DE3242447C2 - Photoelectric control device for quality control of a moving material web - Google Patents

Abstract

To check the surface of a textile web, several rod-shaped fluorescent tubes arranged axially parallel to one another are used, which illuminate scanning tracks of several individual light scanning devices from different angles that complement the entire scanning width.

Description

The invention relates to a photoelectric control device according to the preamble of patent claim 1.

In the production of web material, for example continuously produced textile webs, an ongoing Quality control required to identify material manufacturing defects. For example

for example, textile fixation inserts, which are used in the clothing industry to stiffen shirts, suits, etc. are used are made in the form of coated textile webs. Web-shaped textile material is included coated with a thermoplastic adhesive coherent covering applied but mostly in the form of, for example, applied in a grid-like manner individual glue points. On the textile web, these points have uniformly the shape of gelatinous, translucent shiny elevations, while the textile webs, depending on the intended use, all color shades * have stanchions from white to black.

Faults in the coating system either lead to defects in the coating, with such Places the adhesive on the textile web is missing, or they lead to thickening of the adhesive, with such places too much glue has been applied. Such defects in the coated surface can be human

Eye and / or an optical control system cannot be recognized without aids, as sometimes none at all For this reason there is a color contrast between the adhesive dots and the textile web to be coated An optical brightener is added to the adhesive, which emits blue light when exposed to UV light emitted and thus creates the necessary color contrast

So far, however, it has not been possible to use conventional surface control systems to automatically detect surface defects in the previously mentioned coated textile webs. This is because the conventional surface control systems are designed for other materials and control tasks and are used for Control of such coated textile webs cannot be used satisfactorily.

All known automatic surface control systems have in common that they have a lighting device that shines the test light onto the material web to be controlled, and also a light scanning device that is reflected or reflected by the material web / iitr ^ knalArirene «Dv * tf1ts * h> ot «fv * imrvi + iinr) im nllrvan-ιαΐηΑη uui vugvtuddvitvd χ a uiuviti utatialutitit uiiu tut ttii ^ wili ^ uiwti a photo transducer means comprises so; e an evaluation device which evaluates the output signal of the light scanning device and triggers an error display when predetermined types of errors are detected conventional surface control systems can be divided into static and dynamic systems. at In the static systems, a lighting device with a constant light beam direction is used, while in dynamic systems the surface of the material web is illuminated with a cell-shaped moving (scanning) light spot

A first example of a static surface control system is as a lighting device A number of incandescent lamps, for example halogen lamps, are provided on one side of the material web to be checked. The material web is through on the the same or on the opposite side of the material web in a line arranged side by side optical lenses imaged on photodiodes. This known control system is designed for material webs made of paper, sheet metal or plastic and has proven itself as unsatisfactory in the surface control of textile webs, in particular the coated textile webs mentioned above. Also with this known surface control system DC power supplies required, which must be designed for high DC power in the range of up to 1 KW. In addition, there is a great deal of heat generation. the Illumination of the surface to be checked is not homogeneous. Since there is only one in the light bulb spectrum If the proportion of UV light is low, UV-sensitive brighteners could be used in textile coatings hardly stimulate the emission of visible contrast light. With this known surface control system, surface defects only generate defect signals on the photodiodes when the material web is moving. Resting webs of material are therefore not controllable. aside from that the shapes and thus the frequency spectra of the signals emitted by the photodiodes depend on the feed speed of the material web. Such control systems require amplifiers with a very large frequency bandwidth of about 10 Hz to about 100 kHz, and such amplifiers are expensive.

In another example of a static surface control system known from DE-AS 20 09 001, which is used for the control of paper webs and Sheet metal webs is designed, the material web is with Illuminated with the help of a fluorescent tube attached transversely to the material feed direction Material web received axially parallel to the fluorescent tube mounted light guide rod. This light guide The rod is encased except for a longitudinal slot and directs incoming light through this longitudinal slot by means of total reflection to the end faces of the light guide rod, where it is fixed by means of a photo converter device.

! 0 can be gestured.

The fluorescent tube must be operated with direct current, since alternating voltage operation would simulate surface defects. However, direct current operation reduces the service life of a fluorescent tube. Except- the light intensity along the fluorescent tube axis does not remain stable. Therefore there is an hourly polarity reversal the fluorescent lamp required. Since light transmitted through the material web is received along the entire axis of the light guide rod, this results in a high proportion of constant light from PrüfFrht and extraneous light With this known Kcntrciisysieni practically only very large holes can be recognized in a textile web will.

