DE10360563A1 - Fluid contamination measurement procedure records selectively filtered transmitted and scatter light in visible, UV and IR regions as function of contamination level - Google Patents

Abstract

A fluid (8) contamination measurement procedure records selectively filtered transmitted (11) or scattered (13) light (5), ultraviolet (6) or infrared (7) emitting diode emissions to compare their difference, sum and quotient as a function of contamination level. Independent claims are included for equipment using the procedure.

Description

The The invention relates to a method for characterizing the contaminant state as well as its change a liquid, here preferably of water, which is the spectra of white light emitting diodes exploited so that the intensity ratios in the region of the peaks the injection luminescence at about 470 nm and the photoluminescence registered at about 565 nm such that both the transmission through a container, e.g. a bypass or a stand and / or flow tube, as well the scattering of the two bands in a different direction from the Uses transmission in the liquid container, where the two bands of two white LED or better an LED and staggered optical fiber (LWL) separated by selective filters and their integral intensities become a photoreceiver, here preferably a quadrant photodiode.

According to the invention can thereby the wavelength dependence of the LED light of one LED in absolute relation to each other be registered.

aim The invention is the impurity state and corresponding Changes this State quickly detect, so that as a result of the measuring process also in situ monitoring of Processes and process optimization are possible. This problem is significant for all technical-chemical processes, which work with sufficiently transparent liquids and one Comparison with the initial state or a pure liquid enable.

requirement for the Invention is the dependency the optical data of the liquids of impurities. Such optical data includes the Absorption (Beer-Lambert's law), the light scattering (Rayleigh and Mie scattering) and the selective excitation for fluorescence (Stokesscher Case).

This is based on the spectra of white LEDs, such as those produced by NICHIA and OSRAM (see [1] to [4]), which result from the emission of both the injection luminescence of the GaInN pn junction and the photoluminescence of a composed over the semiconductor chip phosphor layer, which absorbs a portion of the injection luminescence and re-emitted according to Stokes law in the long wavelength with a lying below 1 quantum efficiency (s. 1 ). Since over a large flow range, a direct proportionality between the injection current density and the injection luminescence 1 in 1 (In this particular case, the blue band is the injection luminescence 1 relatively weak, since much of its intensity was absorbed in the conversion layer) and the intensity of photoluminescence 2 in 1 (The yellow band is very intense with this LED and has a long-wave tail to the end of the visible spectral range), there is thus a light source according to 1 at different injection conditions, allows a direct comparison of the two intensities with respect to the ratios of the peak values and the integral intensities of the bands, and in a sense represents a self-calibrating system, considering the two bands separately.

Since the wavelength peaks λ _p are separated from one another by about 100 nm and the emission bands also have a different half-width Δλ and a different structure, it is possible spectroscopically to detect the finest changes after passage of the light through a liquid. The exploitation of these modifications of the spectra are set forth in the application [5], in which case a fiber optic compact spectrometer was used.

With This application will analyze the entire spectrum of white LEDs as well as its modifications made by the liquid and evaluated. Therefore many processes sensors needed which are in a normal relationship to the price of the plant stand (for example, as a sensor in a washing machine or dishwasher), you have to in the foreseeable future for reasons of cost on a fiber optic Unfortunately, compact spectrometers do without.

Out For this reason, the invention was based on a cheap receiver, which has a high sensitivity but low spectral resolution. Such a receiver For example, a Si quadrant photodiode or the like position sensitive receiver and under certain circumstances also be a set of simple discrete photodiodes that are multi-channel Processing various optical signals allowed. In case of a Quadrant photodiode are recorded four individual photosignals and can be compared with each other.

If one characterizes the photocurrents of the quadrants with 1 to 4 and _observes that all photocurrents could be measured to ground, one can consequently also accurately detect each individual photocurrent I _ph1 to I _ph4 to ground. In addition, the differential _{photocurrents} between the individual quadrants can be registered, ie the permutations of the differential _{photocurrents} I _ph12 , I _ph13 , I _ph14 , I _ph23 , I _ph24 and I _ph34 . Thus, with appropriate outputs for all quadrants and for the common ground, the differentiated evaluation of four input signals are available, if one then in the It is able to take all photocurrents at the outputs so that they can be detected individually and do not influence each other. In addition, the sums of the individual photocurrents to each other and the total sum of all photocurrents still come with appropriate evaluation circuit. Under certain circumstances, a quotient of the photocurrents is also well evaluated. The application will be discussed in more detail in the embodiments.

