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86-590: Indoor lighting is usually accomplished using light fixtures , and is a key part of interior design . Lighting can also be an intrinsic component of landscape projects . With the discovery of fire , the earliest form of artificial lighting used to illuminate an area were campfires or torches . As early as 400,000 years ago, fire was kindled in the caves of Peking Man . Prehistoric people used primitive oil lamps to illuminate surroundings. These lamps were made from naturally occurring materials such as rocks, shells, horns and stones, were filled with grease , and had

172-542: A fiber wick . Lamps typically used animal or vegetable fats as fuel. Hundreds of these lamps (hollow worked stones) have been found in the Lascaux caves in modern-day France , dating to about 15,000 years ago. Oily animals (birds and fish) were also used as lamps after being threaded with a wick. Fireflies have been used as lighting sources. Candles and glass and pottery lamps were also invented. Chandeliers were an early form of " light fixture ". A major reduction in

258-463: A lens ), an outer shell or housing for lamp alignment and protection, an electrical ballast or power supply , and a shade to diffuse the light or direct it towards a workspace (e.g., a desk lamp). A wide variety of special light fixtures are created for use in the automotive lighting industry, aerospace , marine and medicine sectors. Portable light fixtures are often called lamps , as in table lamp or desk lamp . In technical terminology ,

344-417: A "gray body" emissivity ε ≤ 1 ( P / A = εσT ). The rate of decrease of the temperature of the emitting body can be estimated from the power radiated and the body's heat capacity . This approach is a simplification that ignores details of the mechanisms behind heat redistribution (which may include changing composition, phase transitions or restructuring of the body) that occur within

430-409: A 60 W incandescent light bulb produces about the same amount of light as a 13 W compact fluorescent lamp . Each of these technologies has a different efficacy in converting electrical energy to visible light . Visible light output is typically measured in lumens . This unit only quantifies the visible radiation, and excludes invisible infrared and ultraviolet light. A wax candle produces on

516-493: A Dosimeter. Dosimeters measure an individual's or an object's exposure to something in the environment, such as light dosimeters and ultraviolet dosimeters. In order to specifically measure the amount of light entering the eye, personal circadian light meter called the Daysimeter has been developed. This is the first device created to accurately measure and characterize light (intensity, spectrum, timing, and duration) entering

602-433: A black body in thermal equilibrium with its environment is called black-body radiation . The name "black body" is given because it absorbs all colors of light. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions." A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation

688-422: A black body originally was introduced by Gustav Kirchhoff in 1860 as follows: ...the supposition that bodies can be imagined which, for infinitely small thicknesses, completely absorb all incident rays, and neither reflect nor transmit any. I shall call such bodies perfectly black , or, more briefly, black bodies . A more modern definition drops the reference to "infinitely small thicknesses": An ideal body

774-404: A black body, and electromagnetic radiation emitted from these bodies as black-body radiation . The figure shows a highly schematic cross-section to illustrate the idea. The photosphere of the star, where the emitted light is generated, is idealized as a layer within which the photons of light interact with the material in the photosphere and achieve a common temperature T that is maintained over

860-463: A black hole there is a mathematically defined surface called an event horizon that marks the point of no return . It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, making it almost an ideal black body (radiation with a wavelength equal to or larger than the diameter of the hole may not be absorbed, so black holes are not perfect black bodies). Physicists believe that to an outside observer, black holes have

946-414: A black surface is a small hole in a cavity with walls that are opaque to radiation. Radiation incident on the hole will pass into the cavity, and is very unlikely to be re-emitted if the cavity is large. Lack of any re-emission, means that the hole is behaving like a perfect black surface. The hole is not quite a perfect black surface—in particular, if the wavelength of the incident radiation is greater than

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1032-523: A body forms an interface with its surroundings, and this interface may be rough or smooth. A nonreflecting interface separating regions with different refractive indices must be rough, because the laws of reflection and refraction governed by the Fresnel equations for a smooth interface require a reflected ray when the refractive indices of the material and its surroundings differ. A few idealized types of behavior are given particular names: An opaque body

1118-420: A building: Specification of illumination requirements is the basic concept of deciding how much illumination is required for a given task. Clearly, much less light is required to illuminate a hallway compared to that needed for a word processing work station. Generally speaking, the energy expended is proportional to the design illumination level. For example, a lighting level of 400 lux might be chosen for

