DE10016892A1 - Optoelectronic device for scanning reflective light includes first and second transmitters to emit transmitting light-induced impulses alternately. - Google Patents

Abstract

First (3) and second (4) transmitters emit transmitting light-induced impulses (2,2') alternately. Impulses (2) from the first transmitter are not polarized. Impulses (2') from the second transmitter are polarized in a first polarization direction. The transmitting light-induced impulses intersperse a transparent object, pass through a polarization filter (8) with a second polarization direction rotated with regard to the first polarization direction and are then picked up by a receiver (6) as received light-induced impulses (5). During the emission of each impulse a received signal is integrated at the receiver's outlet in an analyser (9). For every two consecutive transmitting light-induced impulses a difference (Ud) in integrated receiver signals is generated and compared with a threshold value.

Description

The invention relates to an optoelectronic device according to the Ober Concept of claim 1.

Such optoelectronic devices are used in particular as reflections light sensor trained. In this case, the transmission light rays e centering transmitter and the receiving light receiving receiver one side of the monitoring area and form with an evaluation unit to evaluate the received signals at the receiver sor. Is on the opposite side of the surveillance area a reflector on which the emitted light beams are free leads are. The reflector then transmits the light beams as a reception light rays reflected back to the receiver. With the help of a reflection light buttons become objects in a monitoring area between the sensor and the reflector is detected in that the transmitted-received light beams pass through the object can be steamed. To faulty switching by reflecting object to suppress surfaces, crossed polarization filters are used, which are arranged in front of the reflector and the receiver.

The disadvantage of such reflection light scanners is that they are transparent and even highly reflective objects cannot be reliably detected.

Another optoelectronic device is known from DE 42 38 116. This optoelectronic device also forms a reflection light scanner, in contrast to conventional diffuse sensors two receivers ger are provided. A polarization filter is arranged upstream of each receiver. The polarization directions of the polarization filters are at 90 ° to each other  turned. A polarization filter is also arranged upstream of the reflector. The two Receivers with the upstream polarization filters deliver with a free beam lengang receive signals of different amplitudes at their outputs.

The difference in reception is used to generate a binary switching signal signals rated with a threshold.

The disadvantage of this device is that transparent objects, in particular special plastic films can only be reliably recognized if their depolaris effect is sufficiently large. It can also be highly reflective Objects are faked a free beam path. Finally, through External light irradiation causes incorrect switching depending on the direction of radiation become.

The invention has for its object an optoelectronic device to create the type mentioned that solves the above disadvantages and a reliable detection of transparent objects guaranteed.

The features of claim 1 are provided to achieve this object. Advantageous embodiments and expedient further developments of the Erfin tion are described in the subclaims.

In the optoelectronic device according to the invention, in addition to the A second transmitter is provided in the first transmitter, said transmitter alternately transmitting emit light impulses, and being the transmission emitted by the first transmitter light pulses not polarized and the transmission emitted by the second transmitter light pulses are polarized in a first polarization direction.

The transmitted light pulses passing through a transparent object are over one Polarization filter with a second polarization direction, which with respect to the first direction of polarization is rotated, guided and hit as reception light impulses on the receiver.  

In an evaluation unit, during the emission of each transmission light pulses integrated the received signal at the output of the receiver.

The difference U _{d of} the integrated received signals U ₁ , U ₂ according to U _d = U ₁ -U _{2 is} formed for two successive transmitted light pulses and compared with a threshold value S ₁ (S ₁ <0).

Object detection is present if U _d <S ₁ . Otherwise there is a free beam path.

By scanning the objects with non-pola emitted by the first transmitter emitted light pulses and with polar emitted by the second transmitter based transmitted light pulses, their direction of polarization with respect to the polar sationsfilter is rotated, transparent and non-transparent ob equally secure objects. By integrating the at the exit received signals from the receiver become particularly high Preserve sensitivity, so that transparent objects are almost independent depending on how strong the polarization of the transmission light impulses are changed when passing through these objects.

