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Structure of a photometer: lamp, slit , monochromator , sample, detector

A photometer or photometer is an instrument used to measure photometric quantities (see photometry ), e.g. B. the luminance (unit: cd / m²) or luminous intensity (unit: cd). In astronomy it is used to measure the brightness of celestial bodies . In analytical chemistry , UV / VIS spectroscopes are used to determine concentrations in solutions according to the Lambert-Beer law . In photography, the photometer is used as a light meter .

Lux meter

Lux meter with external measuring cell

The illuminance is measured with a lux meter. This value indicates how bright it is at the measuring point (regardless of the extent and direction of the light source). The illuminance is the incident luminous flux per illuminated area and is specified in the unit lux (formerly phot ).

Classic lux meter

As a measuring cell in classical Luxmeters a frequently used silicon - photodiode . In some devices, the measuring cell is also used to supply energy, so that these devices function without an additional energy source. Lux meters are used, for example, to measure the illuminance at workplaces or street lighting.

Smartphone as a lux meter

Smartphone as a lux meter. The light sensor is marked red

Most smartphones have a built-in light sensor that can be used to measure the illuminance. This is possible with some smartphones without an additional app, as the illustration shows.

You can also access the built-in light sensor of the smartphone via an app , for example with the free and open source app phyphox (physical phone experiments) , which can also be used to measure other physical parameters. As it is available at all times, a smartphone can also be used as a lux meter to optimize lighting levels in the home.

A high (absolute) accuracy of the illuminance measured with the smartphone should not be expected. The sensor does not yet seem to evaluate the incident radiation as it corresponds to the brightness sensitivity curve of the human eye for daytime vision . A smartphone is still quite suitable for the occasional comparison of illuminance levels from different light sources.

General photometer

Light measurement terms

Photometers have accuracies from a few percent to well below one percent (corresponds to 0.01 size classes). Brightness estimates by eye using the step method by Friedrich Argelander from the 19th century are five to ten percent accurate. The measuring principle of most photometers is based on the knowledge that the brightness at the measuring point is inversely proportional to the square of its distance from the light source.

The principle of the photometer

A constant source sends light through a cuvette (for example) with an aqueous measurement solution. Depending on the intensity of the color, part of the light is absorbed and the light passing through is measured in a light-sensitive cell. The amount of light that has passed through allows a clear determination of the measured water value, which can be read from already finished measurement tables, which concentration of the material to be measured corresponds to.

If the light of an optical image is to be measured with a highly sensitive, extensive radiation detector , such as a photocell or a photomultiplier , the beam path can be appropriately widened with the aid of a Fabry lens (according to Charles Fabry ).

Rumford's photometer

Rumford's photometer

According to Rumford, an opaque stick c is set up a short distance in front of a white wall, which casts two shadows d and e on the wall when it is illuminated by the two light sources to be compared.

If one now removes the stronger light source f from the wall until both shadows are equally dark, then, according to the above sentence, the light intensities of the two flames behave like the squares of their distances from the wall.

Ritchie's photometer

According to Wilhelm Ritchie , the light sources to be compared are used to illuminate the two sides of a prism p covered with white paper, which is located in a box blackened on the inside, the sides of which opposite the prism surfaces are provided with openings oo.

Through a tube p in the upper wall of the box one can see at the same time both sides of the prism r, which can be brought to the same brightness by shifting the light sources.

Bunsen's photometer

Bunsen's photometer

The Bunsen photometer or the fat spot photometer was much more precise and more frequently used for technical purposes in the 19th century .

It essentially consists of a paper umbrella with a grease stain made with wax or stearin in the middle. This appears light on a dark background if the screen is more strongly illuminated from the rear than from the front. When observing, one shifts the light sources until the spot on the front disappears. The device that carries the screen and the light sources to be compared, the so-called optical bench aa, is divided in such a way that the numbers that indicate the distances do not have to be squared first.

Desaga has given this apparatus the following form: At one end of the divided horizontal rail aa there is the flame b, which provides the standard for the comparison (the normal flame), at the other end the flame d to be tested. The gas meter c indicates the hourly gas consumption. A cylindrical housing can be moved on the split rail, the rear wall of which is completely opaque, while a diaphragm with the grease stain is located in the front wall. A small gas flame burns in the housing.

One approaches the same up to 20 cm of the normal flame and then regulates the small gas flame so that the grease stain facing the normal flame disappears. Then turn the housing 180 ° and, without changing the size of the small flame, move it closer to the flame to be tested until the grease stain on the diaphragm disappears again. The distance found gives the light intensity of the flame according to the well-known theorem.