1204-594: A clear view of the area being scanned. Ultrasonic sensors transmit sound above the range of human hearing and monitor the time it takes for the sound waves to return. A break in the pattern caused by any motion in the area triggers the control. Ultrasonic sensors can see around obstructions and are best for areas with cabinets and shelving, restrooms, and open areas requiring 360-degree coverage. Some occupancy sensors utilize both passive infrared and ultrasonic technology, but are usually more expensive. They can be used to control one lamp, one fixture or many fixtures. Daylighting

1290-534: A consequence, Kirchhoff's perfect black bodies that absorb all the radiation that falls on them cannot be realized in an infinitely thin surface layer, and impose conditions upon scattering of the light within the black body that are difficult to satisfy. A realization of a black body refers to a real world, physical embodiment. Here are a few. In 1898, Otto Lummer and Ferdinand Kurlbaum published an account of their cavity radiation source. Their design has been used largely unchanged for radiation measurements to

1376-824: A dark nightclub it could be considered general lighting. In a movie theater , steps in the aisles are usually marked with a row of small lights for convenience and safety, when the film has started and the other lights are off. Traditionally made up of small low wattage, low-voltage lamps in a track or translucent tube, these are rapidly being replaced with LED based versions. Street Lights are used to light roadways and walkways at night. Some manufacturers are designing LED and photovoltaic luminaires to provide an energy-efficient alternative to traditional street light fixtures. Floodlights can be used to illuminate work zones or outdoor playing fields during nighttime hours. The most common type of floodlights are metal halide and high pressure sodium lights. Beacon lights are positioned at

1462-544: A dilute gas. At temperatures below billions of Kelvin, direct photon–photon interactions are usually negligible compared to interactions with matter. Photons are an example of an interacting boson gas, and as described by the H-theorem , under very general conditions any interacting boson gas will approach thermal equilibrium. A body's behavior with regard to thermal radiation is characterized by its transmission τ , absorption α , and reflection ρ . The boundary of

1548-413: A long period of time. Some photons escape and are emitted into space, but the energy they carry away is replaced by energy from within the star, so that the temperature of the photosphere is nearly steady. Changes in the core lead to changes in the supply of energy to the photosphere, but such changes are slow on the time scale of interest here. Assuming these circumstances can be realized, the outer layer of

1634-428: A lower emissivity, independent of frequency, is often referred to as a gray body. Constructing black bodies with an emissivity as close to 1 as possible remains a topic of current interest. In astronomy , the radiation from stars and planets is sometimes characterized in terms of an effective temperature , the temperature of a black body that would emit the same total flux of electromagnetic energy. The idea of

1720-417: A nearly ideal Planck spectrum at a temperature of about 2.7 K. It departs from the perfect isotropy of true black-body radiation by an observed anisotropy that varies with angle on the sky only to about one part in 100,000. The integration of Planck's law over all frequencies provides the total energy per unit of time per unit of surface area radiated by a black body maintained at a temperature T , and

1806-424: A non-zero temperature and emit black-body radiation , radiation with a nearly perfect black-body spectrum, ultimately evaporating . The mechanism for this emission is related to vacuum fluctuations in which a virtual pair of particles is separated by the gravity of the hole, one member being sucked into the hole, and the other being emitted. The energy distribution of emission is described by Planck's law with

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1892-481: A perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater, particularly in space or in a vacuum where convective heating is unavailable. They are also useful in telescopes and cameras as anti-reflection surfaces to reduce stray light, and to gather information about objects in high-contrast areas (for example, observation of planets in orbit around their stars), where blackbody-like materials absorb light that comes from

1978-633: A stage lighting designer to produce the desired effects. A set of lighting cues are prepared so that the lighting operator can control the lights in step with the performance; complex theatre lighting systems use computer control of lighting instruments. Motion picture and television production use many of the same tools and methods of stage lighting. Especially in the early days of these industries, very high light levels were required and heat produced by lighting equipment presented substantial challenges. Modern cameras require less light, and modern light sources emit less heat. Measurement of light or photometry

2064-492: A surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function , a standardized model of human visual brightness perception. In English, "lux" is used in both singular and plural. Visual comfort often entails the measurement of subjective evaluations. Several measurement methods have been developed to control glare resulting from indoor lighting design. The Unified Glare Rating (UGR),

