What unit of measurement is used to measure light intensity?
Luminous intensity
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.
Luminosity
In astronomy, "luminosity" is the amount of electromagnetic energy a body radiates per unit of time. When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities (). A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. A star also radiates neutrinos, which carry off some energy (about 2% in the case of our Sun), contributing to the star's total luminosity. The IAU has defined a nominal solar luminosity of to promote publication of consistent and comparable values in units of the solar luminosity.
Luminosity function
The number 683 is connected to the modern (1979) definition of the candela, the unit of luminous intensity. This arbitrary number made the new definition give numbers equivalent to those from the old definition of the candela.
Ultraviolet germicidal irradiation
The UV intensity is specified for each lamp at a distance of 1 meter. UV intensity is inversely proportional to the square of the distance so it decreases at longer distances. Alternatively, it rapidly increases at distances shorter than 1m. In the above formula the UV intensity must always be adjusted for distance unless the UV dose is calculated at exactly 1m from the lamp. Also, to ensure effectiveness the UV dose must be calculated at the end of lamp life (EOL is specified in number of hours when the lamp is expected to reach 80% of its initial UV output) and at the furthest distance from the lamp on the periphery of the target area. Some "shatter-proof" lamps are coated with a fluorated ethylene polymer to contain glass shards and mercury in case of breakage; this coating reduces UV output by as much as 20%.
Densitometer
A densitometer is a device that measures the degree of darkness (the optical density) of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determines the density of a sample placed between the light source and the photoelectric cell from differences in the readings. Modern densitometers have the same components, but also have electronic integrated circuitry for better reading.Some modern types of German manufacture are capable of both types of measurements selectable by a switch. They are used in film photography to measure densities of negatives with the switch in the "T" (Transmission) position and the saturation of a resulting print in the "R" position. Such measurements enable the photographer to choose the right photo paper and the correct exposure, obviating experiments with test strips. Once the papers and darkroom have been calibrated, the first print from a previously measured negative is a success at once.Industrial Tomography Systems (ITS) has collaborated with a leading European dredging contractor to develop their radiation and gamma free Dens-itometer based upon Electrical resistance Tomography. The system utilises electrical conductivity, to provide a greener, simpler, more cost effective alternative to conventional nuclear based systems. ITS pipe based sensors provide real time visual data on the processes occurring within the pipe.
Spectrophotometry
Spectrophotometry is a tool that hinges on the quantitative analysis of molecules depending on how much light is absorbed by colored compounds. Spectrophotometry uses photometers, known as spectrophotometers, that can measure a light beam's intensity as a function of its color (wavelength). Important features of spectrophotometers are spectral bandwidth (the range of colors it can transmit through the test sample), the percentage of sample-transmission, the logarithmic range of sample-absorption, and sometimes a percentage of reflectance measurement.
Turbidimetry
Turbidimetry (the name being derived from "turbidity") is the process of measuring the loss of intensity of transmitted light due to the scattering effect of particles suspended in it. Light is passed through a filter creating a light of known wavelength which is then passed through a cuvette containing a solution. A photoelectric cell collects the light which passes through the cuvette. A measurement is then given for the amount of absorbed light.
Light
Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.
Optical spectrometer
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light or a unit directly proportional to the photon energy, such as reciprocal centimeters or electron volts, which has a reciprocal relationship to wavelength.
Luminosity (scattering theory)
In scattering theory and accelerator physics, luminosity ("L") is the ratio of the number of events detected ("N") in a certain time ("t") to the interaction cross-section (σ):
It has the dimensions of events per time per area, and is usually expressed in the cgs units of cm·s or the non-SI units of b·s. In practice, "L" is dependent on the particle beam parameters, such as beam width and particle flow rate, as well as the target properties, such as target size and density.
Photometry (optics)
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 light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background. Because of the ways in which light propagates through three-dimensional space — spreading out, becoming concentrated, reflecting 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.
Dobson ozone spectrophotometer
The ratio is determined by turning the R-dial, which can be rotated a full 300°, on the instrument. The spectrometer compares two different wavelength intensities, UVB (305 nm) and UVA (325 nm), in order to calculate the amount of ozone. When turned, the R-dial filters and blocks out the light of the UVC wavelength until the intensity of the two wavelengths of light are equal. The ratio of the two wavelengths at incidence can be calculated once the filtered intensities are the same. The results are measured in Dobson Units, equal to 10 µm thickness of ozone compressed to Standard conditions for temperature and pressure (STP) in the column. If all of the ozone in the atmospheric column one was measuring were compressed to STP, the thickness of the compressed atmosphere in mm would equal an answer in Dobson Units divided by 100.
SMC AB8
The simplest way to measure the luminosity of a star is to observe its radiated output at all wavelengths (the spectral energy distribution or SED) and sum them together. Unfortunately this is impractical for AB8 because the majority of the radiation occurs in the far ultraviolet. A more common method is to measure the visual luminosity and apply a bolometric correction to give the total luminosity at all wavelengths, although the size of the bolometric correction is extremely sensitive to the effective temperature. Modelling the atmospheres gives luminosities for the WR and O component of and respectively. Deriving the relative luminosities of the two components from the profile of the O resonance line gives a luminosity of for the primary, but this would imply an unreasonably low temperature.
