Optoelectronic sensor and method for transmission monitoring of a windshield

Abstract

Optoelectronic sensor (10), in particular a laser scanner, for detecting objects in a monitoring area (20), comprising: - a front glass (38), - a light emitter (12) for emitting a light beam (16), - a movable deflection unit (18) for periodically scanning the monitoring area (20) with the light beam (16), - a light receiver (26) for generating a received signal from the light beam (22) reflected by the objects, to which a receiving lens (24) is arranged, - at least one test light emitter (42), at least one test light receiver (44) and at least one test light reflector (48) which form a test light path (46a-b) through the front glass (38), - and an evaluation unit (32) which is configured to obtain information about the objects in the monitoring area (20) from the received signal and to detect impaired light transmission of the front glass (38) from a test light signal recognize,that the test light receiver (44) generates from test light emitted by the test light transmitter (42) and reflected at the test light reflector (48), characterized in that the receiving lens (24) has a near-range zone (54a-b) which is significantly larger than the occlusion by a standardized covering object.

Description

The invention relates to an optoelectronic sensor and a method for monitoring the transmission of a front window of an optoelectronic sensor according to the preamble of claim 1 and 11 respectively. Laser scanners are frequently used for optical surveillance. In these scanners, a laser beam, deflected by a beam generator, periodically sweeps across a monitored area. The light is reflected from objects within the monitored area and analyzed by the scanner. The angle of the beam generator is used to determine the object's position, and the distance of the object from the laser scanner is calculated from the light's travel time using the speed of light. Two fundamental principles are known for determining the light's travel time. In phase-based methods, the transmitted light is modulated, and the phase shift of the received light compared to the transmitted light is analyzed. In pulse-based methods, which are preferred in security technology, the laser scanner measures the travel time until a transmitted light pulse is received again. One important application is the safeguarding of hazards in safety engineering. The laser scanner monitors a protective field that operators are not permitted to enter while the machine is running. Because the laser scanner acquires angular and distance information, it can determine the two-dimensional positions of objects within the monitored area and thus also within the protective field. If the laser scanner detects an unauthorized intrusion into the protective field, such as an operator's leg, it triggers an emergency stop of the machine. Sensors used in safety technology must operate with exceptional reliability and therefore meet stringent safety requirements, such as the EN13849 standard for machine safety and the EN61496 standard for non-contact protective devices. Meeting these safety standards requires a number of measures, including reliable electronic evaluation through redundant, diverse electronics, functional monitoring, and / or the provision of individual test targets with defined reflectances that must be detectable at the corresponding scan angles. Another safety requirement is the detection of any impairment of the transmission of the laser scanner's front lens. If detection capability is limited, a safety-related shutdown must be triggered. This includes detecting both homogeneous contamination, such as dust, and localized interference from small objects exceeding a defined minimum size. Deliberate manipulation by covering areas of the front lens with small obscuring objects must also be prevented. To detect such interference, a laser scanner typically uses optical test channels that examine different positions of the windshield area by means of transmission. In a solution known, for example, from DE 43 45 446 C2, a large number of independent optical test channels are distributed across the entire angular range of the windshield. These channels transmit light through different areas of the windshield to detect impaired transmission. The distribution of the test channels must be dense enough to reliably detect even the smallest contaminants or tampering objects required by the standard, despite the detection being limited to specific points. A large number of test channels naturally increases manufacturing costs and the required installation space. Furthermore, the test channels are located quite close to the outer contour of the laser scanner to avoid interference from the rotating deflection unit.This makes them susceptible to interference from ambient light and reflectors that may be nearby, either accidentally or intentionally placed for manipulation. To obtain defined test areas, the optoelectronic components must be partially shaded using elaborate apertures. With biaxial laser scanners, meaning those with parallel transmitting and receiving beams, an additional problem arises with small obstructions on the windshield. This doesn't inherently affect the windshield's transmittance, but is conventionally detected by windshield monitoring. The receiving lens of a laser scanner is typically a multi-zone lens with a near-range zone in the center to ensure reasonably constant signal levels regardless of object distance, thus achieving the most consistent signal dynamics possible. If this near-range zone