SLS Dictionary

 

A

Absorption: The process in which incident flux is converted to another form of energy, typically heat.
Accent Lighting: Directional lighting used to emphasize a particular object or surface feature, or to draw attention to a part of the field of view.
Actinic: Applied to radiation capable of producing a photochemical effect.
Ambient Lighting: Lighting throughout an area that produces general illumination.
Arc: A discharge light source, usually a carbon arc.
Arc Discharge: An electric discharge characterized by high cathode current densities and a low voltage drop at the cathode.
Arc Lamp: A discharge lamp in which the light is emitted by an arc discharge or by its electrodes.
Area Lighting: The practice of lighting the acting space in separately controlled areas.
Astronomical Time Clock: A time clock with automatic compensation for the annual change in length of day such that the switching operation is coordinated with daylight conditions.
Average Luminance: Luminance is the property of a geometric ray. Luminance as measured by conventional meters is averaged with respect to two independent variables, area, and solid angle, both must be defined for a complete description of a luminance measurement.

B

Back Light: Illumination from behind an object to produce a highlight along its edge and consequent separation between the object and its background.
Baffle: A single opaque or translucent element to shield a source from direct view at certain angles, to absorb or block unwanted light, or to reflect and redirect light.
Ballast: A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current, and wave form) for stating and operating.
Ballast Factor: The fractional lumens of a fluorescent lamp operated on a ballast compared to the lumens when operated on the standard reference ballasting specified for rating lamp lumens.
Beam Angle: The angle between the two directions for which the intensity is 50 percent of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rational symmetry, the beam angle is generally given for two planes at 90 degrees, typically the maximum and minimum angles.
Blackbody: A temperature radiator of uniform temperature who radiant exitance in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it will absorb all the radiant energy that falls upon it. All other temperature radiators may be classed as non-blackbodies. They radiate less in some or all wave-length intervals than a blackbody of the same size and same temperature.
Blue-Light Hazard: Potential for photo-chemically induced injury to the retina of the eye, resulting from radiant exposure at wavelengths principally between 400 nm to 500 nm.
Bollard: Luminaires having the appearance of a short, thick post, used for walkway and grounds lighting. The optical components are usually top-mounted.
Bracket (mast arm): An attachment to a lamp post or pole from which a luminaire is suspended.
Brightness: Attribute of a visual sensation according to which an area appears to emit more or less light.
Brightness Contrast Threshold: When two patches of color are separated by a brightness contract border as in the case of a bipartite photometric field or in case of a disk-shaped object surrounded by its background, the border between the two patches is a brightness contrast border. The contrast which is just detectable is known as the brightness contract threshold.
Brightness of a Perceived Aperture Color: The attribute by which an area of color of finite size is perceived to emit, transmit, or reflect a greater or lesser amount of light. No judgement is made as to whether the light comes from a reflecting, transmitting, or self-luminous object.
Brightness Ratio: Ratio of maximum to minimum luminance’s occurring within a scene.
Bulb: (see Lamp)

