SENSOR ARRANGEMENT

Description

[0001] The invention relates to a sensor arrangement according to the preamble of claim 1.

[0002] Known sensor arrangements of this type are designed as retroreflective light barriers, which comprise a transmitter arranged at a first edge of a monitoring area that emits transmitted light beams, a receiver that receives received light beams, and an evaluation unit. A reflector in the form of a retroreflector is located at the opposite edge of the monitoring area. The transmitter emits polarized transmitted light beams, which is achieved, for example, by a polarization filter assigned to the transmitter. In addition, a polarization filter adapted to the transmitter can be assigned to the receiver.

[0003] Object detection in the evaluation unit generally occurs by thresholding the amplitudes of the receiver's received signals. If the monitoring area is clear, the transmitted light beams reach the reflector unhindered and are then at least partially depolarized as received light beams and returned to the receiver, so that the amplitude of the received signals is above the threshold. If an object enters the monitoring area, the transmitted light beams are usually slightly depolarized by reflection from this object and are generally reflected non-directionally, so that the amplitude of the received signals is below the threshold.

[0004] Depending on this threshold value evaluation, a binary switching signal is generated in the evaluation unit, the switching states of which indicate whether an object is in the monitoring area or not.

[0005] Known reflective light barriers can have two different basic structures, which are described in the Figures 1a and 1b are shown.

[0006] Figure 1a shows a single-lens design of a reflective light barrier 1a with a transmitter 3 emitting transmitted light beams 2 and a receiver 5 receiving received light beams 4. In this embodiment, the transmitted light beams 2 and received light beams 4 are guided coaxially into the monitoring area by a beam splitter 6 via a common transmitting and receiving optics 7, i.e. via only one lens.

[0007] Figure 1bshows a two-lens design of a reflective light barrier 1a. In this embodiment, the transmitter 3 and the receiver 5 are arranged side by side, so that the beam axes of the transmitted light beams 2 and the received light beams 4 are offset from one another. A transmitting optic 7a in the form of a lens is assigned to the transmitter 3, and a receiving optic 7b in the form of a lens is assigned to the receiver 5. The lenses are spaced apart from one another, forming the two-lens reflective light barrier 1a.

[0008] In both cases, the monitoring area is limited by a retroreflector 9a.

[0009] Since the transmitted light beams 2 are reflected back into themselves as received light beams at the retroreflector 9a with only a slight lateral offset, as Figure 2ashows, in the single-lens reflection light barrier 1a, the receiving light spot 40 is guided over its entire surface onto the transmitting / receiving aperture I of the transmitting and receiving optics 7.

[0010] A completely different picture emerges for the two-lens reflective light barrier 1a, as Figure 2b shows. Since the transmitted light beams are reflected back as received light beams at the retroreflector 9a with only a slight offset in a clear surveillance area, the majority of the received light spot 40 is reflected back onto the transmit aperture II of the transmit optics, whereas only a small portion of the received light spot 40 hits the receive aperture III. Only this small portion of the received light spot 40 is therefore available for object detection.

[0011] This is particularly problematic when the retro-reflective sensor is designed to detect transparent objects, such as beverage bottles. While these transparent objects can be detected relatively well with single-lens retro-reflective sensors, they cannot be detected relatively well with dual-lens retro-reflective sensors.

[0012] When detecting transparent objects, the following effects must generally be considered. On the one hand, the received signal at the receiver can be reduced by reflection losses on the surface of the transparent object. Material stresses within the transparent object, particularly in beverage bottles, can increase the received signal through additional depolarization. Light-deflecting properties of the transparent object can also enhance the received signal.

[0013] These last two effects can lead to increased false detections in dual-lens retro-reflective photoelectric sensors, since only a small amount of received light reaches the receiver when the monitoring area is clear, and this signal level defining the free monitoring area can easily be exceeded by the received signal-enhancing effects of the transparent object, thus creating the illusion of a free monitoring area.

