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Terms Used in Lighting Studies

Some of the terminology used in lighting practice and analysis, and in studying the environmental effects of light, are not commonly used elsewhere, or have different meanings when used in this context. Some terms are commonly used when talking about lighting practice, but have different meanings to different people; this is a frequent source of talking cross-purposes in our field.

On this page, we provide some definitions and explanations of some of this terminology. We have done our best to include the most proper and fitting definitions of the terms listed here, and in the cases where certain terms are commonly used (or misused) to mean multiple things, to explain our definition(s) of choice. In any case, when we use any of these terms elsewhere on this website or in our other publications, the definitions below indicate our intended meanings.

Link to Term Index

See Vanity lighting.

Originally generally used by photographers/cinematographers to describe the existing light in a scene (as opposed to light which they add). In artificial lighting engineering, the term is also sometimes used to describe indirect artificial light; that is light which is being added, but which carries into parts of the area in question by reflection and other scattering, and by leakage from more distant luminaires, rather than shining right from a luminaire which is directly illuminating a particular spot. In other words, a city block may have many separate streetlights which each are brightly (directly) illuminating their immediate areas, but all together, the light they each generate ends up creating an ambient light level which suffuses throughout the area (and perhaps beyond).
 
This effect is demonstrated on a city block where one streetlight is not functioning; the space around that fixture won't be totally void of all light -- generally, a notable amount of "ambient light" will be found to spread in from the brighter areas on the street.

Illumination of buildings, facades, structures, or other architectural features. Sometimes only including exterior lighting directed on to those areas, but in other cases also including internal illumination through translucent exterior surfaces. In the latter instance, "architectural lighting" generally refers to illuminated architectural features other than windows where indoor illumination is escaping, but if interior illumination is being left turned on specifically to modify the exterior appearance of a building, it may be considered architectural lighting, too. See also: Vanity lighting.

The American Society of Heating, Refrigerating and Air Conditioning Engineers. Founded in 1894 as the American Society of Heating and Ventilating Engineers, ASHRAE is a non-profit trade organization with the mission statement "To advance the arts and sciences of heating, ventilating, air conditioning and refrigerating to serve humanity and promote a sustainable world", with the vision of becoming "the global leader, the foremost source of technical and educational information, and the primary provider of opportunity for professional growth in the arts and sciences of heating, ventilating, air conditioning and refrigerating" (source: www.ashrae.org). Apparently perceiving a lack of leadership in lighting, ASHRAE has also started to issue its own standards and practices for illumination, too.

Many light fixtures are designed to be mounted above and to the side of an area of intended illumination (such as streetlights mounted on poles on the side of a roadway). In such installations, the light output needs to be directed "forward" from the pole. Backlight refers to light which is emitted "behind" the fixture (such as into the house side of a streetlight), often into areas which do not need illumination. See also Luminaire Classification System.

See Shield.

High intensity discharge and fluorescent lamps require this electrical or electronic device to alter electrical voltage and/or frequency in order to start the lamps' operation. Ballasts draw some additional current beyond what the lamp itself draws; therefore, the total wattage of one of these fixtures is more than the wattage of the lamp(s) within it.

(1) A light placed on or around the top of a tall structure, to make it evident to aircraft pilots at night. (2) A street light placed at an intersection (or other potential hazard), serving more to notify approaching drivers of the intersection's presence than to illuminate the roadway itself. Many street lights in residential and rural areas without heavy pedestrian traffic primarily serve this valid safety purpose. If the roadway areas between the intersections are unlighted, using overly bright lamps in these installations is a mistake, because drivers' eyes should be allowed to adapt to dimmer headlight-only conditions for increased safety.

"Best management practices" is based in the concept that there are techniques, methods, processes which are more effective than others at producing a desired outcome. In various fields, including environmental conservation, and engineering, lists and descriptions of these practices will be created by various bodies, professional groups, or agencies; these lists are then published. In turn, other agencies, laws, and regulations may refer to the published lists in their own standards.

See Color temperature.

Originally, a post along a dock, used to tie up ships. Adopted to describe a strong post, often concrete or metal, placed beside pavement to keep vehicles from leaving the pavement (and perhaps traveling up onto a sidewalk, or running into a building or other hazard). Some traffic bollards have illumination built into them, but using the term bollard for simple pathway post lights (as is being increasingly done) is confusing, since it implies that they can also serve as functional safety barriers.

A nine-level numeric scale for measuring the intrinsic brightness of the cloudless night sky from a particular location, based on the limiting magnitude -- the brightness of the faintest stars visible to the naked eye. Provides a simple quantification of the amount of manmade skyglow over an area, with values ranging from one (a sky totally free of manmade light interference) to nine (a sky so brilliant with manmade light that only the Moon and a few of the brightest stars and planets can be seen with the unaided eye).

Acronym for Backlight, Uplight, Glare. An offshoot of the IESNA's Luminaire Classification System (see), BUG is a "simplified" rating system of luminaire performance as far as sending light into these three main zones of light trespass concern. A BUG rating consists of three numbers; a zero to five rating for each the zones: backlight, uplight, and glare. A rating of zero in any of the zones indicates a level of lumens being emitted into that zone which the IESNA considers to be minimal; higher numbers indicate increasing levels of emission, again based on IESNA preferences. The IESNA tables of how BUG rating numbers are awarded by zonal lumens are available HERE.

A measure of luminous intensity; that is the visible light energy emitted by a light source in one particular direction. A common wax candle's flame emits approximately one candela in any given direction; this does not change whether the light going in other directions is shining into space or absorbed by some barrier. But if a reflective surface is placed behind the candle flame, the light emitted in the opposite direction might be almost doubled by that reflector to two candela. As a standard, the candela is measured in green light at 555 nanometers (around the peak of human color vision).

Initialism for Compact Fluorescent Lamp; see Fluorescent lamp.

The Commission Internationale de l´Eclairage (International Commission on Illumination). Formed in 1913, the non-profit CIE is recognized by the International Organization for Standardization (ISO) as an international standardization body on matters related to the light and lighting, color and vision, and image technology. The CIE's seven active divisions are: 1. Vision and Color, 2. Measurement of Light and Radiation, 3. Interior Environment and Lighting Design, 4. Lighting and Signalling for Transport, 5. Exterior Lighting and Other Applications, 6. Photobiology and Photochemistry, 8. Image Technology (source: www.cie.co.at).

A periodicity, roughly on a 24-hour cycle, in the biochemical, physiological, and/or behavioral processes of living organisms. Circadian rhythms have been demonstrated to occur in organisms in all kingdoms, including single-celled organisms, plants, fungi and animals. While the circadian rhythm is internally generated in most organisms, it is usually entrained (i.e., has its clock set) by external cues; the most common of these is the light/dark cycle of day and night. The physical and behavioral effects of the circadian cycle are numerous in most organisms; basically, the physiology of the organism shifts in a pre-set pattern to prepare for different activities at different times of day, whether it be feeding, resting, growth and healing, reproduction, respiration, sleeping, etc.

 
A ubiquitous style of street light fixture, mounted on a curbside pole, with a (generally narrow) arm extending out over the roadway and a (generally rounded) luminaire at its end. Named for its superficial resemblance to the cobra snake when the latter rises up and flares out its neck, they began being widely adopted in the U.S. in the late 1950s and are still very common today. The actual luminaires on cobra-style lights vary widely in design and performance; original styles all featured drop-bowl diffusers, and tend to scatter light for long distances; some newer versions feature flat lenses, which generally provide a cutoff somewhere below the horizontal (if the lens/fixture is mounted horizontally).