C

Candela: The SI unit of luminous intensity. One candela is one lumen per steradian (lm/sr). Formerly, candle.
Candlepower: Luminous intensity expressed in candelas.
Ceiling Area Lighting: A general lighting system in which the entire ceiling is, in effect, one large luminaire.
Central Visual Field: The region of the visual field which corresponds to the foveal portion of the retina.
Chromatic Adaptation: The process by which the chromatic properties of the visual system are modified by the observation of stimuli of various chromaticities and luminances.
Chromatic Color: Perceived color possessing a hue. In everyday speech, the word color is often used in this sense in contradistinction to white, grey, or black.
Chromaticity of a Color: 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.
Circadian: Variations with a cycle of approximately 24 hours.
Cold-Cathode Lamp: An electric-discharge lamp whose mode of operation is that of a glow discharge, and that has electrodes so spaced that most of the light comes from the positive column between them.
Color: The characteristic of light by which a human observer can distinguish between two structure-free patches of light of the same size, shape, and brightness.
Color Comparison or Color Grading: The judgement of equality, or of the amount and character of difference, of the color of two objects viewed under identical illumination.
Color Correction: The adjustment of a color reproduction process to improve the perceived-color conformity of the reproduction to the original.
Color Discrimination: The perception of differences between two or more colors.
Colorfulness of a Perceived Color: The attribute according to which it appears to exhibit chromatic color. For a stimulus of a given chromaticity, colorfulness normally increases as the absolute luminance is increased.
Colorimetric Shift: The change of chromaticity and luminance factor of an object color due to change of the light source.
Colorimetry: The measurement of the wavelength and the intensity of electromagnetic radiation in the visible region of the spectrum. It is used extensively for identification and determination of concentrations of substances that absorb light.
Color Matching: That means that two people in the same room looking at the same object could see slightly different colors, merely due to the angle at which they are viewing it.
Color Matching Functions: The amounts of primaries needed to match the monochromatic test color at the wavelength shown on the horizontal scale.
Color Rendering: Describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed.
Color Rendering Index: (CRI) is a way to measure a light source’s distinctive attributes. It is an assessment of how the light source shows object colors “naturally” when compared to a familiar basis of reference, either incandescent light or daylight.
Color Temperature of a Light Source: The temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin (K).
Color Wheel: A color wheel or color circle is an abstract illustrative organization of color hues around a circle, which shows the relationships between primary colors, secondary colors, tertiary colors etc.
Compact Fluorescent Lamp: (also called compact fluorescent light, energy-saving light, and compact fluorescent tube) is a fluorescent lamp designed to replace an incandescent light bulb; some types fit into light fixtures designed for incandescent bulbs.
Constant Current Transformer: A transformer designed to maintain the current in the secondary circuit within narrowly defined limits, whatever may be the variations of the impedance of the secondary circuit, or the variations of the voltage applied to the primary circuit.
Cove Lighting: A form of indirect lighting built into ledges, recesses, or valances in a ceiling or high on the walls of a room. It directs light up towards the ceiling and down adjacent walls. It may be used as primary lighting, or for aesthetic accent, especially to highlight decorative ceilings.
Cut-Off Angle: (of a luminaire) means the angle, measured from the lowest point between a vertical line from the center of the lamp extended to the ground and the first line of sight at which the bare source is not visible.

D

Daylight: The combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected by Earth and terrestrial objects, like landforms and buildings.
Daylight Factor: The ratio of the light level inside a structure to the light level outside the structure.
Diffused Lighting: (or soft light) Light that’s filtered by something, such as sunlight through a sheer curtain.
Diffuser: Any material that diffuses or scatters light in some manner to transmit soft light. Diffused light can be easily obtained by reflecting light from a white surface, while more compact diffusers may use translucent material, including ground glass, teflon, opal glass, and greyed glass.
Dim: Means to reduce the output of a lamp or lighting fixture.
Dimmer: A device connected to a light fixture used to lower the brightness of the light. By changing the voltage waveform applied to the lamp, it is possible to lower the intensity of the light output.
Dimmer Board: Lighting consoles that control lighting dimmers for conventional lights and new LED lighting through the DMX 512 protocol.
Direct Glare: The result of high brightness from a light source in the field of vision. For example, the sun in front of a person’s eye.
Direct-Indirect Lighting: The amount of the light that’s absorbed and reflected by the surface itself. Direct lighting from the sun and sky. Light bouncing off a surface, illuminating other objects is called indirect lighting. It arrives at surfaces from everything except light sources.
Directional Lighting: Emit parallel light rays in a single direction but the light reaches out into infinity.
Downlight: A light fixture that is installed into a hollow opening in a ceiling. When installed it appears to have light shining from a hole in the ceiling, concentrating the light in a downward direction as a broad floodlight or narrow spotlight.
Duty Cycle: (or power cycle) is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio.