[0014] This interference effect is hardly noticeable with single-lens retro-reflective light barriers, since in a clear monitoring area almost all of the received light hits the receiver, so that the received signal-enhancing effects of a transparent object do not have a noticeable effect.

[0015] Therefore, single-lens retro-reflective sensors are generally used to detect transparent objects such as beverage bottles, as these can reliably detect transparent objects, unlike double-lens retro-reflective sensors.

[0016] A disadvantage of single-lens retro-reflective sensors, however, is that they have a more complex design than dual-lens retro-reflective sensors, resulting in higher manufacturing costs. This additional expense arises from the required beam splitter. Furthermore, single-lens retro-reflective sensors require transmitting and receiving optics with low internal stresses to ensure defined polarization behavior. This requires the transmitting and receiving optics to be constructed, particularly in the form of expensive glass lenses, whereas dual-lens retro-reflective sensors can utilize cost-effective plastic lenses.

[0017] WO 01 / 13144 A1 relates to a light barrier arrangement comprising at least one light source for emitting a parallel beam and at least one detector element arranged in the beam path of the beam. A beam splitter is arranged in the beam path of the beam to spread the beam into a disk-shaped, flat or bowl-shaped, curved beam field. The at least one detector element is optically aligned toward the beam splitter within the beam field.

[0018] DE 198 01 632 A1 relates to a retro-reflective light barrier, in particular for detecting transparent, polarizing materials, comprising a transmitter and receiver located in close proximity to one another, and a reflector spaced therefrom. The electromagnetic waves emitted by the transmitter are reflected by the reflector onto the receiver, which emits an output signal dependent on the intensity of the received light. This output signal is used to detect objects introduced into the area between the transmitter or receiver and the reflector, the detection area. A polarizer that creates a defined polarization state PS is located in front of the transmitter output. Alternatively, the transmitter emits polarized light of polarization PS, while a polarizer that allows light of a defined polarization PE to pass through to the receiver is located in front of the receiver.In front of the reflector there is a polarizer that allows the reflected light to pass through its polarization state PR.

[0019] The invention is based on the object of providing a sensor arrangement of the type mentioned above which enables reliable detection of transparent objects with little construction effort.

[0020] To achieve this object, the features of claim 1 are provided. Advantageous embodiments and expedient developments of the invention are described in the dependent claims.

[0021] The invention relates to a sensor arrangement for detecting transparent objects in a surveillance area, comprising a sensor at a first edge of the surveillance area and a reflector at a second edge of the surveillance area opposite the first edge. The sensor comprises a transmitter emitting transmitted light beams, a receiver with a receiving aperture, and an evaluation unit for evaluating received signals from the receiver. The sensor arrangement is a two-lens

[0022] A retroreflective light barrier with a transmitter and receiver arranged at a distance from one another. The transmitter is assigned a transmitting optic, and the receiver is assigned a receiving optic arranged at a distance from the transmitting optic. The reflector is designed as a beam-splitting reflector. Transmitted light beams striking the beam-splitting reflector are reflected as received light beams in such a way that the received light beams are split into two partial beams, each of which is angularly offset from the transmitted light beams. The beam-splitting reflector has a triple arrangement with an entrance surface having a roof structure, the triple arrangement being adapted to the roof structure. The transmitted light beams striking the entrance surface enter the triple arrangement and reflect a triple onto reflection surfaces.Transmitted light beams striking the entrance surface are refracted by the roof structure so that the reflected received light beams are split into two partial beams. The beam-splitting reflector is positioned relative to the sensor so that, when the monitoring area is clear, one partial beam is directed to the receiver.

[0023] With the sensor arrangement according to the invention, transparent objects can be reliably detected, whereby the sensor arrangement has a simple, cost-effective design.

[0024] According to the invention, the sensor arrangement is designed like a two-lens reflective light barrier. The transmitter and receiver are spaced apart, so that the beam axes of the transmitted light beams emitted by the transmitter and the received light beams directed to the receiver are spatially offset from each other.