A measure of the efficiency of both a luminaire and the engineering of its placement, being the fractional amount of the total lumen output of the luminaire's lamp(s) which reaches the work plane which the luminaire is intended to illuminate. In simple terms, if the lamp in a luminaire emits 5000 lumens, and 2500 lumens actually reach the area which that fixture was installed to light up, the coefficient of utilization in that instance is 0.5 (or 50%). The CU determination includes local conditions; if, for instance, a fixture is mounted on a wall, and the wall surface reflects some of the light output back into the area to be illuminated, that will increase the CU figure for that installation. See also: Target Efficacy Rating.

The gamut of colors which the human visual system is able to perceive is dependent on the quality of the light which is illuminating a scene. Broad spectrum sunlight will enable a person with normal vision to see 64 different colors in a box of crayons (or about 10,000,000 different colors, if one has a much bigger crayon selection), but if the box is illuminated with just one wavelength of light, every crayon will appear as a different brightness of that one color (a very "unnatural" appearance). Different artificial light sources vary in their ability to make scenes appear in their natural, daylight colors to the human observer.
 
Lighting engineers use the Color Rendering Index (CRI) to rate a light source's ability to render object colors. The higher a light's CRI (on a 0-100 scale), the richer and more natural the colors of illuminated objects will appear. CRI ratings for different light sources may be compared, but numerical comparison is only valid if the lights are close in color temperature (see).
 
Some nocturnal activities, such as walking or operating a vehicle, may not require the accurate perception of surface colors, while others, such as making product selections while shopping, benefit from better color rendition.

In physics, how hot objects create light is modeled through "blackbody radiation". When you increase the temperature of a theoretical "blackbody" enough, it starts to emit visible light in a continuous spectrum; at lower temperatures, the peak of this radiation is in the red end of the spectrum, and the peak moves toward the blue end as the temperature rises. Any object which radiates light because of its high temperature will give off light with a color peak directly corresponding to its temperature. The filament of a 60-watt incandescent lamp heats to about 3000°F, so the lamp puts out light with a color temperature of about 3000°F, which makes things appear somewhat yellowish (redder). The surface of our Sun is at about 10,000°F, so that's what the color temperature of its light is (it has much more blue in it than the light from the 60W bulb does).

The colors shown with the temperature scales above aren't the only ones emitted; they're the peak of the blackbody emission curve, and show about what the relative tint of the light will appear like to the eye. The color temperature of many lighting products will be given in degrees Kelvin, but we've included degrees Fahrenheit in the above examples for continuity with our U.S. view of the universe.

Many of our artificial light sources do not create light by heating up a material until it glows from simple incandescence. Instead of creating a continuous "rainbow" spectrum, like a blackbody does, they generate a complex assortment of color emission lines scattered around the spectrum.

A blackbody color temperature cannot be directly given to this non-blackbody type of emission, which is what is generated by both high intensity discharge and fluorescent lighting (see both). For those sources, a "correlated color temperature" (CCT) value is given, based on what blackbody color temperature their light is perceived by the average human eye to most resemble. A "cool white" fluorescent lamp may have a CCT of 4100°K, and a high pressure sodium streetlight a CCT in the range of 2000-3000°K, but their actual light spectra are quite different than a blackbody of those temperatures would have.
 
While the effect which these spectral differences have on human visual perceptions might be relatively minor, some other effects may not be. For instance, if we are trying to avoid light in the green and blue wavelengths to reduce the strong effects which those shorter wavelengths have on some biological systems, or on skyglow, a 2500°K incandescent light will have low emission on that end of its blackbody spectrum, while a 2500°K CCT high pressure sodium lamp may have fairly strong green, blue, violet and ultraviolet emission lines "buried" in its output (see: spectral power distribution).

Initialism for Crime Prevention Through Environmental Design; see our Key Issues > Light, Safety & Crime page for a discussion.

Pertaining to or occurring during twilight (dusk or dawn); especially referring to animals which are active at those times. (See also: Diurnal, Nocturnal.)

 
In itself, "cutoff" is a simple concept. For any light source which illuminates less than the full sphere around itself, there will be an edge between the illuminated area(s) and the non-illuminated area(s) on that sphere. That edge is the cutoff. It can be a sharp line between bright and dark (a motion picture projector has sharp cutoff at the edge of its beam), or a "soft", gradual line of dimming (in a total solar eclipse, the cutoff line of sunlight on Earth's surface is a bright-to-dark gradient hundreds of miles wide). Most all light fixtures feature some sort of cutoff, at least where the shadow of the fixture itself is cast.
 
Using "cutoff" as a term for classifying luminaires is much more problematic. In 1972, the IESNA (see) issued a standardized set of parameters for classifying luminaire performance using the term "cutoff"; several classes were described, based on how much light each allowed to shine in different directions, especially concerning light shining upward. In that system, "full cutoff" was defined as having no light emitted at or above an angle of 90° above nadir and the luminous flux emitted in the band between 80° and 90° above nadir in all directions as no more than 10% of the total luminous flux for the luminaire. The IESNA has since dropped the "cutoff" designation system, adopting a more detailed system to more accurately address luminaire performance (see: Luminaire Classification System), but the term "full cutoff" has ended up in many different publications and regulations, often included with varying definitions. Because of the many different ways it has been used, we find its meaning to be very imprecise now, and its value too depreciated to use as a specific term for luminaire classification.
 
But "cutoff" itself remains a key concept in analyzing luminaire performance.

In some high intensity discharge lamps, as the lamps age, they require more voltage to maintain the arc within their discharge tubes. Eventually, the maintenance voltage required can exceed what the ballast can provide, and the lamp will start up, approach full brightness, and then shut off when the arc fails. After cooling down (see: Restart time), the ballast will then start the process over again. This (worn out lamps) is a common cause for outdoor lights seen to repeatedly start up and then shut off. Some ballasts are equipped with circuits to shut the fixture down after a few repeated cycles.

Referring to the process in which the human eye shifts from being optimized for daylight vision to being optimized for low-light vision. Most familiar in everyday experience from when we switch off a room's lights at night: At first, we can see very little, but as time passes, more becomes visible to us as our eyes "get used to the dark".
 
While dark adaptation begins to occur within seconds of a drop in light level, to reach their full low-light-level sensitivity can take the eyes thirty minutes or more. At any point in that time that there is exposure to bright light, the process is not only interrupted-- when the light level drops again, the dark adaptation process basically has to start over from the beginning. This poses real quandaries about the uses and engineering of artificial illumination at night: The only practical levels of illumination which can be achieved in many outdoor situations are low enough that the eye will need to be somewhat dark adapted to see well, but then the eye is also very susceptible to being dazzled by brighter light sources which ruin its dark adaptation. (See also: Photopic vision)

Pertaining to or occurring during daylight; especially referring to animals which are active during the day. (See also: Crepuscular, Nocturnal.)

The evening portion of twilight (see), (as opposed to the pre-sunrise twilight). When used technically, generally equivalent to civil twilight.

See Uplighting.

In lighting, this term usually refers to the number of hours a lamp will operate and deliver a suitable level of light output, and retain color quality and energy efficiency close to their original values. For this use, Economic Life may be around 60-75% of the rated life for high intensity discharge lamps and fluorescent lamps. In large-scale installations, Economic Life may refer to the length of time before it is economically and esthetically advisable to group re-lamp all of the fixtures in the installation.