With dynamic surface control systems, at

which illuminates the surface to be checked with a cell-shaped moving, scanning light spot gas lasers or semiconductor laser diodes, more rarely high-pressure lamps, are used as light sources or incandescent lamps. Transmit from the material web tated and / or reflected light is mostly with Light guide rods that extend over the entire width of the material, or with mirror chambers through which light reaching a light entry surface is reflected to photomultipliers, received, rarely in car collimation.

Laser light sources require complex and expensive power supply units, especially when using gas lasers. The lasers themselves are also expensive components. In addition, such light sources in specially adapted Manufacturing centering mounts must be arranged so that replacement and adjustment is only possible by specialists Generate extraneous light. High luminance levels, especially with lasers, require work-hindering measures for eye safety

Light guide rods and mirror chambers accept transmitted and / or reflected light as light receivers. extraneous light along the entire scanning width. Such extraneous light causes a photo converter a DC voltage level and an increased proportion of noise, which limits the working range of the photo converter. Therefore, extraneous light shielding is required, but it hinders the handling of the control device.

All known and in practical use dynamic surface control devices with scan ^ nenden light spot are for large scanning widths with ho hen sampling frequencies are designed Typical frequency components of error signals generated by surface defects are 100 kHz and above. this leads to a sensitivity to electrical interference, against which special measures are taken Need to become.

The surface to be checked is illuminated with a scanning light spot only in the narrow area of the light incidence angle between

see the normal of the controlled surface and the optical axis of the light beam. Also the reception of the reflected or transmitted light through the light receiving device always takes place in a narrow angular range. Material webs with shiny surfaces must therefore be guided very precisely and must not flutter or form waves, otherwise shadows or intensified Reflex error signals are simulated.

The beginning and end of each scan line of the scanning light spot are marked by signals from additional photo transducers.

The invention is based on the object of achieving in a cost-effective manner that one can use AC voltage-operated fluorescent flip-flop lighting A very uniform illumination is achieved both spatially and temporally.

The solution to this problem is given in claim 1 and can according to the further claims be advantageously trained.

This combination of features leads to a very good, spatially homogeneous and temporally uniform illumination of the entire width of the material web also at fluttering or wave-forming points, so that at the exit of the light scanning device a very smooth and uniform Basic signal is obtained, from which even small material web defects can be safely distinguished and recognized without incorrect assessment.

The operation with several fluorescent lamps also leads to the considerable advantage that even when Failure of all fluorescent tubes except for one is still a control option, and with at least the same control quality as is achieved with conventional control devices that are equipped with only a single fluorescent tube.

Because the control device according to the invention has made it possible to operate the fluorescent tubes with alternating voltage, one arrives to a significantly longer service life of the fluorescent tubes and one saves the hourly polarity reversal.

From the BBC book »Ballasts and Switching

Amount and in the range of 70 °, in particular at 71.5 °, for the other angle different therefrom.

In a particularly preferred manner, two light scanning devices with scanning tracks parallel to one another can be applied one behind the other in the longitudinal direction of the material web put in order. In particular, it is advantageous to use these two light scanning devices in combination with three fluorescent tubes. One preferably chooses such a mutual assignment that the ίο Scanning track of one light scanning device from the one outer and the middle fluorescent lamp each from an angle of 45 ° and from the other outer fluorescent lamp from an angle of 71.5 ° and the Scan track of the other light scanning device from the other outer fluorescent lamp and the middle one Fluorescent lamp from an angle of 45 ° and one outer fluorescent lamp from an angle illuminated from 71.5 °. In this way, the generation of false error signals can be very high

Dimensions can be turned off.