[0025] The transmitting and receiving optics can each consist of a cost-effective plastic lens. Furthermore, no complex optical components such as beam splitters are required.

[0026] An essential component of the sensor arrangement for the reliable detection of transparent objects is the beam-splitting reflector, which splits the incoming transmitted light beams into exactly two partial beams, each of which has a different angular offset to the incoming transmitted light beams.

[0027] In contrast to a retroreflector, in the beam-splitting reflector, the reflection from the incident transmitted light beams on the reflector surface does not produce a received light beam running parallel to the transmitted light beams, but rather at least two partial beams which have an angular offset to the transmitted light beams, i.e. the partial beams each run at a predetermined angle to the transmitted light beams.

[0028] According to the invention, this property of the beam-splitting reflector is exploited to position the beam-splitting reflector relative to the sensor in such a way that a partial beam of light hits the receiver, i.e. the amount of received light of this partial beam of light largely hits the receiver.

[0029] In the event that the received light spot is larger than the receiving aperture of the receiver, the partial beam guided to the receiver in a clear monitoring area completely illuminates the receiving aperture of the receiver.

[0030] In the event that the received light spot is smaller than the receiving aperture of the receiver, the partial beam guided to the receiver in a clear monitoring area hits the evaluation unit of the receiver completely.

[0031] If the transmitted light beams are split into exactly two partial beams at the beam-splitting reflector, this ensures that, in a clear monitoring area, 50% or almost 50% of the light reflected back from the beam-splitting reflector reaches the receiver. Compared to dual-lens retro-reflective light barriers that use retroreflectors, the amount of received light reaching the receiver is significantly increased in a clear monitoring area.

[0032] Thus, the sensor arrangement according to the invention, although it has a two-lens design, can detect transparent objects safely and reliably.

[0033] To detect transparent objects, an amplitude evaluation of the receiver's received signals is carried out in the evaluation unit.

[0034] The amplitudes of the received signals are evaluated using a threshold value. The amplitudes of the received signals are above the threshold value when the monitoring area is clear, and below the threshold value when a transparent object is detected.

[0035] As a result, a binary switching signal is generated in the evaluation unit depending on the amplitude evaluation of the received signals.

[0036] According to an advantageous embodiment, the sensor of the sensor arrangement according to the invention operates with polarized light. For this purpose, a first polarization filter is arranged downstream of the transmitter as a polarizer. Furthermore, a second polarization filter is arranged upstream of the receiver as an analyzer. Generally, the transmission directions of the analyzer and polarizer are offset by 90° from each other. The sensor can operate with linearly or circularly polarized light. This arrangement of polarization filters prevents incorrect sensor switching when detecting shiny and reflective objects.

[0037] Since, according to the invention, the amount of light from a partial beam is guided almost entirely to the receiver by the beam-splitting reflector when the monitoring area is clear, effects of a transparent object that would otherwise only increase the light reflected back to the receiver, such as light-deflecting properties or depolarizing properties, do not have an effect, or not so strong as to cause the received signal amplitude to rise above the threshold value. This means that there is no erroneous influence on the switching behavior of the sensor arrangement, since the reflective properties of the transparent object cannot simulate an unobstructed monitoring area. This allows transparent objects to be detected accurately and reliably.

[0038] In particular for transparent objects in the form of rotationally symmetric bodies such as beverage bottles that can be filled with transparent liquids, the sensor arrangement according to the invention provides more reliable object detection than a single-lens reflective light barrier.

[0039] With a single-lens retro-reflective sensor, the transmitted and received light beams are coaxial in a clear monitoring area. If a beverage bottle enters the monitoring area, it is detected centrally, i.e., centrally, by the transmitted light beams. Due to Fresnel reflection losses and transmission losses in the bottle material, the level of the received signals is only relatively slightly reduced compared to the clear monitoring area, so object detection can easily be distorted by interference or is generally relatively unreliable.

