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(fōtŏm`ətrē), branch of physics dealing with the measurement of the intensity of a source of light, such as an electric lamp, and with the intensity of light such a source may cast on a surface area.

Photometric Units of Measurement

The intensity of electric lights is commonly given as so many candlepower, i.e., so many times the intensity of a standard candle. Since an ordinary candle is not a sufficiently accurate standard, the unit of intensity has been defined in various ways. It was originally defined as the luminous intensity in a horizontal direction of a candle of specified size burning at a specified rate. Later the international candle was taken as a standard; not actually a candle, it is defined in terms of the luminous intensity of a specified array of carbon-filament lamps. In 1948 a new candle, about 1.9% smaller than the former unit, was adopted. It is defined as 1-60 of the intensity of one square centimeter of a blackbodyblackbody,
in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, crudely approximates an ideal blackbody; a material consisting of a
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 radiator at the temperature at which platinum solidifies (2,046°K;). This unit is sometimes called the new international candle; the official name given to it by the International Commission of Illumination (CIE) is candela.

Other quantities of importance in photometry include luminous flux, surface brightness (for a diffuse rather than point source), and surface illumination. Luminous flux is the radiation given off in the visible range of wavelengths by a radiating source. It is measured in lumens, one lumen being equal to the luminous flux per unit solid angle (steradian) emitted by a unit candle. Surface brightness is measured in lamberts, one lambert being equal to an average intensity of 1/π candle per square centimeter of a radiating surface. The intensity of illumination, also called illuminance, is a measure of the degree to which a surface is illuminated and is thus distinguished from the intensity of the light source. Illumination is given in footcandles, i.e., so many times the illumination given by a standard candle at 1 ft. Another unit of illumination is the lux, one lux being equal to one lumen incident per square meter of illuminated surface. One lux equals 0.0929 footcandle.

Photometric Instruments

Instruments used for the measurement of light intensity, called photometers, make possible a comparison between an unknown intensity and a standard or known intensity. They are based on the inverse-square law, which states that as a light source is moved away from a surface it illuminates, the illumination decreases in an amount inversely proportional to the square of the distance. Thus the illumination of a surface by a source of light 2 ft away is 1-4 of the illumination at 1 ft from the source. Conversely, for two light sources, one at 1 ft from a surface and the other at 2 ft, to give the same illumination to the surface, it would be necessary for the source at 2 ft to have an intensity 4 times that of the source at 1 ft.

A photometer measures relative rather than absolute intensity. The Bunsen photometer (named for R. W. Bunsen) determines the light intensity of a source by comparison with a known, or standard, intensity. The two light sources (one of known, one of unknown intensity) are placed on opposite sides of the surface (a disk of paper) to be illuminated. In the center of this surface is a grease spot that, when illuminated equally from both sides, will appear neither lighter nor darker than the paper but will become almost invisible. Using the inverse-square law, the intensity of the unknown light source can be easily determined when the relative distances at which the two sources produce equal illumination are known. The Rumford photometer (named for Count Rumford), or shadow photometer, compares intensities of light sources by the density of the shadows produced. In the Lummer-Brodhun photometer, an opaque screen is placed between the two sources, and a comparison is made possible by an ingenious arrangement of prisms.


See E. Budding and O. Demircan, Introduction to Astronomical Photometry (2007); E. F. Milone and C. Sterken, Astronomical Photometry: Past, Present, and Future (2011).


That branch of science which deals with measurements of light (visible electromagnetic radiation) according to its ability to produce visual sensation. Specifically, photometry deals with the attribute of light that is perceived as intensity, while the related attribute of light that is perceived as color is treated in colorimetry. See Color

The purely physical attributes of light such as energy content and spectral distribution are treated in radiometry. Sometimes the word photometry is used to denote measurements that have nothing to do with human vision, but this is a mistake according to modern usage. Such measurements are properly referred to as radiometry, even if they are performed in the visible spectral region. See Radiometry

The relative visibility of a fixed power level of monochromatic electromagnetic radiation varies with wavelength over the visible spectral region (400–700 nanometers). The relative visibility of radiation also depends upon the illumination level that is being observed. The cone cells in the retina determine the visual response at high levels of illumination, while the rod cells dominate in the dark-adapted eye at very low levels (such as starlight). Cone-controlled vision is called photopic, and rod-controlled vision is called scotopic, while the intermediate region where both rods and cones play a role is called mesopic.