The fluorescent tubes are advantageously operated by means of an alternating voltage, the frequency of which is greater than the highest frequency component of the smallest from the control device to detected error caused error signal. An operating AC voltage with a Frequency of at least 20 kHz. This measure also avoids the lifespan of Fluorescent tubes reduce direct current operation and the generation of false error signals is prevented, such as would still occur, for example, when the fluorescent lamps were operated with the usual mains alternating voltage frequency. Regardless of whether you look for the lighting in front of direction one or more or even one When using fluorescent tube, it is particularly preferable to use a dynamic light scanning device with a cell-shaped moving scanning point. By this measure can be used in spite of the static use Lighting devices reduce the influence of and greatly reduce the amount of extraneous light. A dynamic light scanning device is particularly preferred, which has a polygon mirror wheel that is around a transverse axis running to the scanning track with its circumference

gene for low-voltage discharge lamps "by C. H. 45 can be rotated over the scanning track, whereby in Reflexions-Sturm, page 91, it is known per se that the common light beam path of the polygon mirror wheel is a diaphragm me light from three fluorescent lamps in three-phase and a photo converter arranged downstream of this circuit has a lower light ripple number and wherein between the material to be scanned than the light of a single fluorescent lamp. Track and the polygon mirror wheel an optical system is arranged by the fact that one is in the Kon- so net according to the invention, by means of which, with the interposition of the potroll device, several fluorescent lamps and the polygon mirror wheel create a real, moving image with different phases of an alternating voltage

operates as well as arranges it on different sides of the scanning track, it has been possible to detect false errors of all kinds, namely those caused by irregular lighting as well as those caused by shadow formation or reflections through faults, hollows, fluttering of the Textile web or the like, are greatly reduced or eliminated. This enables a very exact detection of real errors.

In a particularly preferred embodiment, three axially parallel fluorescent tubes are used, two of which equate the scanning track from angles.

Material web part is mapped onto the panel.

In a particularly preferred embodiment, there are several lights in the direction transverse to the material web scanning devices arranged in series, their scanning tracks each scan a preferably equal part of the width of the material web and are aligned with one another. This measure avoids the disadvantages of a single spread over the entire width of the materi albahn extending dynamic light scanning device tion, namely large size, high weight and a high scanning speed.

In order to ver scanning gaps when a plurality of light scanning devices are arranged next to one another in series

illuminate the amount and avoid the third scanning track 65, has a particularly preferred embodiment

illuminated from a different angle. two rows with several next to each other

Particularly preferred angles are in the area of arranged light scanning devices, the

in particular at 45 ° for the two angles the same light scanning devices of the one row opposite

the light scanning devices of the other row are offset from one another in the direction of the width of the material web in such a way that scanning gaps which occur between the light scanning devices of one row are in the scanning range of the light scanning devices of the other Series fall.

The invention will now be explained in more detail on the basis of embodiments. In the accompanying drawings shows

F i g. 1 is a schematic representation of a material web which is illuminated by three axially parallel fluorescent tubes and by two rows of light scanning devices is scanned;

FIG. 2 illumination angles occurring in the embodiment according to FIG. 1;

3 shows a schematic representation of the structure and operation of a dynamic light scanning device;

F i g. 4 on at the output of the light scanning device according to FIG. 3 available output signal;

F i g. 5 spectral representations to explain the mode of operation according to the invention; and

F i g. 6 is a schematic circuit diagram of an evaluation device according to the invention.

F i g. 1 shows an exemplary embodiment of a control device with which the quality of a surface coating of a textile web 4 is to be checked. This is a textile web for the production of textile fixing inserts, as mentioned in the introduction. That is, the one to be controlled Fabric web has any shade of gray, including white or black, and the adhesive consists of gelatinous, translucent glossy Material. In order to make the adhesive visible, a brightener is added to it, which when exposed to UV light emits visible light in the blue area.

N4it the control device sous'. Störu ^ ⁰ "* · ip d ° r coating system are detected, leading to Beschichtungsiücken or coating thickening.

The control is normally carried out on the moving textile web, but can also be carried out on if necessary the unmoved textile web.

The surface to be controlled is with this Embodiment with three rod-shaped fluorescent tubes 1, 3 and 5 illuminated, which are equidistant from each other and parallel to each other in the transverse direction to the normally moving textile web 4 · and extend over their entire width. The fluorescent tubes 1, 3 and 5 illuminate the textile web 4 with UV light, preferably in the range from 315 nm to 400 nm.