[0040] However, in the sensor arrangement according to the invention, a partial beam of the received light is reflected at a specific angular offset from the reflector when the monitoring area is clear, with the components of the sensor arrangement being optimized or adjusted such that the partial beam at least largely hits the receiver when the monitoring area is clear. If a beverage bottle, in particular one filled with a transparent liquid, is located in the monitoring area, the beam path of the partial beam is changed by refraction at the beverage bottle, or the transmitted light rays striking the beverage bottle are reflected back in the beverage bottle at an angle that differs from the angular offset of the partial beam reflected back at the reflector when the monitoring area is clear.

[0041] Therefore, when the beverage bottle enters the monitoring area, the received signal level is significantly lower than when the monitoring area is clear. This is especially true if the beverage bottle is moved perpendicular to the propagation direction of the transmitted light beams, so that the transmitted light beams no longer hit the center of the beverage bottle.

[0042] As a result, for the sensor arrangement according to the invention, a higher difference in the level of the received signal compared to the level of the received signal in a free monitoring area is obtained for bottle detection compared to a single-lens reflective light barrier, thereby ensuring more reliable object detection.

[0043] The beam-splitting reflector used in the sensor arrangement according to the invention has a simple and compact design so that it can be manufactured efficiently and cost-effectively.

[0044] According to the invention, the beam-splitting reflector has an entrance surface with a roof structure. On the entrance surface, incident transmitted light beams are refracted by the roof structure such that the reflected received light beams are split into at least two partial beams.

[0045] The beam-splitting reflector has a triple arrangement to which the roof structure is adapted.

[0046] Advantageously, the roof structures are formed from a multiple arrangement of roof elements whose widths are the same as those of the triples of the triple arrangement or differ only slightly from them.

[0047] If the widths of the roof elements correspond to the widths of the triples and the roof elements have a triangular cross-section, the roof structure thus formed generates two partial beams on both sides of the transmitter aperture of the transmitter.

[0048] However, if the widths of the roof elements are slightly larger or smaller than the widths of the triples, a total of three partial beams are generated with a triangular cross-section of the roof elements.

[0049] If the widths of the roof elements correspond to the widths of the triples and the roof elements do not have a triangular but rather a polygonal cross-section, the roof structure formed in this way produces four partial beams which are symmetrical to the transmitter.

[0050] These examples show that by specifying the geometry of the roof elements, the number and spatial distribution of the generated partial beams can be variably specified.

[0051] For the sensor arrangement according to the invention, it is particularly advantageous if the beam-splitting reflector splits the transmitted light beams into only two partial beams, since then, when the monitoring area is clear, a large percentage of the light reflected by the beam-splitting reflector is guided to the receiver.

[0052] The invention is explained below with reference to the drawings. They show: Figure 1a, 1bSchematic representations of prior art retroreflective light barriers. Figure 2a, 2bTransmitting / receiving apertures for the retroreflective light barriers according to the Figures 1a, 1b . Figure 3Embodiment of the sensor arrangement according to the invention. Figure 4Representations of a triple of the beam-splitting reflector for the sensor according to Figure 1: 4a without roof structure, 4b with a first roof element as a component of a roof structure, 4c with a second roof element as a component of a roof structure. Figure 5 First embodiment of a triple arrangement of a beam-splitting reflector with a roof structure: 5a in a cross-sectional view, 5b in a plan view. Figure 6 Second embodiment of a triple arrangement of a beam-splitting reflector with a roof structure: 6a in a cross-sectional view, 6b in a plan view. Figure 7 Schematic representation of the partial beams generated with the beam-splitting reflector relative to the transmitting and receiving aperture of the sensor arrangement according to Figure 3 with optimized distance between the reflector and the sensor. Figure 8Arrangement according to Figure 7 : a) with increased distance between reflector and sensor, b) with reduced distance between reflector and sensor.

[0053] Figure 3schematically shows an embodiment of the sensor arrangement 1 according to the invention. The sensor arrangement 1 has a sensor 10 at one edge of a monitored area. A beam-splitting reflector 9 is located at the opposite edge of the monitored area.