Originally, photometry was carried out by using the human visual sense as the detector of light. As a result, photometric measurements were subjective. In order to put photometric measurements on an objective basis, and to allow convenient electronic detectors to replace the eye in photometric measurements, the Commission Internationale de l'Eclairage (CIE; International Commission on Illumination) has adopted two relative visibility functions as standards. These internationally accepted functions are called the spectral luminous efficiency functions for photopic and scotopic vision, and are denoted by V(λ) and V(λ), respectively. See Luminous efficiency

Thus photopic and scotopic (but not mesopic) photometric quantities have objective definitions, just as do the purely physical quantities. However, there is a difference. The purely physical quantities are defined in terms of physical laws, whereas the photometric quantities are defined by convention. In recognition of this difference the photometric quantities are called psychophysical quantities.

According to the International System of Units, SI, the photometric units are related to the purely physical units through a defined constant called the maximum spectral luminous efficacy. This quantity, which is denoted by Km, is the number of lumens per watt at the maximum of the V(λ) function. Km is defined in SI to be 683 lm/W for monochromatic radiation whose wavelength is 555 nanometers, and this defines the photometric units with which the photometric quantities are to be measured.

At various times, the photometric units have been defined in terms of the light from different standard sources, such as candles made according to specified procedures, and blackbodies at the freezing point of platinum. According to these definitions, Km was a derived, rather than defined, quantity. See Light, Physical measurement, Units of measurement


(foh-tom -ĕ-tree) The measurement of the brightness or intensity of a source of light or other electromagnetic radiation. In astronomy, photometric measurements are taken over particular ranges of frequency or wavelength (these ranges are called wavebands) and take account of how brightness varies with both waveband and time. Various properties of the source, including temperature, can then be inferred. At optical, infrared, and near-ultraviolet wavelengths, astronomers measure brightness or luminous intensity in terms of apparent magnitude. At other wavelengths measurements of flux density are made.

In some cases the eye can make comparisons, with a fair degree of accuracy (to 0.1 magnitude) between the known brightness of a reference star and the unknown brightness of the source under study. The greater accuracies now required in astronomy, however, are achieved with electronic instruments (photometers) or specially prepared photographic emulsions, leading to photoelectric or photographic photometry. A photometric system defines standard wavebands and sets of standard sources, measured with these wavebands, that are well-distributed around the sky. Different systems define different wavebands.

Photoelectric broadband systems, such as the UBV system (see magnitude), use a combination of a suitable filter and a device such as a CCD or photomultiplier, to convert the radiation selected by the filter into an equivalent electrical signal, which is then amplified and measured. The U, B, and V magnitudes are determined over three relatively broad wavebands in the ultraviolet, blue, and yellow-green (visual) spectral regions. Magnitudes can also be measured for various infrared bands. In narrow-band photometry, narrower wavebands are used, as in the uvby system. Extremely narrow bands, now obtainable over the entire optical and infrared regions, can be studied by means of interference filters: the distribution of, say, hydrogen in a source can be determined by isolating a particular spectral line of hydrogen.

Photoelectric photometry can be used to study an isolated star, region, etc., or with the latest detector arrays a narrow angle of sky containing several stars. In photographic photometry, measurements can be made of objects on a photographic image of a wide angle of sky; the photographs are taken at wavelengths varying from near-infrared to near-ultraviolet (depending on emulsion). Highly resolved features can be registered, often of much greater quality than can be obtained electronically. The image, for a given photographic exposure time, varies in density (and size) by an amount that, within relatively wide limits, depends on the original brightness. Densities can be measured accurately by means of microdensitometers, etc.


The science of measuring visible light in a luminaire, with units that are weighted according to the sensitivity of the human eye.



the branch of physical optics that deals with the energy characteristics of optical radiation emitted by sources, propagating in various media, or interacting with bodies. In photometry, the energy of electromagnetic waves in the optical range is averaged over short time intervals that are substantially longer than the period of the waves. Photometry encompasses the experimental methods and instruments used to measure photometric quantities as well as the theories and calculations relating to such quantities.

The basic energy concept in photometry is the radiation flux Φe, which is the average power transmitted by electromagnetic radiation. The spatial distribution of Φe is described by radiometric quantities, which are derivatives of the radiation flux with respect to area, solid angle, or both. In pulse photometry, integrals over time of photometric quantities are also used. In a narrow sense, photometry is sometimes defined as the measurement and calculation of quantities belonging to the most widely used system of photometric quantities, namely, the system of “visual” photometric quantities. Such quantities include illuminance, luminous intensity, luminance, luminous pulse emittance, and luminous exitance. The corresponding radiometric quantities include radiant flux density, radiant intensity, and radiance. The “visual” quantities are photometric quantities that are reduced in accordance with the spectral sensitivity of the average light-adapted human eye, the eye being the most important optical detector for human activity (seePHYSIOLOGICAL ADAPTATION and ). Other systems of reduced photometric quantities used include erythemal, bactericidal, and photosynthetic quantities. The study of the dependence of photometric quantities on radiation wavelength and on the spectral concentration of radiometric quantities is the subject of spectrophotometry and spectroradiometry, respectively. Photometric methods are widely used in astronomy to study celestial sources of radiation in various spectral regions (seeASTRONOMICAL PHOTOMETRY and COLOR INDEX). It is erroneous to restrict photometry only to measurements of “visual” photometric quantities.