Light scanning devices 2 are provided above the fluorescent tubes 1, 3, 5, each with a strip-shaped Uchi-scanning characteristic running in the transverse direction of the textile web 4. The light scanning devices 2 are arranged in two rows 7 and 9 parallel to the fluorescent tubes 1, 3, 5 and to themselves, in such a way that the scanning tracks of the light scanning devices 2 of each row are in one line and result in a total scanning width A. The light scanning devices 2 of the second row 9 are offset from the light scanning devices 2 of the first row 7 in such a way that scanning gaps between the light scanning devices 2 of the first row 7 fall within the scanning range of the scanning devices 2 of the second row 9. The scanning line formed by the light scanning devices 2 of the first row 7 lies between the lines in FIG F i g. 1 foremost fluorescent tube 1 and the middle fluorescent tube 3, while the scanning line formed by the light scanning devices 2 of the rear row 9 lies between the middle fluorescent tube 3 and the rear fluorescent tube 5. The scanning tracks a of the light scanning devices 2 of the front row 7 form a scanning line A 7, while the scanning tracks a of the light scanning devices 2 of the rear row 9 form a scanning line A 9.

ίο The mutual assignment of the fluorescent lamps 1, 3.5 and the light scanning devices 2 is based on FIG. 2 explained. The front fluorescent lamp 1 and the middle fluorescent lamp 3 are positioned on both sides of the scan line A 7 that they Illuminate touch line A 7 from an angle χ , which is preferably about 45 °. This scanning line A 7 is illuminated by the rear fluorescent tube 5 from a shallower angle β , which is preferably approximately 71.5 °.

The scanning line A 9 formed by the light scanning devices 2 of the second row 9, which lies between the middle fluorescent tube 3 and the rear fluorescent tube 5, is illuminated by these two fluorescent tubes each from the angle λ , while they are illuminated by the front fluorescent lamp 1 is illuminated from the flat angle β.

This arrangement of the fluorescent tubes opposite the light scans ensures homogeneous illumination of the textile web to be scanned. so that false detection of defects due to shadow formation or reflections, caused for example by warping, troughing or fluttering of the textile web, is eliminated. An embodiment of one of the in FIGS. 1 and 2 The light scanning devices 2 shown will now be described with reference to FIG. 3 explains a rotating polygon mirror wheel J1 rotates about an axis 13 running transversely to the scanning track a in such a way that its polygonal circumference is aligned with the scanning track a in the reflection rays gang of the polygon mirror! ades 11 there is a Pho to converter 13, preferably in the form of a silicon photodiode or a silicon phototransistor. By means of an optical system 15 arranged between the textile web 4 and the polygon mirror wheel 11, an image 4 ′ becomes a Part of the surface of the textile web 4 is imaged onto a diaphragm arranged in front of the photo converter 13 The aperture has an opening of 2 mm, for example. This results in a practical embodiment on the surface of the textile web a scanning spot with a

Size of 10 mm.

Every time the image of the textile web surface on the photo converter occurs over an edge of the polygonal control wheel 11, the photo converter "sees" both edges of the scan track a, making it short term there is a light pulse-shaped increase in the intensity of the imaging light impinging on the photo converter, as shown in FIG. 3 is indicated by the overlap of two scanning track images a \ a ' Determine electrical light pulses with which the beginning and end of the respective scanning track a can be signaled.

An example of an electrical signal as can be obtained at the output of the photo transducer 13 is shown in FIG F i g. 4 shown. This signal has light pulses 19 occurring at a time period spacing ta , which signal the boundaries of the scanning tracks a. Since these bright pulses are caused by double images of the

Textile web surface arise, their amplitude can be a maximum of twice as large as the basic signal value 21, which comes from the single image of a flawless textile web surface. Faults in the textile web coating are shown in the signal curve in FIG. 4 can be recognized by light defect pulses 23 or dark defect pulses 25. Bright pulses which are clearly above the beginning and end of the scanning track a signaling bright pulses 19 are caused by thickening of the adhesive. Gaps in the glue, where the brightener is missing, appear in the signal curve as dark error pulses 25. Material web defects such as holes, tears, dirt and overlapping folds also lead to dark defect pulses.

The fluorescent tubes 1, 3, 5 are operated with high-frequency alternating voltage, the frequency of which is at least 20 kHz. In this operating mode, fluorescent tubes have their most favorable values with regard to Stability, luminous efficacy, efficiency and service life. At such a high operating frequency that much higher than the highest frequency component of the error signal of the smallest detectable error, is the modulation of the basic signal caused by the AC voltage operation of the fluorescent tubes 21 negligible.