E

Efficacy: (See Luminous Efficacy of a Source of Light)
Efficiency: (See Luminaire Efficiency)
Egress: (See Means of Egress)
Electromagnetic Radiation: A kind of radiation including visible light, radio waves, gamma rays, and X-rays, in which electric and magnetic fields vary simultaneously.
Electromagnetic Spectrum: Describes all the kinds of light, including those the human eye cannot see.
Emergency Exit: A special exit for emergencies such as a fire: the combined use of regular and special exits allows for faster evacuation, while it also provides an alternative if the route to the regular exit is blocked.
Emergency Lighting: A battery-backed lighting device that switches on automatically when a building experiences a power outage.
Emittance: The energy radiated by the surface of a body per second per unit area.
Equipment Operating Factor: The ratio of the lumens of a high intensity discharge (HID) lamp-ballast-luminaire combination in each operating position to the lumens of the lamp-luminaire combination a) operated in the position for rating lamp lumens; and b) using the standard (reference) ballasting specified for rating lamp lumens.
Exit Sign: A pictogram or short text in a public facility denoting the location of the closest emergency exit to be used in case of fire or other emergency that requires rapid evacuation. Most relevant codes require exit signs to be permanently lit.
Explosion-Proof Luminaire: A type of fixture which can be exposed to certain elements, such as chemicals, gases, or high heat, that is designed to be protected against the fixture itself exploding from being exposed to these harsher conditions and environment.
Exterior Lighting Applications: Temporary or permanent lighting that is installed, located, or used in such a manner to cause light rays to shine outside.
Externally Illuminated Exit Sign: Any exit sign that requires a source of illumination outside of the sign itself.

F

Flood: The flooding of large surfaces with light through powerful projectors, where the light sources are concentrated into narrow light beams using specific reflectors
Floodlight: A broad-beamed, high-intensity artificial light. They are often used to illuminate outdoor playing fields while an outdoor sports event is being held during low-light conditions. More focused kinds are often used as a stage lighting instrument in live performances such as concerts and plays.
Floodlighting: (See Floodlight)
Fluorescence: The emission of light by a substance that has absorbed light or other electromagnetic radiation.
Fluorescent Lamp: (or fluorescent tube) A low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow.
Flush Mounted or Recessed: Flush mount, or semi-flush mount light fixtures are typically encased in glass housing which allows light to diffuse or spread throughout and entire space. Recessed lighting, on the other hand, provides vertical downlighting, and can require more fixtures to achieve the same level of brightness within a room.
Footcandle: defined as one lumen per square foot
Footcandle Meter: An instrument for measuring illuminance on a plane.

G

Gaseous Discharge: An essential characteristic of gas discharge lamps used in various lighting applications. When electric power is applied across the electrodes present inside a discharge tube, the electrodes give out electrons which bombard with the gas atoms introduced inside the discharge tube.
Gas-Filled Lamp: An incandescent lamp in which the filament operates in a bulb filled with one or more inert gases
General Color Rendering Index: A quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with a natural or standard light source.
General Illumination: (see Ambient Lighting)
Germicidal Lamp: An electric light that produces ultraviolet C light.

H

High Bay Lighting: Used to light up spaces whose ceilings are higher than 20ft from the floor and around 40ft.
High Intensity Discharge Lamp: A type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube.
High-Mast Lighting: A tall pole with lighting attached to the top pointing towards the ground, usually but not always used to light a highway or recreational field.
High Pressure Sodium Lamp: High intensity light bulbs that put out large amounts of light generally needed for street lighting and security lighting. The combination of metals and gasses within inside the glass tube produces an orange-white light commonly found in streetlights.
Hot-Cathode Lamp: (or thermionic cathode) is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This contrasts with a cold cathode, which does not have a heating element