Gravimetry
Gravity is usually measured in units of acceleration. In the SI system of units, the standard unit of acceleration is 1 metre per second squared (abbreviated as m/s). Other units include the gal (sometimes known as a "galileo", in either case with symbol Gal), which equals 1 centimetre per second squared, and the "g" ("g"), equal to 9.80665 m/s. The value of the "g" approximately equals the acceleration due to gravity at the Earth's surface (although the value of "g" varies by location).
Incandescent light bulb
Luminous "efficacy" of a light source may be defined in two ways. The radiant luminous efficacy (LER) is the ratio of the visible light flux emitted (the "luminous flux") to the total power radiated over all wavelengths. The source luminous efficacy (LES) is the ratio of the visible light flux emitted (the "luminous flux") to the total power input to the source, such as a lamp. Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye. The units of luminous efficacy are "lumens per watt" (lpw). The maximum LER possible is 683 lm/W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye.
Gamma correction
The term intensity refers strictly to the amount of light that is emitted per unit of time and per unit of surface, in units of lux. Note, however, that in many fields of science this quantity is called luminous exitance, as opposed to luminous intensity, which is a different quantity. These distinctions, however, are largely irrelevant to gamma compression, which is applicable to any sort of normalized linear intensity-like scale.
Energy conversion efficiency
The wall-plug efficiency differs from the "luminous efficiency" in that wall-plug efficiency describes the direct output/input conversion of energy (the amount of work that can be performed) whereas luminous efficiency takes into account the human eye's varying sensitivity to different wavelengths (how well it can illuminate a space). Instead of using watts, the power of a light source to produce wavelengths proportional to human perception is measured in lumens. The human eye is most sensitive to wavelengths of 555 nanometers (greenish-yellow) but the sensitivity decreases dramatically to either side of this wavelength, following a Gaussian power-curve and dropping to zero sensitivity at the red and violet ends of the spectrum. Due to this the eye does not usually see all of the wavelengths emitted by a particular light-source, nor does it see all of the wavelengths within the visual spectrum equally. Yellow and green, for example, make up more than 50% of what the eye perceives as being white, even though in terms of radiant energy white-light is made from equal portions of all colors (i.e.: a 5 mw green laser appears brighter than a 5 mw red laser, yet the red laser stands-out better against a white background). Therefore, the radiant intensity of a light source may be much greater than its luminous intensity, meaning that the source emits more energy than the eye can use. Likewise, the lamp's wall-plug efficiency is usually greater than its luminous efficiency. The effectiveness of a light source to convert electrical energy into wavelengths of visible light, in proportion to the sensitivity of the human eye, is referred to as luminous efficacy, which is measured in units of lumens per watt (lm/w) of electrical input-energy.
Spectrophotometry
There are two major classes of devices: single beam and double beam. A double beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample. A single-beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. Although comparison measurements from double-beam instruments are easier and more stable, single-beam instruments can have a larger dynamic range and are optically simpler and more compact. Additionally, some specialized instruments, such as spectrophotometers built onto microscopes or telescopes, are single-beam instruments due to practicality.
Image intensifier
The gain of a tube is typically measured using one of two units. The most common (SI) unit is cd·m·lx, i.e. candelas per meter squared per lux. The older convention is Fl/Fc (foot-lamberts per foot-candle). This creates issues with comparative gain measurements since neither is a pure ratio, although both are measured as a value of output intensity over input intensity. This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively pi or approximately 3.142x. This means that a gain of 10,000 cd/m²/lx is the same as 31.42 Fl/Fc.
Nephelometer
A nephelometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam (source beam) and a light detector set to one side (often 90°) of the source beam. Particle density is then a function of the light reflected into the detector from the particles. To some extent, how much light reflects for a given density of particles is dependent upon properties of the particles such as their shape, color, and reflectivity. Nephelometers are calibrated to a known particulate, then use environmental factors (k-factors) to compensate lighter or darker colored dusts accordingly. K-factor is determined by the user by running the nephelometer next to an air sampling pump and comparing results. There are a wide variety of research-grade nephelometers on the market as well as open source varieties.
Relative intensity noise
Relative intensity noise is measured by sampling the output current of a photodetector over time and transforming this data set into frequency with a fast Fourier transform. Alternatively, it can be measured by analyzing the spectrum of the photodetected signal using an electrical spectrum analyzer. Noise observed in the electrical domain is proportional to electric current squared and hence to optical power squared. Therefore, RIN is usually presented as relative fluctuation in the square of the optical power in decibels per hertz over the RIN bandwidth and at one or several optical intensities. It may also be specified as a percentage, a value that represents the relative fluctuations per Hz multiplied by the RIN bandwidth.
Sound intensity
Sound intensity level also known as acoustic intensity is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.
