DE10062724B4 - Methods and devices for determining the inherent optical parameters scattering coefficient, forward scattering coefficient, backward scattering coefficient and absorption coefficient of a suspension - Google Patents

Abstract

Method for determining the inherent optical parameter backscatter coefficient of a suspension, in which
a) Collimated light emitted by a first radiation source (11) onto a scattering volume (31) in the focal point region of a parabolic mirror (13) and
b) is deflected from the scattering volume (31) into the rear half space;
c) the deflected light beam is transformed into a parallel beam
d) the parallel light beam is limited by means of an adjustable diaphragm arrangement (24, 25) to scattering angles Ψ in a range from Ψ _min to approximately 180 °, wobei _{m in being} determined by the diameter of the alend arrangement and being a minimum of 90 ° and
e) focused on a receiving element (22) and detected there, and
f) the phase function for backward scattering is derived therefrom by differentiating the measurement signal after Ψ _min .

Description

The invention relates to methods and means for determining the inherent optical parameters Scattering coefficient, forward scattering coefficient, Backscatter coefficient and absorption coefficient of a suspension.

The sizes absorption, scatter, backscatter, Vorwärsstreuung and attenuation and their respective coefficients are considered inherent optical Parameters of a suspension. They hang as material constants only from the physical properties of a liquid and their ingredients; they are important monitoring parameters in water optics Assessment of water quality. From the spectral characteristics of the absorption, Art the ingredients and their concentrations are excluded the spectral course of attenuation on its grain size distribution. The degree of turbidity is reflected in the scattering coefficient. The backscatter coefficient is before everything for biooptical modeling in remote water sensing is required.

In 1 the various processes in the passage of a light beam through an infinitesimal volume dV of a suspension are illustrated. Scattering is commonly referred to when light is deflected in one medium by molecules or suspended particles from its original direction into another. The angular dependence of the scatter is described by its phase function β (Ψ):

Here dΦ _s (Ψ) denotes the portion of the incident radiation flux Φ _i deflected per infinitesimal distance dr by an angle Ψ into the solid angle element dΩ. The scattering coefficient b is formed from β (Ψ) by integration across all spatial directions:

In order to obtain the backscatter coefficient b _b , the integral is not carried out over the entire spherical surface 4π, but only over the rear half space (Ψ> 90 °):

wobei dΦ_a der pro Wegstrecke dr absorbierte Strahlungsfluß ist.Absorption is the conversion of light into thermal energy or chemical bonding. The absorption coefficient a is defined as:

where dΦ _{a is} the radiation flux absorbed per path dr.

The attenuation coefficient c of a light beam results from the sum of the absorption and scattering coefficients a and b: c = a + b (5)

A quantitatively accurate determination of the backscatter coefficient b _b is only possible by measuring the phase function β (Ψ) over the angular interval 90 ° <Ψ <180 °. For this purpose, the suspension to be examined is illuminated with a parallel beam. With conventional instruments, the receiver is moved on a circular arc around the sample volume.

In 2 a known arrangement is shown, which was used to determine the backscatter coefficient b _b . With these in situ instruments, the entire measuring device including the mechanics for moving the receiver is immersed; a measuring cell limiting the sample volume is omitted. The construction of such submersible scattered light photometers is very complex. The transmitter and receiver must be housed in separate housings and moving parts must be sealed against the measuring medium.

Due to a scanning mechanism in which the receiver, which is attached to an arm, surrounds the Streuvo lumen is rotated, the device dimensions are relatively large. Furthermore, the conventional measuring method is sensitive to misalignment of the transmitter and receiver fields of view relative to one another, which can easily be caused by shocks and vibrations during use in the field. So far, only occasional in situ measurements of the phase function have been carried out. A commercial measuring device for this is not available.

To avoid the complicated device design, an indirect measurement method is used as standard, for example, which is based on an approximate proportionality between the backscatter coefficient b _b and the phase function β (Ψ) measured at a fixed scattering angle. The best correlation was found for Ψ = 120 °. A regression showed on average: b _b = 7.5 β (120 °). However, the proportionality factor varies with the type of water and its ingredients.

A commercially available immersion probe was developed based on this measurement specification. In order to simplify their construction, the measuring angle of this submersible probe is 140 °, which can lead to an incorrect determination of the backscatter coefficient b _b , especially in coastal and inland waters that are rich in suspended matter. The size of the deviations caused by the indirect measurement with a non-optimal scattering angle is not yet known.