In all photometric examinations, the walls of the room must reflect as little light as possible, which is why it is best to blacken them. If the flames are unevenly colored, the comparison with all photometers is more or less uncertain. The choice of the normal candle (for example Hefner candle ) also presents a great difficulty . As such it has in Germany usually wax - or stearin , in England Walratkerzen used; But there was so much disagreement about the size of the candles and the nature of the material that until the end of the 19th century all photometric investigations could hardly be compared with one another. Lamps presented greater rather than lesser difficulties and, moreover, did not provide a constant light.

The advances in electric lighting have created the need for a photometer that can measure the luminosity of an electric lamp by comparing it to a standard candle. With the early photometers, for example the Bunsen, one had to bring the strong light source an uncomfortably large distance from the screen in order to make the illumination of the screen by electric light the same as that by a normal candle.

Dispersion photometer

Ayrton and Perry caused in their scattering photometer weakening by a concave lens ( negative lens ); Otherwise the apparatus agrees with Rumford's photometer. Scattered by the concave lens, the rays of the electric lamp strike a white paper screen with approximately the same divergence as those of the normal candle and cast on it the shadow of a thin rod placed in front of it; the normal candle creates a second shadow of the stick.

If you make the brightness of the two shadows equal to each other, which is done by roughly setting the candle and finer setting the lens, the light intensity in normal candles can be read on the scale. The observer makes the shadows the same by first seeing through green, then through red glass. Since the electric light is whiter than the light of a candle because of its relatively larger content of more breakable rays, its luminosity as a whole is not directly comparable with that of the normal candle, but only the luminosity for certain colors; For example, the ratio of the luminosity for the breakable green rays is greater than for the less breakable red rays. By measuring these two different colors, one therefore also obtains a numerical expression for the quality of the light; the more different the luminosity for these two colors, the more the electric light surpasses the candlelight in terms of whiteness.

Bothes tangent photometer

In addition to the photometers described, a few other instruments should be mentioned which have some advantages. Bothes tangent photometer is very noteworthy , in which the comparison of the two light sources is also made by looking at a partially transparent strip of paper.

The light sources are not in a straight line, but send their rays below each other at right angles to the paper screen, which is irradiated diagonally by both. It is well known that the intensity of the illumination, apart from the distance from the light source, is dependent on the angle of incidence, namely it is proportional to the cosine of this angle. From this it follows that with the same strength and distance of the lights to be compared, the screen must halve the right angle of the rays coming from both sides in order to be equally brightly illuminated on both sides, as well as a rotation of the screen to one or the other side brings about a change on both sides at the same time without the need to change the distance of a light source.

If the light intensities are unequal, it must be possible to bring about the point by turning the screen where both lights have the same effect, and then the tangent of the angle read gives the ratio of the light intensities.

Dove used the microscope and gained the advantage of being able to compare both strong and weak light sources. The microscopic photograph of writing on glass appears dark on a light background when viewed through the microscope if the lighting is stronger from below than from above, but light on a dark background when the lighting is stronger from above than from below. If the lighting is the same, the writing disappears.

To compare the flames, they are removed from the mirror of the microscope until the constant lighting from above causes the writing to disappear; this gives the brightness ratio from the distance in a known manner. For transparently colored bodies, for example glasses, the opening in the table of the microscope is covered by these glasses from below until the compensation is obtained. In the same way, opaque bodies of different colors are compared by compensating for the light falling from them at an oblique incidence with that coming from above.

In order to determine the brightness of different parts of a room, the microscope, the mirror of which is directed towards the sky, is removed from the window until the upper and lower lighting is balanced. In order to weaken the illumination entering from below as desired, a Nicol's prism can be inserted under the object and a rotatable one at the back can be inserted into the eyepiece.

Wheatstones photometer

Wheatstones photometer

Wheatstone's photometer consists of a cylindrical brass canister about 5 cm in diameter; by means of the crank K the disc S can be set in rotation in such a way that the polished steel ball T attached to its edge describes a path.

Wheatstones photometer

If you now bring the instrument between two light sources, you can see two separate light curves when the crank is turned rapidly because of the after-effect of the light impression in the eye; the instrument is now removed from the stronger light source until both light curves appear equally strong, the distance between the light sources and the ball T is measured and the ratio of the light intensities is calculated in a known manner.

Babinets polarization photometer

Jacques Babinet used the polarization apparatus as a polarization photometer . The light sources to be compared are placed in such a way that the rays of one of them pass through inclined glass plates, the other of which is reflected by them in order to reach the observer's eye.

When a rock crystal and a calcite crystal are placed in front of the eye , the familiar colors of polarized light appear when the two illuminations are unequal. However, the colors disappear when both lights are made the same by shifting one light source appropriately. This photometer is important because it uses the very property of the eye to recognize color nuances.