The individual photocurrents are then picked up by an analog-to-digital converter and with a Micro-processor processed further. Because the spectral sensitivity such a Si receiver in the wavelength range to be used between about 350 nm and 950 nm with the wavelength at most by a factor of 6 varies, however, in the range of the spectrum of the white LED only changes by a factor of about 2.5 (here is also almost a linear function history!), Must For absolute measurements, this partial linear receiver dependence consider. Within the scope of the invention, however, comparisons of spectral modifications are always made or the initial states registered as reference values, so that this wavelength dependence hardly bothers.

Around to supply the light signals uniquely to the quadrant of the receiver According to the invention optical waveguide then positioned locally on the receiver in each quadrant can be. As the optoelectronic receiver Only integral light signals can convert into a photocurrent, the Gangs of white LED injection and photoluminescence be separated. This happens because small-area color filter glasses (edge filters) such as long-pass filters from Schott (OG 495 or OG 515) or Optima Inc. (JB 490) or yellow glasses, which are used as starting material dichroic polarizing filter are used (see [6]), such Use find them on the plastic fiber optic cables (POF = plastic optical fiber) or on the individual quadrants of the photodiode be put on. So you can totally cut off the short-wave band.

For cutting off The long-wavelength band of photoluminescence in the optical fiber can violet glass filters such as ZB 3 of Optima Inc. are used. such Filters have transmission curves that guarantee that one only the band of injection luminescence and the other only the band of photoluminescence is allowed to pass through, leaving the same LED both spectra separated and fixed ratio by means of POF to the desired Quadrants can be steered. A selectivity can also be achieved by photodiodes of SiC or GaP are used, which are partially "blind" in the visible and thus enable to separate the UV spectrum or the blue band from the yellow spectrum. Then needed just a simple Si photodiode for the yellow band and one for the Visible "blind" photodiode can without any switching process of light emitting diodes or a multiplexer for fiber optic circuits made a distinction between differently contaminated waters become.

The Invention is based on embodiments explained. In the drawing show

1 a typical spectrum of a white LED with phosphorus conversion layer: therein

1

the Spectrum of the injection luminescence band and

2

the Spectrum of the photoluminescent band

2 a typical arrangement of the invention with a white LED, a UVED and an IRED on a container with photoreceptor. The individual elements are:

3

tax calculator and diode supply

4

control lines

5

white LED

6

UVED

7

IRED

8th

Container with liquid

9

container wall

10

Tinted container wall

11

selected direct Rays of the LED

12

selected rays scattered radiation

13

window in the container

14

versions the optical fiber at the Behälterfenstemr

15

Optical fiber, for example POF

16

Quadrant photodiode

17

quadrant for the short-wave bands of injection luminescence

18

quadrant for the long wavelength bands of photoluminescence

19

electrical Supply lines to the DA converter

20

DA converter

3 Spectrum modifications of white LEDs through different waters, namely upper curve for tap water, lower curve for contaminated water through defined soiled textiles

21

blueness Band of injection luminescence

22

yellow Band of photoluminescence

4 Spectral modification of a white LED and a UVED by Berlin tap water (thin curve) and contaminated water (tidal area) of a northwest German river (thick curve)

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UV band of tap water

24

UV band of the river water

25

blueness Gang of tap water

26

blueness Gang of river water

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yellow Gang of tap water

28

yellow Gang of river water

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Second Order of the UV band of tap water

30

Second Order of the UV band of river water

31

Second Trim of blue gang of tap water

32

Second Order of the blue gang of river water,

5 Scattered light spectra in 90 ° direction for two different washing times using Persil anti-gray forms / as detergent (curve 33 after a washing time of 66 minutes; Curve 34 after a washing time of 57 min.),

6 Characterization of different polluted water through the spectrum of a white LED. The labels mean:

35

spectrum the Berlin tap water

36

spectrum of tap water with defined contaminated textile according to [8], Storage for 1 h at room temperature,

37

spectrum of tap water with defined contaminated textile according to [8], Storage for 24 h at room temperature,

38

like curve 37 , but with red wine additive,

7 Influence of water quality on the spectra of a white LED and a UVED; thick drawn curve for low water (NW) of a northwest German river at low tide; thin curve for flood (HF) of a northwest German river at high tide (curves were normalized to the maximum of the yellow band of the thick curve).
The labels mean:

39

UV band of the HW's

40

UV band of the NW

41

blueness Gang of the HW's

42

blueness Gang of the NW's

43

yellow Gang of the HW'S

44

yellow Gang of the NW's

45

Second Order of the UV band of the HW

46

Second Order of the UV band of the NW

47

Second Order of the blue ribbon of the HW.