Flicker (light)
For objective assessment of flicker, a widely applied and IEC-standardized metric, the short-term flicker indicator ("P") is used. This metric is derived from the short term flicker severity metric "P" that is applied in the power quality domain for testing electrical apparatus for their potential to cause flicker via voltage fluctuations on the electrical mains network (see the IEC publications IEC 61000-3-3 and IEC 61000-4-15). The short-term flicker indicator "P" is implemented in a light flickermeter which processes the light measured by means of a light sensor. The light flickermeter consists of four processing blocks which include weighting filters to account for frequency dependency of the visibility of TLMs as well as statistical processing to enable assessing a-periodic TLMs. The specification of the light flickermeter and the test method for objective flicker assessment of lighting equipment is published in IEC technical report IEC TR 61547-1.
Carcel
The Carcel is a former French unit for measuring the intensity of light. The unit was defined in 1860 as the intensity of a Carcel lamp with standard burner and chimney dimensions, which burnt colza oil
Motion analysis
The camera measures the intensity of light at each image point, a light field. In practice, a digital camera measures this light field at discrete points, pixels, but given that the pixels are sufficiently dense, the pixel intensities can be used to represent most characteristics of the light field that falls onto the image plane. A common assumption of motion analysis is that the light reflected from the scene points does not vary over time. As a consequence, if an intensity "I" has been observed at some point in the image, at some point in time, the same intensity "I" will be observed at a position that is displaced relative to the first one as a consequence of the apparent motion. Another common assumption is that there is a fair amount of variation in the detected intensity over the pixels in an image. A consequence of this assumption is that if the scene point that corresponds to a certain pixel in the image has a relative 3D motion, then the pixel intensity is likely to change over time.
Intensity (heat transfer)
Spectral intensity formula_14 is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength.
Louis Essen
During World War II, Essen worked on radar and developed a number of instruments, including the cavity resonance wavemeter. It was this work that suggested to Essen the possibility of a more precise measurement of the speed of light. In 1946, in collaboration with A.C. Gordon-Smith, he used a microwave cavity, of precisely known dimensions, and exploited his expertise in time-measurement to establish the frequency for a variety of its normal modes. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. Their result, 299,792±3 km/s, was substantially greater than that from the prevailing sequence of optical measurements that had begun around the start of the 20th century and Essen had to withstand some fierce criticism and disbelief. Even NPL director Sir Charles Galton Darwin, while supporting the work, observed that Essen would get the "correct result" once he had perfected the technique. Moreover, W.W. Hansen at Stanford University had used a similar technique and obtained a measurement which was more consistent with the conventional (optical) wisdom. However, a combination of Essen's stubbornness, his iconoclasm and his belief in his own skill at measurement (and a little help with calculations from Alan Turing) inspired him to refine his apparatus and to repeat his measurement in 1950, establishing a result of 299,792.5±1 km/s. This was the value adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. Most subsequent measurements have been consistent with this value. In 1983, the 17th Conférence Générale des Poids et Mesures adopted the standard value, 299,792.458 km/s for the speed of light.
Crab (unit)
A Crab is a standard astrophotometrical unit for measurement of the intensity of Astrophysical X-ray sources. One Crab is defined as the intensity of the Crab Nebula at the corresponding X-ray photon energy.
Bicycle lighting
The lumen (lm) is the SI unit for luminous flux, the total amount of light emitted by a source, weighted according to the sensitivity of the human eye to various colours of light. Lumens per watt is a common measure of the efficacy of a light source. The luminous flux is of less value for bicycle lighting due to the importance of directionality. Luminous intensity is much more useful, but lumens per watt is a handy way to compare the output of otherwise similar lights.
Phase problem
Light detectors, such as photographic plates or CCDs, measure only the intensity of the light that hits them. This measurement is incomplete (even when neglecting other degrees of freedom such as polarization and angle of incidence) because a light wave has not only an amplitude (related to the intensity), but also a phase, which is systematically lost in a measurement. In diffraction or microscopy experiments, the phase part of the wave often contains valuable information on the studied specimen. The phase problem constitutes a fundamental limitation ultimately related to the nature of measurement in quantum mechanics.
Fluorometer
A fluorometer or fluorimeter is a device used to measure parameters of fluorescence: its intensity and wavelength distribution of emission spectrum after excitation by a certain spectrum of light. These parameters are used to identify the presence and the amount of specific molecules in a medium. Modern fluorometers are capable of detecting fluorescent molecule concentrations as low as 1 part per trillion.
Brightness temperature
where
Photometer
The principle of spectrophotometers and filter photometers is that (as far as possible) monochromatic light is allowed to pass through a container (cell) with optically flat windows containing the solution. It then reaches a light detector, that measures the intensity of the light compared to the intensity after passing through an identical cell with the same solvent but without the coloured substance. From the ratio between the light intensities, knowing the capacity of the coloured substance to absorb light (the absorbency of the coloured substance, or the photon cross section area of the molecules of the coloured substance at a given wavelength), it is possible to calculate the concentration of the substance using Beer's law.
Candela (disambiguation)
The candela is a unit of luminous intensity.
