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Radiometry

Radiometry

In 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

Telecommunication

Telecommunication refers to communication over long distances. In practice, something of the message may be lost in the process. Telecommunication covers all forms of distance and/or conversion of the original communications, including radio, telegraphy, television, telephony, data communication and computer networking. The elements of a telecommunication system are a transmitter, a medium (line) and possibly a channel imposed upon the medium (see baseband and broadband as well as multiplexing), and a receiver. The transmitter is a device that transforms or encodes the message into a physical phenomenon; the signal. The transmission medium, by its physical nature, is likely to modify or degrade the signal on its path from the transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits of signal degradation. Sometimes, the final "receiver" is the human eye and/or ear (or in some extreme cases other sensory organs) and the recovery of the message is done by the brain (see psychoacoustics.) Telecommunication can be point-to-point, point-to-multipoint or broadcasting, which is a particular form of point-to-multipoint that goes only from the transmitter to the receivers. One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium, and the statistical properties of the message in order to design the most effective encoding and decoding mechanisms. When systems are designed to communicate through human sensory organs (mainly those for vision and hearing), physiological and psychological characteristics of human perception must be taken into account. This has important economic implications and engineers must research what defects can be tolerated in the signal and not significantly degrade the viewing or hearing experience.

Examples of human (tele)communications

In a simplistic example, consider a normal conversation between two people. The message is the sentence that the speaker decides to communicate to the listener. The transmitter is the language areas in the brain, the motor cortex, the vocal cords, the larynx, and the mouth that produce those sounds called speech. The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener, there might be other devices that do or do not introduce their own distortions of the original vocal signal (for example a telephone, a HAM radio, an IP phone, etc.) The receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will "decode" the signal into meaningful information and filter out background noise. All channels have noise. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech.

See also


- History of telecommunication
- ITU
- Federal Standard 1037C for a glossary of telecommunications terms.
- Public utility
- Lists of public utilities
- Internet traffic engineering

External links


- [http://web.archive.org/web/20040413074912/www.ericsson.com/support/telecom/index.shtml Ericsson's Understanding Telecommunications] at archive.org (Ericsson removed the book from their site in Sep 2005)
- [http://www.carrieraccessbilling.com/telecommunications-glossary-a.asp Intec Telecom Systems' Telecom Dictionary]
- [http://www.mobile-phone-directory.org/Glossary/ Mobile Phone Directory Telecommunications Glossary]
- [http://www.tiaonline.org Telecommunications Industry Association (TIA)]
- [http://www.aronsson.se/hist.html Aronsson's Telecom History Timeline]
- [http://www.alcatel.com/atr Alcatel Telecommunications Review] Telecom magazine published since 1922
- [http://www.teleclick.ca Telecommunications Industry News]
- [http://www.bt.com BT] British Telecommunications company
- Category:Digital Revolution ms:Telekomunikasi ja:電気通信 th:โทรคมนาคม



Measurement

In classical physics and engineering, measurement is the process of estimating or determining the ratio of a magnitude of a quantitative property or relation to a unit of the same type of quantitative property or relation. A process of measurement involves the comparison of physical quantities of objects or phenomena, or the comparison of relations between objects (e.g. angles). A particular measurement is the result of such a process, normally expressed as the multiple of a real number and a unit, where the real number is the ratio obtained from the measurement process. For example, the measurement of the length of an object might be 5 m, which is an estimate of the object's length, a magnitude, relative to a unit of length, the meter. Measurement is not limited to physical quantities and relations but can in principle extend to the quantification of a magnitude of any type, through application of a measurement model such as the Rasch model, and subjecting empirical data derived from comparisons to appropriate testing in order to ascertain whether specific criteria for measurement have been satisfied. In addition, the term measurement is often used in a somewhat looser fashion than defined above, to refer to any process in which numbers are assigned to entities such that the numbers are intended to represent increasing amount, in some sense, without a process that involves the estimation of ratios of magnitudes to a unit. Such examples of measurement range from degrees of uncertainty to consumer confidence to the rate of increase in the fall in the price of a good or service. It is generally proposed that there are four different levels of measurement, and that different levels are applicable to different contexts and types of measurement process. In scientific research, measurement is essential. It includes the process of collecting data which can be used to make claims about learning. Measurement is also used to evaluate the effectiveness of a program or product (known as an evaluand). :: A measurement is a comparison to a standard. -- William Shockley :: By number we understand not so much a multitude of Unities, as the abstracted Ratio of any Quantity to another Quantity of the same kind, which we take for Unity -- Sir Isaac Newton