2150-532: A table is general lighting, while the desk lamp is considered task lighting. Magnifier lamps are also task lighting. The illuminated ceiling was once popular in the 1960s and 1970s but fell out of favor after the 1980s. This uses diffuser panels hung like a suspended ceiling below fluorescent lights, and is considered general lighting. Other forms include neon, which is not usually intended to illuminate anything else, but to actually be an artwork in itself. This would probably fall under accent lighting, though in

2236-516: A temperature T : where c is the speed of light , ℏ is the reduced Planck constant , k B is the Boltzmann constant , G is the gravitational constant and M is the mass of the black hole. These predictions have not yet been tested either observationally or experimentally. The Big Bang theory is based upon the cosmological principle , which states that on large scales the Universe

2322-410: A wall switch. Fixtures require an electrical connection to a power source, typically AC mains power, but some run on battery power for camping or emergency lights. Permanent lighting fixtures are directly wired. Movable lamps have a plug and cord that plugs into a wall socket. Light fixtures may also have other features, such as reflectors for directing the light, an aperture (with or without

2408-404: A wide variety of styles for various functions. The most important functions are as a holder for the light source, to provide directed light and to avoid visual glare . Some are very plain and functional, while some are pieces of art in themselves. Nearly any material can be used, so long as it can tolerate the excess heat and is in keeping with safety codes. An important property of light fixtures

2494-414: A work environment involving meeting rooms and conferences, whereas a level of 80 lux could be selected for building hallways. If the hallway standard simply emulates the conference room needs, then much more energy will be consumed than is needed. Lighting control systems reduce energy usage and cost by helping to provide light only when and where it is needed. Lighting control systems typically incorporate

2580-410: Is an uplight intended for ambient lighting. It is typically a floor lamp but may be wall-mounted like a sconce. Further interior light fixtures include chandeliers, pendant lights, ceiling fans with lights, close-to-ceiling or flush lights, and various types of lamps The portable or table lamp is probably the most common fixture, found in many homes and offices . The standard lamp and shade that sits on

2666-447: Is emitted according to Planck's law , meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition. An ideal black body in thermal equilibrium has two main properties: Real materials emit energy at a fraction—called the emissivity —of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity ε = 1 . A source with

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2752-447: Is generally concerned with the amount of useful light falling on a surface and the amount of light emerging from a lamp or other source, along with the colors that can be rendered by this light. The human eye responds differently to light from different parts of the visible spectrum, therefore photometric measurements must take the luminosity function into account when measuring the amount of useful light. The basic SI unit of measurement

2838-406: Is homogeneous and isotropic. According to theory, the Universe approximately a second after its formation was a near-ideal black body in thermal equilibrium at a temperature above 10 K. The temperature decreased as the Universe expanded and the matter and radiation in it cooled. The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature". It has

2924-550: Is known as the Stefan–Boltzmann law : where σ is the Stefan–Boltzmann constant , σ  ≈  5.67 × 10  W⋅m ⋅K ‍ To remain in thermal equilibrium at constant temperature T , the black body must absorb or internally generate this amount of power P over the given area A . The cooling of a body due to thermal radiation is often approximated using the Stefan–Boltzmann law supplemented with

3010-473: Is now defined, called a blackbody . A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation. This section describes some concepts developed in connection with black bodies. A widely used model of

3096-400: Is one for which all incident radiation is reflected uniformly in all directions: τ = 0, α = 0, and ρ = 1. For a black body, τ = 0, α = 1, and ρ = 0. Planck offers a theoretical model for perfectly black bodies, which he noted do not exist in nature: besides their opaque interior, they have interfaces that are perfectly transmitting and non-reflective. Kirchhoff in 1860 introduced

3182-442: Is one that transmits none of the radiation that reaches it, although some may be reflected. That is, τ = 0 and α + ρ = 1. A transparent body is one that transmits all the radiation that reaches it. That is, τ = 1 and α = ρ = 0. A grey body is one where α , ρ and τ are constant for all wavelengths; this term also is used to mean a body for which α is temperature- and wavelength-independent. A white body

3268-408: Is selected and arranged to create dramatic effects. Stage lighting uses general illumination technology in devices configured for easy adjustment of their output characteristics. The setup of stage lighting is tailored for each scene of each production. Dimmers, colored filters, reflectors, lenses, motorized or manually aimed lamps, and different kinds of flood and spot lights are among the tools used by