With the optoelectronic device according to the invention, in particular reliable detection of plastic films and plastic panes with little enables damping of the transmission. At the same time it is also a safe one Detection of reflective surfaces as well as a good suppression of extraneous light guaranteed. The device according to the invention advantageously has a simple and inexpensive construction with standard components, with no high demands on the orientation of the individual compo must be provided.

Furthermore, the optoelectronic device can also be used as an angle sensor and for speed measurement. In this case, the polarization filter with the reflector is applied to an object rotating about an axis of rotation. The reflector is preferably applied to the object lying in the axis of rotation. To determine the angular position and / or speed of the object, in addition to the difference U _{d of} the received signals, their sum U _{S is also} formed, which is kept constant by regulating the transmit level. The difference U _d is then a measure of the angle between the reflector on the object to be measured and the optoelectronic device.

Another application is the detection of clear glass bottles. When moving a bottle past the optoelectronic device, there are typical profiles of the sum and difference signals U _S and U _d , which are used to count bottles even if there is no distance from the following bottle. When radiating at the edge of the bottle, the sum and difference signals U _S , U _d decrease, while a reflection in the middle of the bottle causes the sum signal U _S to rise sharply and the difference signal U _{d to} assume negative values.

Another application is the protection of passageways and machines intervention openings for personal protection. The functions the optoelectronic device is cyclically tested by an auxiliary receiver ger decoupled from the area of the peripheral radiation of the transmitted light pulses Detects light and thus controls the transmission functions. The auxiliary recipient can be located in the vicinity of the two transmitters or via one Light deflecting device that transfers part of the edge radiation to the auxiliary receiver decoupled be supplied. The recipient checks himself if the Beam path by recognizing the unpolarized transmitted light impulses of the first transmitter the entire light path of the optoelectronic Device with all optical components. Because this control was A device with self-monitoring is possible at the end of each transmission pulse period Realization in the system response time.

The invention is explained below with reference to the drawings. It demonstrate:  

Fig. 1: Block diagram of an embodiment of the optoelectronic device according to the invention.

Fig. 2: Structure of the optical components of the optoelectronic Before direction of FIG. 1.

Fig. 3: Second embodiment of the structure of the optical components of the optoelectronic device.

Fig. 4: Third embodiment of the structure of the optical components of the optoelectronic device.

Fig. 5: Fourth embodiment of the structure of the optical components of the optoelectronic device.

Fig. 6: Fifth embodiment of the structure of the optical compo nents of the optoelectronic device.

Fig. 7: First embodiment of the diagrams of the transmit and Emp start signals of the optoelectronic device.

Fig. 8: Typical level of the received signals of the optoelectronic device before the detection of various objects.

Fig. 9: embodiment of the structure of the evaluation unit of the optoe-electronic device.

Fig. 10: Second embodiment of the diagrams of the transmit and Emp start signals of the optoelectronic device.

Fig. 1 shows schematically the structure of an embodiment of the inventive optoelectronic device 1 for detecting objects in a surveillance area. In this case, the optoelectronic device 1 is designed as a reflection light barrier. It has two transmit light pulses 2 , 2 'emitting transmitter 3 , 4 and a receive light pulse 5 receiving receiver 6 , which are arranged on one side of the monitoring area.

On the opposite side of the monitoring area there is a reflector 7 with a polarization filter 8 arranged upstream thereof. When the optical path of the optoelectronic device 1 is free , the transmission light pulses 2 , 2 'emitted by the sensors 3 , 4 ' reach the reflector 7 unhindered and are reflected back there as reception light pulses 5 to the receiver 6 . If there is a non-transparent object in the beam path of the optoe-electronic device 1 , the transmitted light pulses 2 , 2 'are reflected back by the object as received light pulses 5 to the receiver 6 . If there is an at least partially transparent object in the beam path, the transmitted light pulses 2 , 2 'penetrate the object and are reflected on the reflector 7 . The received light pulses 5 reflected on the reflector 7 enforce the object again and then hit the receiver 6 .