Most frequently, the efficiency of a lamp at converting electrical energy into visible light (light which the human eye can "see by"). Efficacy is usually measured in lumens per watt (lumens of light produced per watt of electricity consumed, abbreviated LPW or lm/w). For Luminare Efficacy, see Target Efficacy Rating and Fitted Target Efficacy.

For the efficiency of a lamp, see Efficacy. For the efficiency of an installed luminaire, see Coefficient of utilization.

In exterior illumination, a value determined by defining the coefficient of utilization within a defined target area of illumination for a luminaire. Part of the calculation of Target Efficacy Rating (see).

A standards program, initially created by the U.S. Environmental Protection Agency, but later adopted by the E.U. and some other nations, designed to recognize models of appliances and other energy-consumption-related products which are designed to save energy, compared to other similar products. In the U.S., Energy Star standards are set by either the E.P.A. or the Department of Energy.

The placement of windows and openings in buildings and other structures. In lighting practice, this includes any transparent or translucent openings in exterior or interior walls or ceilings which let light travel from one area to another.

A metric for luminaire efficacy (a corollary of energy efficiency), created by the U.S. Department of Energy for use by the Energy Star program to rate solid-state outdoor lighting (primarily LEDs at this time). The product of the metric is a lumens per watt rating for a luminaire, similar to that of the Target Efficacy Rating metric (see). It is sometimes reported that the main difference between the FTE and TER metrics is that FTE includes consideration of the uniformity of illumination provided by the luminaire being measured; actually, the largest difference between the two metrics is the manner in which the target zone of illumination is defined. In TER, the target zone is based on the standard IESNA streetlight type classification system (see). In FTE, the target area is determined by the luminaire being rated: whatever area it lights up is its target area, by default. The FTE system, therefore, makes direct comparison of the performance of multiple luminaires problematic. The DOE introduction to FTE is available here.

A mercury vapor gas discharge lamp, modified to emit various colors of visible light by the addition of various phosphorescent compounds. The mercury plasma within the lamp, excited by an electrical current passing through it, emits large amounts of invisible ultraviolet light. The phosphors are placed as a coating on the inside of the lamp tube; they absorb the ultraviolet light, and re-emit its energy in spectral lines in the visible portion of the spectrum. Different phosphors can be mixed in various combinations to achieve different "colors" of light (see: Color temperature). In general, most fluorescent lamps will have a higher efficacy (see) than an incandescent lamp of the same wattage. Fluorescent lights are most commonly used to illuminate indoor areas like schools and offices, and outdoors for sign lighting (many "neon" lights are actually fluorescent lamps, not neon gas discharge).
 
The "compact fluorescent lamp" (CFL) has gotten large amounts of press for its higher efficacy (aren't you glad you learned that term?) than the old standard incandescent lamps which have been used to illuminate our homes for decades. CFLs have problems with not handling short on/off cycling well (short on-times may reduce lamp life dramatically; the U.S. Energy Star program recommends leaving CFLs on for a minimum of 15 minutes at a time, or face lamp lifetimes no longer than those of incandescents), with warm-up time, with their mercury content, with the inability of most models to be put on dimmer circuits, and with several other issues. Watch for light-emitting diodes (LEDs) to displace this "replacement" before long; they face none of these problems, and will be longer-lived and have higher-yet efficacies.

See Luminous flux.

A measure of illuminance, the amount of light falling onto a surface. One lumen of light, shining evenly across one square foot of surface, illuminates that surface to one foot-candle (sometimes also written footcandle or foot candle). The European & scientific standard unit for measuring illuminance is the lux; one foot-candle equals about 10.764 lux. Here is a calculator which will convert between the two units:

See Cutoff.

Creating general classifications for luminaire performance is difficult. The Luminare Classification System (see) of the IESNA provides a straightforward method for rating a number of directions of light distribution by a given luminaire, but it does not provide a standard set of luminaire "types" (see: Cutoff). We saw the need to provide a name for the general design of luminaire which maximizes energy efficiency and minimizes stray light and glare by utilizing optimized reflective surfaces and other optics to deliver as much as possible of its lamp's total light output to the area of intended illumination; we have chosen the term "Fully-focused" for this use. (See our Lighting Studies > Fixtures page for illustrations.)
 
At present, this term does not have any strict performance standards associated with it; if we end up applying it in specific recommendations or regulations, adding numerical standards will be necessary. In theory, the term could be used with luminaires having varying angles of cutoff; the important factors will be having a high coefficient of utilization, and a low limit to illumination outside of the specified cutoff line.

In general terms, glare is defined as light filling a small angular area of a person's field of vision which is notably more intense than the average illumination of the majority of their field of vision; this smaller, more intense source of light dazzles the viewer's eye, which is adapted to the brightness of the general scene. Glare can occur at any ambient light level, whether it be sunlight glinting off glass in the daytime or intense artificial light sources at night.
 
In engineering, intensity of glare is often divided into three classes: Discomfort Glare is distracting, uncomfortable, and/or triggers an instinctive desire to look away from the source. Disability Glare is more intense, and directly effects a person's visual perception enough to handicap their performance of visual tasks. Blinding Glare is so intense that, after it has been removed from the field of vision, there is an appreciable length of time before normal vision returns.
 
The terms Veiling Luminance, Veiling Reflection or Veiling Glare apply to situations where light coming into an area actually reduces the visibility in that zone. An example of veiling glare or reflection would be where the lane marking lines painted on a roadway may be visible on a dry night, but when the roadway surface is wet, glare reflecting off its shiny surface may obscure the visibility of those same lines. An example of veiling luminance would be where a business along a roadway is more brightly lit than the roadway itself; while the roadway lighting on its own would be sufficient for safe travel, the added commercial light by the roadside reduce drivers' ability to see the dimmer roadway.

See Photometrics.

A class of lamps which create light energy from electrical energy by running electrical current through a tube of various gases and metal salts. The heat from the current produces a plasma (a hot, partially ionized gas), which emits light in the characteristic spectral lines of its constituent gasses and metals.
 
The vast majority of outdoor illumination is presently done using high intensity discharge lamps; they have replaced incandescent lamps in outdoor (and much large-volume indoor lighting) because of their higher efficacy (see). Mercury vapor, low pressure and high pressure sodium, and metal halide lamps are all high intensity discharge types (although low pressure sodium lamps are sometimes relegated to the broader class of "gas discharge lamps", because some folks don't consider their intensity to be "high" enough to be "high intensity" discharge; 22,000 lumens off a 135 watt lamp seems pretty high intensity to us). Fluorescent lamps (see) also generate light via a mercury vapor plasma, but they differ from HID lamps in that the ultraviolet light which the plasma produces is then used to excite phosphors which coat the inside of their tubes; instead of the plasma itself, those phosphors are what create the visible light we see. (Most HID lamps also create notable amounts of ultraviolet light; many are protected by an envelope of glass which is formulated to absorb much of the potentially harmful UV radiation.)

Synonym of Period fixture (see).

The Illuminating Engineering Society (of North America). Formed in 1907, the IES is a non-profit trade organization of lighting engineers and businesses. Their mission statement reads: "The IESNA seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public." Their goals include efforts to "develop standards, design guides, technical memoranda, lighting energy management materials, guidelines and lighting measurement, testing and calculation guides" (source: www.ies.org).

The total luminous flux (see) falling on a surface, measured per unit surface area. Illuminance could be measured right down to the photon of visible light (something like photons/cm²) but in the U.S. our common unit of measure in discussions of lighting is the foot-candle (see).