The scattering coefficient b can be, similar to the backscatter coefficient b _b, on genauesten by measuring the phase function β (Ψ) and subsequent integration with respect to G1. (2) determine. The conventional measuring arrangement corresponds to that in 2 shown, the limits of the target interval 0 ° <Ψ <180 ° must be excluded from the detectable scattering angle range. If the scattering angle is too large, the receiver shades the incident beam in the backward direction. If the scattering angle is too small, the receiver detects directly transmitted light, which would lead to a serious falsification of the measurement signal.

The scattering coefficient can be indirect b by correlation with a value of the measured in the forward direction Phase function β (Ψ) fixed be advantageous, a measuring angle Ψ <15 ° is. The accuracy is strongly dependent on the optical properties of the Suspension dependent and therefore not too high.

Furthermore, the scattering coefficient b by measuring the attenuation coefficient c and the absorption coefficient a can be determined. The scattering coefficient b results from G1. (5) from the difference (c - a). On In-situ measuring device based on this principle is known. The result is, however, affected by the errors of the two measured variables (c and a). In front absorption is difficult to determine and shows in the Usually a relatively large one Inaccuracy.

The object of the invention is a accurate, reliable Determination of the inherent optical parameters backscatter coefficient, Forward scattering coefficient, Carry out scattering coefficient and absorption coefficient of a suspension and robust, compact and for Suitable devices in situ for determining these coefficients specify.

According to the invention, this object is by methods for determining the inherent optical parameters Backscatter coefficient, Forward scattering coefficient, Scattering coefficient and absorption coefficient of a suspension with the features of the claims 1, 4, 5, 6 or 9 reached.

Furthermore, according to the invention are a device and further developments of this facility for carrying out the Method according to the above claims.

With the method according to the invention and the device according to the invention as well as their further training to carry out the procedures , Phase function β (Ψ) in one Range from 90 ° to nearly 180 ° determined become. Since in the facilities of the invention and their developments the essential moving parts, such as for example an iris diaphragm and a drive assigned to it inside a housing of the receiving part are arranged in this way robust in situ instrument for Establish routine measurements that can withstand rough field operations. The control of the measuring process and the data is recorded, for example, using an internal one Microcontroller or an external computer. Knowing the Phase function in the reverse direction allows the use of an algorithm that can be derived analytically for one Connection between the for the remote sensing used size reflection and the material properties scattering and absorption of the ingredients.

By simplifying the measuring device as well as their further training in such a way that apart from one if necessary required tiltable deflecting mirror no moving parts are required an exact determination of the coefficients specified above in detail allows.

The device according to the invention and its further developments are also characterized by the fact that the distances that the Light in the medium travels, are kept very short. Thus the attenuation of the suspension works less on the measurement signal back than conventional ones Scattered light photometers. If the radiation is attenuated by the suspension not negligible the attenuation can be determined with a transmissometer and taken into account in the data evaluation become.

The invention is explained in detail below with reference to the attached drawings. It shows gene:

1 schematically different processes in the passage of a light beam through an infinitesimal volume of a suspension;

2 a known arrangement for determining the backscatter coefficient;

3 a preferred embodiment of a device for determining the phase function, and

4 is a schematic representation of an arrangement of a scattered light reception in a transmissiometer.

In 3 is a preferred embodiment of a submersible device for determining the backscatter coefficient is shown schematically, the one in a first housing 10 housed transmitter part 1 and one in a second housing 20 housed receiving part 2 having. The area between the two housings 10 and 20 is filled with liquid or the measuring medium in the form of a suspension to be determined. The two housings 10 . 20 can for example by means of - in 3 - Not shown - connecting elements, such as bolts that do not hinder the flow of the measuring medium, are firmly connected. As a first source of radiation 11 can be used for example lasers or LEDs. White light sources, such as halogen lamps, gas discharge lamps, allow spectrally resolved measurements. To illuminate a suspension, this is from the radiation source 11 emitted light by means of a collimator 12 bundled, the incident light output must be stabilized or measured.

By means of one in the housing 10 fixed parabolic mirror 13 with a central passage opening, this becomes a scattering volume 31 light deflected at angles from 90 ° to approximately 180 ° in the form of parallel beams 4 on a converging lens 21 in the housing 20 displayed. The spreading volume 31 is in the focal point area of the parabolic mirror 13 , The radiation in the measuring medium 3 The distance traveled is therefore independent of the scattering angle.