is obscured by an object on the windshield, the laser scanner suffers a significant loss of detection capability in the close range. In coaxial systems, such manipulation is immediately noticeable because the transmitting beam is also blocked, manifesting itself, for example, through strong stray light.In a biaxial system, however, the transmitted light beam does not pass through the near-field zone. It is possible to give the windshield monitoring system the additional function of detecting such shadowing objects. For this, however, particularly closely spaced test channels must be used. EP 2 237 065 A1 discloses a laser scanner in which the entire measuring unit, including the light source and detector, rotates. A test light source and a test detector are also housed on the corresponding rotor, while a reflector element is located outside the housing. Thus, the test light source and test detector scan the front panel during rotation with the aid of the reflector element. However, such systems with a rotating measuring head have the disadvantage that a data connection and power supply must be established between the stationary part and the rotating measuring head. Furthermore, the test light detector is necessarily directed outwards and can therefore be relatively easily interfered with by ambient light. From EP 2 508 914 A1, a laser scanner is known which has a tubular channel separator that shields the transmit path from the receive path. A passive optical element, for example a retroreflector or a prism film, is attached to the channel separator. To monitor the degree of soiling of the windshield, monitoring light radiation from a transmitter passes through the optical element to a receiver and thereby passes through the windshield twice. US patent 2008 / 0158555 A1 discloses another windshield monitoring system for a laser scanner, in which a multitude of pairs of light emitters and light receivers are distributed around the outside of the windshield. A retroreflector is attached to the rotating measuring head of the laser scanner, which completes the test light path of these pairs in appropriate rotational positions. The purpose of the invention is therefore to improve windscreen monitoring. This problem is solved by an optoelectronic sensor and a method for monitoring the transmission of a windshield of an optoelectronic sensor according to claim 1 and 11, respectively. A test light transmitter, a test light receiver, and a test light reflector define a test channel or test light path to check the light transmission of the windshield. The invention is based on the fundamental idea of ​​using a test channel to monitor the transmission not just at a single point, but over a larger area of ​​the windshield. For this purpose, the test light reflector can be moved along with the deflection unit. If the deflection unit is, as is often the case, a rotating mirror, the test light reflector is preferably attached to it and thus follows the rotational movement. The invention offers the advantage of enabling spatially resolved transmission measurement using simple means. This allows for the detection of unacceptable losses, for example, due to contamination or obstructions, thus ensuring reliable detection. Because of the moving test light reflector, significantly fewer test light paths are required, making the system less complex, more cost-effective, and smaller. Furthermore, an internal light path on the test light path and an inward-facing receiving axis result in increased robustness against interference from ambient light and reflectors. This allows for greater flexibility in the positions monitored on the windshield. The small surface area of ​​the test light reflector eliminates the need for a mechanical component for shading and channel separation. The test light transmitter and receiver are preferably not arranged to move with the deflection unit. This means that the test light transmitter and receiver are located in the stationary part of the sensor, such as a base or housing in which the movable deflection unit is mounted. This significantly reduces the system's complexity, as no power supply or data exchange with a moving or rotating system is required. On the other hand, some sensors already incorporate a movable measuring head for the light transmitter and receiver as the deflection unit. In principle, the connection problem is therefore manageable, and such a connection can also be added for transmission monitoring or even used in a sensor with a movable measuring head without significant additional effort.Consequently, it is quite conceivable to also move the test light transmitter and test light receiver, for example via a kind of balcony attached to the movable deflection unit, in order to keep the test light transmitter and test light receiver moving along with the test light, or alternatively, in reverse of the light path, the test light reflector. Advantageously, several test light transmitters and test light receivers are arranged circumferentially around the deflection unit. Even more preferably, each test light transmitter and test light receiver forms a pair, and several such pairs are distributed around the circumference to monitor a rotating windshield with spatial resolution in multiple areas. Since the test light reflector moves along with the unit, a single test light reflector is sufficient to form the corresponding test channels with the multiple pairs. By controlling the test channels in a specific time frame during operation, different positions on the windshield can be scanned. Of course, it is also