Photometer

A photometer or photometer is an instrument for measuring photometric quantities, such as the luminance (unit: cd / m²) or light intensity (unit: cd). In astronomy, it is used for measuring brightness of the heavenly bodies. In analytical chemistry used to determine concentrations in solutions according to the Lambert -Beer law. In photography, the photometer is used as a light meter.

Here is the photometer the investigation of a light source or reflective surface in the foreground, while is measured by the light meter, how bright it is at the measuring point (regardless of extent and direction of the light source). Lux meters are used for example to measure the illuminance in workplaces or street lighting. With lux meters of the incident light flux Φ per unit area in the unit is lux (formerly phot ) detected. The measuring cell is often used when a light meter silicon photodiode. In some devices, the measuring cell is also used to supply energy, so that these devices work without additional energy source.

General photometer

Photometer have accuracies of a few percent to well below one percent ( equivalent to 0.01 magnitudes ). Brightness estimates with the eye to the scale method of Friedrich Argelander from the 19th century are five to ten percent accurate. The measuring principle of most of the photometer is based on knowing that the brightness of the measuring point is inversely proportional to the square of the distance from the light source.

The principle of the photometer

A constant source sends light through a cuvette (for example) with aqueous measurement solution. Depending on the intensity of coloring a part of the light is absorbed, and a light-sensitive cell, the transmitted light is measured. The amount of transmitted light allows for an unambiguous determination of the water measured value that can be read on the basis of already finished measuring tables, which corresponds to this concentration of the measured material.

Rumford's photometer

After Rumford one is at a short distance in front of a white wall, an opaque strips on c, which raises two shades of d and e on the wall, when it is illuminated by two light sources to be compared.

It now removes the stronger light source f as long as the wall until both are the same shade dark, so behave according to the theorem above, the luminous intensity of the flames as the squares of their distances from the wall.

Ritchie's photometer

By William Ritchie is illuminated with the light sources to be compared the two sides of a paper coated with white prism P which is located in an internally blackened box having the prism surfaces opposing sides are provided with openings oo.

P by a tube in the top wall of the box is overlooked at the same time the two sides of the prism, r, to be brought by displacement of the light sources of the same brightness.

Bunsen photometer

Much more accurate and more often for technical purposes in the 19th century was in use by the photometer or Bunsen grease spot photometer.

It consists essentially of a paper umbrella, a must with wax or stearic grease spot is in the middle. This appears bright on a dark background when the screen from the back is more enlightened than from the front. In the observation shifting the light sources until the stain on the front side disappears. The device that transmits the screen to be compared and the light source, the so called optical bench aa is divided so that one does not have the numbers indicating the first distance to raise the squared.

Desaga has given this apparatus the following form: the flame b, which provides the standard for comparison is located at one end of the split horizontal rail aa ( the normal flame ) at the other, however, the test flame d are the gas meter c the hourly gas consumption of. On the split rail a cylindrical housing is movable, its rear wall is completely opaque while there is a diaphragm with the grease spot in the front wall. In the case burns a small gas flame.

Is approached to the same 20 cm of the normal flame, and then regulates the small gas flame, so that the normal flame facing grease stain disappears. Then turn the housing 180 °, and, without changing the size of the small flame, approaching it to be inspected flame until the grease spot disappears on the diaphragm again. The distance in this case found results in accordance with the known rate the intensity of the flame.

In all photometric studies, the walls of the room must reflect as little light as possible, therefore they are blackened most beneficial. Are the flames colored unequal, the comparison at all photometers is more or less uncertain. A major difficulty also offers the choice of standard candle (eg, Hefner candle). As such, it has usually wax or stearin, used Walratkerzen in Germany in England; but it was so divided over the size of the candles and the nature of the material that by the end of the 19th century all photometric studies were only slightly comparable. Lamps offered rather more than less difficulties and also gave no constant light.

The progress of electric lighting have raised the need for a photometer that can measure the luminosity of an electric lamp, by comparison with a normal candle. In the early photometers, such as the Bunsen, you had to make the illumination of the screen by electric light equal to that of a normal candle, bring the strong light source in an uncomfortable large distance from the screen.

Scattering photometer

Ayrton and Perry resulted in their dispersion photometer the attenuation by a concave ( diverging lens ); in the rest of the apparatus is consistent with the Rumford 's photometer. Scattered through the concave lens, the rays of the electric lamp meet approximately the same divergence as those of normal candle on a white paper shade and design on him a shadow of a previously employed thin rod; the normal candle creates a second shadow of the stick.

