminance of the emitting surface due to the attenuation of the transmitting media.

Note In common usage the term *brightness* usually refers to the strength of sensation that results from viewing surfaces or spaces from which light comes to the eye. This sensation is determined in part by the definitely measurable luminance defined above and in part by conditions of observation such as the state of adaptation of the eye. In much of the literature, brightness, when used alone, refers to both luminance and sensation. The context usually indicates which meaning is intended. Previous usage not withstanding, neither the term brightness nor the term photometric brightness should be used to denote the concept of luminance.

luminance coefficient (LC) The ratio of average wall or ceiling cavity luminance to the total lamp flux (lumens) divided by the floor area. This requires the luminance in the nonstandard units of footlamberts, where floor area is in square feet. See exitance coefficient for current terminology.

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 = |L1L/L1|, where L1 and L2 are the luminances of the background and object, respectively. The form of the equation must be specified. The ratio L1L/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 contrast. See note under *luminance.*

luminance factor, B 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 exitance, M = d。/dA the areal density of luminous flux leaving a surface at a point. Formerly luminous emittance (deprecated). Note This is the total luminous flux emitted, reflected, and transmitted from the surface and is independent of direction.

luminous flux, φ radiant flux (radiant power); the time rate of flow of radiant energy, evaluated in terms of a standardized visual response:

where

φv = lumensφe, λ = watts per nanometer λ = nanometers

V(*λ*) = the spectral luminous efficiency

Km = the maximum spectral luminous efficacy in lumens per watt Unless otherwise indicated, the luminous flux is defined for photopic vision. For scotopic vision, the corresponding spectral luminous efficiency V(*λ*) and the corresponding maximum spectral luminousefficacy Km are substituted in the above equation. Km and K'm are derived from the basic SI definition of luminous intensity and have the values 683 lm/W and 1754 lm/W, respectively.

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.

luminous intensity, I = dφ/dm (of a point source of light in a given direction) the luminous flux per unit solid angle in the direction in question. Hence, it is the luminous flux on a small surface centered on and normal to that direction divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity can be expressed in candelas or in lumens per steradian (lm/sr). Note Mathematically a solid angle must have a point as its apex; the definition of luminous intensity, therefore, applies strictly only to a point source. In practice, however, light emanating from a source whose dimensions are negligible in comparison with the distance from which it is observed can be considered as coming from a point. Specifically, this implies that with change of distance (1) the variation in solid angle subtended by the source at the receiving point approaches 1/distance2, and that (2) the average luminance of the projected source area as seen from the receiving point does not vary appreciably. For extended sources see equivalent luminous intensity of an extended source at a specified distance. The word *intensity* as defined above is used to designate luminous intensity (or *candlepower*). It is also widely used in other ways, either formally or informally, in other disciplines. Stimulus intensity can be used to designate the retinal illuminance of a proximal stimulus (see *proximal stimuli*) or the luminance of a distal stimulus (see *distal stimuli*). Intensity is used in the same sense with respect to other modulates such as audition. Intensity has been used to designate the level of illuminance on a surface or the flux density in the cross section of a beam of light. In physical optics, "intensity" usually refers to the square of the wave amplitude.

luminous intensity distribution curve† See intensity distribution curve.

luminous reflectance any of the geometric aspects of reflectance in which both the incident and the reflected flux are weighted by the spectral luminous efficiency of radiant flux, V(*λ*). Note Unless otherwise qualified, the term *reflectance* means luminous reflectance.

luminous transmittance any of the geometric aspects of transmittance in which the incident and transmitted flux are weighted by the luminous efficiency of radiant flux, V(*λ*). Note Unless otherwise qualified, the term transmittance means luminous transmittance.

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

lux meter† See illuminance (lux or footcandle) meter.