Second
A second is part of other units, such as frequency measured in hertz (inverse seconds or second), speed (meters per second) and acceleration (meters per second squared). The metric system unit becquerel, a measure of radioactive decay, is measured in inverse seconds. The meter is defined in terms of the speed of light and the second; definitions of the metric base units ampere and candela also depend on the second. Of the 22 named derived units of the SI, only three: degree Celsius, radian, and steradian, do not depend on the second. Many derivative units for everyday things are reported in terms of larger units of time, not seconds, such as clock time in hours and minutes, velocity of a car in miles per hour or kilometers per hour, kilowatt hours of electricity usage, and speed of a turntable in rotations per minute.
Kilogram
With the magnitude of the primary units of electricity thus determined by the kilogram, so too follow many others, namely the coulomb, volt, tesla, and weber. Even units used in the measure of light would be affected; the candela—following the change in the would in turn affect the lumen andlux.
Lighting
Typical measurements of light have used a Dosimeter. Dosimeters measure an individual's or an object's exposure to something in the environment, such as light dosimeters and ultraviolet dosimeters.
Skyglow
Amateur astronomers have used the Bortle Dark-Sky Scale to measure skyglow ever since it was published in "Sky & Telescope" magazine in February 2001. The scale rates the darkness of the night sky inhibited by skyglow with nine classes and provides a detailed description of each position on the scale. Amateurs also increasingly use Sky Quality Meters (SQM) that measure in astronomical photometric units of visual (Johnson V) magnitudes per square arcsecond. Professional astronomers and light pollution researchers use various measures of luminous or radiant intensity per unit area, such as (nano-)Lamberts, magnitudes per square arcsecond, or (micro-)candela per square meter. All-sky maps of skyglow brightness are produced with professional-grade imaging cameras with CCD detectors and using stars as calibration sources.
Radiant intensity
In radiometry, radiant intensity is the radiant flux emitted, reflected, transmitted or received, per unit solid angle, and spectral intensity is the radiant intensity per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. These are "directional" quantities. The SI unit of radiant intensity is the watt per steradian (), while that of spectral intensity in frequency is the watt per steradian per hertz () and that of spectral intensity in wavelength is the watt per steradian per metre ()—commonly the watt per steradian per nanometre (). Radiant intensity is distinct from irradiance and radiant exitance, which are often called "intensity" in branches of physics other than radiometry. In radio-frequency engineering, radiant intensity is sometimes called radiation intensity.
Luminosity
The magnitude of a star, a unitless measure, is a logarithmic scale of observed visible brightness. The apparent magnitude is the observed visible brightness from Earth which depends on the distance of the object. The absolute magnitude is the apparent magnitude at a distance of 10 parsecs, therefore the bolometric absolute magnitude is a logarithmic measure of the bolometric luminosity.
Image intensifier
The sensitivity of an image intensifier tube is measured in microamperes per lumen (µA/lm). It defines how many electrons are produced per quantity of light that falls on the photocathode. This measurement should be made at a specific color temperature, such as "at a colour temperature of 2854 K". The color temperature at which this test is made tends to vary slightly between manufacturers. Additional measurements at specific wavelengths are usually also specified, especially for Gen2 devices, such as at 800 nm and 850 nm (infrared).
National Physical Laboratory of India
The unit of luminous intensity, candela, is realized by using an absolute radiometer. For practical work, a group of tungsten incandescent lamps are used.
Light
The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The cone cells in the human eye are of three types which respond differently across the visible spectrum, and the cumulative response peaks at a wavelength of around 555 nm. Therefore, two sources of light which produce the same intensity (W/m) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw power by a quantity called luminous efficacy, and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and charge-coupled devices (CCD) tend to respond to some infrared, ultraviolet or both.
Measurement
There are two types of SI units, base units and derived units. Base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units, for example, the watt, i.e. the unit for power, is defined from the base units as m·kg·s. Other physical properties may be measured in compound units, such as material density, measured in kg/m.
Static light scattering
To measure the average molecular weight directly without calibration from the light scattering intensity, the laser intensity, the quantum efficiency of the detector, and the full scattering volume and solid angle of the detector needs to be known. Since this is impractical, all commercial instruments are calibrated using a strong, known scatterer like toluene since the Rayleigh ratio of toluene and a few other solvents were measured using an absolute light scattering instrument.
Exposure value
Illuminance is measured using a flat sensor; if the common value of is used, an EV of zero (e.g., an aperture of "f"/1 and a shutter time of 1 sec) for corresponds to an illuminance of (). At EV = 15 (the "sunny sixteen" amount of light) the illuminance is (). For general photography, incident-light measurements are usually taken with a hemispherical sensor; the readings cannot be meaningfully related to illuminance.
Conventional electrical unit
Four significant steps were taken in the last half century to increase the precision and utility of measurement units. In 1967 the Thirteenth General Conference on Weights and Measures (CGPM) defined the second of atomic time in the International System of Units as the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1983, the seventeenth CGPM redefined the metre in terms of the second and the speed of light, thus fixing the speed of light at exactly . And in 1990, the eighteenth CGPM adopted conventional values for the Josephson constant and the von Klitzing constant, fixing the conventional Josephson constant at exactly , and the conventional von Klitzing constant at exactly (again, these volts and ohms are not precisely the same as the SI definitions but very nearly equivalent).