According to the invention, the transmitters 3 , 4 alternately emit transmission light pulses 2 , 2 'with a predetermined pulse-pause ratio. The transmit clock is specified via an evaluation unit 9 , to which both transmitters 3 , 4 are connected.

The evaluation unit 9 is formed by a microcontroller or the like.

The first transmitter 3 emits unpolarized transmitted light pulses 2 and is formed by a light emitting diode. The second transmitter 4 emits transmission light pulses 2 'which are polarized in a predetermined direction of polarization. In the present exemplary embodiment, the transmitter 4 is formed by a light-emitting diode with a downstream polarization filter 10 . Alternatively, the transmitter 4 can be formed by a polarized light emitting laser. The receiver 6 is preferably formed by a photodiode.

The polarization direction of the emitted by the second transmitter 4 Sendelichtim pulse 2 'is rotated relative to the polarization direction of the reflector 7 vorgeord Neten polarization filter 8 , the angular offset preferably being 90 °.

The received light pulses 5 impinging on the receiver 6 generate photo currents I _e as received signals, which are read into a signal processing stage 11 . The signal processing stage 11 is connected to the evaluation unit 9 . From the evaluation unit 9 4 a periodically varying clock signal U _T is the light transmission of the transmitter 3, input to the signal conditioning stage. 11

By means of this transmit clock U _t , the pending reception signals I _e pending at the output of the receiver 6 are integrated during the duration of the emission of a transmission light pulse 2 or 2 '. From the thus formed integrated reception signals U ₁ , U ₂ , each of which is obtained during the emission of a transmission light pulse 2 or 2 'emitted by the first and second transmitters 3 , 4 , the difference U _{d of} these integrated reception signals is in accordance with

U _d = U ₁ - U ₂

obtained and read into the evaluation unit 9 . In addition, the sum signal U _S = = U ₁ + U ₂ is expediently formed and read into the evaluation unit 9 . There, the difference U _{d is} compared with a threshold value S ₁ (S ₁ <0) in order to generate an object message. An object detection exists if U _d <S ₁ . Otherwise there is a free beam path.

The switching signal thus formed is output via a switching output 12 connected to the evaluation unit 9 . Via a parameter input 13 connected to the evaluation unit 9 , parameter values, such as the height of the threshold value S _{1, can be} read into the evaluation unit 9 . These parameter values are stored in a parameter memory 14 .

FIG. 2 schematically shows the structure of the optical components of the optoelectronic device 1 according to FIG. 1. The unpolarized transmission light pulses 2 of the first transmitter 3 pass through a beam splitter mirror 15 and thus reach the monitoring area. The transmitted light pulses 2 'of the second transmitter 4, which are polarized by means of the polarization filter 10, are deflected at the beam splitter mirror 15 , so that their beam axis runs parallel to or at a small distance from the beam axis of the transmitted light pulses 2 emitted by the first transmitter 3 . The transmitted light pulses 2 , 2 'emitted by the transmitters 3 , 4 ' penetrate a transparent object, which in the present case is formed by a transparent film 16 . Then the transmitted light pulses 2 , 2 'are reflected on the reflector 7 with the upstream polarization filter 8 and, as the received light pulses 5, once again pass the transparent film 16 before they strike the receiver 6 .

FIG. 3 shows another exemplary embodiment of the optical components of the optoelectronic device 1 according to the invention. While the optoelectronic device 1 according to FIG. 2 forms a reflection light barrier, the optoelectronic device 1 according to FIG. 3 works according to the light barrier principle. The arrangement of the transmitters 3 , 4 with the beam splitter mirror 15 corresponds to the arrangement according to FIG. 2. In contrast to the exemplary embodiment according to FIG. 3, the receiver 6 is located on the opposite side of the monitoring area, the polarization filter 8 being arranged upstream of the receiver 6 whose polarization direction is in turn rotated relative to the polarization direction of the polarization filter 10 arranged downstream of the second transmitter 4 . The transmitted light pulses 2 , 2 'emitted by the transmitters 3 , 4 ' pass through the transparent film 16 and then reach the receiver 6 .