3354-442: Is the candela (cd), which describes the luminous intensity, all other photometric units are derived from the candela. Luminance for instance is a measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle . The SI unit for luminance is candela per square metre (cd/m). The CGS unit of luminance

3440-401: Is the luminous efficacy or wall-plug efficiency , meaning the amount of usable light emanating from the fixture per used energy, usually measured in lumen per watt . A fixture using replaceable light sources can also have its efficiency quoted as the percentage of light passed from the "bulb" to the surroundings. The more transparent the lighting fixtures are, the higher efficacy. Shading

3526-478: Is the stilb , which is equal to one candela per square centimetre or 10 kcd/m. The amount of useful light emitted from a source or the luminous flux is measured in lumen (lm). The SI unit of illuminance and luminous emittance , being the luminous power per area, is measured in Lux . It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through

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3612-508: Is the oldest method of interior lighting. Daylighting is simply designing a space to use as much natural light as possible. This decreases energy consumption and costs, and requires less heating and cooling from the building. Daylighting has also been proven to have positive effects on patients in hospitals as well as work and school performance. Due to a lack of information that indicate the likely energy savings, daylighting schemes are not yet popular among most buildings. Unlike electric lighting,

3698-468: Is within the area being scanned can control lighting. When motion can no longer be detected, the lights shut off. Passive infrared sensors react to changes in heat, such as the pattern created by a moving person. The control must have an unobstructed view of the building area being scanned. Doors, partitions, stairways, etc. will block motion detection and reduce its effectiveness. The best applications for passive infrared occupancy sensors are open spaces with

3784-565: The 1939 World's Fair ) or rope light , occasionally with neon lighting , and recently with LED strip lighting . It is a form of backlighting. Soffit or close to wall lighting can be general or a decorative wall-wash, sometimes used to bring out texture (like stucco or plaster ) on a wall, though this may also show its defects as well. The effect depends heavily on the exact type of lighting source used. Recessed lighting (often called "pot lights" in Canada , "can lights" or 'high hats" in

3870-463: The Sun has an effective temperature of 5780 K, which can be compared to the temperature of its photosphere (the region generating the light), which ranges from about 5000 K at its outer boundary with the chromosphere to about 9500 K at its inner boundary with the convection zone approximately 500 km (310 mi) deep. A black hole is a region of spacetime from which nothing escapes. Around

3956-569: The U-B index, which becomes more negative the hotter the star and the more the UV radiation. Assuming the Sun is a type G2 V star, its U-B index is +0.12. The two indices for two types of most common star sequences are compared in the figure (diagram) with the effective surface temperature of the stars if they were perfect black bodies. There is a rough correlation. For example, for a given B-V index measurement,

4042-574: The US ) is popular, with fixtures mounted into the ceiling structure so as to appear flush with it. These downlights can use narrow beam spotlights, or wider- angle floodlights , both of which are bulbs having their own reflectors . There are also downlights with internal reflectors designed to accept common 'A' lamps (light bulbs) which are generally less costly than reflector lamps. Downlights can be incandescent, fluorescent, HID (high intensity discharge) or LED . Track lighting , invented by Lightolier ,

4128-593: The lamp is the light source, which, in casual terminology, is called the light bulb . Both the International Electrotechnical Commission (IEC) and the Illuminating Engineering Society (IES) recommend the term luminaire for technical use. Fixture manufacturing began soon after production of the incandescent light bulb . When practical uses of fluorescent lighting were realized after 1924,

4214-477: The United States, and Americans spent more on gas and kerosene than on whale oil. The final death knell for whale oil was in 1859, when crude oil was discovered and the petroleum industry arose. Gas lighting was economical enough to power street lights in major cities starting in the early 1800s, and was also used in some commercial buildings and in the homes of wealthy people. The gas mantle boosted

4300-593: The Visual Comfort Probability, and the Daylight Glare Index are some of the most well-known methods of measurement. In addition to these new methods, four main factors influence the degree of discomfort glare; the luminance of the glare source, the solid angle of the glare source, the background luminance, and the position of the glare source in the field of view must all be taken into account. To define light source color properties,

4386-449: The benefit of a proposed design. In some instances, the materials used on walls and furniture play a key role in the lighting effect. For example, dark paint tends to absorb light, making the room appear smaller and more dim than it is, whereas light paint does the opposite. Other reflective surfaces also have an effect on lighting design. Lighting illuminates the performers and artists in a live theatre, dance, or musical performance, and