I

Ignitor: A starting device that generates voltage pulses to start a discharge lamp.
Illuminance: The total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.
Illumination: Lighting or light
Incandescence: The emission of electromagnetic radiation from a hot body as a result of its high temperature.
Incandescent Filament Lamp: (or incandescent light bulb, incandescent lamp, or incandescent light globe) is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxidation.
Indirect Lighting: (see Direct-Indirect Lighting)
Infrared Radiation: (or infrared light) is electromagnetic radiation with wavelengths longer than those of visible light. It is therefore invisible to the human eye.
Instant Start Fluorescent Lamp: A fluorescent lamp designed for operation with a ballast that provides a low voltage winding for preheating the electrodes to initiate the arc without a starting switch or the application of high voltage.
Internally Illuminated Exit Sign: Signs that require no external light to shine on the sign for it to be visible. Internally illuminated exit signs usually consist of one or more LED light bulbs contained inside a plastic or aluminum housing. The light shines through a colored film to illuminate the word “EXIT”
Iris: A metal frame housing designed with an adjustable shutter assembly (an iris). The iris is placed inside fixtures through a specifically designed opening, the accessory (or iris) slot that lets you reduce the size of a circle of projected light. Used commonly in stage lighting.

 

L

Lamp: A device for giving light, either one consisting of an electric bulb together with its holder and shade or cover, or one burning gas or a liquid fuel and consisting of a wick or mantle and a glass shade.
Lamp Burnout Factor: The ratio of the illuminance (or exitance or luminance) provided when a lighting system is allowed to operate with a set number of lamps burned out to that provided with all lamps operating.
Lamp Failure: The point where a lamp stops operating.
Lamp Lumen Depreciation: A factor used by lighting designers to predict the depreciation in light output for a specific light source over a defined period.
LED Die: A semiconductor device made of gallium nitride (GaN). When electric current passes through the die, it emits blue light. One or more die are then mounted to a substrate commonly made of aluminum or ceramic.
LED Control Circuitry: An electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED.
LED Driver: A self-contained power supply which has outputs that are matched to the electrical characteristics of the LED(s).
LED Driver (Class II): are listed or part of a listed system comply with UL 1310 Standard for Safety for Class 2 Power Units which requires the output voltage to pose no risk of fire or electric shock which makes secondary circuit protection unnecessary.
LED Luminaire: A complete lighting unit consisting of LED-based Light emitting elements and a matched driver together with parts to distribute light, to position and protect the light emitting element, and to connect the unit to a branch circuit. The LED-based lighting emitting elements may take the form of LED.
LED Package: A plastic casing that carries an LED chip and phosphor.
LED Array or Module: Refers to an assembly of LED packages (components) or dies (or chips) on a printed circuit board or substrate, oftentimes with optical elements whereby light generated by the at least an LED can have a desired pattern of distribution.
LED Lamp: An electric light that produces light using light-emitting diodes.
Light: (or visible light) is electromagnetic radiation that can be perceived by the human eye.
Light Adaptation: The process including contraction of the pupil and decrease in rhodopsin by which the eye adapts to conditions of increased illumination.
Light Control: The ability to regulate the level and quality of light in each space for specific tasks or situations.
Light Distribution: How the light moves from the fixture out to the area around it, typically on the ground.
Light Emitting Diode: (or LED) a semiconductor device that emits light when current flows through it.
Lighting: Equipment in a home, workplace, studio, theater, or street for producing light. Also, the arrangement or effect of lights.
Lighting Calculation: (or the Lumen Method, Light Flux Method) A calculation used to figure out how much light you need in each room and how many fixtures you’ll need.
Light Loss Factor: (or LLF) 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 may be either initial or maintained.
Light Meter: (see Footcandle Meter)
Light-Source Color: The lower the color temperature of the light source, the warmer or redder the source will be. Inversely, the higher the. color temperature of the source, the cooler or bluer it will be.
Linear Light: A linear shape luminaire (opposed to square or round).
Long-Arc Lamp: A lamp that produces light by an electric arc.
Louver: A window blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain and direct sunshine.
Low-Bay Lighting: A light intended for use in a ceiling under 20 feet in height from the floor. A low bay ceiling is the most common type of ceiling commonly seen in homes, most retail businesses, and public buildings.
Low Pressure Mercury Lamp: A discharge lamp designed to have a low mercury vapor pressure when lit to efficiently emit the mercury resonance lines.
Low Pressure Sodium Lamps: A specific type of gas-discharge light (also known as a high intensity discharge, HID or arc light). The bulb principally contains solid sodium metal inside a borosilicate glass tube that vaporizes once the lamp is turned on.
Lumen: The unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time.
Luminaire: A complete lighting unit containing an electric lamp that provides illumination.
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.
Luminaire Dirt Depreciation: The efficiency of a new / clean luminaire to the efficiency of the luminaire due to accumulation of dirt on the luminaires’ reflecting and refracting surfaces.
Luminaire Efficiency: How efficient the entire lighting fixture is. It measures the efficiency ratio of the total amount of lumens output by the luminaire to the amount of electricity required to power the light fixture.
Luminance: The intensity of light emitted from a surface per unit area in a given direction.
Luminous Flux: (or Luminous Power) is the measure of the perceived power of light.
Lux: The unit of illuminance, or luminous flux per unit area, in the International System of Units. It is equal to one lumen per square meter.