Candela
Like most other SI base units, the candela has an operational definition—it is defined by a description of a physical process that will produce one candela of luminous intensity. Since the 16th General Conference on Weights and Measures (CGPM) in 1979, the candela has been defined as:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency and that has a radiant intensity in that direction of  watt per steradian.
The definition describes how to produce a light source that (by definition) emits one candela, but does not specify the luminosity function for weighting radiation at other frequencies. Such a source could then be used to calibrate instruments designed to measure luminous intensity with reference to a specified luminosity function. An appendix to the SI Brochure makes it clear that the luminosity function is not uniquely specified, but must be selected to fully define the candela.
Compact fluorescent lamp
Because the eye's sensitivity changes with the wavelength, the output of lamps is commonly measured in lumens, a measure of the power of light as perceived by the human eye. The luminous efficacy of lamps is the number of lumens produced for each watt of electrical power used. The luminous efficacy of a typical CFL is 50–70 lumens per watt (lm/W) and that of a typical incandescent lamp is . Compared to a theoretical 100%-efficient lamp (), CFL lamps have lighting efficiency ranges of 7–10%, versus 1.5–2.5% for incandescents.
Intensity interferometer
An intensity interferometer is the name given to devices that use the Hanbury Brown and Twiss effect. In astronomy, the most common use of such an astronomical interferometer is to determine the apparent angular diameter of a radio source or star. If the distance to the object can then be determined by parallax or some other method, the physical diameter of the star can then be inferred. An example of an optical intensity interferometer is the Narrabri Stellar Intensity Interferometer. In quantum optics, some devices which take advantage of correlation and anti-correlation effects in beams of photons might be said to be intensity interferometers, although the term is usually reserved for observatories.
Exposure (photography)
If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriate spectral sensitivity, the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye). Only in this appropriately weighted case does the "H" measure the effective amount of light falling on the film, such that the characteristic curve will be correct independent of the spectrum of the light.
Brightness temperature
formula_3 (the Intensity or Brightness) is the amount of energy emitted per unit surface area per unit time per unit solid angle and in the frequency range between formula_1 and formula_5; formula_6 is the temperature of the black body; formula_7 is Planck's constant; formula_1 is frequency; formula_9 is the speed of light; and formula_10 is Boltzmann's constant.
Intensity interferometer
An intensity interferometer is built from two light detectors, typically either radio antenna or optical telescopes with photomultiplier tubes (PMTs), separated by some distance, called the baseline. Both detectors are pointed at the same astronomical source, and intensity measurements are then transmitted to a central correlator facility. A major advantage of intensity interferometers is that only the measured intensity observed by each detector must be sent to the central correlator facility, rather than the amplitude and phase of the signal. The intensity interferometer measures interferometric visibilities like all other astronomical interferometers. These measurements can be used to calculate the diameter and limb darkening coefficients of stars, but with intensity interferometers aperture synthesis images cannot be produced as the visibility phase information is not preserved by an intensity interferometer.
Decibel
In spectrometry and optics, the blocking unit used to measure optical density is equivalent to −1 B.
Candela
The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. The human eye, when adapted for bright conditions, is most sensitive near this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength "λ" is given by
where "I"("λ") is the luminous intensity, "I"("λ") is the radiant intensity and formula_2 is the photopic luminosity function. If more than one wavelength is present (as is usually the case), one must integrate over the spectrum of wavelengths to get the total luminous intensity.Prior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these was the English standard of candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp.
Light effects on circadian rhythm
Historically light was measured in the units of luminous intensity (candelas), luminance (candelas/m) and illuminance (lumen/m). After the discovery of ipRGCs in 2002 additional units of light measurement have been researched in order to better estimate the impact of different inputs of the spectrum of light on various photoreceptors. However, due to the variability in sensitivity between rods, cones and ipRGCs and variability between the different ipRGC types a singular unit does not perfectly reflect the effects of light on the human body.
Metrology
The International System of Units (SI) defines seven base units: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. By convention, each of these units are considered to be mutually independent of each other; however, in reality they are interdependent given some definitions contain other base SI Units. All other SI units are derived from the seven base units.
Lighting
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 the eye that affects the human body's clock.
Intensity (physics)
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, and in heat transfer.
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph. Typically a light meter will include either digital or analog electronic circuit, which allows the photographer to determine which shutter speed and f-number should be selected for an optimum exposure, given a certain lighting situation and film speed.
International System of Units
The International System of Units (SI, abbreviated from the French ") is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.
High-intensity radiated field
An RF electromagnetic wave has both an electric and a magnetic component (electric field and magnetic field), and it is often convenient to express the intensity of the RF environment at a given location in terms of units specific to each component. For example, the unit "volts per meter" (V/m) is used to express the strength of the electric field (electric "field strength"), and the unit "amperes per meter" (A/m) is used to express the strength of the magnetic field (magnetic "field strength"). Another commonly used unit for characterizing the total electromagnetic field is "power density." Power density is most appropriately used when the point of measurement is far enough away from an antenna to be located in the "far-field" zone of the antenna.