In general terms, the deliverance of visible light into an environment. Illumination may be either natural (skylight, sunlight, moonlight, etc.) or manmade (artificial).

A lamp which creates visible light by heating an internal filament to a temperature high enough to create blackbody radiation in the visible wavelengths. Electric current is passed through the filament, where electrical resistance causes the generation of heat. Exposed to outside air, the filament (normally made from the metallic element tungsten), would rapidly oxidize and "burn out", so the glass housings of incandescent lamps are sealed, and filled with inert gases such as argon.
 
Incandescent lamps emit light in a continuous "rainbow" spectrum with a color temperature (see) equal to the temperature of their filament, which will vary depending on the type and wattage of lamp.
 
Quartz halogen lamps are a type of incandescent light. They tend to have higher filament temperatures; the halogen gas filling the lamp tube supports the higher temperatures better than the gasses used in "regular" incandescent bulbs. They also tend to be smaller in volume than regular incandescent lamps; the quartz shell is more heat resistant than a glass one, so it can be stable closer to the hot filament than a glass housing could be. Running at higher temperatures gives them a higher color temperature; a "whiter" light than a standard incandescent. The higher temperature also tends to give them a higher efficacy at turning electrical energy into visible light.
 
In outdoor illumination, incandescent bulbs have been mostly supplanted by high intensity discharge lamps, which generally have notably higher efficacy. For homeowners, where HID lamp use isn't practical or necessary, we're being told to replace incandescent with CFLs (see fluorescent lamp), but those have their own set of problems. LEDs will likely be what finally replaces Edison's invention.

A type of fluorescent lamp (see) which, instead of using an electrical current flowing through the lamp tube, uses radio-frequency or microwave radiation beamed through the tube to stimulate the gasses and metals within to emit light. Just as in the other fluorescent lamps, the majority of this light is in ultraviolet wavelengths, and is used to stimulate a phosphor coating which then emits the desired wavelengths of visible light. Since induction lamps lack the internal electrodes of regular fluorescent lamps, they generally have notably longer lifetimes (see: Economic life).

Many types of lamps (see) will produce reduced amounts of light as they age. "Initial lumens" is, as it sounds, the rating of how much light a lamp will put out when it is first used. Lamp specifications often list both initial lumens and "average lumens"; the latter, as its name implies, is the average light output of a lamp through its economic life (see).

Cells in the retina which are sensitive to light, but do not contribute to visual image forming. Research in just the past decade has demonstrated that in humans, the stimulation of these cells by light (most effectively in the visible blue wavelengths) sends signals directly to the "timekeeping" center of the brain, the suprachiasmatic nucleus of the hypothalamus. It is through this mechanism that the circadian rhythm of the individual is synchronized to the cyclic variation between daylight and dark-of-night.
The ipRGC (sometimes also called Photosensitive ganglion cells, or Photosensitive Retinal Ganglion Cells, or melanopsin-containing ganglion cells) also serve some other light sensing functions, such as controlling the dilation/constriction of the pupils, and perhaps some other "subconcious" awareness of the brightness/darkness of one's surroundings (even when asleep). See also: photopic vision, melanopsin.

See Photometrics.

See Watt.

In common terms, what is called a "light bulb"; lamp is the proper engineering term for the specific part which actually takes electrical energy from a socket in a light fixture and generates light. Also in common terms, "lamp" will refer to the whole package, such as in "table lamp", but in engineering, just the light bulb is the "lamp" and the rest of the setup is the "fixture", and the lamp and fixture together are a "luminaire" (see).

Initialism for light-emitting diode, an electronic light source. Unlike most of the types of lamps which we use for general illumination, LEDs do not create light by heating a material. Instead, light is generated in a semiconductor, by the process of combining electrons with "electron holes" (electrons missing from energy levels around the nucleus of an atom).
 
LEDs emit light in one or more specific wavelengths; this may be in the ultraviolet, visible, or infrared. "White" light can be created in two main ways. If red, green, and blue LEDs are clustered together, the resulting combined light appears "white" to the human eye. Or, the light from a short-wavelength LED (usually ultraviolet or blue) can be used to excite a phosphorescent compound which is generally placed as a coating right on top of the semiconductor material within the LED package.
 
LED technology is advancing rapidly. It holds the promise of achieving higher efficacy than the lighting systems we are currently using, plus longer lamp economic life. LEDs are also, by their nature, focused light sources (as opposed to most other lamps, which emit light in all directions); this feature eliminates the need for luminaire design incorporating all sorts of optics to gather and re-focus misdirected light.

As it sounds, any sort of illumination present after the end of evening twilight and before its morning counterpart. When referred to in ecological or light pollution contexts, often meaning only manmade illumination which is of a greater intensity than or different spectral quality from the natural illumination found in the area in question.

See Luminaire.

In lighting engineering, a sum of all the factors which contribute to the lowering of light output from a luminaire; these can include dirt buildup, reflector degradation, lamp depreciation, etc.

A general term for any manmade light being "dumped" where it isn't wanted or where it can do environmental harm. Just as fertilizer is of benefit when applied to a garden, but is a pollutant when it is introduced into a stream, nighttime illumination which is helpful in one area can be harmful in a neighboring one. "Light pollution" does not have any more of a specific, generally accepted definition, though, and our group tends to avoid the use of the term. Since it was first used in a widespread sense in the astronomy community, it tends to mainly mean "bright skies which hamper astronomical observing" to many people, leaving out the other problems stemming from irresponsible outdoor lighting. Other people just have problems with the term, pointing out that air pollution means making the air dirty, and water pollution the water dirty, but light pollution doesn't make the light dirty.

See Trespass.

In astronomy, a measure of the sky's brightness based on the brightness (stellar magnitude) of the faintest object visible against the sky brightness with a specific instrument. When applied to measuring manmade skyglow, the default is often to use the unaided human eye as the measuring instrument, observing in an area near the zenith; i.e., if the faintest stars visible overhead to the naked eye from a particular location were of magnitude 4.0, the location's Limiting Magnitude would commonly be referred to as 4.0. (See also Bortle Scale, magnitudes per square arc second.)

A unit of measure of luminous flux (see), or visible light. This is a measure of light energy, basically related to the total number of photons of visible light present. It differs from a measure of illuminance (see) or brightness, in that brightness depends on the size of the area over which the light energy is spread. Lamps are generally rated in lumens; this is the total amount of visible light energy which they are emitting at any one moment of operation.

See Efficacy.

A complete lighting unit, including lamp(s), socket(s), ballast, reflectors and lenses, housing, etc. Often also referred to as a fixture, but "fixture" may or may not include the lamp(s), while "luminaire" always does.

 
The Luminaire Classification System (LCS) is a product of the IESNA (see), adopted in 2007 as a replacement for their older Cutoff Classification System (see: Cutoff). The LCS divides the sphere of light emission around a luminaire into three main zones: Up, Forward/Front (down), and Back (down). Those zones are then subdivided further. The concept is to provide a standard tool for measuring where a luminaire sends its light output; luminaires can be rated by the measured percentage of their output which they shine into each zone, and/or to the actual amount of lumens shining into a zone. In this graphic of the zones, you can see the divisions: Up Low (UL) covers from 90°-100°; Up High above that (Up zones colored pink). Front (blue) and Back (green) are separated, because many fixtures are mounted to the side of the area which they are intended to illuminate (some other fixtures will have identical front and back performance). Front and Back are both divided into different elevations: Low (0°-30°), Medium (30°-60°), High (60°-80°), and Very High (80°-90°).
 