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4472-490: The body while it cools, and assumes that at each moment in time the body is characterized by a single temperature. It also ignores other possible complications, such as changes in the emissivity with temperature, and the role of other accompanying forms of energy emission, for example, emission of particles like neutrinos. If a hot emitting body is assumed to follow the Stefan–Boltzmann law and its power emission P and temperature T are known, this law can be used to estimate

4558-661: The built environment is known as 'architectural lighting design'. Lighting of structures considers aesthetic elements as well as practical considerations of quantity of light required, occupants of the structure, energy efficiency, and cost. Artificial lighting takes into account the amount of daylight received in a space by using daylight factor calculations. For simple installations, hand calculations based on tabular data are used to provide an acceptable lighting design. More critical or complex designs now routinely use computer software such as Radiance for mathematical modeling, which can allow an architect to quickly and accurately evaluate

4644-418: The close order of 13 lumens, a 60 watt incandescent lamp makes around 700 lumens, and a 15-watt compact fluorescent lamp produces about 800 lumens, but actual output varies by specific design. Rating and marketing emphasis is shifting away from wattage and towards lumen output, to give the purchaser a directly applicable basis upon which to select a lamp. Lamp types include: Lighting design as it applies to

4730-404: The color temperature of the lamps will be selected for the best overall lighting effect. Lighting is classified by intended use as general, accent, or task lighting, depending largely on the distribution of the light produced by the fixture. Forms of lighting include alcove lighting, which like most other uplighting is indirect. This is often done with fluorescent lighting (first available at

4816-478: The correct light intensity and color spectrum for each task or environment. Otherwise, energy not only could be wasted but over-illumination can lead to adverse health and psychological effects. Light fixture A light fixture (US English), light fitting (UK English), or luminaire is an electrical lighting device containing one or more light sources, such as lamps , and all the accessory components required for its operation to provide illumination to

4902-457: The cost of lighting occurred with the discovery of whale oil . The use of whale oil declined after Abraham Gesner , a Canadian geologist, first refined kerosene in the 1840s, allowing brighter light to be produced at substantially lower cost. In the 1850s, the price of whale oil dramatically increased (more than doubling from 1848 to 1856) due to shortages of available whales, hastening whale oil's decline. By 1860, there were 33 kerosene plants in

4988-460: The curves of both most common sequences of star (the main sequence and the supergiants) lie below the corresponding black-body U-B index that includes the ultraviolet spectrum, showing that both groupings of star emit less ultraviolet light than a black body with the same B-V index. It is perhaps surprising that they fit a black body curve as well as they do, considering that stars have greatly different temperatures at different depths. For example,

5074-413: The diameter of the hole, part will be reflected. Similarly, even in perfect thermal equilibrium, the radiation inside a finite-sized cavity will not have an ideal Planck spectrum for wavelengths comparable to or larger than the size of the cavity. Suppose the cavity is held at a fixed temperature T and the radiation trapped inside the enclosure is at thermal equilibrium with the enclosure. The hole in

5160-418: The distribution of daylight varies considerably throughout the entire year inside a building. In recent years light-emitting diodes (LEDs) are becoming increasingly efficient leading to an extraordinary increase in the use of solid state lighting . In many situations, controlling the light emission of LEDs may be done most effectively by using the principles of nonimaging optics . It is valuable to provide

5246-438: The enclosure will allow some radiation to escape. If the hole is small, radiation passing in and out of the hole has negligible effect upon the equilibrium of the radiation inside the cavity. This escaping radiation will approximate black-body radiation that exhibits a distribution in energy characteristic of the temperature T and does not depend upon the properties of the cavity or the hole, at least for wavelengths smaller than

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5332-411: The entrance to a property. These lights are installed for safety, security, and for decoration. Underwater accent lighting is also used for koi ponds, fountains, swimming pools and the like. Neon signs are most often used to attract attention rather than to illuminate. Vehicles typically include headlamps and tail lights. Headlamps are white or selective yellow lights placed in the front of

5418-425: The environment. All light fixtures have a fixture body and one or more lamps. The lamps may be in sockets for easy replacement—or, in the case of some LED fixtures, hard-wired in place. Fixtures may also have a switch to control the light, either attached to the lamp body or attached to the power cable. Permanent light fixtures, such as dining room chandeliers , may have no switch on the fixture itself, but rely on