In the first embodiment of the 2 the sensor is explained using only one white LED. On an air- or gas-filled or liquid-filled container 8th , which can be directly connected to a running technical process and also be a normal riser or flow tube and in a simplified way has a cylindrical shape, is the white LED 5 at the window 13 the otherwise blackened container 9 . 10 appropriate.

The LED 5 emits lightly divergent rays, of which only a selected ray 11 is marked in the container. This jet penetrates the container in a straight line or is partly due to the contaminants in the liquid of the container 8th scattered, wherein the scattered in the 90 ° direction partial beam 12 is drawn. The straight beam 11 and the scattered partial ray 12 arrive again at a window 13 on the outside of which are the sockets of optical fibers 14 that can be populated with filters. The straight beam and also the scattered sub-beam reach the respective quadrants via the optical fibers after filtering, which allows either the short-wave or the long-wave band to pass through 17 . 18 the photodiode 16 ,

From the photodiode 16 The individual signals are transmitted through electrical connection lines 19 in an AD converter 20 who leads them to a computer after conversion 3 , The computer 3 evaluates the measurements, registers them, revises the data and controls the LED 5 to 7 via connecting lines 4 , Thus, the photocurrents measured by the blue band of the direct or scattered light in the quadrant are measured 17 be caused.

The optical fiber is analogous to this 15 after filtering the photons of the yellow band, which are then in the quadrant 18 Cause signals of direct or scattered light. The scatter signals, which depend heavily on the contamination of the liquid, are recorded here in the 90 ° direction. There are also other angular arrangements possible. The filters are either on the fiber 14 contained or on the surface of each quadrant of the photodiode 16 appropriate.

With this arrangement, one is thus able to separate the blue and yellow bands by two long-pass filters or two violet glass filters. The change in the spectra of a white LED due to contamination of defined soiled cloths of the testing institute EMPA from St. Gallen (storage of the lobes 24 h in water, lower curve in 3 shows dirty water additionally mixed with a little red wine, upper curve in 3 is tap water) comes in the 3 well expressed.

So takes in this example, the short-wave peak 21 by a factor of 4.49, while the long-wave peak 22 decreases by a factor of 2.79, and due to the effect of the red wine also a shift of the yellow spectrum in the long-wave from 555 nm to 569 nm occurs, while the short-wave band has a peak shift from 665 nm to 467.7 nm, which should still be in the error range. The decrease in the integrals of the bands from the clear tap water to the contaminated water with a lower or upper integration limit at 495 nm gives a ratio of 4.3 for the blue band and 2.5 for the yellow band due to absorption and scattering, respectively Trap of the blue band is stronger than in the case of yellow, and what is ultimately detected by the quadrant photodiode.

conditioned by the wavelength-dependent refractive index For example, a different weakening of water occurs Light in dependence from the wavelength on. The effects are within the scope of the white LED spectrum not very large in the case of pure water (see Mobley [7]), especially as well the container dimensions and thus the optical path lengths are relatively small and at most a few cm. Strong changes However, they occur when the spectral measuring range expands will be that also infrared emitter diodes (IRED) and UV emitter diodes (UVED) consideration find what's next Application example to be discussed.

decisive Modifications of the LED spectrum occur when selective absorptions caused by the action of impurities in the liquid. This can be done in water by chemical substances (for example by detergents) or by organic substances (for example by chlorophyll) respectively. Then these changes detectable by the photosignals.

decisive changes Occur even when contaminants are present in the liquid are, the light scattering, mostly through the Rayleigh or through describable, cause the Mieshe theory.

Rayleigh light scattering naturally also occurs on the molecules of pure water (density inhomogeneities), but requires clear optical path lengths for unambiguous detection which are not present in the container considered here. Now, if impurities are added to the liquid whose size is still clearly smaller than the wavelength of the light λ, then there are well detectable light scattering at the appropriate concentration. The intensity of the scattered light can be calculated according to Rayleigh's theory from equation (1): I (λ) = C * 2 V 2 I O (1 + cos 2 θ) / (R 2 λ 4 ) (1)

In this equation, C * represents a constant, V the scattering volume, I _o the incident light intensity, R the observer's distance from the scattering location, and θ the scattering angle. Multiple scattering is not considered. The scattered light also has a preferential polarization which, under our conditions, should not be considered further. At a fixed angle θ there is therefore a dependence of the intensity I on the wavelength λ ^-n , where exponent n is normally between 4 and 4.15, but under certain conditions it can also assume much smaller values. The size C * contains both the number of scattering centers and some other constant quantities. The angle dependence is given by a cosine distribution and varies only by a factor of 2 for the straight ahead and backward directions from the 90 ° direction. Both the intensity and the angle dependence with the wavelength can be registered as changes in the spectrum of the white LED.