Units and systems of measurement

:Main articles: Units of measurement and Systems of measurement Because measurement involves the estimation of magnitudes of quantities relative to particular quantities, called units, the specification of units is of fundamental importance to measurement. The definition or specification of precise standards of measurement involves two key features, which are evident in the Système International d'Unités (SI). Specifically, in this system the definition of each of the base units makes reference to specific empirical conditions and, with the exception of the kilogram, also to other quantitative attributes. Each derived SI unit is defined purely in terms of a relationship involving itself and other units; for example, the unit of velocity is 1 m/s. Due to the fact that derived units make reference to base units, the specification of empirical conditions is an implied component of the definition of all units. The measurement of a specific entity or relation results in at least two numbers for the relationship between the entity or relation under study and the referenced unit of measurement, where at least one number estimates the statistical uncertainty in the measurement, also referred to as measurement error. Measuring instruments are used to estimate ratios of magnitudes to units. Prior comparisons underlie the calibration, in terms of standard units, of commonly used instruments constructed to measure physical quantities.

Metrology

Metrology is the study of measurement. In general, a metric is a scale of measurement defined in terms of a standard: i.e. in terms of well-defined unit. The quantification of phenomena through the process of measurement relies on the existence of an explicit or implicit metric, which is the standard to which measurements are referenced. If I say I am 5, I am indicating a measurement without supplying an applicable standard. I may mean I am 5 years old or I am 5 feet high, however the implicit metric is that.

History

:Main article: History of measurement Laws to regulate measurement were originally developed to prevent fraud. However, units of measurement are now generally defined on a scientific basis, and are established by international treaties. In the United States, commercial measurements are regulated by the National Institute of Standards and Technology NIST, a division of the United States Department of Commerce. The history of measurements is a topic within the history of science and technology. The metre (us: meter) was standardized as the unit for length after the French revolution, and has since been adopted throughout most of the world. The United States and the UK are in the process of converting to the SI system. This process is known as metrication.

Difficulties in measurement

Measurement of many quantities is very difficult and prone to large error. Part of the difficulty is due to uncertainty, and part of it is due to the limited time available in which to make the measurement. Examples of things that are very difficult to measure in some respects and for some purposes include social related items such as:
- A person's knowledge (as in testing, see also assessment)
- A person's feelings, emotions, or beliefs.
- A person's senses (qualia). Even for physical quantities gaining accurate measurement can be difficult. It is not possible to be exact, instead, repeated measurements will vary due to various factors affecting the quantity such as temperature, time, electromagnetic fields, and especially measurement method. As an example in the measurement of the speed of light, the quantity is now known to a high degree of precision due to modern methods, but even with those methods there is some variability in the measurement. Statistical techniques are applied to the measurement samples to estimate the speed. In earlier sets of measurements, the variability was greater, and comparing the results shows that the variability and bias in the measurement methods was not properly taken into account. Proof of this is that when various group's measurements are plotted with the estimated speed and error bars showing the expected variability of the estimated speed from the actual number, the error bars from each of the experiments did not all overlap. This means a number of groups incorrectly accounted for the true sources of error and overestimated the accuracy of their methods.

See also


- Conversion of units
- Dimensional analysis
- Dimensionless number
- Econometrics
- History of measurement
- Instrumentation
- Levels of measurement
- Measurement in quantum mechanics
- Orders of magnitude
- Systems of measurement
- Thermometry
- Timeline of temperature and pressure measurement technology
- Timeline of time measurement technology
- Uncertainty in measurement
- Uncertainty principle
- Units of measurement
- Weights and measures

Miscellaneous

Measuring the ratios between physical quantities is an important sub-field of physics. Some important physical quantities include:
- Speed of light
- Planck's constant
- Gravitational constant
- Elementary charge (electric charge of electrons, protons, etc.)
- Fine-structure constant

References

Newton, I. (1728/1967). Universal Arithmetic: Or, a Treatise of Arithmetical Composition and Resolution. In D.T. Whiteside (Ed.), The mathematical Works of Isaac Newton, Vol. 2 (pp. 3-134). New York: Johnson Reprint Corp.

External links


- [http://www.unc.edu/~rowlett/units/index.html A Dictionary of Units of Measurement]
- [http://www.geocities.com/qubestrader/conversion.html Conversion Calculator]
- [http://www.euromet.org/docs/pubs/docs/Metrology_in_short_2nd_edition_may_2004.pdf 'Metrology In Short', 2nd Edition] ja:測定 simple:Measurement