Photometry (astronomy)
Historically, photometry in the near-infrared through short-wavelength ultra-violet was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light onto a photosensitive cell. These have largely been replaced with CCD cameras that can simultaneously image multiple objects, although photoelectric photometers are still used in special situations, such as where fine time resolution is required.
National Physical Laboratory of India
The standard unit of length, metre, is realized by employing a stabilized Helium - Neon laser as a source of light. Its frequency is measured experimentally. From this value of frequency and the internationally accepted value of the speed of light (299,792,458 metres/second), the wavelength is determined using the relation:
Ambiguity
The term "intensity" is ambiguous when applied to light. The term can refer to any of irradiance, luminous intensity, radiant intensity, or radiance, depending on the background of the person using the term.
Irradiance
In radiometry, irradiance is the radiant flux (power) "received" by a "surface" per unit area. The SI unit of irradiance is the watt per square metre (). The CGS unit erg per square centimetre per second () is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity.
Daily light integral
The daily light integral (DLI) is a function of photosynthetic light intensity and duration (day) and is usually expressed as moles of light (mol) per square meter (m) per day (d), or: mol·m·d. In other words, DLI measures the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day.
Photometry (optics)
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.
Metric system
The International System of Units is the modern metric system. It is based on the Metre-Kilogram-Second-Ampere (MKSA) system of units from early in the 20th century. It also includes numerous coherent derived units for common quantities like power (watt) and irradience (lumen). Electrical units were taken from the International system then in use. Other units like those for energy (joule) were modeled on those from the older CGS system, but scaled to be coherent with MKSA units. Two additional base units, "degree Kelvin" equivalent to degree Centigrade for thermodynamic temperature, and "candela", roughly equivalent to the international candle unit of illumination, were introduced. Later, another base unit, the "mole", a unit of mass equivalent to Avogadro's number of specified molecules, was added along with several other derived units.
Eborac Island Light
The current light characteristic is two flashes, separated by two seconds, every 10 seconds, colored white, red or green depending on direction (Fl.(2)W.R.G. 10s). White, visible for , is shown at 267°30′-281°, 288°30′-000° and 135°-252°. Green, visible for , is shown at 252°30′-267°30′ and 000°-135°. Red, visible for , is shown at the small middle sector, 281°-288°30′.
The apparatus is a Chance Brothers 400 mm focal length Fresnel lens. The light source is a solar powered 12 Volt 35 Watt Halogen lamp and the intensity is 
3,700 cd for the white light and 700 cd for the red and green ones.
Speed of light
In 2011, the CGPM stated its intention to redefine all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. It proposed a new, but completely equivalent, wording of the metre's definition: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly when it is expressed in the SI unit ." This is one of the proposed changes to be incorporated in the next revision of the SI also termed the "New SI".
Lux
The lux is one lumen per square metre (lm/m), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m). There is no single conversion factor between lx and W/m; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.
Luminous intensity
Like other SI base units, the candela has an operational definition—it is defined by the description of a physical process that will produce one candela of luminous intensity. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 watts per steradian in a given direction, that light source will emit one candela in the specified direction.
Glossmeter
The glossmeter provides a quantifiable way of measuring gloss intensity ensuring consistency of measurement by defining the precise illumination and viewing conditions. The configuration of both illumination source and observation reception angles allows measurement over a small range of the overall reflection angle. 
The measurement results of a glossmeter are related to the amount of reflected light from a black glass standard with a defined refractive index. The ratio of reflected to incident light for the specimen, compared to the ratio for the gloss standard, is recorded as gloss units (GU).
Absolute magnitude
Absolute magnitude is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared on a magnitude scale. As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength ranges corresponding to specified filter bands or passbands; for stars a commonly quoted absolute magnitude is the absolute visual magnitude, which uses the visual (V) band of the spectrum (in the UBV photometric system). Absolute magnitudes are denoted by a capital M, with a subscript representing the filter band used for measurement, such as M for absolute magnitude in the V band.
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m). A non-SI term for the same unit is the nit. The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m.
Ultraviolet–visible spectroscopy
The instrument used in ultraviolet–visible spectroscopy is called a UV/Vis spectrophotometer. It measures the intensity of light passing through a sample (formula_3), and compares it to the intensity of light before it passes through the sample (formula_6). The ratio formula_7 is called the "transmittance", and is usually expressed as a percentage (%T). The absorbance, formula_8, is based on the transmittance:
Luminous efficacy
Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source.
Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a source or overall luminous efficacy.
Multiangle light scattering
Measurements were generally expressed as scattered intensities or scattered irradiance. Since the collection of data was made as the detector was placed at different locations on the arc, each position corresponding to a different scattering angle, the concept of placing a separate detector at each angular location of interest was well understood, though not implemented commercially until the late 1970s. Multiple detectors having different quantum efficiency have different response and hence needs to be normalized in this scheme. An interesting system based upon the use of high speed film was developed by Brunsting and Mullaney in 1974. It permitted the entire range of scattered intensities to be recorded on the film with a subsequent densitometer scan providing the relative scattered intensities. The then-conventional use of a single detector rotated about an illuminated sample with intensities collected at specific angles was called "differential light scattering" after the quantum mechanical term "differential cross section", "σ(θ)" expressed in milli-barns/steradian. Differential cross section measurements were commonly made, for example, to study the structure of the atomic nucleus by scattering from them nucleons, such as neutrons. It is important to distinguish between differential light scattering and dynamic light scattering, both of which are referred to by the initials DLS. The latter refers to a technique that is quite different, measuring the fluctuation of scattered light due to constructive and destructive interference, the frequency being linked to the thermal motion, Brownian motion of the molecules or particles in solution or suspension.