The fundamental law for photometry E = I/l2 states that the illumination E varies inversely as the square of the distance I from a point source with a luminous intensity I; the law was formulated by J. Kepler in 1604. However, P. Bouguer should be regarded as the founder of experimental photometry. In 1729, Bouguer published a description of a visual method for the quantitative comparison of light sources, namely, the balancing of the luminances of adjacent surfaces by varying the distances to the sources and the use of the eye as the instrument. Up to the present time, the methods of visual photometry have been used in individual cases; as a result of the research of Soviet scientists who introduced the concept of equivalent brightness, the visual methods have been extended to low levels of brightness. Depending on the methods employed to measure photometric quantities, photometry may be classified as, for example, visual, photographic, photoelectric, or photochemical.

The development of theoretical photometric methods, which was initiated by J. Lambert, was generalized in the theory of light fields. The theory was systematized by the Soviet scientist A. A. Gershun in the 1930’s. Contemporary theoretical photometry has been extended to turbid media. Theoretical photometry is based on the relationship dΦe = LedG, which expresses the inverse-square law in differential form; here, Le is the radiance, dΦe is the differential of the radiation flux of a unit pencil of rays, and dG—the differential of the geometrical factor—is the measure of a set of rays. The photometric properties of substances and bodies are characterized by transmission factors τ, reflection coefficients ρ, and absorption factors α. For a particular body, the relationship of the transmission factor, reflection coefficient, and absorption factor is given by the expression τ + ρ + α = 1. The attenuation of the radiation flux of a narrow beam during propagation in a substance is described by the Bouguer-Lambert-Beer law.

Experimental photometric methods are based on absolute or relative measurements of a radiation flux by selective or nonselective radiation detectors, that is, by detectors whose response depends on or is independent of the radiation wavelength, respectively. To determine dimensional photometric quantities, either photometers that permit a direct comparison of unknown and known fluxes or photometers that are already calibrated in the units of the appropriate radiometric quantities or reduced photometric quantities are used. In particular, to assign values to “visual” quantities, standard and working standard lamps checked against state photometric standards are usually employed; such lamps are sources with known photometric properties. Photometry of laser radiation is based mainly on the use of standard and working standard nonselective radiation detectors that have been checked against state standards for the power and energy of laser radiation. The dimensionless quantities T and p are measured with photometers by using relative methods in which the ratio of the responses of a linear radiation detector to the respective fluxes is recorded. Methods are also employed in which the responses of a linear or nonlinear radiation detector are balanced by varying the radiation fluxes being compared; the fluxes are varied a known number of times in accordance with a specified law.

The theoretical and experimental methods of photometry are used in illuminating engineering and signal-system engineering, in astronomy and astrophysics, in the computation of radiative transfer in gas-discharge and stellar plasmas, in the chemical analysis of substances, in pyrometry, in calculations of radiative heat transfer, and in many other areas of science and industry.


Bouguer, P. Opticheskii traktat o gradatsii sveta. Moscow, 1950. (Translated from French.)
Gershun, A. A. Izbr. trudy po fotometrii i svetotekhnike. Moscow, 1958.
Meshkov, V. V. Osnovy svetotekhniki, parts 1–2. Moscow-Leningrad, 1957–61.
Tikhodeev, P. M. Svetovye izmereniia v svetotekhnike (Fotometriia), 2nd ed. Moscow-Leningrad, 1962.
Vol’kenshtein, A. A. Vizual’naia fotometriia malykh iarkostei. Moscow-Leningrad, 1965.
Sapozhnikov, R. A. Teoreticheskaia fotometriia, 2nd ed. Leningrad, 1967.
Gurevich, M. M. Vvedenie v fotometriiu. Leningrad, 1968.



The calculation and measurement of quantities describing light, such as luminous intensity, luminous flux, luminous flux density, light distribution, color, absorption factor, spectral distribution, and the reflectance and transmittance of light; sometimes taken to include measurement of near-infrared and near-ultraviolet radiation as well as visible light.


The measurement of quantities associated with light.


1. the measurement of the intensity of light
2. the branch of physics concerned with such measurements
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