The converging lens 21 focuses the radiation on a receiving element 22 , for example a photodiode or a commercially available miniature spectrometer. The flat side of the converging lens is used to improve the optical image 21 to the receiving element (detector) 22 aligned. Here, the converging lens is arranged inside the housing and in front of it a glass plate is to be attached as a window, which is in 3 is not shown in detail.

Radiation which is the scattering volume 31 passes through without distraction, is absorbed by a light trap, which is what in 3 is not shown, centered on the converging lens 21 is appropriate. If radiation scattered back from the light trap influences the measurement signal, the non-scattered radiation can alternatively be used with a deflection mirror 23 be removed from the measuring arrangement. The deflecting mirror 23 can be made smaller than a corresponding light trap. A larger scattering angle range can thereby be detected. However, scattered light that arises in the area in which the beam runs parallel to the lens surface must then be blocked by a diaphragm best applied to the lens; because all scattered light from this area, which strikes the converging lens perpendicularly, contributes to the measurement signal.

To block false light on the detector 22 would hit, further screens or optical elements can be inserted into the beam path. The influence of daylight on the measurement signal is switched off in that the transmitter part is switched off 1 measured light is subtracted from the measurement signal. The penetration of ambient radiation into the area between the two housings 10 . 20 can be prevented by blinds.

By arranging all of the imaging optical elements on that by the radiation source 11 in the broadcasting section 1 and the receiving element 22 in the receiving part 2 fixed axis, it is now possible to use the phase function β (Ψ) by means of an electric motor 25 driven and therefore adjustable iris diaphragm 24 to measure, whose center is also on this axis. The iris diaphragm 24 is in the housing 20 between converging lens 21 and detector element 22 arranged. Through the inner diameter of the aperture hole of the iris diaphragm 24 is the opening angle of the in the scattering volume 3 backwards deflected beam and thus to the receiving element 22 transmitted light output fixed.

The phase function β (Ψ) results from differentiating the measurement signal according to the maximum detectable scattering angle specified by the respective aperture position. The backscatter coefficient b _b corresponds to the measurement signal at the setting at which the iris diaphragm 24 Blocks scattering angle less than 90 °. Because the size of the litter volume 3 what light from the focal spot of the parabolic mirror 13 on the receiving element 22 can change with the scattering angle Ψ, a calibration of the phase function β (Ψ) is necessary. This can be done in two different ways.

On the one hand, this can be done by comparing the measurement signals with a conventional scattered light measuring device 2 possible, for which calibration methods are known. On the other hand, calibration of the measuring device can be achieved in a known manner by using a fluorescent liquid instead of the suspension; the phase function of the fluorescence can be derived from the theory. The weighting curve for the phase function obtained by the calibration can be included in the software. It should be noted that the size of the scattering volume is dimensioned such that larger particle fractions are not underrepresented.

Is the calibration function for a measuring device that detects the phase function 3 known, the weighting curve as a function of the scattering angle can also be mechanically simulated by means of a fixed diaphragm; in such a case is the iris 24 then no longer necessary to determine the backscatter coefficient. Instead, a fixed diaphragm can be introduced into the beam path, for example also on the flat side of the converging lens 21 be applied. Here, the aperture must be dimensioned such that each scattering angle range dΨ in the integral of G1. (3) is automatically weighted correctly.

The backscatter coefficient b _b is thus obtained with a single measurement. Using the parabolic mirror 13 the scattered radiation from almost the entire rear half-space is detected. Only a small area around Ψ = 180 ° can due to the impairment of the beam path by the deflecting mirror 23 or a light trap cannot be detected. According to Eq. (3), this scattering angle range only contributes little to the backscattering coefficient due to its low weighting with sinΨ. The resulting error is very small and can easily be corrected by assuming a plausible course for the phase function.

With the establishment after 3 a much more precise measurement of the backscatter coefficient can thus be carried out than with the indirect method which has been customary to date, in which only the signal at a single scattering angle is used for extrapolation to the backscatter coefficient b _b . The measurement result is also no longer dependent on the composition of the ingredients in the suspension.

Will the in 3 the beam path of the illuminating light bundle reversed, the device can be used to investigate the forward scattering behavior (Ψ <90 °) of the suspension. Analogous to the methods described above, either the phase function in the forward direction or without an adjustable iris diaphragm, only the forward scattering coefficient can be determined.