If one makes the brightness of the shadow equal to each other, which is done by setting the candle coarse and finer adjustment of the lens, so the light intensity can be read in normal candles on the scale. The observer makes equal the shadow by only by green, then looks through red glass. Since the electric light namely by virtue of its relatively larger content of refrangible rays whiter than the light of a candle, so is not directly comparable to its luminosity as a whole with that of the normal candle, but only the luminosity of certain colors; example, it is the ratio of the luminosities for the breakable green rays greater than for the less refrangible red. By measuring these two different colors, therefore you also get a cipher regular expression for the quality of light; the electric light exceeds the candlelight all the more white, the different, the luminosities for these two colors.

Bothe tangent photometer

In addition to the described photometers are still some other instruments to mention, which have some advantages. Very noteworthy is Bothe's tangent photometer in which the comparison of the two light sources is also carried out by considering a partially transparent paper strip.

The light sources are, however, not in a straight line, but their beams under at right angles to the paper screen, which is irradiated obliquely from both. Is now well known, the intensity of the light, apart from the removal of the light source, depending on the incident angle, namely, it is proportional to the cosine of this angle. It follows that for the same thickness and distance of the lights to be compared, the screen must bisect the right angle of the coming of the two beams to be the same light illuminated on both sides, and that a rotation of the screen to one side or the other side change at the same time produces on both sides, without being necessary to change the distance of a light source.

If they are not the light levels so you have also by rotation of the screen may cause the point where both lights exert the same effect, and then the tangent of the angle read gives the ratio of the light intensities.

Dove used the microscope, thereby gaining the advantage of being able to compare both strong and weak light sources together. Microscopic photograph of a writing on glass would indeed appear when viewed through the microscope dark on a light background when the illumination from below more than from above, while light on a dark background when the lighting is stronger than from the top from the bottom. Given equality of lighting the font disappears.

For comparison of the flames they are removed from the mirror of the microscope, to the constant illumination from above causes the disappearance of the font; thus, the brightness ratio removal results in a known manner. For transparent colored body, such as glasses, the opening in the stage of the microscope is obscured by so these glasses from below so that the compensation is obtained. Similarly, opaque bodies of different colors are compared by the incident of them under oblique incidence of light is compensated by entering from above.

To determine the brightness of different points of a room, the microscope, the mirror is directed against the sky, so far away from the window until the balance of the upper and lower illumination is produced. In order to weaken the entering of down lighting as desired, one can use a Nicol prism at the object and a rear rotatable in the eyepiece.

Wheatstone's photometer

Wheatstone's photometer consists of a cylindrical brass sleeve of about 5 cm in diameter; by means of the crank K, the slice S are put into rotation so that the attached at its edge polished steel beads T describes a path.

If at this point the Instrumentchen between two light sources, one safeguarded in rapid revolution of the crank because of the after-effect of the light impression in mind two separate light curves; it now removes the Instrumentchen from the stronger light source until both light curves appear equally strong, measures the distance of the light sources from the beads T and calculates in a known manner, the ratio of light intensities.

Babinet polarization photometer

Jacques Babinet has brought the polariscope as a polarization photometer in use. Want to compare light sources are placed so that the rays of a pass through slanted glass plates, the others of which bounce off these to get into the eye of the observer.

Occur then, if before the eye a rock crystal and a calcite crystal are placed, the familiar colors of polarized light when the two lights are not equal. However, the colors disappear when both lights are made ​​equal by a suitable shift of a light source. This photometer is important because it uses just the one property of the eye to detect color shades.

Becquerel polarization photometer

The polarization photometer by Becquerel consists of two telescopes with communal eyepiece, in each of which two Nicol prisms are attached. Brings you to the comparative light sources in front of the lenses, so the two halves of the visual field appear enlightened unequal. By rotation of one Nicols in the telescope outwardly of the stronger light source to bring the two halves of the field of the same brightness, and reads the rotation angle from a portion of a circle. The raised cosine of this angle then pressed from the ratio of intensities of the weaker and the stronger light source. The Berek gap photometer is an evolution of Becquerelschen polarization photometer.

Publican Astro photometer

To measure the brightness of stars is the best publican Astro photometer ( see Astrophotometrie ). From a flame falls through a round opening the light on a biconcave lens, passes through these three and nicols and by a rock crystal plate and finally through a biconvex lens. The broken by the latter rays fall on an inclined glass plate and are reflected by it.

The glass plate but is located in a telescope and allows the falling into the lens rays of a star the passage, so that you can now side by side sees the image of the flame and the image of the star in the telescope. The front prisms between which the rock crystal plate is located, are rotatable and allow you to change the intensity of artificial light as desired.

The amount of rotation is read on a circular arc, and it is therefore easy for the brightness of different stars to compare them. Since the rotation of the front prism alone alters the color of the produced in the telescope image of artificial light, one can also determine the colors of the stars and compare their intensities more secure with each other. For the measurement and comparison of the intensity of the colored light is operated Vierordt of the spectroscope. The light of a kerosene lamp passes through a side, provided with an adjustable slit tube on the back surface of the prism and is reflected by the observation herein tube.

