Working toward legislation to curb light pollution in Illinois.
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
Accent lighting -
See Vanity lighting.
Ambient light -
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).
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.
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.
Beacon light -
(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 -
"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.
Blackbody radiation -
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.
Bortle Scale -
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 lŽEclairage (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).
Circadian rhythm -
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.
Coefficient of utilization (CU) -
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.
Color rendering -
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.
Color temperature -
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.
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 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.
Dark adaptation -
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".
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.
Economic life -
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.
Energy Effectiveness Factor -
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).
Energy Star -
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.
Fitted Target Efficacy (FTE) -
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.
Fluorescent lamp -
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).
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:
Full 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.)
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.
High intensity discharge (HID) -
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.
"Historical" luminaire -
Synonym of Period fixture (see).
IESNA (or IES) -
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).
Incandescent lamp -
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.
Induction lamp -
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).
Initial lumens -
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).
Intrinsically photosensitive retinal ganglion cells (ipRGC)-
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.
Kilowatt hour -
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).
Light at night (LAN) -
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.
Light fixture -
Light loss factor -
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.
Light pollution -
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.
Light trespass -
Limiting Magnitude -
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.
Lumens per watt -
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.
Luminaire Classification System -
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).
Luminous flux -
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).
Luminous intensity -
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.
Magnitudes per Square Arc Second -
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.
Mesopic vision -
See Photopic vision.
Motion sensor -
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.
Mounting height -
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").
Outdoor lighting -
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.
PAR lamp -
Part-night lighting -
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.
Period fixture -
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.
Photoelectric switch -
In lighting engineering, the study of the distribution of light output from luminaires and luminaire installations.
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.
Photopic vision -
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).
Pole height -
See Mounting height.
Post top -
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.
Power density -
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.
Quartz halogen lamp -
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.
Restart time -
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.
Scotopic vision -
See Photopic vision.
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").
Spectral power distribution -
Spill Light -
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.
Streetlight (also Street light) -
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.
Streetlight type classification -
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.
Target Efficacy Rating (TER) -
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.
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.
Task Lighting -
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.
"Task lumens per Watt" -
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:
Uniformity ratio -
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.
Vanity lighting -
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".
Veiling luminance -
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.
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).
A: Accent Lighting - Ambient Light -
Architectural Lighting - ASHRAE
light pollution Illinois Chicago Cook County DuPage County Will County Springfield energy enviromnent global warming anti light pollution legislation lighting ordinances