The LCS gives the lighting engineer the ability to look on paper in reasonably good detail at some common "problem" problem areas: Backlight, Uplight, and Glare (glare and light trespass often originating from light in the front and back Very High zones, and in some installations in their High zones, and lower yet Back zones); it is incorporated into the BUG (acronym for Backlight, Uplight, Glare) system for specifying fixture performance (see BUG).

A measure of the brightness of a surface which is emitting light. The unit of measurement most commonly used is candelas per square meter, often referred to as nits in the U.S. (1 nit = 1 cd/m²). The nocturnal appearance and environmental effect of objects such as internally lit signs may be analyzed both by total light output (lumens) and by their surface brightness (nits).

The visibly useful light energy emitted by a light source. The total amount of light emitted would be the radiant flux, but luminous flux is balanced according to the sensitivity of the human eye to different wavelengths (colors) of light. Measured in lumens (the "lumens" rating for a lamp is a measure of the total quantity of visible light it puts out).

The amount of visible light leaving a source in one specific direction; measured in candella. We cannot measure luminous intensity directly; instead, we must measure illuminance at a known distance from the light source, and calculate the luminous intensity using the inverse square law (illuminance = luminous intensity/distance squared).

A measure of illuminance (see); see Foot-candle for further discussion.

A scientific unit of measurement of the brightness of the observed surface of an astronomical object. When applied to judging the brightness of skyglow in a location, a photometric measure of magnitudes per square arc second is more objective than a visual measure of limiting magnitude, because the latter depends on the visual acuity of an observer's eyes. With the magnitudes per square arc second scale, the sky brightness decreases as the numerical value increases, because the scale is based on stellar magnitudes (which increase numerically as brightness decreases); a sky brightness of 21 mag/arc-sec² is very dark, while a value of 16 mag/arc-sec² would be 100 times brighter, and similar to what is found over many light-polluted communities.

A photopigment found in human Intrinsically photosensitive retinal ganglion cells (see). First discovered in 1998 in light-sensitive cells in frog skin, this photopigment is more "primitive" than those used for image forming in the vertebrate eye; apparently it has been retained in the ipRGC bundles in the human retina to perform the simple tasks of signaling whether it is day or night and helping trigger pupil dilation and contraction.

A naturally occurring hormone found in most animals, and some organisms of other kingdoms, including algae. Melatonin levels are important in the regulation of circadian rhythm (see); it is also a potent antioxidant and plays particular roles in the protection of nuclear and mitochondrial DNA. In animal testing, melatonin has been demonstrated to prevent the damage to DNA caused by some carcinogens, stopping the mechanism by which they cause cancer. In humans, some studies have demonstrated that melatonin levels reduced by exposure to light at night lead to increased risk of the development of some forms of cancer, including those of the breast and prostate.

See Photopic vision.

An electronic device designed to detect the presence of moving objects; most commonly in lighting practice, a sensor which, when it detects the movement of a warm body (such as a person or vehicle) within its operating range, closes a switch. The switch may operate light fixtures, so that when someone enters an area, the light or lights in that zone go on. Most such switches are adjustable, holding the light on either only while motion continues to be detected, or keeping it on for a pre-set time after motion ceases, then shutting off. Some light fixtures have motion detectors built into them.

The elevation off the ground at which a luminaire is installed; preferably measured to the level of the highest part of the luminaire where any light is emitted, but sometimes to the lowest point of the joint where the luminaire is attached to pole or wall. In free-standing fixtures, also referred to as "pole height", but sometimes different than the vertical dimension of the lamp post/pole/standard itself.

A perfectly vertical straight line drawn through any point on Earth's surface heads in two directions: The downward one, aimed at the point where the force of gravity is pulling, is directed at the nadir; the upward side, heading straight "overhead", is directed at the zenith.

The National Electrical Manufacturers Association, a U.S. trade group with the vision statement goal of being "the trade association of choice through which the electroindustry develops and promotes positions on standards and government regulations and through which members acquire information on industry and market economics" (source: www.nema.org).

The Next Generation Lighting Industry Alliance, formed in 2003 as a partnership of the U.S. government and the lighting industry to promote advancement of solid state lighting systems (primarily LEDs, see). See www.nglia.org for more information.

A unit of measure of luminance (see).

Pertaining to or occuring at night. (See also: Crepuscular, Diurnal.)

Not transparent; i.e. the property of a material which does not allow any visible light to pass through it.

Either: The branch of physics dealing with the nature and property of light, or more colloquially, any hardware which is used to manipulate light, such as lenses or mirrors (short for "optical components", as in: "The optics of the fixture consist of a polished, spun aluminum reflector and a Fresnel lens diffuser").

While this term could be used to describe any light fixtures which are mounted outside of a building, we prefer to have it also include any artificial lighting which reaches the out-of-doors. This definition then also includes interior lighting which may leave buildings via windows or other transparent or translucent surfaces.

 
A style of incandescent lamps; PAR stands for the Parabolic Aluminized Reflector which is formed by the back of the lamp glass body itself, and takes the light produced by the lamp filament and focuses it out the front face of the lamp. PAR lamps are made in a number of sizes, specified by a two digit number, and are made in both floodlight (wide beam) and spotlight (narrow beam) configurations. Most PAR lamps have a heavy, rugged glass shell and are suitable for outdoor use. However, as they are commonly use outside of homes, pointed up at fairly high angles to illuminate yards, they "miss the boat" by a long way; their light trespasses into neighboring areas, and they often defeat any intended security effect by creating so much glare that they actually make it impossible for neighbors to see if anything is amiss.

As opposed to all-night lighting, which, as the term implies, would operate from around sunset to around sunrise, part-night lighting is only run some of the nighttime hours, or may be run all night, but dimmed for some of the hours. Parking lots, and even some roadways may need full illumination through the busy hours of the night (perhaps from sunset until closing hours, or 10, 11, or 12:00pm), but then can have their illumination safely dimmed or extinguished. Part-night lighting can cut electricity consumption greatly when used in place of all-night lighting.

A contemporary light fixture which is designed to mimic the appearance of a light fixture of an earlier century. Usually, this involves a modern light source (such as an HID lamp) in a fixture which was originally created for a much dimmer light source, such as a gas flame or an Edison incandescent lamp. While cleverly designed fixtures can pull this off, generally by placing the modern lamp in a different part of the fixture than where the light source was located in the original design, just replacing the light source in a fixture with one which may be hundreds or even thousands of times brighter than the original is generally a recipe for glare and energy waste (see our Lighting Studies > Style & Fashion page for illustrations).

The scientific study of the interactions of light and living organisms. The American Society for Photobiology divides the study into five major divisions: 1) Photochemistry, Photophysics and Phototechnology; 2) Photosensory and Circadian Biology; 3) Photosynthesis, Bio- and Chemiluminescence; 4) Photomedicine; and 5) Environmental Photobiology and UV Effects.

See Photoelectric switch.

 
A device for turning an electrical appliance (usually a light fixture) on and off based on the ambient light level. Commonly used with outdoor lighting installations, such as the streetlight luminaire shown here, this switch allows automatic operation of the fixture, turning the lamp on "after dark", and off in the daylight. Consists of a solid-state photocell, which "measures" the level of ambient light, and circuitry which operates either a mechanical or solid-state relay to switch the power supply to the lamp (or its ballast) on and off.
 