Fig. 4 shows a further arrangement of the optical components of the apparatus 1 optoe lektronischen. In this arrangement, the transmitters 3 , 4 , the receiver 6 and a reflector 7 , which the polarization filter 8 is arranged, are arranged on one side of the monitoring area. The transmitted light pulses 2 emitted by the first transmitter 3 in turn pass through the beam splitter mirror 15 , while the transmitted light pulses 2 ′ emitted by the second transmitter 4 are deflected at the beam splitter mirror 15 . A mirror 17 is arranged on the opposite side. The transmitted light pulses 2 , 2 'averaged by the transmitters 3 , 4 e pass through the transparent film 16 and strike the mirror 17 . From there, the transmitted light pulses 2 , 2 'are guided to the reflector 7 and from there reflected back in the same direction to the mirror 17 , so that the transparent film 16 is penetrated twice more by the transmitted light pulses 2 , 2 '. The then again reflected on the mirror 17 transmitted light pulses 2 , 2 'pass through as the received light pulses 5, the transparent film 16 for the fourth time and finally hit the receiver 6 . Since the transparent film 16 is penetrated several times by the transmitted light pulses 2 , 2 ', these are significantly attenuated even with a very high transparency of the films 16 , which considerably increases the sensitivity of detection when detecting the transparent objects of this type.

The arrangement of the optical components shown in FIG. 5 essentially corresponds to the arrangement according to FIG. 2. In contrast to the arrangement according to FIG. 2, in the arrangement according to FIG. 5 the polarization filter 8 is not arranged in front of the reflector 7 but in front of the receiver 6 .

Fig. 6 shows a further arrangement of the optical components of the optoe electronic device 1 , in which a substantially coaxial beam guidance of the transmitted light pulses 2 , 2 'of both transmitters 3 , 4 and the received light pulses 5 is obtained. The polarized transmitter light pulses 2 'emit de transmitter 4 sits in a central bore of a lens 18 , through which the transmitter light pulses 2 of the first transmitter 3 and the received light pulses 5 are guided. The receiver 6 with the upstream polarization filter 8 is located behind a beam splitter mirror 19 , the received light pulses 5 passing through the beam splitter mirror 19 and then hitting the receiver 6 . The transmitted light pulses 2 emitted by the transmitter 3 are partially reflected at the beam splitter 19 and guided through the lens 18 into the monitoring area. A part of the transmitted light pulses 2 emitted by the first transmitter 3 passes through the beam splitter mirror 19 . An absorbing surface 20 is provided behind the beam splitter mirror 19 so that this portion of the transmitted light does not lead to undesired reflections.

In a modified embodiment of the arrangement according to FIG. 6, the polarization filter 8 can be arranged in front of the reflector 7 instead of in front of the receiver 6 .

Fig. 7 shows the curves of the transmit and receive signals for an exemplary embodiment of the optoelectronic device 1 according to the invention.

The transmission signals U _S1 U _S2 form voltage signals with which the first and second transmitters 3 , 4 are controlled. The temporal profiles of the transmission signals U _S1 U _S2 correspond to the temporal profiles of the transmitted light pulses 2 , 2 'emitted by the two transmitters 3 , 4 . As can be seen from FIG. 7, the transmitters 3 , 4 emit transmission light pulses 2 , 2 'at the same frequency with a predetermined pulse-pause ratio. The transmission signals U _S1 and thus the unpolarized transmission light pulses 2 of the first transmitter 3 have a lower signal amplitude than the transmission light pulses 2 'emitted by the second transmitter 4 .