[0054] The sensor 10 has a housing 10a in which a transmitter 3 emitting transmitted light beams 2 and a receiver 5 are arranged. A transmitting optics 7a and a polarization filter (not shown) are arranged downstream of the transmitter 3. A receiving optics 7b and a polarization filter (not shown) are arranged upstream of the receiver 5. The transmission directions of the polarization filters are offset by 90° from each other. The provision of the polarization filters is optional and prevents false detections of objects with shiny or reflective surfaces.

[0055] The transmitter 3 can, for example, be formed by a light-emitting diode, while the receiver 5 can be formed by a photodiode. The transmitting and receiving optics 7a, 7b each consist of a lens, in particular a plastic lens.

[0056] The transmitter 3 and the receiver 5 as well as the transmitting optics 7a and the receiving optics 7b are each spaced apart from each other, so that the optical axes of the transmitting optics 7a and the receiving optics 7b are offset from each other. In this respect, the sensor arrangement 1 according to Figure 3 the two-lens reflective light barrier 1a according to Figure 1b .

[0057] The sensor 10 further comprises an evaluation unit 8 for evaluating the received signals present at the output of the receiver 5. The evaluation unit 8 can be formed by a microprocessor or the like.

[0058] In the evaluation unit 8, a binary switching signal is generated based on the received signals from the receiver 5. The switching states indicate whether an object is within the monitoring area or not. The amplitudes of the received signals are evaluated using a threshold value. The switching state corresponding to a clear monitoring area receives the switching signal if the amplitude of the received signal is above the threshold. Conversely, the amplitude of the received signals is below the threshold if an object is within the monitoring area and the transmitted light beams 2 from the transmitter 3 impinge on this object.

[0059] According to the invention, a beam-splitting reflector 9 is used as a reflector, wherein the Figures 4b, 4c Embodiments of the beam-splitting reflector 9. The design of the beam-splitting reflector 9 according to the invention according to the Figures 4b, 4cis compared below with a known from the state of the art, in Figure 4a The reflector shown is explained.

[0060] Figure 4a shows a schematic sectional view of a triple 11 of a reflector known from the prior art, which consists of a multiple arrangement of such triple 11s. This triple 11 has a flat surface element 11a as the entrance surface, onto which the transmitted light beams 2 impinge. The entrance surface simultaneously forms the exit surface of the triple 11 for the received light beams. The triple 11 is preferably made of transparent plastic.

[0061] As from Figure 4aAs can be seen, the incident light rays Ia, IIa strike the entrance surface perpendicularly and are then reflected by a first inclined surface of the triple 11, so that they are deflected transversely as light rays IIIa and then reflected again by the second or third inclined surface of the triple 11. As a result, the outgoing light rays Ib, IIb are offset parallel to the incident light rays Ia, IIa.

[0062] In the triples 11 according to the invention according to the Figures 4b, 4c The entry or exit surface forms a roof element 12a, 12b. In the embodiment of the Figure 4b Two surface segments extend at an angle α towards a raised central edge, so that the roof element 12a is designed as a gable roof with a triangular cross-section. In the embodiment of the Figure 4ctwo surface segments run at an angle α towards a recessed central edge, so that the roof element 12b is groove-shaped with a triangular cross-section.

[0063] The roof elements 12a, 12b according to the Figures 4b and 4c The incident light rays Ia, IIa are refracted by these, resulting in an angular offset δ for the outgoing light rays IIb, IVb. This leads to a splitting of the received light rays into two partial beams, which are symmetrical to the transmitting aperture II.

[0064] The Figures 5a and 5b show a first embodiment of a reflector with a roof structure consisting of the roof elements 12a and 12b. The three reflection surfaces of each individual triple are square. Figures 6a and 6b show a further embodiment of the beam splitting reflector 9.