conceivable to mount several test light reflectors at different angles on the deflection unit. The evaluation unit is preferably designed to derive information about the position of the deflection unit from the test light signal. The test paths are linked to the deflection unit via the moving test light reflector. Therefore, it is also possible to obtain at least rough information about the respective position, especially the angular position of the deflection unit, from the identity of the currently active test channel and possibly also from the expected intensity of the test light, which depends on the position of the deflection unit. This allows a conventional encoder for detecting the angular position of the deflection unit to be supplemented or even replaced. At the very least, this provides a diverse and redundant second source of information about the angular positions, which can be used, particularly in safety engineering, for checking and validating the encoder. The test light reflector preferably exhibits retroreflective properties. For this purpose, the test light reflector can be a retroreflector, for example, with structures similar to a cat's eye. Alternatively, the test light reflector may have one or more convex or concave mirror surfaces, or several planar mirror surfaces positioned at an angle to each other, which reflect at least a portion of the incident test light back in the direction of incidence. The reflection behavior of such a mirror, not just a retroreflector in the strict sense, is also considered a retroreflective property here. The retroreflective properties cause the test light to return to the test light receiver over a wide angular range during the movement of the test light reflector, thus forming a test light path.This allows the windshield to be tested for transmission over a corresponding angular range, and significantly fewer test channels are needed to cover the windshield. Without retroreflective properties, such as a simple mirror surface, the windshield can only be tested with the test light reflector over a smaller area, as a complete test light path is only formed within a limited angular range. However, due to the finite extent of the mirror surface, more than just a point area is tested, so even a simple mirror surface offers significant advantages over a conventional static test path. When using a retroreflector, the test light source and receiver must be very close together. With a flat mirror, on the other hand, there is the freedom to position the test light receiver at a different location than the test light source. The test light reflector preferably has several sub-reflectors arranged one above the other and / or tilted relative to each other. With such staggered sub-reflectors, the test light path is spread out, enabling, in particular, multiple scans of the windshield at different heights. A simple test light receiver can only evaluate these multiple scans in aggregate, but can nevertheless detect any impairment of transmission. A spatially resolved test light receiver also detects at which height the transmission is altered. The windscreen preferably has the shape of a solid of revolution and, in cross-section with respect to its central axis, an inwardly directed contour. The test light transmitter and receiver are arranged in a region of the windscreen where it has a small radius. Examples of this windscreen shape are a truncated cone or a segment of a sphere. The inwardly directed contour of the windscreen, compared to a corresponding cylinder, leaves a space in which the test light transmitters and receivers can be arranged. Since the test light reflector is located at the deflection unit, a very inward-facing test light path is created, which is hardly disturbed by ambient light and is therefore correspondingly robust. The front lens preferably has a surrounding base. This quickly returns the radius of the inward-facing contour to the outside. The test light receivers and transmitters can be mounted beneath this base, thus remaining protected within the sensor. The test light path then first passes through the base to the outside and subsequently, for the actual transmission test, through the front lens itself to the test light reflector. The light transmitter and receiver of the optical distance measuring system are preferably arranged side-by-side in a biaxial configuration with parallel optical axes. Traditionally, a coaxial configuration is often chosen instead, in which the transmitting beam, with its smaller cross-section, lies centrally within the receiving beam. However, a biaxial arrangement is simpler, especially for very small devices. A receiving lens is preferably positioned in front of the light receiver, which has a near-field detection zone that is significantly larger than the area obscured by a standardized obscuring object. As explained earlier, the near-field detection zone serves to adjust the signal dynamics in the near field as desired. Conventionally, it is possible to obscure the near field with small objects, and therefore a correspondingly complex transmission measurement of the windshield with a large number of test channels must be performed. An enlarged near-field detection zone is insensitive to this interference or manipulation with small objects, as it can no longer be obscured so easily. Consequently, significantly fewer test channels are required for the transmission measurement, and the detection reliability is increased.Examples of enlarged near-field zones include the provision of several separate or distributed near-field zones, or, for