M

Maintenance Factor: (see Light Loss Factor)
Median Life: The total operating time at which, under normal operating conditions, 50 percent of any large group of initially installed lamps is expected to still be operating.
Mercury-Fluorescent Lamp: An electric discharge lamp having a high-pressure mercury arc in an arc tube, and an outer envelope coated with a fluorescing substance (phosphor), which transforms some of the ultraviolet energy generated by the arc into light.
Mercury Lamp: A gas-discharge lamp that uses an electric arc through vaporized mercury to produce light.
Mesopic Vision: (or Twilight Vision) A combination of photopic and scotopic vision under low-light conditions.
Metal Halide Lamp: An electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides.
Mounting Height: The distance from finished grade to the bottom most portion of a light fixture, (e.g. lamp, surface of lens).

N

Nanometer: (See Spectral Parameters)
Narrow-Angle Diffusion: That diffusion in which flux is scattered at angles near the direction that the flux would take by regular (specular) reflection or regular transmission.
Narrow-Angle Luminaire: A luminaire that concentrates the light within the cone of a comparatively small solid angle.

O

Overhang: In roadway lighting: 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.

P

Parallel: (or bed) In television, film and theater lighting: Platform usually suspended from above to provide lighting positions on motion picture stages.
PAR Lamp: A designation that means “parabolic aluminized reflector,” most often applied to pressed hard glass, two-part (lens and reflector) reflector lamps.
Perceived Light-Source Color: Colors can look different depending on the spectral composition of the light source under which a color or object is viewed.
Perimeter Lights: Wall and Ceiling mounted LED luminaires used to define interior or exterior architectural perimeters as well as create patterns of light in space.
Phosphorescence: Light emitted by a substance without combustion or perceptible heat.
Photocontrol or Light Activated Switch: Electrical timers and switches that use ambient light levels to trigger the power supply to light a given area.
Photopic Vision: The vision of the eye under well-lit conditions
Pipe: Clear tubes that transmit light from a light source, such as an LED to a user interface. Designed to carry light short distances with high efficiency.
Power Source: The origin of incoming electricity.
Power Supply: An electrical device that supplies electric power to an electrical load.
Pre-burning Time: (See Stabilization Time.)
Preheat Fluorescent: 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.
Projection Lamp: A lamp with physical and luminous characteristics suited for projection systems (e.g., motion picture projectors, slide projectors, and microfilm viewers).
Pulsed Lamp: As a light source description for risk group classification (of lamps): A lamp that delivers its energy in the form of a single pulse or a train of pulses, where each pulse shall have duration of less than 0.25 s.
Pupil: The opening of the iris that admits light into the eye.