5504-501: The eye that affects the human body's clock. The small, head-mounted device measures an individual's daily rest and activity patterns, as well as exposure to short-wavelength light that stimulates the circadian system. The device measures activity and light together at regular time intervals and electronically stores and logs its operating temperature . The Daysimeter can gather data for up to 30 days for analysis. Several strategies are available to minimize energy requirements for lighting

5590-452: The fixtures on the track or rod with 12 or 24 volts, instead of each light fixture having its own line-to-low voltage transformer. There are traditional spots and floods, as well as other small hanging fixtures. A modified version of this is cable lighting , where lights are hung from or clipped to bare metal cables under tension . A sconce is a wall-mounted fixture, particularly one that shines up and sometimes down as well. A torchère

5676-400: The incoming light in the spectral range from the ultra-violet to the far-infrared regions. Other examples of nearly perfect black materials are super black , prepared by chemically etching a nickel – phosphorus alloy , vertically aligned carbon nanotube arrays (like Vantablack ) and flower carbon nanostructures; all absorb 99.9% of light or more. A star or planet often is modeled as

5762-405: The intersection of two roads to aid in navigation. Sometimes security lighting can be used along roadways in urban areas, or behind homes or commercial facilities. These are extremely bright lights used to deter crime. Security lights may include floodlights and be activated with PIR switches that detect moving heat sources in darkness. Entry lights can be used outside to illuminate and signal

5848-409: The lamp. An incandescent bulb has a color temperature around 2800 to 3000 kelvins; daylight is around 6400 kelvins. Lower color temperature lamps have relatively more energy in the yellow and red part of the visible spectrum, while high color temperatures correspond to lamps with more of a blue-white appearance. For critical inspection or color matching tasks, or for retail displays of food and clothing,

5934-412: The light will normally decrease efficacy but increase the directionality and the visual comfort probability . Color temperature for white light sources also affects their use for certain applications. The color temperature of a white light source is the temperature in kelvins of a theoretical black body emitter that most closely matches the spectral characteristics ( spectral power distribution ) of

6020-519: The lighting industry predominantly relies on two metrics, correlated color temperature (CCT), commonly used as an indication of the apparent "warmth" or "coolness" of the light emitted by a source, and color rendering index (CRI), an indication of the light source's ability to make objects appear natural. However, these two metrics, developed in the last century, are facing increased challenges and criticisms as new types of light sources, particularly light-emitting diodes (LEDs), become more prevalent in

6106-467: The lights on and off can occur, particularly during unstable weather conditions or when daylight levels are changing around the switching illuminance. Not only does this disturb occupants, it can also reduce lamp life. A variation of this technology is the 'differential switching or dead-band' photoelectric control which has multiple illuminances it switches from so as not to disturb occupants as much. Occupancy sensors to allow operation for whenever someone

6192-569: The luminosity of utility lighting and of kerosene lanterns. The next major drop in price came about in the 1880s with the introduction of electric lighting in the form of arc lights for large space and street lighting , followed by incandescent light bulb -based utilities for indoor and outdoor lighting. Over time, electric lighting became ubiquitous in developed countries. Segmented sleep patterns disappeared, improved nighttime lighting made more activities possible at night, and more street lights reduced urban crime. Lighting fixtures come in

6278-619: The market. For example, in order to meet the expectations for good color rendering in retail applications, research suggests using the well-established CRI along with another metric called gamut area index (GAI). GAI represents the relative separation of object colors illuminated by a light source; the greater the GAI, the greater the apparent saturation or vividness of the object colors. As a result, light sources which balance both CRI and GAI are generally preferred over ones that have only high CRI or only high GAI. Typical measurements of light have used

6364-594: The present day. It was a hole in the wall of a platinum box, divided by diaphragms, with its interior blackened with iron oxide. It was an important ingredient for the progressively improved measurements that led to the discovery of Planck's law. A version described in 1901 had its interior blackened with a mixture of chromium, nickel, and cobalt oxides. See also Hohlraum . There is interest in blackbody-like materials for camouflage and radar-absorbent materials for radar invisibility. They also have application as solar energy collectors, and infrared thermal detectors. As

6450-410: The removable and replaceable part of a light fixture, which converts electrical energy into electromagnetic radiation . While lamps have traditionally been rated and marketed primarily in terms of their power consumption, expressed in watts , proliferation of lighting technology beyond the incandescent light bulb has eliminated the correspondence of wattage to the amount of light produced. For example,