Decisive influence on the light of the white LED has the Mie scattering, which occurs when the diameter D of the scattering centers with the wavelength λ of the light is comparable or greater. The mathematically demanding Mie theory yields approximately the following intensity dependence: I (λ, n, α) = {I O λ 2 ) / 8π 2 R 2 } I { 1 (n, α, θ) + i 2 (n, α, θ)} (2)

Here, i _{1,2 represent} the intensity functions for the vertical and parallel component of the scattered polarized light. Furthermore, n is the refractive index of the scattering center and α is the dimension of the scattering center (α = dπ / λ with d as the diameter of the scattering center). Otherwise, the choice of sizes is as in equation (1). The scatter functions i _1,2 represent mathematically complicated expressions and are difficult to determine because the rows contained therein converge slowly. Thus, the scattering intensity is weakly dependent on the wavelength (well illustrated by white clouds versus the blue sky caused by Rayleigh scattering), but it varies greatly with the size of the scattering centers. Thus, the forward scattering can increase by orders of magnitude with increasing diameter d of the scattering centers, which is explained by the general description of the diffraction and decay sorption phenomena of Mie theory is understandable.

In So this theory also takes into account particle effects, if that Scattering center selectively absorbs and diffracts light. These phenomena change that as well Light of the white Diode depending strong and able by the concentration and type of impurities to register the sensor. These are the photocurrents of the two bands in 90 ° direction evaluated.

In depends on both types of litter the scatter intensity from the concentration of the scattering centers linearly, if you multiply scattering processes excludes.

The light so modified by different impurities passes after leaving the container in plastic fiber optics with selective filters are. Thus, in the case of one liquid, only slightly different spectra are obtained on the individual quadrants, in the case of heavily modified spectra significantly different intensities. This arrangement thus has the direct transmission, which also Proportions of the forward direction may contain scattered light.

The Resulting stray light is with the optical waveguides in 90 ° direction again directed to the remaining quadrants, separated by filters after gangs. Thus, the scattering intensities reach the other two Quadrant of the photoreceiver, They then as a photocurrent (or depending on the circuit the photodiode as a photovoltage) and ultimately provides them via a Analog to digital converter to a computer. The control of the LED, which can be switched on or with a larger flow stream or even with a pulse current can be operated, which is due their exponential flow characteristic over almost two orders of magnitude variable light intensity could result the for heavily contaminated liquids is given, also takes place from the computer. Thus stand the Evaluation electronics Photovoltage signals of the four quadrants of the Photodiode available, which all have their origin in the one white LED. Besides, they are thus scattering phenomena even better detectable (especially by the radiation of the UVED) and Transmission controls of heavily contaminated liquids more clearly measurable (in particular by the radiation of the IRED).

Puts one, as already noted above, that the injection and the photoluminescence are directly proportional to the flow, then the modified Partial all in a certain relationship to each other, if you on the initial state or that of the pure liquid and save it in the computer. So for example only the intensity the short-wave band of injection luminescence changed and the photoluminescence remains constant, you can immediately on the Close pollution effect.

The same meets the intensities the scatters too. If the dispersion is very strong, theirs can intensities possibly greater than those of the directly transmitted radiation; even happens a wavelength-dependent dispersion as discussed above based on the different intensities on the concentration and close the type of scattering centers.

in the Principle can the permutations of the channels, As described above, the number of comparison options and thus provide in-situ measurement of the contaminant state. Because opposite the high-resolution Spectrometer measurement for a pixel of the photodiode array or the CCD in the spectrometer essential less photons than for One quadrant of the photodiode available, you get at Using a quadrant photodiode a much greater detection sensitivity, because the dark currents One quadrant of a typical quadrant photodiode is lost 1 nA.

By the use of integrated amplifiers for each quadrant an even better signal-to-noise ratio can be realized. This is another advantage of the present invention.