Luminaires may have their own photoelectric switches, like the one shown here, or groups of fixtures may be operated by one common switch. Most such switches feature either an electronic adjustment of their on & off light levels, or a mechanical adjustment of a shutter over the photocell window to achieve the same variability.

In lighting engineering, the study of the distribution of light output from luminaires and luminaire installations.
 
When designing a lighting installation, it is important for a lighting engineer to be able to model what the achieved levels of illumination will be across the target areas. In order to be able to do this, he/she must know what the pattern of light output is for each of the luminaires installed, and plug that information into a model of the actual space of the installation.
 
The first step of the process begins with the luminaire manufacturer. They submit a sample model of each fixture to testing with a tool called a goniophotometer, which takes a reading of the luminous intensity of the light output at a large number of points on a spherical shell surrounding the operating luminaire. The table of numbers of these multiple readings is put into a standard format, which the fixture manufacturer then supplies to lighting engineers and other customers. In the U.S., the standard used is the IESNA's LM-63; there are other standard photometric file formats used in Europe and elsewhere.
 
 
The lighting engineer then takes the file(s) of photometric data for the luminaire(s) chosen, and computes the actual lighting levels achieved across the "3-D" area of the planned installation. This step is most often done using modeling software, which reads the photometric data, does the computational modeling, and outputs the results in a graphic form known as an isoplot. The isoplot shows the area being modeled, overlayed with "isobars"; outlined zones of varying levels of illuminance. In this simple example, the isoplot is just the output from a single focused light fixture on to a flat plane; the zones indicate the percentage of illuminance compared to that of the central ("100%") zone, which is listed elsewhere for a given throw distance.
 
This methodology allows the lighting engineer to model what real-world illumination will end up looking like while installations are still on the drawing board. CAD software can also take photometric data files for luminaires and render walk-through simulations of what an area will look like when illuminated by the specified fixtures.
 
Isoplots can also be created for existing installations, by doing a point-by-point measuring of the actual illuminance on a grid plan, and plotting the figures obtained on a plat and drawing in appropriate contour lines.

The physiological reaction of living organisms to the length of light and dark periods. Under natural lighting, the 24-hour day is divided into a light period (day) and a dark one (night). The ratio of hours of light to hours of dark varies throughout the seasons in all parts of the globe away from the equator; many organisms synchronize to the "time of year" by the changing light/dark ratio. In plants, effects include the triggering of blooming, and the move to dormancy in the autumn. In animals, day/night length changes trigger functions such as migration, reproduction, seasonal changes in coloring, and hibernation.
 
Manmade light at night can (and does) cause organisms to misjudge the onset and length of night, disrupting any and all functions regulated by photoperiodism.

The retina of the human eye contains two types of vision cells. The cone cells are come in three different types, which, working together with the image center in the brain, provide us with our color vision. These cells need a decent amount of light to function; moonlight, for instance, does not illuminate the landscape enough for them to operate, so we lack our color vision under it. The second type of vision cell is the rod cell; while it is much more sensitive to faint light, it does not provide color perception; this is why we can see in the moonlit landscape, but it all appears as "black and white" (shades of gray).
 
Photopic vision is the term for the sight which our cone cells provide; basically our "daytime vision". The rod cells provide our Scotopic vision -- our "nighttime vision".
 
For a long time, physiologists thought that at any given time our eyes were either operating photopically or scotopically. But more recent research has shown that under moderate light levels, where we still have color perception, the rod cells are also contributing a substantial amount of visual information to the brain. This combined rod/cone sight is called Mesopic vision. Mesopic vision is at its strongest under "twilight" light levels; this happens to be the lighting level we frequently encounter in artificially illuminated environments, such as when operating vehicles, or in outdoor areas which are lit for basic nocturnal activities. So, an understanding of how the eye responds under mesopic conditions is important when discussing outdoor illumination.

See Mounting height.

A class of exterior luminaires, designed (as their name implies) to be mounted directly on top of a lamp post (light pole, standard). This places the lamp(s) within directly over the pole, as opposed to fixtures which hang on the side of the pole, or on arms extending out from the pole.
 
Post top luminaires come in many designs, with widely differing efficiencies and effects on the environment around them. On the "poor" end of the scale would be translucent shells surrounding otherwise naked lamps, and allowing the output from the lamps to shine in all directions with no effective effort to focus the light where it is needed. On the "good" end would be fixtures which focus a healthy majority of the light output of the lamps where it is needed, and send little or none elsewhere.
 
Post top is often associated with period fixtures, but there are modern-styled ones, too.

In lighting, the electrical power consumed divided by the area (usually in a horizontal plane) of intended illumination. In U.S., normally measured in watts per square foot. For example: If you are consuming 100 watts of electricity (including the draw from the lamp, ballast, and any other luminaire or control parts) to illuminate a target area of 100 square feet, the power density of your installation is 1 watt/ft².

See Incandescent lamp.

In lighting, the percentage of incident visible light which bounces off of a surface, rather than being absorbed. Scientifically, albedo is a more accurate term for the visible reflectivity of surfaces and areas which have diffuse reflection (like much of the world around us). Shiny specular surfaces, like mirrors, polished metal, and the surface of water have very low reflectivity at most angles of lighting except at the direct angle of reflection.

Some types of lamps, including most high intensity discharge models, cannot be shut off and then instantly re-started; they need a cooling-down period which may take from a few to ten or more minutes before they can re-light. This is caused by the fixture's ballast's inability to "strike" an new arc in a lamp which is already under the high temperature and pressure of operation, so sometimes this delay is called "restrike time".

A new term (2003) to describe the particular study of biological systems which are directly and specifically affected by darkness; particularly to the dark portion of a day/night cycle.

See Photopic vision.

 
One of the most problematic words in this glossary, "shield" and its various forms and uses (shielded, shielding, "fully shielded", etc.) are often used to mean one thing by one person, and something else by others.
 
In its simplest interpretation, anything in, on, or around a luminaire which stops the light output of the lamp(s) from going out in a particular direction is a "shield". But in our interpretation of the word, we tend to not think of parts which serve other purposes, like ballasts, sockets, wiring, basic housings, reflectors, etc., as "shields"; we'd prefer to use this term for parts which are added on to a luminaire specifically to block the light from shining in some particular direction(s). "Shield" also, in our view, implies something external to a luminaire; an internal shield is better termed a baffle.

Lastly, we prefer to use "shield" to describe a component which does this light-blocking action in a non-constructive manner; either absorbing the misdirected light, or diffusing it and/or sending it in yet another (probably misdirected) direction. If a component is added to a luminaire to take misdirected light and actively focus it on the area which the fixture is meant to illuminate, then that component (reflector, etc.) is an active optic (see) of the luminaire, rather than just a "shield".
 
This all points out the difficulty in using the word shield in the broad classification of luminaires. Some users picture a "fully-shielded" fixture as having optics to focus the lamp's output in the "right" direction, but the term itself could apply as well to a lamp in a box with a black interior and just one open side (some light escaping from the open side, but all of the rest going to waste). To question the term even further, an armored car which is covered in steel plate on five sides, and paper on the sixth, isn't "fully shielded"; doesn't it seem logical in a linguistic sense that the only way to have a fully-shielded lamp would be to seal it in an opaque box?

Any light shining from the nighttime sky (after the end of astronomical twilight in the evening, and before the beginning of morning astronomical twilight) other than that from external celestial objects. Some natural skyglow is created by atmospheric phenomena, but at very low levels; the term is more commonly used in the discussion of environmental issues to refer to manmade light which is being projected up into the sky, and reflected back down by air molicules or dust, moisture, and other particulates in the atmosphere (on "clear nights"), or by clouds (on "cloudy nights").
 