Corresponding to the sequence of the transmitted light pulses 2 , 2 'emitted by the transmitters 3 , 4 , the course of the received signal I _e shown in the third diagram in FIG. 7, obtained at the output of the receiver 6 as a photocurrent, is obtained when the optoelectronic device 1 is free becomes. The course of the received signal I _e is pulse-shaped, the pulse frequency corresponding to the transmission frequency of the transmitters 3 , 4 . Although the transmitted light pulses 2 'emitted by the second transmitter 4 have a greater amplitude than the transmitted light pulses 2 emitted by the first transmitter 3 , the amplitude of the received signal I _e , which originates from a transmitted light pulse 2 from the first transmitter 3 , is greater than the received signal I _e , that comes from the second transmitter 4 . The reason for this is that the the second transmitter 4 downstream of the polarization filter is rotated 10 relative to the polarization filter 8 in front of the receiver 6, so that a correspondingly low transmission light amount to the catcher Emp impinges. 6 In the present exemplary embodiment, the polarization filters 8 , 10 are rotated relative to one another by an angle that is close to 90 °, so that with a free beam path from the transmitted light emitted by the second transmitter 4, only a small part hits the receiver 6 .

7 also the timing of the clock signal U is shown in FIG. _T, which is input from the evaluation unit 9 in the signal conditioning stage 11. Within each period T of the clock signal V _t varying in the transmission clock, a transmitter light pulse 2 , 2 'is emitted by each transmitter 3 , 4 . The received signal I _e generated during the emission of the transmitted light pulse 2 of the first sensor 3 is integrated in the signal conditioning stage 11 and forms the integrated received signal U ₁ . Correspondingly, during the emission of the transmitted light pulse 2 'emitted by the second transmitter 4 , the received signal I _e generated thereby is integrated and forms the integrated received signal U ₂ . Then in the signal processing stage 11, the difference between the integrated received signals U ₁ , U ₂ according to the relationship

U _d = U ₁ - U ₂

educated.

In the exemplary embodiment shown in FIG. 7, the integration time is selected such that it not only includes the emission of a transmission light pulse 2 or 2 'but also the subsequent transmission pause. Thus, in addition to the integrated reception signal U ₁ obtained during the emission of a first transmission light pulse 2 , the portion of the integrated reception signal U _p obtained during the following transmission break is added. The same applies to the reception signal I _e registered in the transmission pause following the transmission light pulse 2 'of the second transmitter 4 . This received signal is also integrated and added to the value of U ₂ as an integrated received signal U _p .

This results in the course of U _d shown in the lower diagram of FIG. 7 with a free beam path ("free path"). The value of U _d is read into the evaluation unit 9 and compared there with the threshold value S ₁ , which is selected such that U _d lies above S ₁ . Accordingly, the switching output 12 assumes the "no object present" switching state. If an object penetrates the beam path of the optoelectronic device 1 , the value U _d falls below S ₁ , so that the switching output 12 assumes the switching status "object present".

If there is a non-transparent object in the beam path of the optoelectronic device 1 , the value of the integrated received signal U ₁ drops to a value close to zero, so that U _{d is} in any case less than S ₁ . If there is an at least partially transparent object in the beam path of the optoelectronic device 1 , the level of the integrated received signal U _{1 is} at least slightly reduced. At the same time, the level of U ₂ increases due to the depolarizing effect of the transparent object, so that in this case too, the difference U _d lies below S ₁ , so that an object message is sent via the switching output 12 . Thus, with the optoelectronic device 1 according to the invention, both transparent and non-transparent objects are detected.

Fig. 8 shows in the upper diagram of typical level of the integrated reception signals U _1, U ₂ for different objects in the beam path. The lower diagram of FIG. 8 shows the course of U _d obtained from these received signals.