The second embodiment also works 2 back. In this case, in addition to the white LED 5 another ultraviolet diode 6 (UVED 6 ), as made for example by NICHIA, with an emission band around 370 nm and an infrared diode 7 (IRED 7 ), which should have a band around 870 nm. These emitters are through the control unit in the computer 3 switched on individually after the measurement with the white LED 5 finished.

The measured value is recorded by the same fiber optic cable 14 and 15 as in the first embodiment, and in such a way that the UVED radiation is passed through the optical fiber, which filter out the yellow emission and allow only radiation below about 495 nm to pass. This is an integral measurement of both the direct light 11 as well as the scattered light 12 the UVED 6 possible.

Will the IRED 7 switched on, then the fiber optic 14 and 15 used for the measurement recording, the light can pass after filtering about 495 nm. Thus, both the direct and the possibly scattered radiation are available for the evaluation. Consequently, the transmission and the spread over a wavelength range of about 350 nm up to 900 nm, the minimum being the absorption for pure water, the blue band, for heavily contaminated water the yellow band can preferably be used.

It can be expected, for example, in the case of Rayleigh scattering the intensity of the scattered light at the transition from the steps of 860 nm to 560 nm, to 470 nm and to 370 nm, respectively by the factors 5,6, 2,01 and 2,6, so in total by the factor 29.2 increases by setting the intensity at 1 at 860 nm. Alone between the blue band around 470 nm and the ultraviolet Gang would take the scattered intensity for the same particle type and concentration by a factor of 2.6, a value which is clearly detectable with a photodiode.

In contrast, it can be expected that for normal cases the Rayleigh scattering in the infrared band range should be very small. However, if the scattering centers change and cause Mie scattering, then so should the IRED 7 It is possible to use the two additional diodes, with the same receiver, the data of the white LED 5 clearly to supplement.

Here, too, the measured values for the individual photocurrents, the differences and totals and the quotients are available for evaluation. Advised by the Steuerge 3 For example, the dynamic range, which is determined at a fixed photon flux density by the dynamic range of the photoreceptor and is about four orders of magnitude, can be extended to at least six orders of magnitude as a result of the flux control. This means that heavily soiled liquids can still be detected and corresponding monitoring and control systems can be realized.

in the third embodiment should a white one LED available stand, which is a very big one intensity the blue band and an approximately equal or slightly smaller intensity of the yellow Gang owns. Then the integrated surfaces of the two bands have a "normal" relationship to each other.

In this case you only need two yellow glass filters. One then measures the overall intensity of the two diode spectra, ie, the optical fiber 14 and 15 have either no or one yellow glass filter. Then, using the yellow glass filters, only the intensity of the direct or scattered intensity is registered by the corresponding quadrant. The evaluation is then again as already described above. Also in this case, only a white LED is used, so again resorted to a quasi selbststeichendes system.

Around to get a very good signal-to-noise ratio, can in inventive method the LED with different sizes DC currents, or operated with pulses or with modulated forward voltage become. Thus, the detection sensitivity for impurities can considerably be improved (AC technology).

In The following figures will give some details of typical measurements for better understanding cited become.

Out 4 For example, the influences of impurities on the spectrum of a white LED and a UVED according to the second embodiment can be well recognized. The UV spectrum 23 , the blue gang 25 , the yellow gang 27 , the second order of the UVED 29 and the second order of the blue gang 31 belong to the transmission of radiation by normal tap water.

The second curve with the UV spectrum 24 , with the blue ribbon 26 , the yellow gang 28 and the respective second orders 30 . 32 represents the transmission of radiation through river water under the same conditions as for tap water. It can be seen depending on the wavelength correspondingly strong changes in the peak wavelengths and the surface areas of the transmission spectra. However, the surface areas are converted into photocurrents, which allows very sensitive detection.

In 5 90 ° scatter spectra are included, which were recorded on water samples during a laundry washing operation at 60 ° with a Persilwaschmittel. You can see between the curves 33 and 34 strong changes in the scattered light intensity and the band structure, although the curve 34 after a washing time of 57 min. and the upper curve 33 after a washing time of 66 min. were recorded. Here are elementary processes of the cleaning process, which are very accurately registered by the sensor.

Finally, in 6 Seeing the course of a solution process when using defined contaminated textiles at different times. Starting from the upper curve for tap water 35 are the decreasing curves in intensity 36 to 38 Assign different levels of contamination, so that again corresponding ratios of the surface integrals and thus the photocurrents result in a good evaluation.