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.) :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = -\frac \mathbf :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf \mathbf=\mathbf=\mathbf is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf Where \mathbf can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf These types of equations are identified as linear wave equations with wave speed \frac. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space \epsilon_0, the permeability of free space \mu_0, and the speed of light itself: c = \frac. 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. :\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right) Here \mathbf_0 is the constant amplitude, f is any second differentiable function, \hat is a unit vector in the direction of propagation, and \hat is a position vector. We observe that f\left( \hat \cdot \mathbf - c t \right) is a generic solution to the wave equation. In other words :\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right), for a generic wave traveling in the \hat 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? :\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0 :\mathbf \cdot \hat = 0 The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf :\mathbf = \frac \hat \times \mathbf The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of \mathbf,\mathbf. 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, \mathbf_0 = c \mathbf_0. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as \mathbf \times \mathbf. 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

Visible light

The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. There are no exact bounds to the optical spectrum, but there are to the visible spectrum. A typical human eye will respond to wavelengths from 400 to 700 nm, although some people may be able to perceive wavelengths from 380 to 780 nm. A light-adapted eye typically has its maximum sensitivity at around 555 nm, in the green region of the optical spectrum. Wavelengths visible to the eye also pass through the "visible window", the region of the electromagnetic spectrum which passes largely unattenuated through the Earth's atmosphere (although blue light is scattered more than red light, which is the reason the sky is blue). The response of the human eye is defined by subjective testing (see CIE), but the atmospheric windows are defined by physical measurement. The "visible window" is so called because it overlaps the human visible response spectrum; the near infrared (NIR) windows lie just out of human response window, and the Medium Wavelength IR (MWIR) and Long Wavelength or Far Infrared (FIR or LWIR) are far beyond the human response region. The eyes of many species perceive wavelengths different than the spectrum visible to the human eye. For example, many insects, such as bees, can see light in the ultraviolet, which is useful for finding nectar in flowers. flower into the colors of the optical spectrum.]]

Historical use of the term

Sir Isaac Newton first used the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of white sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different colored bands. Newton hypothesized that light was made up of "corpuscles" (particles) of different colors, and that the different colors of light moved at different speeds in transparent matter, with red light moving more quickly in glass than violet light. The result is that red light was bent (refracted) less sharply that violet light as it passed through the prism, creating a spectrum of colors. It is now known light is composed of photons (which display some of the properties of a wave and some of the properties of a particle), and that all light travels at the same speed (the speed of light) in a vacuum. The speed of light within a material is lower than the speed of light in a vacuum, and the ratio of speeds is known as the refractive index of the material. In some materials, known as non-dispersive, the speed of different frequencies (corresponding to the different colors) does not vary, and so the refractive index is a constant. However, in other (dispersive) materials, the refractive index (and thus the speed) depends on frequency in accorance with a dispersion relation: glass is one such material, which enables glass prisms to create an optical spectrum from white light.

Spectroscopy

dispersion relation transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.]] The scientific study of objects based on the spectrum of the light they emit is called spectroscopy. One particularly important application of spectroscopy is in astronomy, where spectroscopy is essential for analysing the properties of distant objects. Typically, astronomical spectroscopy utilises high-dispersion diffraction gratings to observe spectra at very high spectral resolutions. Helium was first detected through an analysis of the spectrum of the Sun; chemical elements can be detected in astronomical objects by emission lines and absorption lines; and the shifting of spectral lines can be used to measure the redshift or blueshift of distant or fast-moving objects. The first exoplanets to be discovered were found by analysing the doppler shift of stars at such high resolution that variations in their radial velocity as small as a few metres per second could be detected: the presence of planets was revealed by their gravitational influence on the motion of the stars analysed.

See also


- Frequency
- Rydberg formula
- Wavelength Category:Color Category:Electromagnetic spectrum
- Category:Vision ko:가시광선 ja:可視光線

Photometry (optics)

:This article deals with the usage of this term in optics and lighting. :For the usage of this term in astronomy, see Photometry (astronomy). Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of light in terms of absolute power. =Photometry and the eye= The human eye is not equally sensitive to all wavelengths of light. Photometry attempts to account for this by weighting the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength. The eye's response to light as a function of wavelength is given by the luminosity function. Note that the eye has different responses as a function of wavelength when it is adapted to light conditions (photopic vision) and dark conditions (scotopic vision). Photometry is based on the eye's photopic response, and so photometric measurements will not accurately indicate the percieved brightness of sources in dim lighting conditions. =Photometric quantities= Many different units of measure are used for photometric measurements. People sometimes ask why there need to be so many different units, or ask for conversions between units that can't be converted (lumens and candelas, for example). We are familiar with the idea that the adjective "heavy" can refer to weight or density, which are fundamentally different things. Similarly, the adjective "bright" can refer to a lamp which delivers a high luminous flux (measured in lumens), or to a lamp which concentrates the luminous flux it has into a very narrow beam (candelas). Because of the ways in which light can propagate through three-dimensional space, spread out, become concentrated, reflect off shiny or matte surfaces, and because light consists of many different wavelengths, the number of fundamentally different kinds of light measurement that can be made is large, and so are the numbers of quantities and units that represent them.

