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Photometry is the science of the
measurement Measurement is the quantification (science), quantification of variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependen ...

measurement
of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
, in terms of its ''perceived''
brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

brightness
to the
human eye The human eye is a that reacts to and allows . and in the are photoreceptive cells which are able to detect and convey this information to the . Eyes signal information which is used by the brain to elicit the perception of colour, shape, ...

human eye
. It is distinct from
radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the ...
, which is the science of measurement of
radiant energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
(including light) in terms of absolute power. In modern photometry, the radiant power at each wavelength is weighted by a
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way.


Photometry and the eye

The
human eye The human eye is a that reacts to and allows . and in the are photoreceptive cells which are able to detect and convey this information to the . Eyes signal information which is used by the brain to elicit the perception of colour, shape, ...

human eye
is not equally sensitive to all
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

wavelength
s of
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
. Photometry attempts to account for this by weighing the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength. The standardized model of the eye's response to light as a function of wavelength is given by the luminosity function. The eye has different responses as a function of wavelength when it is adapted to light conditions (
photopic vision Photopic vision is the vision of the human eye, eye under well-lit conditions (luminance level 10 to 108 Candela per square metre, cd/m2). In humans and many other animals, photopic vision allows color vision, color perception, mediated by cone cel ...
) and dark conditions (
scotopic vision Scotopic vision is the vision of the eye under low-light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usual ...
). Photometry is typically based on the eye's photopic response, and so photometric measurements may not accurately indicate the perceived brightness of sources in dim lighting conditions where colors are not discernible, such as under just moonlight or starlight. Photopic vision is characteristic of the eye's response at luminance levels over three candela per square metre. Scotopic vision occurs below 2 × 10−5 cd/m2.
Mesopic vision Mesopic vision is a combination of photopic vision Photopic vision is the vision of the human eye, eye under well-lit conditions (luminance level 10 to 108 Candela per square metre, cd/m2). In humans and many other animals, photopic vision allows c ...
occurs between these limits and is not well characterised for spectral response.


Photometric quantities

Measurement of the effects of electromagnetic radiation became a field of study as early as the end of 18th century. Measurement techniques varied depending on the effects under study and gave rise to different nomenclature. The total heating effect of
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

infrared
radiation as measured by thermometers led to development of radiometric units in terms of total energy and power. Use of the human eye as a detector led to photometric units, weighted by the eye's response characteristic. Study of the chemical effects of
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

ultraviolet
radiation led to characterization by the total dose or actinometric units expressed in photons per second. 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
candela The candela ( or ; symbol: cd) is the of in the (SI); that is, luminous power per unit emitted by a point light source in a particular direction. Luminous intensity is analogous to , but instead of simply adding up the contributions of ever ...

candela
s, 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. For example, offices are typically "brightly" illuminated by an array of many recessed fluorescent lights for a combined high luminous flux. A
laser pointer A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode file:Diode laser.jpg, A packaged laser diode shown with a penny for scale file:Laser diode chip.jpg, The laser diode chip remove ...

laser pointer
has very low luminous flux (it could not illuminate a room) but is blindingly bright in one direction (high luminous intensity in that direction).


Photometric versus 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 Luminance is a measure of the per of travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given . is the for the ''subjective'' ...

Luminance
(photometric) and
radiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of meas ...
(radiometric) *
Luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

Luminous flux
(photometric) and
radiant flux In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
(radiometric) *
Luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...
(photometric) and
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
(radiometric) In photometric quantities every wavelength is weighted according to how sensitive the human eye is to it, 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 greater luminous flux than a red source with the same radiant flux would. Radiant energy outside the visible spectrum does not contribute to photometric quantities at all, so for example a 1000
watt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equa ...

