:: wikimiki.org ::
| Irradiance |
IrradianceIrradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. The other two terms are used interchangably for radiation emerging from a surface. The SI units for all of these quantities are watts per square metre (W/m2). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units.
All of these quantities characterize the total amount of radiation present, at all frequencies. It is also common to consider each frequency in the spectrum separately. When this is done for radiation incident on a surface, it is called spectral irradiance, and has SI units W/m3, or commonly W·m-2·nm-1.
If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source.
=See also=
- Illuminance
Category:Radiometry
Category:Physical quantity
ja:放射照度
RadiometryIn telecommunications and physics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power.
=External links=
- [http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page (University of Arizona).
Category:Optics
Electromagnetic radiation
Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation.
The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable.
Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.
Physics
Theory
Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields.
According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.
Properties
Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form.
In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism.
The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism.
EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.
Wave model
An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction.
A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes.
Waves can also be described by their radiant energy.
Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.
Particle model
In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy.
A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference.
The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies.
Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.
Speed of propagation
Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10-34 J·s is Planck's constant, and ν is the frequency of the wave.
One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)
Electromagnetic spectrum
Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.
Light
EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light.
If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit.
In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.
Radio waves
Radio waves carry information by varying amplitude and by varying frequency within a frequency band.
When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.
Derivation
Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.)
:
:
:
:
is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus.
:
Where can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following.
:
:
These types of equations are identified as linear wave equations with wave speed . Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space , the permeability of free space , and the speed of light itself: . Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism.
But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field.
:
Here is the constant amplitude, is any second differentiable function, is a unit vector in the direction of propagation, and is a position vector. We observe that is a generic solution to the wave equation. In other words
:,
for a generic wave traveling in the direction. The proof of this is trivial.
This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field?
:
:
The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates.
:
:
The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of .
Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, . The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as .
Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.
See also
- Electromagnetic wave equation
- Electromagnetic spectrum
- Electromagnetic radiation hazards
- Radiant energy
- Light
- Electromagnetic pulse
- Control of electromagnetic radiation
- Klystron
References
-
-
-
External links
; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy]
; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component
ko:전자기파
ja:電磁波
SI
The International System of Units (abbreviated SI from the French language name Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science.
The older metric system included several groupings of units. The SI was developed in 1960 from one of these, the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants.
The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses.
With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the US and UK) have redefined most of their traditional, non-SI units in terms of SI units.
History
:See main articles: metre, kilogram, second, ampere, Kelvin, and candela.
The metric system was officially adopted in France after the French Revolution. During the history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional measurement systems.
By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial and American systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.
Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name: Le Système International d'Unités. The seventh base unit, the mole, was added in 1970 by the 14th CGPM.
The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).
Units
:Main articles: SI base unit, SI derived unit, SI prefix
The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI.
A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.
SI writing style
- Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original abbreviation "l" is dangerously similar to "1". The NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM. The cursive "ℓ" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre.
- Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs". Pluralization would be language dependent; "s" plurals (as in French and English) are particularly undesirable since "s" is the symbol of the second. Other cases may be marked in a language-dependent manner, e.g. Finnish 25 kg:lla = 25 kilogrammalla "with 25 kg".
- Symbols do not have an appended period (.).
- It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
- A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×102 m2", "22 °C" [http://physics.nist.gov/Pubs/SP811/sec07.html]. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
- Spaces should be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000).
- The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". In practice, the full stop is used in English, and the comma in most other European languages.
- Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m.
- Symbols formed by division of two units are joined with a solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s-1", "m·s-1" or . A solidus should not be used if the result is ambiguous, i.e. "kg·m-1·s-2" is preferable to "kg/m/s2".
Spelling variations
- Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
- The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.
Cultural issues
The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system.
Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR. In some countries, the informal cup measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendent, SI. Engineers work with volts, amperes, ohms, farads, and coulombs, which are of great practical utility, while the centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. People with diabetes test their plasma glucose level regularly. In the U.S., measurement are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/liter (mmol/L).
The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research do not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.
See also
- Units of measurement
- Weights and measures
- Mesures usuelles
- Metrified English unit
- History of measurement
- Other systems of measurement:
- Imperial units
- U.S. customary units
- Metre-tonne-second system of units
- Chinese system of units
- Planck units
- Atomic units
- Geometrized units
- CODATA
- Metrication
- Metric system in the United States
- Metrology
- UTC (Coordinated Universal Time)
- Binary prefixes - used to quantify large amounts of computer data
- Orders of magnitude
- ISO 31
External links
Official
- [http://www.bipm.fr/en/si/ BIPM (SI maintenance agency)] (home page)
- [http://www.bipm.org/en/si/si_brochure/ BIPM brochure] (SI reference)
- [http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=5448&ICS1=1 ISO 1000:1992 SI units and recommendations for the use of their multiples and of certain other units], with its price tag of 99 Swiss francs for a 22 page, coverless pamphlet showing why the public is sometimes a little slow to pick up on their recommendations.