Photometric vs. radiometric quantities

There are two parallel systems of quantities known as photometric and radiometric quantities. Every quantity in one system has an analogous quantity in the other system. Some examples of parallel quantities include:
- Luminance (photometric) and radiance (radiometric)
- Luminous flux (photometric) and radiant flux (radiometric)
- Luminous intensity (photometric) and radiant intensity (radiometric) In photometric quantities every wavelength is weighted according to how visible it is, while radiometric quantities use unweighted absolute power. For example, the eye responds much more strongly to green light than to red, so a green source will have higher luminous flux than a red souce with the same radiant flux would. Light outside the visible spectrum does not contribute to photometric quantities at all, so for example a 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but as a light source it puts out very few lumens (because most of the energy is in the infrared, leaving only a dim red glow in the visible).

Watts (radiant flux) vs. lumens (luminous flux)

A comparison of the watt and the lumen illustrates the distinction between radiometric and photometric units. The watt is a unit of energy. We are accustomed to thinking of light bulbs in terms of power in watts. But power is not a measure of the amount of light output. It tells you how quickly the bulb will increase your electric bill, not how effective it will be in lighting your home. Because incandescent bulbs sold for "general service" all have fairly similar characteristics, power is a guide to light output, but only a rough one. Watts can also be a measure of output. In a radiometric sense, an incandescent light bulb is about 80% efficient; 20% of the energy is lost (e.g. by conduction through the lamp base) The remainder is emitted as radiation. Thus, a 60 watt light bulb emits a total radiant flux of about 45 watts. Incandescent bulbs are, in fact, sometimes used as heat sources, (as in a chick incubator), but usually they are used for the purpose of providing light. As such, they are very inefficient, because most of the radiant energy they emit is invisible infrared. There are compact fluorescent bulbs that say on their package that they "provide the light of a 60 watt bulb" while consuming only 15 watts. The lumen is the photometric unit of light output. Although most consumers still think of light in terms of power consumed by the bulb, in the U. S. it has been a trade requirement for several decades that light bulb packaging give the output in lumens. The package of a 60 watt incandescent bulb indicates that it provides about 900 lumens, as does the package of the 15 watt compact fluorescent. The lumen is defined as amount of light given into one steradian by point source of one candela strength; while the candela, a base SI unit, is defined as the luminous intensity of a source of monochromatic radiation, of frequency 540 terahertz, and a radiant intensity of 1/683 watts per steradian. (540 THz corresponds to about 555 nanometres, the wavelength, in the green, to which the human eye is most sensitive. The number 1/683 was chosen to make the candela about equal to the standard candle, the unit which it superseded). Combining these definitions, we see that 1/683 watt of 555 nanometre green light provides one lumen. The relation between watts and lumens is not just a simple scaling factor. We know this already, because the 60 watt incandescent bulb and the 15 watt compact fluorescent both provide 900 lumens. The definition tells us that 1 watt of pure green 555 nm light is "worth" 683 lumens. It does not say anything about other wavelengths. Because lumens are photometric units, their relationship to watts depends on the wavelength according to how visible the wavelength is. Infrared and ultraviolet radiation, for example, are invisible and do not count. One watt of infrared radiation (which is where most of the radiation from an incandescent bulb falls) is worth zero lumens. Within the visible spectrum, wavelengths are light are weighted according to a function called the "photopic spectral luminous efficiency." According to this function, 700 nm red light is only about 4% as efficient as 555 nm green light. Thus, one watt of 700 nm red light is "worth" only 27 lumens. =Photometric measurement techniques= = Non-SI photometry units =

Luminance


- Footlambert
- Millilambert
- Stilb

Illuminance


- Foot-candle
- Phot =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 Category:Lighting

Category:Radiometry

In 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. Category:measurement Category:optics Category:telecommunications

Homerton railway station

Homerton railway station is in the London Borough of Hackney in East London. It is on the North London Line, and the station and all trains serving it are operated by Silverlink. It is in Travelcard Zone 2. The station is near Homerton Hospital and Hackney Marshes. The typical service at the station is 4 trains per hour westbound to Richmond via Hackney, Highbury, Camden Town and Willesden, and 4 trains per hour eastbound to Stratford, of which alternate trains are extended to North Woolwich. An additional shuttle train running between Camden Road and Stratford in the morning and evening peaks increases this number to 5-6 trains per hour in each direction. With the exception of a partial section of wall to the north of the bridge over Barnabas Road, the original station has been demolished. Although of reduced size, the original station building would have been similar to buildings remaining at Hackney Central and Camden Road. The present basic station was built in the 1980s when passenger operation was restored to the North London Line.

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  Category:Hackney

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