Becquerel's polarization photometer

The Becquerel polarization photometer consists of two telescopes with a shared eyepiece, each with two Nicol prisms. If the light sources to be compared are brought in front of the lenses, the two halves of the field of view appear unequally illuminated. By turning one Nicol in the telescope directed towards the stronger light source, the two halves of the field of view are brought to the same brightness and the angle of rotation is read off on a partial circle. The cosine square of this angle then expresses the ratio of the intensities of the weaker and the stronger light source. The Berek slit photometer is a further development of the Becquerel polarization photometer.

Zöllner's astrophotometer

Zöllner's astrophotometer is best used to measure the brightness of the stars (see Astrophotometry ). The light from a flame falls through a round opening onto a biconcave lens, passes through this and three Nicol prisms as well as through a rock crystal plate and finally through a biconvex lens. The rays refracted by the latter fall on an inclined glass plate and are reflected by this.

The glass plate, however, is located in a telescope and allows the rays of a star falling into the lens to pass through, so that the image of the flame and the image of the star can now be seen side by side in the telescope. The front prisms, between which the rock crystal plate is located, can be rotated and allow the intensity of the artificial light to be changed as required.

The magnitude of the rotation is read on an arc, and it is therefore easy to compare the brightness of different stars with one another. Since the rotation of the foremost prism alone changes the color of the image of the artificial light produced in the telescope, one can also determine the colors of the stars and compare their light intensities with one another all the more reliably. Vierordt uses the spectroscope to measure and compare the strength of the colored light . The light from a kerosene lamp falls through a side tube with an adjustable gap onto the rear surface of the prism and is reflected by the observation tube.

The light of the gap is then weakened by placing smoked glasses in a known ratio, until the areas of the field of vision illuminated by the pure spectral colors can no longer be distinguished from the stripes illuminated by the weakened white and the spectral colors. The intensity relationships of the spectral colors result from the known degrees of darkening at which this occurs.

A method of measuring the chemical effect of light has been developed by Bunsen in collaboration with Roscoe to such an extent that it can be used for regular observations in meteorological observatories. It is based on the fact that, within very wide limits, the same products of light intensity and duration of insulation correspond to the same blackening on chlorine silver paper with the same sensitivity. The apparatus used for this essentially consists of a pendulum, which swings at intervals of about 3/4 of a second, and through its oscillations a small sheet of blackened mica is moved back and forth over a horizontal strip of paper impregnated with chlorine silver in such a way that the sheet alternates covers the paper and leaves it free again. The duration of exposure must be calculated for each point on the paper strip and the blackening achieved then gives the magnitude of the chemical effect.

The degree of coloration is determined with sodium light, which contains no chemical rays, and while looking for the place on the paper strip which shows the normal coloration, one can determine with the help of a table how long this part of the paper strip has been exposed. The unit of measurement is that light intensity which produces the normal color from the photographic normal paper in one second.

In Roscoe's simpler apparatus, a strip of paper, blackened in a pendulum photometer, then fixed and graduated according to a non-fixed strip, serves as a scale. A strip of normal photographic paper is now glued to the back of a tape with rubber, in which nine round holes are ejected one behind the other at one point so that the light can only affect the sensitive paper through the latter. The strip is pushed into a flat sheath of sheet brass, open at the top and bottom, with a round hole 10 mm in diameter on one side, which can be easily opened and closed with a slide. Under this hole there must be a hole in the insulation tape when observing, so that if the hole in the vagina is opened for a certain number of seconds, the sensitive paper takes on a certain color.

With very strong light, one would only be allowed to expose for a few seconds, thereby significantly increasing the error that arises from incorrectly reading the time. This can be avoided by rotating a perforated metal disc over the hole in such cases, thereby weakening the lighting effect. One can make nine observations in a row with one strip and then insert a new insulation tape into the vagina.

For this purpose one uses a bag of black silk, open on both sides, in which one can operate with one's hands and expose the sensitive paper without having to fear any change from the light. The blackening obtained can be read off with sodium light concentrated through a converging lens.

Electric photometer from Siemens

The electrical photometer from Siemens is based on the property of selenium that its electrical conductivity increases proportionally to the square roots of the luminous intensity when illuminated. The selenium melted between the windings of two flat wire spirals lying one inside the other is located in a kind of camera obscura , the lens of which collects the rays of the light source on the selenium preparation; the intensity of the light source is deduced from the magnitude of the resistance which it presents to a galvanic current conducted through it during irradiation.

Zollner's scale photometer

Zollner's scale photometer
Zöllner's scale photometer (average)

Zöllner used the radiometer ( light mill ) to construct his scale photometer . The radiometer cross b, which consists of four wings, is located in an empty glass vessel aa on a sufficiently strong cocoon thread . The wings of the same are made of mica, the surfaces of which are coated on one side with soot.