A reversal of the beam path can be achieved in that the radiation source is shifted, for example, to the point at which the incident beam from the loading area between the two housings 1 and 2 exit. This is in 3 by a (second) radiation source shown in dashed lines 11 ' indicated that the collimator lens is also shown in dashed lines 12 ' is subordinate. The scattering volume is now determined by means of this beam path 31 over the deflecting mirror 23 illuminated from the opposite side; Light scattered in the scattering volume is again via the parabolic mirror 13 and the converging lens 21 ultimately on the detector 22 displayed; Undeflected radiation passes through the central opening of the parabolic mirror 13 , To keep their diameter small, the beam from the radiation source 11 ' also be focused on the hole. At the position of the radiation source 11 a light trap must be installed or the beam must be released outdoors. In the latter case, the parabolic mirror can 13 surrounding housing 10 omitted.

By changing the facility to Determine the backscatter coefficient the forward scattering coefficient can also and / or the phase function is determined in the forward direction, without additional moving parts needed become.

Measuring devices for determining the backscatter coefficient b _b and variants for determining the forward scatter are described above. The scattering coefficient and its phase function can be measured from their combination. This can be done, for example, by means of two separate devices for forward and backward scattering, or also by integrating two radiation sources, for example by means of the radiation sources 11 and 11 ' in 3 into one device. The radiation sources 11 and 11 ' are to be arranged as in connection with the measuring device 3 and the modification of which is described.

A much simpler structure can be achieved by expanding the in 3 reach the measuring device shown. In order to be able to determine the phase function also in the forward direction (Ψ <90 °), the deflection mirror is used 23 in 3 tilted with the help of an adjustment unit so that the through the opening of the parabolic mirror 13 emerging beam after crossing the scattering volume 31 is reflected in itself. The particles in the scattering volume 31 are lit from the front and from behind, so to speak. The parabolic mirror 13 In addition to the back scattered radiation, this also collects the forward scattered portion of the reflected beam.

In the position of the deflecting mirror described above 23 becomes perpendicular to the incident beam when moving through the adjustable iris diaphragm 24 after differentiating the measurement signal, the sum of the phase functions from the front and rear half-space β (Ψ) + β (180 ° - Ψ) is detected. Becomes β (Ψ) for the rear area with the deflection mirror inclined at 45 ° 23 Measured separately, the phase function can be determined over almost the entire scattering angle range 0 ° <Ψ <180 °.

At the interval limits, the Phase function not recorded become. The same arguments apply as for the conventional one Measurement methods. However, the mechanical structure of the measuring device is compared to this again much more compact and robust. The calibration of the Instruments are analogous to those of the phase function measuring device for backscattering.

If only the scattering coefficient b and not the phase function β (Ψ) is to be determined, the iris diaphragm can be used 24 and the adjustment option for the deflection mirror 23 omitted. The latter is mounted perpendicular to the optical axis so that the illuminating beam is reflected in itself. As with the device for determining the backscatter coefficient, the calibration is achieved by means of a mechanical diaphragm. While each If the aperture enables the correct weighting of the phase function for each scattering angle, the aperture is used to determine the weighting for the sum β (Ψ) + β (180 ° - Ψ) of the two complementary scattering angles.

This creates no difficulties. Forward scatter dominates backward scatter; thus the aperture 24 be adapted to the forward direction. When illuminating the particles from one side, the same aperture shape and size should be used as when illuminating from the other side, since the scattering volume 31 , from which light from the focal point area of the parabolic mirror 3 on the detector 22 can hit and which thus defines the weighting function, is basically the same size for both beam directions.

Measurement of absorption in cloudy media is unsatisfactorily solved in the prior art. The fundamental The problem is that measuring signal is influenced by scattering in the absorption or measurement volume. The scattered light can hardly be completely absorbed in absorption measuring devices the receiving element are imaged and deceives an increased absorption coefficient in front.

The absorption coefficient a can be determined by measuring the attenuation coefficient c and the scattering coefficient b according to G1. (5). The absorption coefficient a results according to G1. (5) to: a = c - b

This is a precise determination the absorption is achieved, since the size scattering, which is a disturbance in conventional measuring methods recorded directly here becomes. The structure is simplified if the same lighting source for both measured variables is used.

Detection of attenuation coefficient c is carried out with a transmission measuring unit. It determines the decrease in intensity due to absorption and scattering of a parallel Light beam when passing through the medium. The acceptance angle the receiver optics is very small to choose so that no scattered light hits the receiver.