The three fluorescent tubes 1, 3, 5 are each operated with separate power supplies. This achieves different phase positions of the high-frequency alternating voltages of the individual fluorescent tubes to each other, whereby the modulation caused by the AC voltage operation of the entire Reduced luminous flux of the three fluorescent tubes. Because comes through the different phase positions there is a compensation of the residual modulation.

At an operating frequency of 20 kHz there is a signal ratio between bright pulse 19 and residual modulation of the basic signal 21 of more than 30: 1.

As shown schematically in Fig.3, are in the imaging beam path In front of the diaphragm 17, two filters 27, 29 are arranged, with which the largest part is disruptive External light, such as daylight or room lighting, is filtered out. From the points of the web of material 4, which contain no brightener, practically no light reaches the photodetector 13, so that surface spots with coating gaps lead to a clear dark defect pulse, as shown in FIG. 4 with 25 is marked.

The effect of these filters 27, 29 is shown in FIG. 5 shown. F i g. 5 (a) shows that of the fluorescent tubes used 1, 3, 5 emitted light spectrum, which is in the range of about 315 nm to 400 nm in Fig.5 (a) the spectral emission range of the brightener contained in the adhesive material is shown, after which the UV light from the fluorescent tubes triggers an emission in the range of around 400 to 600 nm.

F i g. 5 (b) shows on the one hand the spectral sensitivity of a silicon photodiode and on the other hand the filter characteristics the arranged in front of the aperture 17 optical filters 27 and 29. While the spectral The sensitivity of the silicon photodiode extends over a very large area of the spectral spectrum, one achieves through the optical filters 27 and 29 that essentially only light in the emission range of the brightener is left to the photodiode while on the one hand that emitted by the fluorescent tubes 1,3,5 Light and, on the other hand, a large part of the visible part of daylight reaching up to 780 nm and room illumination light is blocked from the photodiode.

Fig. 6 shows a schematic circuit diagram of an evaluation circuit, with regard to the mutual assignment and interconnection of the individual circuit components expressly refer to the representation in FIG. 6 is referenced.

A photo converter in the form of a photo diode 13 is assigned to each light scanning device 2. Any photodiode 13 an amplifier 31 is connected downstream. With help

ίο in each case a control circuit 33, the light pulses 19 are regulated to a constant value. The evaluation circuit of each light scanning device 2 is adjusted to the same constant value u. A special feature of the control is that the photodiode 13 is arranged in the so-called »Photovoltagew circuit, i.e. parallel to the two inputs of the amplifier 31 designed as an operational amplifier. The control circuit 33 connected between the one input and the output of the operational amplifier 31 has a field effect transistor at the input to obtain a high input resistance.

In Fig. 6 are evaluation circuits for two light scanning devices 2 shown. In practice, however, up to twenty light scanning devices 2 can be used will.

Each amplifier 31 is followed by a light threshold comparator 35 on the one hand and a dark threshold comparator 37 on the other hand. All light threshold comparators 35 are connected with their comparison threshold input to a light threshold potentiometer 39, and the comparison threshold inputs of all dark threshold comparators 37 are connected to a dark threshold potentiometer 41. With the light threshold potentiometer 39, a light error threshold SW is set, while with the dark threshold potentiometer 41 a dark error threshold SD is defined. The light error threshold SH is preferably set to a fixed value, while the dark error threshold SD is continuously adjustable with a multi-turn potentiometer on the front panel of the housing of the evaluation circuit. Because all important defects such as dirt, folds, holes, tears, geometric defects and missing adhesive coating generate dark defect pulses 25 Externally accessible dark threshold potentiometer 41 can thus continuously be set different quality levels for the errors leading to dark error pulses. The dark threshold potentiometer 41 can expediently also be designed as a step switch with resistors as a voltage divider.

The output signal of each comparator 35,37 is given to a digital filter 43, in which a pulse shaping is carried out. In addition, you can set the width of the respective digital filter 43 in each Must have comparator delivered error pulses in order to pass the digital filter 43 can. The outputs of all digital filters 43 connected to light threshold comparators 35 are connected, each with an input of a light error OR logic circuit 54 connected, while the outputs of all digital filters 43, which are a dark threshold comparator 37 are nachgecrdnct, each with one input of a dark error OR logic circuit 47 are connected. The outputs of the two OR interconnection

11 12

circuits 45 and 47 are fed to an output stage 47, by means of which either an acoustic or optical Error display signa! is triggered or an error marking is made on the controlled material web. s

If the entire width of the material web were to be scanned with a single light scanning device 2, this would have to have a very large overall size, would have to work at a high scanning speed and would be quite heavy. If the entire scanning width A is divided into several small scanning tracks a, several small light scanning devices 2 can be used, each of which allows a relatively small overall height, which makes a small overall height of the control device possible.