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.
Quantity of Lighting: Initial lumens indicate how much light is produced once the lamp has stabilized; for fluorescent and high-intensity discharge (HID) lamps, this is typically 100 hours.
Quartz: (See Tungsten-Halogen)
Quartz Lights: Generic term for luminaires of various types that utilize tungsten-halogen lamps.

R

Rapid Start Fluorescent Lamps: A fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes to initiate the arc without a starting switch or the application of high voltage.
Rated Lamp Life: (or half-life) how long it takes for half the light bulbs in a test batch to fail.
Reflectance: The measure of the proportion of light or other radiation striking a surface which is reflected off it.
Reflectance Factor: The ratio of the flux 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 the same way as the sample.
Reflected Glare: Glare resulting from reflections of high luminance in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or from areas near the region being viewed.
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.
Reflector: A device used to redirect the flux from a source 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 in which its speed is different.
Refractor: A device used to redirect the flux from a light source primarily by the process of refraction.
Regulation: The constancy of the voltage applied to the equipment under test.
Resultant Color Shift: The difference between the perceived color of an object illuminated by a test source and the perceived color of the same object illuminated by the reference source, considering the state of chromatic adaptation in each case; i.e., the resultant of colorimetric shift and adaptive color shift.
Retina: A membrane lining the posterior part of the inside of the eye. It comprises visual photoreceptors (cones and rods) and non-visual photoreceptors (intrinsically photosensitive retinal ganglion cells) that are sensitive to light, and nerve cells that transmit signals via the optic nerve to various portions of the brain.
Retinal Burn: A photochemical or thermal retinal lesion.
Retro-Reflector: A device or surface that reflects radiation (usually light) back to its source with minimum scattering.
Risk Group 1 (low risk): In risk group classification (of lamps): The philosophical basis is that the lamp does not pose a hazard, due to normal behavioral limitations on exposure.
Risk Group 2 (moderate risk): In risk group classification (of lamps): The philosophical basis is that the lamp does not pose a hazard, due to the aversion response to very bright light sources or due to thermal discomfort.
Risk Group 3 (high risk): In risk group classification (of lamps): The philosophical basis is that the lamp may pose a hazard even for momentary or brief exposures.
Roadway Lighting: Lighting that provides improved visibility for the various users of the roadways and associated facilities.
Rods: Retinal receptors that continue to respond at levels of luminance below the threshold for cones, and that provide no ability to discriminate colors. Rods are absent in the center of the fovea.