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 versus lumens

Watts are units of radiant flux while lumens are units of luminous flux. A comparison of the watt and the lumen illustrates the distinction between radiometric and photometric units. The watt is a unit of power. We are accustomed to thinking of light bulbs in terms of power in watts. This power is not a measure of the amount of light output, but rather indicates how much energy the bulb will use. Because
incandescent bulb image of the tungsten Tungsten, or wolfram, is a chemical element with the Symbol (chemistry), symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identi ...
s sold for "general service" all have fairly similar characteristics (same spectral power distribution), power consumption provides a rough guide to the light output of incandescent bulbs. Watts can also be a direct 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, mostly in the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

infrared
. 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. A can provide light comparable to a 60 watt incandescent while consuming as little as 15 watts of electricity. 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 The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

steradian
by a
point source A point source is a single identifiable ''localised'' source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can us ...
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
nanometre file:EM Spectrum Properties edit.svg, 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its derived scales. The nanometre is often used to express dimensions on an atomic scale and mostly in the Mo ...
s, 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 can 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 of light are weighted according to a function called the "photopic spectral luminous efficiency." According to this function, 700 nm red light is only about 0.4% as efficient as 555 nm green light. Thus, one watt of 700 nm red light is "worth" only 2.7 lumens. Because of the summation over the visual portion of the EM spectrum that is part of this weighting, the unit of "lumen" is color-blind: there is no way to tell what color a lumen will appear. This is equivalent to evaluating groceries by number of bags: there is no information about the specific content, just a number that refers to the total weighted quantity.


Photometric measurement techniques

Photometric measurement is based on
photodetector Photodetectors, also called photosensors, are sensors A sensor is a device that produces an output signal for the purpose of sensing of a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsyste ...
s, devices (of several types) that produce an electric signal when exposed to light. Simple applications of this technology include switching luminaires on and off based on ambient light conditions, and light meters, used to measure the total amount of light incident on a point. More complex forms of photometric measurement are used frequently within the lighting industry. Spherical
photometer A photometer is an instrument that measures the strength of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is t ...

photometer
s can be used to measure the directional luminous flux produced by lamps, and consist of a large-diameter globe with a lamp mounted at its center. A
photocell Photodetectors, also called photosensors, are sensors In the broadest definition, a sensor is a device, module, machine, or subsystem whose purpose is to detect events or changes in its environment and send the information to other electro ...

photocell
rotates about the lamp in three axes, measuring the output of the lamp from all sides. Lamps and lighting fixtures are tested using
goniophotometer A Goniophotometer is a device used for measurement of the light emitted from an object at different angles. The use of goniophotometers has been increasing in recent years with the introduction of LED-light sources, which are mostly directed light ...
s and rotating mirror photometers, which keep the photocell stationary at a sufficient distance that the luminaire can be considered a point source. Rotating mirror photometers use a motorized system of mirrors to reflect light emanating from the luminaire in all directions to the distant photocell; goniophotometers use a rotating 2-axis table to change the orientation of the luminaire with respect to the photocell. In either case, luminous intensity is tabulated from this data and used in lighting design.


Non-SI photometry units


Luminance

* Footlambert * Millilambert * Stilb


Illuminance

*
Foot-candle A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area Area is the quantity that expresse ...
*
Phot A phot (ph) is a photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an ...


See also

*
List of light sources This is a list of sources of light. Light sources include light bulbs and stars like the Sun. Reflectors (such as the moon, cat's eyes, and mirrors) do not actually produce the light that comes from them. Incandescence Incandescence is the emiss ...
*
Photometria ''Photometria'' is a book on the measurement of light by Johann Heinrich Lambert Johann Heinrich Lambert (, ''Jean-Henri Lambert'' in French language, French; 26 or 28 August 1728 – 25 September 1777) was a Switzerland, Swiss polymath who made im ...
*
Photometry (astronomy) Photometry, from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
*
Radiometer 210px, An example of a Crookes radiometer. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. A radiometer or roentgenometer is a device ...

Radiometer
*
Reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...
*
Spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon ...

Spectrometer
*
Colorimetry Colorimetry is "the science and technology used to quantify and describe physically the human ". It is similar to , but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the and re ...


Notes


References


External links


Photometry
(nist.gov) (archived)

Professor Jim Palmer's Radiometry FAQ page (University of Arizona) (archived)

— Java executable JAR {{DEFAULTSORT:Photometry (Optics) Lighting