Magnitude (astronomy)
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
Apparent magnitude
Magnitude is complicated by the fact that light is not monochromatic. The sensitivity of a light detector varies according to the wavelength of the light, and the way it varies depends on the type of light detector. For this reason, it is necessary to specify how the magnitude is measured for the value to be meaningful. For this purpose the UBV system is widely used, in which the magnitude is measured in three different wavelength bands: U (centred at about 350 nm, in the near ultraviolet), B (about 435 nm, in the blue region) and V (about 555 nm, in the middle of the human visual range in daylight). The V band was chosen for spectral purposes and gives magnitudes closely corresponding to those seen by the light-adapted human eye, and when an apparent magnitude is given without any further qualification, it is usually the V magnitude that is meant, more or less the same as visual magnitude.
Luminous intensity
The luminous intensity for monochromatic light of a particular wavelength λ is given by
Metric system
The standard metre is defined as exactly 1/299,792,458 of the distance that light travels in a second. The realisation of the metre depends in turn on precise realisation of the second. There are both astronomical observation methods and laboratory measurement methods that are used to realise units of the standard metre.
Because the speed of light is now exactly defined in terms of the metre, more precise measurement of the speed of light does not result in a more accurate figure for its velocity in standard units, but rather a more accurate definition of the metre. The accuracy of the measured speed of light is considered to be within 1 m/s,
and the realisation of the metre is within about 3 parts in 1,000,000,000, or an order of 10 parts.
List of unusual units of measurement
It is common in particle physics, where mass and energy are often interchanged, to use eV/c, where eV (electronvolt) is the kinetic energy of an electron accelerated over one volt ( joules), c is the speed of light in a vacuum (from E = mc). This definition is intuitive for a linear particle accelerator when accelerating electrons.
Turbidity
The most widely used measurement unit for turbidity is the Formazin Turbidity Unit (FTU). ISO refers to its units as FNU (Formazin Nephelometric Units). ISO 7027 provides the method in water quality for the determination of turbidity. It is used to determine the concentration of suspended particles in a sample of water by measuring the incident light scattered at right angles from the sample. The scattered light is captured by a photodiode, which produces an electronic signal that is converted to a turbidity. Open source hardware has been developed following the ISO 7027 method to measure turbidity reliably using an Arduino microcontroller and inexpensive LEDs.
Foot-candle
A foot-candle (sometimes foot candle; abbreviated fc, lm/ft, or sometimes ft-c) is a non-SI unit of illuminance or light intensity. One foot-candle represents "the illuminance cast on a surface by a one-candela source one foot away." This unit is commonly used in lighting layouts in parts of the world where SAE units are used.
Radiant intensity
Radiant intensity, denoted "I" ("e" for "energetic", to avoid confusion with photometric quantities, and "Ω" to indicate this is a "directional" quantity), is defined as
where
Luminance (disambiguation)
Luminance is a photometric measure of the density of luminous intensity in a given direction, measured in candela per square metre (cd/m).
Bicycle lighting
The candela (cd) is the SI unit of luminous intensity, that is power per unit solid angle in a given direction, weighted according to the sensitivity of the human eye to various colours of light. A typical candle produces light with about 1 candela of luminous intensity in all directions. A lamp can produce higher luminous intensity either by producing more light, or by focusing it tighter. The luminous intensity of a light depends on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens. Despite its complexity, it is a more useful measure than watts, because it defines how much usable light is shed in a given place: a dynamo headlight designed for road use and focused for seeing the road makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics.
Hefner lamp
The Hefner lamp provided the German, Austrian, and Scandinavian standard for luminosity during the late nineteenth and early twentieth centuries. The unit of light intensity was defined as that produced by the lamp burning amyl acetate with a 40 mm flame height. The light unit was adopted by the German gas industry in 1890 and known as the "Hefnereinheit". In 1897 it was also adopted by the Association of German Electrical Engineers under the name "Hefnerkerze" (HK).
Lux
The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt 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 as both the singular and plural form.
Flashlight
The FL1 standard requires measurements reported on the packaging to be made with the type of batteries packaged with the flashlight, or with an identified type of battery. Initial light output is measured with an integrating sphere photometer, 30 seconds after the light is switched on with fresh (or newly charged) batteries. The total light emitted is reported in lumens. Luminous intensity is determined by measuring the brightest spot in the beam produced by the flashlight, in candelas. Since this is a measure of all the light emitted in a solid angle (the "cone" of light in a particular direction), the beam intensity is independent of distance.