Such a cross always rotates in the same direction under the influence of both luminous and dark heat rays. The scale c consists of a circular paper cylinder, the circumference of which is divided into 100 parts. The index is located in front of a circular opening in a cylindrical movable brass capsule dd, the edge of which is supported by the protruding edge ee of the upper brass piece located underneath and can be easily rotated from the same.

Since the zero point of the scale only assumes a sufficiently fixed position after the instrument has been standing for a long time, the index must be able to move in order to correct the zero point. f is a thick-walled glass cylinder with a matt finish on both sides, which is used to diffuse light and absorb dark heat rays. It stands in a brass cylinder, which laterally has a circular opening g with a plate of frosted glass or matt glass, which can be easily closed by a lid. The instrument has a circular level on its head for vertical positioning. According to the laws of torsion, the number of scale divisions increases proportionally to the angle of rotation, although care must of course be taken that the scale does not rotate several times under the influence of sunlight. It is therefore absolutely necessary to always leave the instrument with the opening closed when it is not in use.

The scale photometer is also suitable for measuring the intensity of scattered daylight for photographic purposes. To use it in this way, the outer brass cylinder is replaced by a conical reflector, silver-plated on the inside, with an opening pointing upwards. If the instrument is then permanently set up in a location inaccessible to the sun, possibly outdoors under the protection of a glass bell attached above, it enables the exposure time to be reliably determined.

The temperature will presumably exert an influence on the sensitivity of the instrument, which, however, should practically be neglected for the fluctuations occurring in inhabited rooms. A thermometer is included with the instrument for precise measurements .

Astronomical photometer

Astronomical photometers are placed behind the eyepiece of a telescope or in the focus of the lens . The amount of radiation arriving from the object is i. A. Measured relatively - by comparison with a calibrated light source, the standard star .

Visual photometer
Visual photometer: the eye serves as a radiation receiver . You compare two neighboring light sources with each other. The artificial star is matched to the star to be measured by a gray filter .
Photoelectric photometer
Photons that strike a metal surface, can there when they are energetic enough to work function overcome the metal, detach electrons (see also Photoelectric Effect ). These electrons can e.g. B. be registered directly as current flow (in so-called vacuum photodiodes ), the electrical current is then directly proportional to the luminous flux. If the illuminance to be measured is very low, the signal can be amplified with a secondary electron multiplier (SEM). This combination of photocathode and SEV is called a photomultiplier . This system can be made so sensitive that it can be used to detect individual photons.
Photoelectric area photometer
Photoelectric area photometer enables the measurement of the areal distribution of radiation on the cathode by mapping onto a fluorescent screen or a storage disk; Image enhancement through intensifying screens is possible.
CCD particle detectors
CCD - detector : based on the internal photoelectric effect of a silicon Scheibchens , on the picture elements ( pixels ) with exposure electrons become free.
Thermoelectric photometer
Thermoelectric photometer: a blackened receiver surface is heated by the radiation; is also effective in the far infrared .
Plate photometer
A plate photometer measures the distribution of density on photo plates ; to calibrate using calibration stars .
Iris diaphragm photometer
Supplementation of the plate photometer with a variable aperture that regulates the light .

Web links

Commons : Photometer  - collection of images, videos and audio files


  • Zöllner: The scale photometer . Leipzig 1879.
  • Krüß: The electrotechnical photometry . Vienna 1886.
  • Manfred Horst: Electronic aids for film and photo. 1st edition, Franzis-Verlag, Munich, 1974, ISBN 3-7723-3371-0 .
  • Wilhelm Gerster: Modern lighting systems for inside and outside. 1st edition, Compact Verlag, Munich, 1997, ISBN 3-8174-2395-0 .
  • DIN 5032-1: Light measurement - Part 1: Photometric methods. Beuth Verlag, Berlin 1999.

Individual evidence

  1. Secret control codes. Android user, accessed October 19, 2018 . If you enter * # 0 * # on the numeric keypad of a Samsung Galaxy smartphone , an extended service menu opens. The current measured values ​​of all available sensors are displayed as an overview via the Sensor button . If you tap on the small gray area Light Sensor , the display opens exclusively for this sensor.
  2. Your smartphone is a mobile laboratory. 2. Physics Institute of RWTH Aachen University , accessed on October 19, 2018 .
  3. Luxmeter app vs. Measuring device: are smartphones suitable for measuring? Dial, accessed October 25, 2018 . If, in the future, a smartphone achieves the accuracy of illuminance meters, which, depending on the manufacturer and accuracy class, cost between 100 and 2,000 euros, this would put an industrial branch under pressure
  4. ^ Fabry-Linse , Wikibook Digital Imaging Methods , accessed on July 21, 2015.