[0065] The Figures 5a and 6aeach show a lateral view into the beam-splitting reflector 9. The areas of the individual triplet lenses as well as the roof structure applied above the individual triplet lenses on the entrance surface can be seen. The roof elements 12a are surfaces with rising flanks, which are Figures 5b and 6b are represented by ascending hatching. The roof elements 12b are surfaces with falling flanks, which are Figures 5b and 6b are represented by descending hatching. Roof elements 12a and 12b have the same width.

[0066] Figure 5bshows a vertical view of the entrance surface of the beam-splitting reflector 9, which consists of three rows of three triple prisms 11 each. The view shows hexagonally delimited individual triples, in which all three surfaces contribute to retroreflection across their entire extent. The width of a roof element 12a together with that of an adjacent roof element 12b corresponds to the width of a single triple prism. The second row of prisms is offset from the first and third rows. The roof structure above the individual triples of the upper and lower rows of prisms has an ascending flank to the left and a descending flank to the right of the vertical axis of symmetry of the hexagonal surface shown as roof element 12b, and thus corresponds to a characteristic as in Figure 4b. The roof structure above the individual triples of the middle row of prisms has a sloping flank on the left as roof element 12b and a rising flank on the right of the symmetry axis as roof element 12a and thus corresponds to a form as in Figure 4c All surface elements of each individual triplet are assigned a different roof element 12a, 12b to the left of the symmetry axis than to the right of the symmetry axis. Therefore, perpendicular irradiation results in a beam splitting into two beams upon reflection.

[0067] Figure 6b shows a vertical view of the entrance surface of a reflector consisting of three columns, each with three triple prisms 11.

[0068] The width of a roof element 12a together with that of an adjacent roof element 12b corresponds to 0.75 0.5< times the width of a single triple prism. The roof structure above the individual triples on the left essentially shows a sloping flank and thus essentially corresponds to a form as in Figure 4b All surface elements of each individual triplet are assigned a different roof element to the left of the symmetry axis than to the right. Therefore, perpendicular irradiation results in a beam splitting into two beams upon reflection.

[0069] By using a beam splitting reflector 9 as shown in the Figures 4b, 4c and 5a, 5b, 6a, 6b described, the sensor arrangement 1 is as follows Figure 3 the course of the partial beams forming the received light beams 4a, 4b shown there is obtained.

[0070] As from Figure 3As can be seen, when the monitoring area is clear, the received light beams 4a hit the receiver 5 as the first partial beam, but not the received light beams 4b of the second partial beam.

[0071] According to the invention, the beam-splitting reflector 9 is adapted to the sensor 10 in such a way that the received light beams 4a largely impinge on the receiver 5, advantageously illuminating it completely. This shows Figure 7The beam-splitting effect of the beam-splitting reflector 9 creates two received light spots 40a, 40b located on either side of the transmitting aperture II. By adapting the sensor 10 to the beam-splitting reflector 9, or vice versa, the received light beams 4a create a received light spot 40a that completely illuminates the receiving aperture III. The received light spot 40a is only slightly larger than the receiving aperture III, meaning that the majority of the received light strikes the receiving aperture III.

[0072] With this beam guidance, transparent objects in the surveillance area can be detected very safely and reliably.

[0073] Since the received light rays 4a are almost completely imaged onto the receiving aperture III when the monitoring area is clear, and thus only a small portion of the received light rays 4a does not reach the receiver 5 when the monitoring area is clear, depolarizing properties or light-deflecting properties of a transparent object in the monitoring area can no longer lead to a strong increase in the received signal, since almost the entire amount of received light from the received light rays 4a already hits the receiver 5 when the monitoring area is clear. This ensures, however, that even for transparent objects, the amplitude of the received signal is always below the threshold value when a transparent object is located in the monitoring area, so that the transparent object is recognized as an object.

[0074] While Figure 7the conditions at an optimally adjusted distance between reflector 9 and sensor 10, shows Figure 8a the situation when the distance is greater than the optimal distance. Accordingly, Figure 8b the situation at a reduced distance.