Spectral flux density
In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light in terms of the brightness to the eye, or photons. In SI units it is measured in W m, although it can be more practical to use W m nm (1 W m nm = 1 GW m = 1 W mm) or W m μm (1 W m μm = 1 MW m), W·m·Hz, Jansky or solar flux units. The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density.
Wave interference
Because the frequency of light waves (~10 Hz) is too high to be detected by currently available detectors, it is possible to observe only the intensity of an optical interference pattern. The intensity of the light at a given point is proportional to the square of the average amplitude of the wave. This can be expressed mathematically as follows. The displacement of the two waves at a point is:
Candela
It became clear that a better-defined unit was needed. Jules Violle had proposed a standard based on the light emitted by 1 cm of platinum at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a black body) effect, and was thus independent of the construction of the device. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared.
Monochromator
An absorption spectrophotometer measures the absorption of light by a sample as a function of wavelength. Sometimes the result is expressed as percent transmission and sometimes it is expressed as the inverse logarithm of the transmission. The Beer–Lambert law relates the absorption of light to the concentration of the light-absorbing material, the optical path length, and an intrinsic property of the material called molar absorptivity. According to this relation the decrease in intensity is exponential in concentration and path length. The decrease is linear in these quantities when the inverse logarithm of transmission is used. The old nomenclature for this value was optical density (OD), current nomenclature is absorbance units (AU). One AU is a tenfold reduction in light intensity. Six AU is a millionfold reduction.
Astronomical unit
In 1983, the International Committee for Weights and Measures (CIPM) modified the International System of Units (SI, or "modern" metric system) to make the metre defined as the distance travelled in a vacuum by light in 1/299792458 second. This replaced the previous definition, valid between 1960 and 1983, which was that the metre equalled a certain number of wavelengths of a certain emission line of krypton-86. (The reason for the change was an improved method of measuring the speed of light.) The speed of light could then be expressed exactly as "c" = , a standard also adopted by the IERS numerical standards. From this definition and the 2009 IAU standard, the time for light to traverse an AU is found to be τ = , more than 8 minutes. By multiplication, the best IAU 2009 estimate was "A" = "c"τ = , based on a comparison of JPL and IAA–RAS ephemerides.
Inductively coupled plasma atomic emission spectroscopy
Within the optical chamber(s), after the light is separated into its different wavelengths (colours), the light intensity is measured with a photomultiplier tube or tubes physically positioned to "view" the specific wavelength(s) for each element line involved, or, in more modern units, the separated colors fall upon an array of semiconductor photodetectors such as charge coupled devices (CCDs). In units using these detector arrays, the intensities of all wavelengths (within the system's range) can be measured simultaneously, allowing the instrument to analyze for every element to which the unit is sensitive all at once. Thus, samples can be analyzed very quickly.
Squeezed states of light
Squeezed light can be used in radiometry to calibrate the quantum efficiency of photo-electric photo detectors without a lamp of calibrated radiance. Here, the term photo detector refers to a device that measures the power of a bright beam, typically in the range from a few microwatts up to about 0.1 W. The typical example is a PIN photo diode. In case of perfect quantum efficiency (100%), such a detector is supposed to convert every photon energy of incident light into exactly one photo electron. Conventional techniques of measuring quantum efficiencies require the knowledge of how many photons hit the surface of the photo detector, i.e. they require a lamp of calibrated radiance. The calibration on the basis of squeezed states of light uses instead the effect, that the uncertainty product formula_40 increases the smaller the quantum uncertainty of the detector is. In other words: The squeezed light method uses the fact that squeezed states of light are sensitive against decoherence. Without any decoherence during generation, propagation and detection of squeezed light, the uncertainty product has its minimum value of 1/16 (see above). If optical loss is the dominating decoherence effect, which usually is the case, the independent measurement of all optical losses during generation and propagation together with the value of the uncertainty product directly reveals the quantum uncertainty of the photo detectors used.
When a squeezed state with squeezed variance formula_41 is detected with a photo detector of quantum efficiency formula_42 (with formula_43), the actually observed variance is increased to 
Optical loss mixes a portion of the vacuum state variance to the squeezed variance, which decreases the squeeze factor. The same equation also describes the influence of a non-perfect quantum efficiency on the anti-squeezed variance. The anti-squeezed variance reduces, however, the uncertainty product increases. Optical loss on a pure squeezed state produces a mixed squeezed state.
Planck constant
Prior to Einstein's paper, electromagnetic radiation such as visible light was considered to behave as a wave: hence the use of the terms "frequency" and "wavelength" to characterise different types of radiation. The energy transferred by a wave in a given time is called its intensity. The light from a theatre spotlight is more "intense" than the light from a domestic lightbulb; that is to say that the spotlight gives out more energy per unit time and per unit space (and hence consumes more electricity) than the ordinary bulb, even though the colour of the light might be very similar. Other waves, such as sound or the waves crashing against a seafront, also have their own intensity. However, the energy account of the photoelectric effect didn't seem to agree with the wave description of light.
Candlepower
Candlepower (abbreviated as cp or CP) is an obsolete unit of measurement for luminous intensity, equal to 0.981 candelas. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. In modern usage, "candlepower" equates directly to the unit of luminous intensity, candela.