6536-465: The reverse gear, warning anyone behind the vehicle that it is moving backwards, or about to do so. Flashing turn signals on the front, side, and rear of the vehicle indicate an intended change of position or direction. In the late 1950s, some automakers began to use electroluminescent technology to backlight their cars' speedometers and other gauges or to draw attention to logos or other decorative elements. Commonly called 'light bulbs', lamps are

6622-407: The same surface flux of energy as the star. If a star were a black body, the same effective temperature would result from any region of the spectrum. For example, comparisons in the B (blue) or V (visible) range lead to the so-called B-V color index , which increases the redder the star, with the Sun having an index of +0.648 ± 0.006. Combining the U (ultraviolet) and the B indices leads to

6708-527: The size of the hole. See the figure in the Introduction for the spectrum as a function of the frequency of the radiation, which is related to the energy of the radiation by the equation E = hf , with E = energy, h = Planck constant , f = frequency. At any given time the radiation in the cavity may not be in thermal equilibrium, but the second law of thermodynamics states that if left undisturbed it will eventually reach equilibrium, although

6794-421: The star is somewhat analogous to the example of an enclosure with a small hole in it, with the hole replaced by the limited transmission into space at the outside of the photosphere. With all these assumptions in place, the star emits black-body radiation at the temperature of the photosphere. Using this model the effective temperature of stars is estimated, defined as the temperature of a black body that yields

6880-505: The theoretical concept of a perfect black body with a completely absorbing surface layer of infinitely small thickness, but Planck noted some severe restrictions upon this idea. Planck noted three requirements upon a black body: the body must (i) allow radiation to enter but not reflect; (ii) possess a minimum thickness adequate to absorb the incident radiation and prevent its re-emission; (iii) satisfy severe limitations upon scattering to prevent radiation from entering and bouncing back out. As

6966-556: The three leading companies to produce various fixtures were Lightolier , Artcraft Fluorescent Lighting Corporation , and Globe Lighting in the United States. Light fixtures are classified by how the fixture is installed, the light function or lamp type. There are various types of devices used to manage the amount of light used: Black body A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation , regardless of frequency or angle of incidence . The radiation emitted by

7052-456: The time it takes to do so may be very long. Typically, equilibrium is reached by continual absorption and emission of radiation by material in the cavity or its walls. Radiation entering the cavity will be " thermalized " by this mechanism: the energy will be redistributed until the ensemble of photons achieves a Planck distribution . The time taken for thermalization is much faster with condensed matter present than with rarefied matter such as

7138-832: The use of time schedules, occupancy control, and photocell control (i.e. daylight harvesting ). Some systems also support demand response and will automatically dim or turn off lights to take advantage of utility incentives. Lighting control systems are sometimes incorporated into larger building automation systems . Many newer control systems are using wireless mesh open standards (such as Zigbee ), which provides benefits including easier installation (no need to run control wires) and interoperability with other standards-based building control systems (e.g. security). In response to daylighting technology, daylight harvesting systems have been developed to further reduce energy consumption. These technologies are helpful, but they do have their downfalls. Many times, rapid and frequent switching of

7224-425: The vehicle, designed to illuminate the upcoming road and to make the vehicle more visible. Many manufactures are turning to LED headlights as an energy-efficient alternative to traditional headlamps. Tail and brake lights are red and emit light to the rear so as to reveal the vehicle's direction of travel to following drivers. White rear-facing reversing lamps indicate that the vehicle's transmission has been placed in

7310-521: The wrong sources. It has long been known that a lamp-black coating will make a body nearly black. An improvement on lamp-black is found in manufactured carbon nanotubes . Nano-porous materials can achieve refractive indices nearly that of vacuum, in one case obtaining average reflectance of 0.045%. In 2009, a team of Japanese scientists created a material called nanoblack which is close to an ideal black body, based on vertically aligned single-walled carbon nanotubes . This absorbs between 98% and 99% of

7396-459: Was popular at one period of time because it was much easier to install than recessed lighting, and individual fixtures are decorative and can be easily aimed at a wall . It has regained some popularity recently in low-voltage tracks, which often look nothing like their predecessors because they do not have the safety issues that line-voltage systems have, and are therefore less bulky and more ornamental in themselves. A master transformer feeds all of

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