In FIG. 8, the level ratios of the integrated reception signals U _1, U ₂ and U _d are to the free beam path ( "free distance") for an in-beam path transparent film 16 ( "film"), for a angeord Neten in the beam path mirror ("Mirror") and for a non-transparent, diffusely reflecting object located in the beam path ("diff. Object"). From FIG. 8 it can be seen that U _{d is} above half of S ₁ only in the case of a free beam path, whereas a value of U _d less than S _{1 is} obtained for each type of object intervention.

In order to optimally set U _d , a teaching process can be carried out on a free path, the transmission level of the first transmitter 3 being regulated in such a way that the differential voltage U _d just reaches a threshold value S ₂ , where S ₂ <S ₁ . The regulation can also be carried out by incrementally changing the transmission level by incrementing or by nesting intervals. Depending on the desired sensitivity, the threshold value S _{1 is} calculated as a fraction of S ₂ and both values are stored in the parameter memory 14 . In order to compensate for a drift due to contamination or temperature, the transmission level of the first transmitter 3 can be tracked at larger time intervals. For this purpose, a counter value w _{1 is} decremented with each transmission pulse if the differential voltage U _d exceeds the threshold value S ₂ , or incremented if U _d lies between S ₁ and S ₂ . If w ₁ exceeds an upper threshold value (e.g. 10 ⁶ ), the transmission level of the first transmitter 3 is increased by one level step, or decremented by a level step if the value falls below a lower threshold value (e.g. -10 ⁶ ) and the counter value w ₁ reset to zero. The transmission frequency of the transmitters 3 , 4 is typically 1-50 kHz. The high amount of the threshold value for w ₁ ensures that the tracking takes place only slowly and is not distorted by object movements. At signal levels U _d below the threshold S _{1 there} is no tracking because there is then no free path. After cleaning the exit window of the optoelectronic device 1, which is not shown, the transmission level of the first transmitter 3 is too high and is automatically reduced by the tracking until U _{d reaches} the threshold value S ₂ . If the optoelectronic device 1 is immediately available after a cleaning process, a teaching process can be triggered by an external signal at the parameter input 13 .

Fig. 9 shows an embodiment of the signal conditioning stage 11. The photodiode forming the receiver 6 is operated in the reverse direction and supplies the photocurrent I _e which flows through the transistor T ₁ when illuminated. The transistors T ₁ to T ₃ form a current mirror, the current in T ₂ feeding the transistor T ₄ , which together with T ₅ forms a further current mirror. The current of the transistor T ₃ or T _{5 reaches} the summation point of the low-pass filter via the diode switch D ₁ to D ₄ . Depending on the clock signal U _t , the diode pair D ₂ , D ₃ , or D ₁ , D _{4 is} conductive, whereby the mirrored photo current through C _{1 is integrated} positively or negatively. By connecting several transistors T ₃ to T ₃ ^{k '} and T ₅ to T ₅ ^{k' in} parallel, the photocurrent is amplified by a factor of k. The low pass filter 2 . Order ensures that the ripple is kept low.

The circuit offers the advantage that the reverse voltage at the photodiode remains constant regardless of the photocurrent and, as a result, the diode capacitance does not have to be recharged, as a result of which short transmission light pulses 2 , 2 'or high transmission frequencies are possible. In addition, there are no circuit-related resistances in the input circuit that could act as sources of noise. It is also advantageous that no coupling capacitors are required which are noticeable as disruptive energy stores in the case of high-energy light pulses. Since the conversion of the photocurrent into an output voltage takes place only in the capacitor C ₁ , the circuit is largely insensitive to interference voltages. Such a design of the signal processing stage 11 is particularly advantageous if it is designed together with the evaluation unit 9 as an ASIC.

Fig. 10 shows an embodiment of the transmission and reception level for another embodiment of the evaluation of the receiver 6 pending the reception signals I _e. Analogously to the exemplary embodiment according to FIG. 7, U _S1 , U _{S2 represent} the transmission signals of the two transmitters 3 , 4 , the transmitters 3 , 4 periodically and alternately emitting transmission light pulses 2 , 2 'of different amplitudes.

