photometry (redirected from photometrist)

photometry (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 black body 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.

---

photometry

Precision measurement of the brightness, colour, and spectrum of stars and other celestial objects to obtain data on their structure, temperature, and composition. About 130 BC Hipparchus used a system that divided the stars into six magnitudes, from brightest to faintest. Beginning in the 17th century, use of the telescope led to the discovery of many fainter stars, and the scale was extended. The use of photographic and, since the 1940s, photoelectric equipment has vastly extended the sensitivity and wavelength range of astronomical photometry. The main (UBVRI) classification system uses wave bands in the ultraviolet, blue, visual, red, and infrared ranges. More elaborate systems can distinguish giant and dwarf stars, detect metals in stars, and determine surface gravity.

---

photometry

1. the measurement of the intensity of light

2. the branch of physics concerned with such measurements

---

Photometry

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 K_m, is the number of lumens per watt at the maximum of the V(λ) function. K_m 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, K_m was a derived, rather than defined, quantity. See Light, Physical measurement, Units of measurement