As a measuring device for determination of the scattering coefficient is due to its robust and compact execution the facility described above. However, as well like conventional ones Devices, none under very large or very small angle scattered radiation can be detected. While the Effects on the accuracy of the scattering coefficient b by the missing angular range around 180 °, as stated above, is small the measurement error due to the undetectable scattering angle range in the forward direction not neglected because the phase function takes on very large values.

That of a scattered light photometer, for example after 3 , but also after 2 Forward area not detected can be measured by expanding the transmissometer by one or more scattered light receivers. These are as in 4 for a scattered light receiver indicated to be arranged laterally offset from the optical axis at the level of the transmitted light receiver. The offset determines the mean detected scattered light angle and the geometric dimensions of the receiver (s) determine the extent of the angular ranges detected.

The calibration of the scattered light receiver can again with a suitable phase function measuring device or a fluorescent one liquid carried out become. The measurement signal obtained is to correct the scattering coefficient measured too small subtract from the transmission value.

As an alternative to recording the scattering coefficient with the simplified structure of a measuring device described above to determine the scattering coefficient and the transmission unit for the only marginally distracted forward scattered light can also the phase function with the described device for Determination of the phase function in the forward and backward scattering range, extrapolated to the undetectable area and then integrated become.

1

sending part

10

first casing

11

radiation source

11 '

radiation source

12

collimator

12 '

collimator

13

parade

2

receive part

20

second casing

21

converging lens

22

receiving element (Detector)

23

deflecting

24

iris

25

Electric motor for 24

3

measuring medium (Suspension)

31

scattering volume

4

parallel beams

Ψ

scattering angle

β (Ψ)

phase function

b

scattering coefficient

b _b

Backscatter coefficient

a

absorption coefficient

c

attenuation coefficient

Claims (16)