example, a ring-shaped near-field zone, particularly at the outer edge of the receiving lens. The sensor is preferably designed as a safety laser scanner, with the evaluation unit configured to compare the position of detected objects with protective fields and, upon detection of an unauthorized intrusion into the protective field, to control a safe output with a shutdown signal. Simplified windshield monitoring is particularly advantageous for such applications. The method according to the invention can be further developed in a similar manner and exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the dependent claims following the independent claims. The invention is further explained below with regard to additional features and advantages by way of example embodiments and with reference to the accompanying drawing. The figures in the drawing show: Fig. 1 a sectional view of an embodiment of a laser scanner; Fig. 2 a three-dimensional view of a front lens of a laser scanner to illustrate a test light path; Fig. 3 a schematic top view of a deflection unit of a laser scanner in two different angular positions with associated test light paths; Fig. 4 a schematic sectional view of a section of a front lens of a laser scanner with test light paths in the case of a split test light reflector for front lens monitoring at several heights; and Fig. 5 a three-dimensional view of a receiving lens with an extended near zone. Fig. 1 shows a schematic sectional view through a laser scanner 10 according to the invention, which is designed in particular as a distance-measuring safety laser scanner. A light transmitter 12, for example a laser in the form of an edge emitter or a VCSEL, emits a light signal, for example in the form of short light pulses or periodically amplitude-modulated light. The emitted light is collimated by a transmitting optic 14 to form an emitted light beam 16, which is directed by a movable deflection unit 18 into a monitoring area 20 and is remitted or reflected there by an object that may be present. A portion of this light returns to the laser scanner 10 as an incident reflected light beam 22 and is deflected by the deflection unit 18 to a receiving optic 24 and from there focused onto a light receiver 26, for example a photodiode or APD (avalanche photodiode). The deflection unit 18 can be designed as a oscillating mirror, but is usually a rotating mirror that rotates continuously by a motor 28. The respective angular position of the deflection unit 18 is detected by an encoder 30. The light beam 16 generated by the light transmitter 12 thus sweeps across the monitoring area 20 created by the movement. If a reflected light beam 22 is received by the light receiver 26 from the monitoring area 20, the angular position of the object in the monitoring area 20 can be deduced from the angular position of the deflection unit 18 by means of the encoder 30. Additionally, the time of flight of light from the emission of the light beam 16 until the reception of the reflected light beam 22 after reflection from the object in the monitoring area 20 is determined. All time-of-flight methods are conceivable for this purpose. Using the speed of light, the distance of the object from the laser scanner 10 is calculated from the time of flight. This evaluation takes place in an evaluation unit 32, which is connected to the light transmitter 12, the light receiver 26, the motor 28, and the encoder 30. The angle and distance then provide two-dimensional polar coordinates of all objects within the monitoring area 20. Two-dimensional protective fields can thus be defined within the monitoring area 20, into which unauthorized objects, such as operators or their body parts, must not enter. If the evaluation unit 32 detects an unauthorized intrusion into the protective field, a safety-related shutdown signal is output via a safe output 34 (OSSD, Output Signal Switching Device) to, for example, stop a monitored hazardous machine or move it to a safe position. Alternatively, measurement data is output via output 34, particularly if the laser scanner 10 is not a safety laser scanner and therefore does not monitor protective fields. All the aforementioned functional components are arranged in a housing 36, which has a circumferential front panel 38 in the area of ​​light emission and light entry. The front panel 38 is frequently, but not necessarily, designed as a body of revolution and, in both cases, does not necessarily have to extend over 360°, so that a certain angular range remains as a dead zone. The windscreen 38 has an inwardly directed contour, which in the example of Fig. 1 is chalice-shaped. However, other curvatures and even a straight contour are also conceivable, which would then result in the overall shape of a truncated cone. At the bottom, the windscreen 38 is closed off by a base 40, which restores the radius that decreases above due to the inwardly directed contour. Beneath this base 40 are arranged a test light transmitter 42 and a test light receiver 44. The test light transmitter 42 is any light source whose spectrum preferably differs from that of the light transmitter 12 to avoid stray light, but which tests the front panel 38 for transmission in a relevant wavelength range. The test light receiver can be a simple photodiode, although spatial resolution is advantageous in some embodiments. Further pairs of test light transmitters and test light receivers are preferably distributed around the circumference of the front panel 38, but are not shown in Fig. 1. The test light from the test light transmitter 42 first