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Claims (13)

Patent claims: 1. Photoelectric control device for quality control of a moving material web, with a lighting device fitted transversely above or below the material web and radiating test light onto the material web, with a photoelectric light scanning device that is reflected or transmitted by the material web and that scans the material web in strips in the transverse direction , and with an evaluation device downstream of the light scanning device for evaluating the output signal of the light scanning device, characterized in that the lighting device has a plurality of rod-shaped fluorescent tubes (1,3,5) which are connected on both sides of the scanning track (A 7; A 9) of the light scanning device (2, 7 ; 9) and are arranged parallel to this and the scanning track (A 7; A -t) from different. Illuminate angular sightings (α, β) , and which are operated with different phases of an alternating voltage. 2. Control device according to claim 1, characterized in that the frequency of the alternating voltage is greater than the highest frequency component of that of the smallest from the controller to be recognized error caused error signal. 3. Control device according to claim 1 or 2, characterized in that the fluorescent tubes (I ₁ 3, 5) were operated with an AC voltage frequency of at least 20 kHz. 4. Control device na; h claim 3, characterized in that two fluorescent tubes (1,3; 3, 5) are provided which illuminate the scanning track (A 7; A 9) from angles (a) of the same amount. 5. Control device according to claim 4, characterized in that the two fluorescent tubes (1, 3; 3, 5) the scanning track (A 7, A 9) each from an angle (cc) of 45 ° with respect to the on the scanning track (A 7 ; A 9) illuminate the incident normal. 6. Control device according to claim 4 or 5, characterized in that a third fluorescent tube (5; 1) is arranged parallel to the other two fluorescent tubes (1,3; 3,5) and the scanning track (A 7; A 9) from one second angle (ß) illuminated. 7. Control device according to claim 6, characterized in that the third fluorescent tube (5; 1) the scanning track (A 7; A 9) from an angle (β) of 71.5 ° with respect to the scanning track (A 7; A 9 ) incident normal illuminated. 8. Control device according to one or more of claims 1 to 7, characterized in that in the direction transversely to the material web (4) several light scanning devices (2) are arranged in series, the scanning tracks (a) each scan a preferably equal part of the material web width and aligned with each other are. 9. Control device according to claim 8, characterized in that two parallel rows (7,9) of a plurality of light scanning devices (2) are provided, the light scanning devices (2) of the one row (7) are offset from the light scanning devices (2) of the other row (9) in such a way that that scanning gaps occurring in a row from the light scanning devices (2) of the other row be keyed. 10. Control device according to claim 1 or 8, characterized in that the photoelectric light scanning device (2) an imaging optics (15), a polygonal mirror wheel (11) which about an axis extending parallel to the material web (13) with its periphery with the scanning track (a ) is aligned rotatable, and has a photo converter (13) 11. Control device according to claim 13, characterized in that when using a lighting device with three fluorescent tubes (1,3,5) the scanning track (A 7 or A 9) of one row (7 or 9) of one of the two outer (1 or 3) and the middle (3) fluorescent tube from the same angle (cc) and the scanning track (A 9 or A 7) of the other row (9 or 7) from the middle (3) and the other (5 or 1 ) the outer fluorescent tube is illuminated from the same angle (cc) . 12. Control device according to claim 10 or 11, characterized in that each light scanning device (2) is assigned an evaluation device which has a marking pulse detector, the electrical pulses (19) supplied by the photo converter (13), which are approximately twice the amplitude of the basic signal level (21 ), and arise at the transfer points of the polygonal mirror wheel (11) where L iht from two adjacent mirror surfaces impinges on the photo converter (13), defined as the start and end points of the scanning track (a) of the respective light scanning device (2) 13. Control device according to claim 12, characterized in that the individual light scanning devices (2) assigned evaluation devices each have a control circuit (33) by means of which the individual light scanning devices (2) associated marker pulses (19) regulated to the same constant amplitude value will.