Method for determining the inherent optical parameter backscatter coefficient of a suspension, in which a) from a first radiation source ( 11 ) emitted light onto a scattering volume ( 31 ) in the focus area of a parabolic mirror ( 13 ) collimated and b) from the scattering volume ( 31 ) is deflected into the rear half-space; c) the deflected light beam is converted into a parallel beam d) the parallel light beam by means of an adjustable diaphragm arrangement ( 24 . 25 ) is limited to the scattering angle Ψ in a range from Ψ _min to approximately 180 °, where Ψ _{m in is} determined by the diameter of the alum arrangement and is a minimum of 90 ° and e) on a receiving element ( 22 ) is focused and detected there, and f) the phase function for backward scattering is derived therefrom by differentiating the measurement signal after Ψ _min . Method according to claim 1, characterized in that, with regard to the dependence of the size of the spreading volume ( 31 ) the phase function β (Ψ) is subjected to a calibration, which is carried out either with the aid of a calibrated conventional measuring device or by means of a fluorescent liquid. Method according to claim 1, characterized in that that this Scattered volume deflected without deflecting light radiation and / or is absorbed. Method for determining the inherent optical parameter forward scattering coefficient of a suspension, in which a ') from a second radiation source ( 11 ' ) emitted light via a deflecting mirror ( 23 ) to a spread volume ( 31 ) in the focus area of a parabolic mirror ( 13 ) collimated and b ') from the scattering volume ( 31 ) is deflected into the parabolic mirror half-space; c ') the deflected light beam is transformed into a parallel beam, d') the parallel light beam by means of an adjustable diaphragm arrangement ( 24 . 25 ) is limited to the scattering angle Ψ in a range from slightly greater than 0 ° to Ψ _max , where Ψ _{max is} determined by the diameter of the diaphragm arrangement and is a maximum of 90 °, e ') on a receiving element ( 22 ) focused and detected there, and f ') the phase function for forward scatter is derived from this by differentiating the measurement signal according to Ψ _max . Method for determining the inherent optical parameter scattering coefficient of a suspension, in which a '') from a first radiation source ( 11 ) emitted light and from a second radiation source ( 71 ' ) delivered via a deflecting mirror ( 23 ) redirected light onto a scattering volume ( 31 ) in the focus area of a parabolic mirror ( 13 ) collimated and b '') from the scattering volume ( 31 ) are deflected into the parabolic mirror half-space; c '') the deflected light bundles are transformed into parallel light bundles, d '') the parallel light bundle by means of an adjustable diaphragm arrangement ( 24 . 25 ) are limited to scattering angles Ψ in a range from Ψ _min to 180 ° - Ψ _min , where Ψ _{min is} determined by the diameter of the diaphragm arrangement and is at least slightly larger than 0 °, e '') on a receiving element ( 22 ) focused and detected there, and f '') the sum of the phase function for forward and backward scattering is derived from this by differentiating the measurement signal after Ψ _min . Method for determining the inherent optical parameter scattering coefficient of a suspension, in which a first radiation source ( 11 ) emitted light onto a scattering volume ( 31 ) in the focus area of a parabolic mirror ( 13 ) and the spread volume ( 31 ) reflected light is reflected in itself by a tiltable mirror aligned perpendicular to the beam path and together with that on the scattering volume ( 31 ) collimated light from the scattering volume ( 31 ) are deflected into the parabolic mirror half-space; c ''') the entire deflected light beam is converted into a parallel beam, d''') the parallel light beam by means of an adjustable diaphragm arrangement ( 24 . 25 ) is limited to the scattering angle Ψ in a range from Ψ _min to 180 ° - Ψ _min , where Ψ _{min is} determined by the diameter of the aperture arrangement and is at least slightly larger than 0 °, e ''') on the receiving element ( 22 ) is focused and detected there, and f ''') the sum of the phase function for forward and backward scattering is derived from this by differentiating the measurement signal after Ψ _min . A method according to claim 5, characterized in that the illuminating beam after passing through the scattering volume ( 31 ) is first reflected in itself by means of the tiltable mirror, which is initially aligned perpendicular to the beam, and is then directed out of the measuring device by 45 ° after tilting the mirror aligned perpendicular to the beam path, so that the scattering coefficient in the first measurement process, the backscatter coefficient in the second measurement process and can be determined from the difference of the forward scattering coefficient. Method according to claim 7, characterized in that by means of the adjustable diaphragm arrangement ( 24 . 25 ) in the first measuring process the sum of the phase functions for forward and backward scattering, in the second measuring process only the phase function for backward scattering and from the difference the phase function for forward heating scattering is determined Method for determining the inherent optical parameter absorption coefficient a of a suspension, wherein the attenuation coefficient c of the suspension is measured by means of a conventional transmission measuring device and the scattering coefficient b is determined according to one of the methods according to one of Claims 5 to 7, and the absorption coefficient a is obtained by subtraction as a = c - b. Device for carrying out the method according to claim 1, characterized in that in a first sealed housing ( 10 ) a radiation source ( 11 ) and a parabolic mirror ( 13 ) with a central passage opening and in a second, at a distance from the first housing ( 10 ) provided and this ( 10 ) stationary assigned housing ( 20 ) a converging lens ( 21 ), an adjustable aperture arrangement ( 29 . 25 ) and a receiving element ( 22 ) housed and on the by the radiation source ( 11 ) in the housing ( 10 ) and the receiving element ( 22 ) in the housing ( 20 ) fixed optical axis are arranged in such a way that from the radiation source ( 11 ) emitted and to a scatter volume ( 31 ) in the focal point area of the parabolic mirror ( 13 ) incident light is deflected into the rear half-space by the scattering volume ( 31 ) Deflected light limited to a scattering angle Ψ between 90 ° and approximately 180 °, on the receiving element ( 22 ) is focused and detected there and thus the backscatter coefficient b _{b is} determined and / or in one. Computing unit by differentiating the measurement signal proportional to the received light, a phase function β (Ψ) corresponding to the detected scattering angle range is formed. Device according to claim 10, characterized in that the adjustable diaphragm arrangement comprises an iris diaphragm ( 24 ), for the adjustment of which an electric motor ( 25 ) is provided. Device according to claim 10, characterized in that at a known calibration for the phase function β (Ψ) instead the adjustable aperture arrangement a fixed mechanical Aperture is provided. Device according to claim 10, characterized in that the fixed diaphragm on the flat surface of the converging lens ( 23 ) is attached. Device according to claim 10, characterized in that in the center of the converging lens ( 21 ) a light trap is provided. Device according to claim 10, characterized in that a mirror ( 23 ) is provided for removing light that is not deflected in the scattering volume. Device for performing the method according to claim 6, wherein in the device according to claim 10 in front of the housing ( 20 ) the tiltable mirror aligned perpendicular to the beam path or in the center of the in the housing ( 20 ) housed converging lens ( 21 ) and thus on the optical axis of the device for reflecting the scattered volume ( 31 ) crossing light in itself a mirror element with an approximately the diameter of the scattering volume ( 31 ) corresponding diameter is provided.