Skyglow is what prevents us from seeing as many stars from urban areas as we can from remote ones (the same Universe is hanging over both). First, the brightness of the sky (and our neighborhoods) prevents our eyes from achieving a good level of dark adaptation (dark adapted eyes being needed to see faint stars). Second, if the sky itself appears brighter to us than the objects "beyond" it, the objects become invisible (the same reason that we can't see any stars other than our Sun in the daytime: the blue sky is too bright).

 
The precise color output of a light source, describing the power (flux) per unit area per wavelength. Generally provided as a Spectral Power Distribution (SPD) curve; a graph plotting power ("intensity") on the y-axis and wavelength ("color") on the x-axis. Measured with a spectrometer.
 
SPD measurement is the only effective way of analyzing what color-related effects a light source will have on environmental and biological systems. Correlated color temperature (see Color temperature) is often mistakenly applied for such considerations, but that product is entirely based on human visual perception of color, which is quite skewed from the actual spectral and energy levels present in light, especially in non-blackbody radiation.

A term for light leaving an intentionally illuminated property or zone; see also trespass.

Besides the usual definition of "something established as a rule or basis of comparison", in lighting standard also refers to a free-standing upright support for a light fixture; i.e., a (lamp)pole or (light)post.

A class of outdoor luminaires designed primarily for illumination of roadways and adjacent parkways; identical or similar fixtures are also used for area lighting in lots and other open spaces.
 
From a lighting engineering point of view, streetlighting is installed for the specific purpose of bringing the roadway, the adjacent parking spaces, curb or shoulder, and/or the sidewalk up to a certain level of illumination which will allow for safe activity at night; or to serve as beacon lighting (see) to delineate intersections and other potential hazards to drivers. But in practice, especially within some municipalities, streetlights are looked on more as illuminated decorations than as energy-consuming appliances installed to achieve safe levels of lighting for nocturnal pursuits. Applying good lighting standards and practices to fixtures which are designed as decorations is difficult. For instance, an inefficient, glaring globe-on-post fixture needs to be quite bright to illuminate the adjacent pavement to a theoretically adequate level, but then the glare from the light ends up making said pavement harder to see; this cannot be remedied by installing a brighter lamp, because that would increase the glare level, too.
 
It is our belief that some luminaires are being specified, marketed, and installed as "streetlights" when they are not actually filling the function of illuminating roadways. If lights are installed as illuminated roadside decorations, or to provide massive ambient light throughout an area, they should be honestly called something other than streetlights.

The IESNA has developed a system for classifying streetlight luminaires by the pattern of distribution of light they output. Different types are specified depending on the nature of the roadway (and possibly parkway) where illumination is needed; on the placement, spacing, and mounting heights of the fixtures; and on the light levels and uniformity ratio to be achieved. A given model of luminaire may be available in different versions, which vary in light distribution type class.

While this system was developed for roadway illumination, it is also used in the specification of luminaires used for other outdoor area lighting.

While Coefficient of Utilization describes the fractional amount of the visible light that is output by the lamp(s) in a luminaire which reaches the area of desired illumination, that figure for a given luminaire will vary depending on its placement. NEMA (see) has developed a protocol (2008) to allow somewhat more direct comparison of general luminaire energy effectiveness, by defining standardized placement criteria for luminaires in its Target Efficacy Rating system.
 
For outdoor fixtures, a use classification is chosen from this selection: a) Area and Site, b) Bollard, c) Border, d) Canopy, e) Historical- post top, and f) Roadway. Each classification then has a standardized method for computing a luminaire's Energy Effectiveness Factor (EEF) based on the published Coefficient of Utilization tables for that luminaire. The EEF, total lamp lumens, ballast factor (a rating of the efficiency of the lamp ballast), and input watts figure for the luminaire are factored together, and the resulting figure is the TER for the luminaire. TER is listed in lumens per watt, like the efficacy rating for lamps; but it is not the total light output of a luminaire's lamp(s) which is being rated for efficiency-- only the light (the lumens) which the luminaire successfully delivers onto the target area (which it will theoretically be installed to illuminate). Use of the TER system will allow direct comparison of the energy performance of different luminaires within the same use classifications, and the setting of performance criteria for luminaires in a systematic, laboratory-tested manner.

UPDATE: NEMA issued a revised version of the Target Efficacy Rating metric in 2009, less than a year after the version described above. The principle change in the system is definition of the target area for many of the exterior types of luminaires. While in LE 6-2008, Types II, III and IV luminaires (see streetlight type classification) only included light falling on the street side of the luminaire in the efficacy calculation, LE 6-2009 (available here) now counts light falling out to one mounting height back on the house side of the luminaire as being efficacious. While, in some installations, a moderate amount of light on the house (or "back") side of a streetlight may be useful for illuminating shoulder or sidewalk, the main effect of this alteration of TER is to increase the efficacy rating of many luminaires without improving their performance.

In indoor lighting, the installation of light fixtures to specifically provide the levels of visible illumination, contrast, and color rendering needed to perform one or more defined activities in a narrowly defined work area or surface (in contrast to providing general ambient lighting throughout an enclosed space). The concept of task lighting translates exceptionally well to outdoor illumination, where providing widespread artificial ambient lighting is usually grossly wasteful of energy, and very disrupting to the surrounding environment; yet the use of concept as a sensible, core practice is not yet widely adopted in the design of outdoor lighting installations.

In theory, the number of lumens (amount of visible light) delivered to a "task zone" (target area) per watt of electrical energy consumed by the illuminating device (luminaire or luminaires). This term is a generalization; it is not usable for performance measurement without having specified a precise metric for measuring the performance, such as one of the NEMA Target Efficacy Ratings, or the Fitted Target Efficacy system (see both). This is akin to the Miles Per Gallon vehicle fuel efficiency concept; without following one precise standard for how MPG is to be measured, the numerical figures posted by vehicle manufacturers would be worthless for use in comparing one vehicle's fuel efficiency to another's.

The property of a material which allows visible light to pass through it. Properly, translucence indicates that light passes through a material, but that the light is diffused so that images cannot be clearly seen through it. A translucent material may include filtering which changes the color of the transmitted light. See also: Opaque and Transparent.

The property of a material which allows visible light to pass through it directly enough that it does not notably interfere with the visibility of objects on the other side. A transparent material may include filtering which changes the color of the transmitted light. See also: Opaque and Translucent.

In lighting, the projection of artificial light from an area of intentional illumination to any areas outside of that zone. Most commonly used in reference to having lighting on one property shine into other properties, or into the sky in general, but light trespass can also occur within a property, where there are areas needing illumination and others which do not benefit from it.

"Dusk", or the time between fully-dark night and both sunrise and sunset. Frequently divided into three portions:
 
  Civil twilight is the period when the sun is less than 6° below the horizon; during this time, enough daylight is often present (depending on cloud cover) for human daytime vision (see Photopic vision) to be somewhat functional.
  Nautical twilight is the period when the sun is from 6° to 12° below the horizon; while the sky brightness may contribute some visibility to the earthly scene, human vision will mainly operate in its nighttime mode (Scotopic vision). When the sun is about 8.5 ° below the horizon, the clear sky provides about as much illumination to the local surroundings as a full moon overhead would.
  Astronomical twilight is the period when the sun is from 6° to 18° below the horizon; during this period, most people would consider the sky to be "dark".
 