For the sake of completeness, a further measurement example should be noted that the modification of the spectra of a white LED and a UV diode by different waters to Aus puts pressure. 7 contains the transmission spectra for two waters of a northwest German river, which were pulled at low tide. The thinly drawn curve corresponds to the flood, the thick drawn to the low water. The marks 39 to 47 characterize the maxima of the respective bands of the light-emitting diodes or of the second order of the prominent bands, which also express the understanding here if, according to the second embodiment, an IR diode is also used. However, the ratio of UV and blue bands for the different waters gives differential photocurrents of 1:10 to about 1: 3. This example also illustrates the inventive use of the method for water characterization in cleaning processes with such a simple sensor.

Claims (11)

Method for determining the contaminant state of liquids, preferably in containers (flow or riser tubes) or in the container of containers, characterized in that the bands of a white light emitting diode ( 5 ) in transmission and in the scattering direction or additional of an ultraviolet light emitting diode ( 6 ) or in addition an infrared light emitting diode ( 7 ) using selective filters individually with an integral receiver depending on the state of contamination of the liquid. Method according to Claim 1, characterized in that only the white light-emitting diode ( 5 ) is used as a basic light source, so that even for different flux currents, the ratio of the blue band of the injection luminescence yellow band of photoluminescence has a certain value both in the transmission direction and in the scattering direction, here preferably in the 90 ° direction, whose changes from the initial state of the liquid and then registered as a function of the course of the process or of the state of contamination and compared with one another. Method according to claim 1 or 2, characterized that the detection of the individual signals of the photocurrents of the direct Blue and yellow gang light, blue scattered light and yellow gang, their differences, sums and quotients the use of a position-sensitive photoreceiver, preferably one Quadrant photodiode, proven and for multi-channel processing an evaluation and control unit to be supplied. Device for carrying out the method according to one of claims 1 to 3, characterized in that the direct radiation ( 11 ) in the straight-ahead direction and the scattered radiation ( 12 ) is provided in a specific direction, preferably in a 90 ° -direction, and by means of optical waveguides ( 14 . 15 ), preferably by means of plastic optical waveguides (POF) and a photoreceptor ( 16 . 17 . 18 ) can be fed. Device according to claim 4, characterized in that the container ( 8th ) opaque walls and at the locations of the light entrance and exit transparent windows, that between the light emitting diodes ( 5 to 7 ) and the signal-carrying optical waveguides ( 14 . 15 ) or between the latter and the photoreceiver ( 16 to 18 ) Color filters are attached, which are permeable either only for the yellow band (from about 495 nm) or only for the blue band (up to about 495 nm), so that the photoreceptor only the photons of the respective band of direct or scattered Light are supplied. Apparatus according to claim 4 or 5, characterized that only one radiation direction is associated with a yellow filter, and that a comparator for the photocurrent of the entire bands of the direct and the scattered Radiation is present, by comparing the photocurrent the yellow band with the blue band a difference signal generated is when the measurement of photocurrents detect contamination power. Device according to one of claims 4 to 6, characterized in that in addition a second white light emitting diode with radiation direction 90 ° to the radiation direction of the first white light emitting diode ( 5 ) and that four optical fibers in the straight-ahead direction of the first white light emitting diode ( 5 ) on the container ( 8th ) are coupled, further for measuring the direct radiation of the first white light emitting diode ( 5 ), the second light emitting diode is turned off and for measuring the scattered in the 90 ° direction radiation of the second light emitting diode, the first light emitting diode is turned off, and that are arranged in the light signal paths filter for separating the two bands in the direct and scattered light. Device according to one of claims 4 to 7, characterized in that in addition to the white light emitting diode ( 5 ) nor an ultraviolet light emitting diode ( 6 ) and / or an infrared light emitting diode ( 7 ) and all the light emitting diodes can be switched on and off individually by a control computer. Method according to one of claims 1 to 3, characterized that for better recognition of scattering phenomena all photocurrents of the integrated Bands on a band, preferably on the yellow band, normalized become. Method according to one of claims 1 to 3 or 9, characterized in that the light emitting diodes ( 5 to 7 ) are operated with different sized direct currents or with pulses or with modulated forward voltage. Method according to one of claims 1 to 3 or 9 and 10, characterized in that as a light emitting diode for the UV range a light-emitting diode which is "blind" in the visible spectral range (eg of SiC or GaP) is used.