The light of the gap is then attenuated by prefixing of smoke glasses in known proportions, to the enlightened of the pure spectral locations of the visual field can no longer be distinguished from the same time enlightened by the attenuated White and the spectral strip. From the known degrees of obscuration at which this occurs, give the intensity ratios of the spectral colors.

A method to measure the chemical action of light, has been developed so far by Bunsen in communion with Roscoe, that they may serve in meteorological observatories to regular observations. It is based on the fact that within very wide limits same products of light intensity and correspond Insolationsdauer same blackening on silver chloride paper of the same sensitivity. The serving this purpose apparatus consists essentially of a pendulum which swings into periods of about 3 /4 second, and by the oscillations a leaflet blackened mica via a horizontal, impregnated with silver chloride strip of paper is passed back and forth that the leaflets alternately the paper covering and uncovering one since. The time duration of exposure must be calculated for each point of the paper strip, and the blackening obtained then gives the size of the chemical effect.

The degree of staining is determined in sodium light, which contains no chemical rays, and while the place you visits on the paper tape, which shows the fixed normal coloring, can be determined using a table, as long as this place the paper strip has been exposed. The unit of measurement that applies light intensity, which produces the normal coloring in one second from the photographic plain paper.

In Roscoe's simpler apparatus serves as a benchmark in the pendulum photometer blackened, then fixed and graded according to a non-fixed strips of paper. Then adhered to a strip of photographic paper with a normal rubber on the back side of a tape, in which one after the other along nine circular holes are discharged in a position so that the light may act on the latter only by the photosensitive paper. The strip is pushed into a top and bottom open flat sheath of brass plate, on one side there is a round hole diameter of 10 mm, which can be easily opened and closed by a slide. Under this hole must be in the observation of a hole Insolationsbandes are, so that when the hole in the vagina a certain number of seconds is opened, the sensitive paper receives a certain color.

With very strong light you would be allowed to expose only a few seconds and thus significantly increase the error arising from incorrect reading of the time. This avoids you by a perforated metal disc is allowed to rotate about the hole in such cases and thereby attenuates the light effect. One can use a strip to perform nine observations one after the other and then bring a new Insolationsband into the vagina.

To this end, use is made of an open on both sides of the bag of black silk, in which you operate with your hands and the delicate paper can expose, without fear of a change by the light. The blackening obtained was read off at a focused by a condenser lens sodium light.

Electric photometer from Siemens

The electric photometer from Siemens is based on the selenium property that its electrical conductivity approximately the square root of the light intensity increases proportionally by lighting. The fused between the turns of two flat, interlocking wire spirals lying selenium is in a kind of camera obscura, the lens collects the rays of the light source on the selenium preparation; from the value of the resistance which it presents a galvanic current passing therethrough during the irradiation, it is concluded that the intensity of the light source.

Publican scale photometer

Publican to the radiometer ( light mill ) has served for the construction of its scale photometer. In an evacuated glass vessel aa is located at a sufficiently strong cocoon thread, which consists of four wings radiometer cross b. Thereof the blades consist of mica, whose side surfaces are coated with carbon black.

Such cross rotates under the influence of both brighter than dark heat rays always in the same direction. C is the scale of a circular cylinder paper whose periphery is divided into 100 parts. The index is in front of a circular hole in a cylindrical movable brass capsule dd, the edge of which is carried by the underneath protruding edge ee of the upper brass piece and can easily be turned from it.

Since the zero point of the scale only after prolonged standing of the instrument occupies a sufficiently fixed position, so the mobility of the index for a correction of the zero point is required. f is a thick-walled on both sides matt polished glass cylinder, which is used for dispersion of light and dark to absorb heat rays. The same is in a brass cylinder, which has a slightly circular opening closable g carries laterally by a cover with a plate of frosted glass or frosted glass. At the top of the instrument carries a circular level to the vertical position. The number of scale divisions grows in accordance with the Torsionsgesetzen proportional to the rotation angle, but this has to naturally make sure that does not take place several revolutions of the scale under the influence of sunlight. It is therefore quite necessary if the instrument is not being used to make always stand with closed opening.

The scale photometer is also suitable for measuring the intensity of scattered daylight for photographic purposes. It replaced, to use it in such a way the outer brass cylinder. Internally by a silver-plated conical reflector with an upwardly opening Then receives the instrument on one of the sun is not accessible location, possibly outside under the protection of a mounted above the glass bell, his permanent installation, so it allows for a reliable determination of the exposure period.

The temperature is likely to have on the sensitivity of the instrument influence which is likely, however, be practically neglected for occurring in inhabited rooms fluctuations. For accurate measurements, the instrument is added to a thermometer.

Astronomical Photometer

Astronomical Photometer be attached behind the eyepiece of a telescope or the focus of the lens. The incoming radiation from the object quantity is measured relative A., - by comparison with a calibrated light source, the standard star.