Information
- [http://physics.nist.gov/cuu/Units/index.html US NIST reference on SI]
- [http://ts.nist.gov/ts/htdocs/200/202/pub814.htm#chart chart]
- [http://www.aticourses.com/international_system_units.htm SI - Its history and use in science and industry]
- [http://www.unc.edu/~rowlett/units/ A Dictionary of Units of Measurement]
- [http://www.unics.uni-hannover.de/ntr/russisch/si-einheiten.html5 Cyrillic transcription of SI symbols]
- Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 ([http://ts.nist.gov/ts/htdocs/200/202/SP%20447.pdf 46 page PDF file])
- [http://www.france-property-and-information.com/metric_conversion_table.htm Metric system and conversion tables (courtesy French property advice)]
- [http://www.metre.info metre-info - an encyclopaedia of all metric units]
Pro-metric pressure groups
- [http://www.ukma.org.uk/ The UK Metric Association]
- [http://www.metric.org/ The US Metric Association]
Pro-customary measures pressure groups
- [http://www.bwmaonline.com/ The British Weights and Measures Association]
Further reading
- I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838.
Category:SI units
Category:Systems of units
Category:International standards
Category:Dimensional analysis
ko:SI 단위계
ja:国際単位系
simple:SI
th:หน่วยเอสไอ
Watt:For other uses, see: Watt (disambig)
The watt (symbol: W) is the SI derived unit of power.
Definition
One watt is one joule of energy per second.
: 1 W = 1 J/s = 1 newton meter per second
Origin
The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures in 1960.
SI multiples
Conversions
- 1 watt ≈ 3.41214163 BTU/h
- 1 horsepower ≈ 745.700 W
- 1 horsepower (electrical British) = 746 W
- 1 horsepower (electrical European) = 736 W
- 1 horsepower ("metric") = 735.498 75 W
Derived and qualified units for power distribution
A watt is a unit of power or the amount of energy per unit time.
Kilowatt-hour, MWd
When paired with a unit of time the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour; it equals 3.6 megajoules (1 hour = 3600 seconds). A megawatt day (MWd or MW·d) is equal to 86.4 GJ (1 day = 86400 seconds). These units are often used in the context of power plants and home energy bills.
For the use of watts as a measurement of transmitter power in radio, see effective radiated power and nominal power.
MWe, MWt
Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe).
Watt thermal (abbreviation: Wt). This is a term that refers to thermal power produced. SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt).
For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity. See nuclear proliferation for discussion of a reactor that generates 200 MWt (50 MWe), and another reactor that generates 800 MWt (200 MWe).
See also
- SI
- Kilowatt hour (kW·h)
- Watt balance
- Conversion of units
- Orders of magnitude (power)
- James Watt
- RMS
- Back to the Future
External links
- Nelson, Robert A., "[http://www.aticourses.com/international_system_units.htm The International System of Units] Its History and Use in Science and Industry". Via Satellite, February 2000.
Category:SI derived units
Category:Units of power
ko:와트
ja:ワット
simple:Watt
Intensity:For other senses of this word, see intensity (disambiguation).
In physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light).
In physics, the word "intensity" is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless as the parameters of those variables do not match.
If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity drops off in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies. The consequence of this is that the net power coming from the source must be constant, thus:
:
where P is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. That P is a constant. If the source is radiating uniformly, i.e. the same in all directions, and we take A to be a sphere centered on the source (so that I will be constant on its surface), the equation becomes:
:
where I is the intensity at the surface of the sphere, and r is the radius of the sphere. ( is the expression for the surface area of a sphere). Solving for I, we get:
:
Anything that can carry energy can have an intensity associated with it.
If the medium is damped (i.e. both sound and light in air slowly lose energy), then the intensity drops off more quickly than the above equation suggests.
Photometry and radiometry
In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where "intensity" can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists.
See also
- Sound intensity
Category:Optics
Category:Radiometry
Category:Physical quantity
ja:放射発散度
IlluminanceIn photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the perceived intensity of the incident light. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface.
In SI derived units, these are both measured in lux (lx) or lumens per square metre (cd·sr·m-2).
Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.
The human eye is capable of seeing somewhat over more than a 2 trillion fold range: The presence of white objects is somewhat discernable under starlight, at 5
- 10-5 lux, while at the bright end, is it possible to read large text at 108 lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.
=See also=
- Irradiance
Category:Physical quantity
Category:Photometry
ja:照度
Category:Physical quantity
This category identifies Physical quantities which can be manipulated or measured by a Physicist
Category:Measurement
Quantity
ko:Category:물리량
ja:Category:物理量
zh-cn:category:物理量 Loss of habitatHabitat destruction is a process of land use change in which one habitat-type is removed and replaced with some other habitat-type. In the process of land-use change, plants and animals which previously used the site are displaced or destroyed. Generally this results in alteration or reduction in biodiversity.
See also: Deforestation, Habitat fragmentation
Category:Ecology
Category:Conservation
warsaw map spielautomaten narty austria heavy metal Sepsa |
|
|
|