Sunrise, sunset, and twilight pose particular problems with visibility and safety issues; the first two periods often bring glare issues from the low sun altitude, and during the twilight periods from the eye's difficulty with adapting to fluctuating illumination levels. Twilight also brings a peak to vehicular collisions with large animals in the US, where many are active at that time of day (see crepuscular).

In an area to be illuminated, either the ratio of the illuminance in the brightest-lit spots to that in the dimmest areas; or of the average illuminance of the whole area to that of the dimmest spots. For most illumination for nocturnal activity, the best results as far as visual acuity, safety, and productivity result when the uniformity ratio is close to one, that is when the area in question is evenly illuminated. The more the light levels between the bright zones and dark zones vary, the more difficult it can be to see across the area. Different activities have different requirements as far as uniformity ratio is concerned. Uniformity ratios may vary as the lamps providing the light to an area age (see Initial lumens); specifications for uniformity should indicate what lamp age (generally initial or average) they refer to.

 
Generally, any instance of intentionally aiming a luminaire so its light output is projected above the horizontal. For instance, a vertical wall could be illuminated by either placing light fixtures at its base, shining upwards (uplighting), or above it, shining down (downlighting). Also written as "up lighting" and "up-lighting".
 
Any type of lighting which projects outside of the area or beyond the surface which it is meant to illuminate is wasteful and can cause harmful trespass, but uplighting is particularly troublesome to anyone who is concerned about the preservation of the nighttime sky. Any light which misses its target may end up shining into the sky, and sometimes reflections from illuminated objects will bounce more in an upward direction when they are illuminated from the bottom, too. While misdirected downlighting is still wasteful, at least much of the stray light can get absorbed by the ground, rather than carrying for miles through the atmosphere.

A general term for exterior lighting which is installed primarily or entirely to draw attention. Unlike signage, which may need to be illuminated at night in order to be read, vanity lighting may illuminate facades or other architectural components (see Architectural lighting), landscaping, water features, etc.  Businesses use vanity lighting as attention-getters; surrogate "signage", as it were (the amount and area of signage allowed is often regulated by municipalities, but simple gross lighting of property often isn't). In residential areas, vanity lighting apparently serves to demonstrate a homeowner's desire to be noticed, too (see our Lighting Studies > Photo Gallery page for illustrations). Known in the lighting industry more by the more marketing-friendly term of "accent lighting".

See Glare.

 
A luminaire built for installation on a vertical wall surface; most often a high intensity discharge lamp, ballast, wiring, housing, lens, etc. all in one ready-to-install "package" (hence the appellation wallpack: "wall" + "pack").
 
The term can be used for any sort of wall-mounted, self-contained fixture, including newer fully-focused, downlighting (see: Uplighting) models, but it often brings to mind the all too common glass-sided units like the one pictured here. While some of its light output makes its way down to the property surrounding the wall it is mounted on, much of it sprays out as glare and light trespass, and more yet up into the sky as pure wasted energy and light pollution.
 
Units such as the one shown, no matter what the efficacy is of the lamp inside, waste a large amount of energy because of their having very low coefficients of utilization in most outdoor installations. See our Lighting Studies > Photo Gallery page for illustrations of good and bad versions.

In outdoor lighting, setting a defined list of criteria or conditions to be met to demonstrate the need for installing and providing artificial illumination in particular locations. Such criteria may include certain volumes of vehicular or pedestrian traffic, recognized hazards such as crosswalks or unprotected intersections, or a previous history of accidents or crime. If the location meets the listed criteria, the installation and operation of additional lighting is warranted; if it does not, light fixture or fixtures will not be installed.
 
Developing a good warranting system is the first step in any well planned out lighting program, especially for plans covering extended areas, such as municipal streetlighting. Warranting can also effectively be applied to existing installations, providing a practical and fair method for analyzing the balance between effectiveness and waste, and determining if decades-old installations are serving any cost-effective purpose.

A unit of power (equal to one joule of energy per second). Commonly used in rating power consumption by electrical devices, including lamps; simply, a 100 watt lamp draws 100 watts of electrical energy every moment it is operational (although note that a fixture with a 100 watt high intensity discharge lamp in it draws more than 100 watts; see Ballast).
 
In lighting, we frequently end up discussing large amounts of electricity, so the standard prefixes are added; 1 kilowatt = 1,000 watts; 1 megawatt = 1,000,000 watts.
 
While the watt is a measure of the power draw of a device at any given moment, we buy our electricity by the total amount consumed over a period of time. The standard unit for this is the kilowatt hour (sometimes written kilowatt-hour, and abbreviated kWh); the product of the amount of power drawn by the time of the consumption. The 100 watt lamp, operated for 10 hours, would draw a total of (100 x 10 =) 1000 watt hours, or one kilowatt hour of electricity.

See Nadir.
 

A: Accent Lighting  -  Ambient Light  -   Architectural Lighting  -  ASHRAE
B: Backlight  -   Baffle  -   Ballast  -  Beacon Light  -   Best Management Practices  -  Bollard  -   Bortle Scale  -   BUG
C: Candella  -   CFL  -  CIE  -   Circadian Rhythm  -   Cobra  -  Coefficient of Utilization  -   Color Rendering  -  Color Temperature  -   CPTED  -  Crepuscular  -   Cutoff  -  Cycling
D: Dark Adaptation  -  Diurnal  -   Downlighting
E: Economic Life  -   Efficacy  -  Efficiency  -   Energy Effectiveness Factor  -   Energy Star
F: Fenestration  -   Fitted Target Efficacy  -   Fluorescent Lamp  -  Flux  -   Foot-candle  -  Full Cutoff  -   Fully Focused
G: Glare  -   Goniophotometer
H: High Intensity Discharge  -   Historical Luminaire
I: IESNA  -   Illuminance  -  Illumination  -   Incandescent Lamp  -  Induction Lamp  -   Initial Lumens  -  Intrinsically Photosensitive Retinal Ganglion Cells  -   Isoplot
K: Kilowatt Hour
L: Lamp  -   LED  -  Light At Night  -   Light Fixture  -  Light Loss Factor  -   Light Pollution  -  Limiting Magnitude  -   Lumen  -  Lumens per watt  -   Luminaire  -  Luminaire Classification System  -   Luminance  -  Luminous Flux  -   Luminous Intensity  -  Lux
M: Magnitudes per Square Arc Second  -  Melanopsin  -   Melatonin  -  Mesopic Vision  -   Motion Sensor  -  Mounting Height
N: Nadir  -  NEMA  -  NGLIA  -   Nit  -  Nocturnal
O: Opaque  -  Optics  -   Outdoor Lighting
P: PAR Lamp  -   Part-night Lighting  -  Period Fixture  -   Photobiology  -  Photocell  -   Photoelectric Switch  -  Photometrics  -   Photoperiodism  -  Photopic Vision  -   Pole Height  -  Post Top  -  Power Density
Q: Quartz Halogen Lamp
R: Reflectance  -   Restart Time
S: Scotobiology  -   Scotopic Vision  -  Shield  -   Skyglow  -  Spectral Power Distribution  -   Spill Light  -  Standard  -   Streetlight  -  Streetlight Type Classification
T: Target Efficacy Rating  -  Task Lighting  -   Task Lumens Per Watt  -   Translucent  -  Transparent  -   Trespass  -  Twilight
U: Uniformity Ratio  -  Uplighting
V-Z: Vanity Lighting  -  Veiling Luminance  -   Wallpack  -  Warranting  -   Watt  -  Zenith

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