In contrast to the exemplary embodiment, the clock signal U _{t is formed} by a periodic signal sequence which, in addition to predefined time intervals t ₁ , t _2, into several random time intervals tz ₁ , tz ₂ ,. , , tz _{n is} divided. In each case within a time interval T ₁ , the emission of a transmission light pulse 2 or 2 'takes place. The integration time t _{1 is} only chosen to be long enough to reliably detect the photo current I _e caused by a transmitted light pulse 2 , 2 '. The remaining time intervals each subdivide the subsequent transmission breaks. Depending on whether the value of U _{t is} positive or negative within one of these time intervals, the value of the received signal U _p integrated during this time interval with a positive or negative sign _is taken into account when forming U _d . In this case, in the formation of U _d to be taken into account before U ₁ and U ₂ with a positive sign with a negative sign in the formation of U _d.

During the transmission pause, which is large compared to the duration of the first transmission light pulse 2 , the direction of integration is continuously switched. In order to suppress extraneous light with the same frequency as the transmission frequency of the transmitters 3 , 4 or an integer fraction thereof, each integration interval pair of two equal random time _intervals t _{zn is formed} by a random value, where t ₂ <t _z <t ₁ . To reverse the phase, the time t _{2 is} inserted after the interval pair t ₁ . The next transmitted light pulse 2 , 2 'is started when the time t ₃ , which corresponds to the pulse pause time of the transmitted light pulses 2 , 2 ', has elapsed and the current pair of intervals has ended. This ensures that the mean value of U _t remains approximately zero over a transmission period, as a result of which uniform light radiation is suppressed.

Reference list

(

1

) Optoelectronic device
(

2

) first transmission light pulse
(

2

') second transmitted light pulse
(

3rd

) first station
(

4

) second transmitter
(

5

) Received light pulse
(

6

) Receiver
(

7

) Reflector
(

8th

) Polarization filter
(

9

) Evaluation unit
(

10th

) Polarization filter
(

11

) Signal processing stage
(

12th

) Switching output
(

13

) Parameter input
(

14

) Parameter memory
(

15

) Beam splitter mirror
(

16

) transparent film
(

17th

) Mirror
(

18th

) Lens
(

19th

) Beam splitter mirror
(

20th

) absorbent surface

Claims (16)