[0075] Due to these distance adjustments, the Figures 8a, 8b In the situations shown, the received light spots 40a no longer completely fall on the receiving aperture III. However, this part of the received light spots 40a impinging on the receiving aperture III is still sufficient for reliable detection of transparent objects. List of reference symbols

[0076] (1)Sensor arrangement (1a)Reflection light barrier (2)Transmitted light beam (3)Transmitter (4)Received light beam (4a)Received light beam (4b)Received light beam (5)Receiver (6)Beam splitter (7)Transmitting and receiving optics (7a)Transmitting optics (7b)Receiving optics (8)Evaluation unit (9)Beam-splitting reflector (9a)Retroreflector (10)Sensor (10a)Housing (11)Triple (12a)Roof element (12b)Roof element (40)Received light spot (40a)Received light spot (40b)Received light spot ISending / receiving aperture IaIncoming light rays IbEmerging light rays IISending aperture IIaIncoming light rays IIbEmerging light rays IIIReceiving aperture IIIaLight rays IVaincident light rays IVboutcanceled light rays

Claims

1. A sensor assembly (1) for detecting transparent objects in a monitoring area, having a sensor (10) at a first edge of the monitoring area and having a reflector (9) at a second edge of the monitoring area opposite the first edge, the sensor (10) having a transmitter (3) emitting transmitted light beams (2), a receiver (5) with a receiving aperture and an evaluation unit (8) for evaluating received signals from the receiver (5), characterised in that the sensor assembly (1) is a two-line retro-reflective light barrier (la), the transmitter (3) and the receiver (5) being arranged at a distance from one another, in that the sensor assembly (1) is a two-line retro-reflective light barrier (la), the transmitter (3) and the receiver (5) being arranged at a distance next to one another, a transmitting optical system (7a) being assigned to the transmitter (3) and a receiving optical system (7b) arranged at a distance from the transmitting optical system (7a) being assigned to the receiver (5), in that the reflector (9) is designed as a beam-dividing reflector (9), wherein transmitted light beams (2) impinging on the beam-dividing reflector (9) are reflected as received light beams (4a, 4b) are reflected at the beam-dividing reflector (9) in such a way that the received light beams (4a, 4b) are divided into two partial beams which each have an angular offset relative to the transmitted light beams (2), in that the beam-dividing reflector (9) has a triple assembly with an entrance surface which has a roof structure, the triple assembly being adapted to the roof structure, and wherein the transmitted light beams impinging on the entry surface enter the triple assembly and are reflected on reflection surfaces of a triple, so that the transmitted light beams (2) impinging on the entry surface are refracted at the roof structure in such a way that the reflected received light beams (4a, 4b) are divided into two partial beams, and that the beam-dividing reflector (9) is positioned relative to the sensor (10) in such a way that when the monitoring area is free, a partial beam is guided onto the receiver (5).

2. A sensor assembly (1) according to claim 1, characterised in that the partial beam guided onto the receiver (5) when the monitoring area is free largely illuminates the receiving aperture of the receiver (5).

3. A sensor assembly (1) according to claim 1, characterised in that the partial beam guided onto the receiver (5) when the monitoring area is free strikes the receiver (5) completely.

4. A sensor assembly (1) according to one of claims 1 to 3, characterised in that an amplitude evaluation of the received signals of the receiver (5) is carried out in the evaluation unit (8) for the detection of transparent objects.

5. A sensor assembly (1) according to claim 4, characterised in that the amplitudes of the received signals are evaluated with a threshold value, the amplitudes of the received signals being above the threshold value when the monitoring area is free and below the threshold value when a transparent object is detected.

6. A sensor assembly (1) according to one of claims 4 or 5, characterised in that a binary switching signal is generated in the evaluation unit (8) as a function of the amplitude evaluation of the received signals.

7. A sensor assembly (1) according to one of claims 1 to 6, characterised in that the transmitter (3) emits polarised transmitted light beams (2).

8. A sensor assembly (1) according to claim 7, characterised in that the transmitter (3) and the receiver (5) are each assigned a polarisation filter.