S

Scotopic Vision: Vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.001 cd/m2.
Seasoning: The process of operating a new lamp for a fixed period. This is sometimes referred to as aging. A lamp is typically seasoned prior to measuring its initial photometric characteristics. It is an accepted industry procedure for obtaining the initial rating of light sources.
Seasoning Time: The length of time a new lamp is operated prior to performing photometric measurements. The photometric data obtained at the end of the seasoning time are referred to as initial data or rated data. The seasoning time is different depending on the type of lamp.
Self-Ballasted Lamps: Any arc discharge lamp of which the current-limiting device is an integral part.
Self-Luminous Exit Sign: An exit sign consisting of phosphor-coated glass tubes filled with radioactive tritium gas. When the radioactive gas bombards the phosphor, the tube emits light (via luminescence) and illuminates the exit legend; typically between 0.2 and 0.7 cd/m2.
Semi-Direct Lighting: Lighting involving luminaires that distribute 60% to 90% of the emitted light downward and the balance upward.
Semi-Indirect Lighting: 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 the daylighting provides a specified illuminance level. It often is stated as a monthly average.
Shielding Angle: The angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible.
Short-Arc Lamp: An arc lamp in which the distance between the electrodes is small (on the order of 1 to 10 mm) This type of lamp (e.g., xenon or mercury) generally has an arc tube containing gas at very high pressure.
Shutters: Adjustable beam-shaping devices (usually four) in a luminaire.
Skylight: A glazed opening on the roof of a building making an angle of 0 to 60 degrees from the horizontal.
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.
Spacing (roadway lighting): The spacing between two light poles should be roughly 2.5–3 times the height of the pole. Shorter light poles should be installed at closer intervals. The density, speed of travel, and the type of light source along a corridor will also determine the ideal height and spacing of roadway lighting.
Spacing-to-Mounting Height Ratio: The spacing between luminaires divided by their height above the horizontal reference plane.
Spectral Lamp: A discharge lamp that emits a significant portion of its radiative power in a line spectrum and that, in combination with filters, may be used to obtain monochromatic radiation.
Spectral Radiance: Spectral concentration of radiance.
Spectral Radiant Flux: Radiant flux per unit wavelength interval at wavelength λ; e.g., watts per nanometer.
Spectral Reflectance: The ratio of the reflected flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, Δ λ, about λ.
Specular Angle: That angle between the perpendicular to the surface and the reflected ray that is numerically equal to the angle of incidence, and that lies in the same plane as the incident ray and the perpendicular but on the opposite side.
Spider: In television, film, and theater lighting: An electrical distribution device; usually breaks down the feed from the source to individual circuits.
Spot: A light beam illuminated to cover only a tiny area hence the name “spot.”
Spotlight: A form of floodlight usually equipped with lens and reflectors to give a fixed or adjustable narrow beam.
Stabilization: The operation of a test lamp for a sufficient period such that the parameters to be tested are constant. Also sometimes called warm-up time or pre-burning time.
Stabilization Time: (or warm-up time) The period required for the parameters of the lamps under test to become constant.
Stray Light: Light from a source that is scattered onto parts of the retina lying outside the retinal image of the source.
Surface Mounted: A luminaire that is mounted directly on a wall or on the ceiling.
Suspended (pendant): A luminaire that is hung from a ceiling by supports.

T

Task Lighting: Lighting directed to a specific surface or area, providing illumination for visual tasks.
Time Clock: (or Time Switch) A clock-operated switch programmed to turn lighting on and off for selected periods.
Top Hat: (or Funnel) In television, film, and theater lighting: A metal tube that can be mounted on the front of a spotlight to control stray light.
Torchere: (or Torchiere) An indirect floor lamp that sends all or nearly all its light upward.
Troffer: A long recessed lighting unit usually installed with the opening flush with the ceiling. The term is derived from “trough” and “coffer.”
Tungsten Halogen Lamp: A gas-filled tungsten-filament incandescent lamp containing halogens or halogen compounds and utilizing the halogen regenerative cycle to prevent blackening of the lamp envelope during life.

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.

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 usually parallel to the wall) to conceal light sources, giving both upward and downward light distribution.
Valance Lighting: Lighting consisting of light sources shielded by a panel parallel to the wall at the top of a window.
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 when observed under standardized viewing conditions and having the same threshold as the given object.
Visibility Level: A contrast multiplier to be applied to the visibility reference function to provide the luminance contrast required at different levels of task background luminance to achieve visibility for specified conditions relating to the task and observer.
Visual Acuity: A measure of the ability to distinguish fine details, usually measured with optotypes* of different sizes when not constrained by contrast. Quantitatively, it is the reciprocal of the minimum angular size in minutes of arc of the critical detail of an object that can just be resolved.
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 may be specified or the combination of the two.

W

Warm-up Time: (See Stabilization Time)
Wide-Angle Diffusion: That diffusion in which flux is scattered at angles far from the direction that the flux would take by regular (specular) reflection or regular transmission.
Wide-Angle Luminaire: A luminaire that concentrates the light within the cone of a comparatively large solid angle.
Work Light: In television, film, and theater lighting: Illumination provided for specific operational purposes, independent of the production lighting, such as for cleaning and set construction.