1. Optoelectronic device for detecting at least partially transparent objects in a surveillance area with at least one transmitter and one receiver, characterized in that in addition to the first transmitter ( 3 ), a second transmitter ( 4 ) is provided, these alternating transmission light pulses ( 2 , 2 '), and the transmitted light pulses ( 2 ) emitted by the first transmitter ( 3 ) are not polarized and the transmitted light pulses ( 2 ') emitted by the second transmitter ( 4 ) are polarized in a first polarization direction such that the transmitted light pulses ( 2 , 2 ') are guided via polarization filters ( 8 ) with a second polarization direction, which is rotated with respect to the first polarization direction, and hit the receiver ( 6 ) as received light pulses ( 5 ), that in an evaluation unit ( 9 ) during the emission of each transmitted light pulse (2, 2 ') receiving signal I _e at the output of the receiver ngers (6) is integrated, that for every two successive transmitting light pulses (2, 2 '), the difference U _d of the integrated reception signals U _1, U ₂ according to U _d = U ₁ - U ₂ is formed, and with a threshold value S ₁ ( S ₁ <0) is compared, and that there is an object detection if U _d <S ₁ and otherwise there is a free beam path. 2. Optoelectronic device according to claim 1, characterized in that in the transmission pauses of the transmitters ( 3 , 4 ) at the output of the receiver ( 6 ) pending reception signal I _{e is} integrated, and that the integrated reception signal U _p with a predetermined sign is added to the integrated received signals U ₁ , U ₂ . 3. Optoelectronic device according to claim 2, characterized in that the sign of the reception signal U _p integrated in a transmission pause corresponds in each case to the sign of the integrated reception signal U ₁ or U ₂ , which emits the emission of the transmission light pulse emitted before the respective Sen pause ( 2 , 2 ') was determined. 4. Optoelectronic device according to claim 2, characterized in that each transmission pause is subdivided into a plurality of time intervals, where the reception signals U _pi determined in successive time intervals are weighted with alternating signs. 5. Optoelectronic device according to claim 4, characterized in net that the lengths of the time intervals are given by a random value ben are. 6. Optoelectronic device according to one of claims 1-5, characterized in that the transmission power of at least one transmitter ( 3 , 4 ) is regulated by means of a control circuit, so that the value of the difference signal U _d a threshold value S ₂ (S ₂ <S ₁ ) corresponds. 7. Optoelectronic device according to one of claims 1-6, characterized in that the second, polarizing transmitter light pulse ( 2 ') emitting transmitter ( 4 ) is formed by a laser. 8. Optoelectronic device according to one of claims 1-6, characterized in that the second transmitter ( 4 ) is followed by a polarization filter ( 10 ) through which the transmitted light pulses ( 2 ') emitted by this transmitter ( 4 ) polarize in the first polarization direction are. 9. Optoelectronic device according to one of claims 1-8, characterized in that the transmitter ( 3 , 4 ) the monitoring area on a first side and the receiver ( 6 ), which is the polarization filter ( 8 ) upstream, the monitoring area on the limit opposite side. 10. Optoelectronic device according to one of claims 1-8, characterized in that the transmitter ( 3 , 4 ) and the receiver ( 6 ) are arranged on one side of the monitoring area, and that the monitoring area on the opposite side from a reflector ( 7 ) is limited, upstream of which the polarization filter ( 8 ) is arranged, the transmitted light pulses ( 2 , 2 ') emitted by the transmitters ( 3 , 4 ) being guided to the receiver ( 6 ) via the reflector ( 7 ) as received light pulses ( 5 ) . 11. Optoelectronic device according to one of claims 1-8, characterized in that on one side of the monitoring area, the sen ( 3 , 4 ), the receiver ( 6 ) and a reflector ( 7 ), which upstream the polarization filter ( 8 ) is, are arranged, and that on the opposite side of the monitoring area a mirror ( 17 ) is arranged so that the emitted by the transmitters ( 3 , 4 ) transmit light pulses ( 2 , 2 ') on the mirror ( 17 ) and reflected are guided to the reflector ( 7 ) and from there via the mirror ( 17 ) to the receiver ( 6 ). 12. Optoelectronic device according to one of claims 1-8, characterized in that the transmitter ( 3 , 4 ) and the receiver ( 6 ), which the polarization filter ( 8 ) is arranged upstream, are arranged on one side of the monitoring area, and that a reflector ( 7 ) is arranged on the opposite side of the monitoring area, so that the transmitted light pulses ( 2 , 2 ') emitted by the transmitters ( 3 , 4 ) via the reflector ( 7 ) as received light pulses ( 5 ) to the receiver ( 6 ) are led. 13. Optoelectronic device according to claim 12, characterized in that the transmitted light pulses ( 2 , 2 ') and received light pulses ( 5 ) run axially in the monitoring area. 14. Optoelectronic device according to one of claims 1-13, characterized in that the difference U _{d is formed} in a signal conditioning stage ( 11 ) connected to the evaluation unit ( 9 ), which has an arrangement of transistors and diodes. 15. Optoelectronic device according to claim 14, characterized in that the evaluation unit ( 9 ) and the signal processing stage ( 11 ) are formed by an ASIC. 16. Optoelectronic device according to one of claims 1-15, characterized in that an auxiliary receiver is provided for checking the function of the transmitter ( 3 , 4 ), with transmitted light in the edge region of the beam diameter of the transmitted light pulses ( 2 , 2 ') being coupled out and onto the Aid recipient is guided.