passes through the base 40 to the outside via a test light path 46a-b and then shines through the front panel 38 into the interior of the laser scanner 10.There, it is reflected by a test light reflector 48, which is attached to the deflection unit 18, and returns to the test light receiver 44, passing through the front glass 38 and base 40 again to complete the test light path 46a-b. Alternatively, the test light receiver 44 is located inside, so that the front glass 38 is only illuminated once. Using the test signal from the test light receiver 44, the evaluation unit 32 can, for example, check whether the front glass 38 is impaired in its light transmission at the points where the test light path 46a-b intersects the test light path by comparing it with a target level. This enables testing for homogeneous soiling or the presence of shadowing objects on the front glass 38. In a safety laser scanner, a safety-related shutdown is triggered if an impairment is detected that no longer guarantees safe operation.A sensor not used in security technology could, for example, simply display a maintenance request. The test light reflector 48 rotates with the deflection unit 18. This also makes the test light path 46a-b dynamic, as one of its endpoints moves with the test light reflector 48. As explained in more detail below with reference to Figures 2, 3 to 4, the windshield 38 can thus be monitored for its transmission properties not only at specific points, but also over its entire range during rotation. The evaluation unit 32 must, of course, take the rotation into account when controlling the test channels formed by the test light transmitters 42 and test light receivers 44 via the test light reflector 48. Conversely, the test channels can also be used to obtain information about the angular position of the deflection unit 18. A rough angular information is already contained in the position of the pair of test light transmitters 42 and test light receivers 44 that are currently able to generate a test light signal via the test light reflector 48.Depending on the arrangement of the test light path 46a-b and the properties of the test light reflector 48, the intensity and point of incidence of the test light on the test light receiver 44 can also vary with the respective position of the test light reflector 48. This allows for the acquisition of more precise angular information, which can be used, for example, to check the plausibility of the measurement information from the encoder 30 or even to replace the encoder 30. The laser scanner 10 according to Fig. 1 has a biaxial arrangement in which the emitted light beam 16 runs alongside the incident reflected light beam 22. However, this is only an example. A transmission test of the windscreen 38 via a moving test light reflector 48 is also possible in any other configuration of a corresponding optoelectronic sensor with a periodically moving deflection unit. Fig. 2 shows another three-dimensional view of the front glass 38 with base 40 and the deflection unit 18 with the moving test light reflector 48. For clarity, only one test channel with a pair of a test light transmitter 42 and a test light receiver 44 is shown again. Fig. 3 is a supplementary related top view showing two exemplary angular positions of the deflection unit 18 and thus also of the test light reflector 48, which are shown with solid and dashed lines respectively. The rotation of the deflection unit 18 shifts the test light path 46a-b, and the point of penetration through the windshield 38 moves over an extended area. Therefore, the transmission test is performed in the same test channel not only at a fixed azimuth angle, but over an entire azimuth angle range. This means that a smaller number of test channels, each with a static pair of test light transmitter 42 and test light receiver 44, are sufficient to test the windshield 38 for all relevant azimuth angles. The test light reflector 48 preferably has retroreflective properties. This allows the test light to be reflected back to the test light receiver 44 at different azimuth angles, as can be seen in Fig. 3. The test light reflector 48 is preferably designed as a retroreflector or as an arrangement of several retroreflectors. Alternatively, a contoured mirror can be used that reflects light from different angles of incidence at least partially due to its inclination or multiple reflections. Transmission testing is also possible with a simple mirror surface as the test light reflector 48. This allows for greater flexibility in the arrangement of the test light transmitter 42 and the test light receiver 44, but only a smaller area of ​​the windshield 38 is tested. Fig. 4 shows a simplified sectional view of the front window 38 and a test channel in a further embodiment. In this embodiment, the test light reflector 48 has two or more partial reflectors 48a-b, which are arranged one above the other and / or tilted at different angles. While the movement of the test light reflector 48 enables scanning of the front window 38 over an azimuth angle range, the partial reflectors 48a-b also allow scanning over different polar angles or altitude ranges. In most cases, it is sufficient to generate only a summed overall signal from all partial reflectors 48a-b in the test light receiver 44, as this is sufficient to detect a safety-critical impairment. However, if contamination or an obstruction is to be located, the test light receiver 44 can also be configured for spatial resolution. The required number of test channels can be significantly reduced by moving the test light reflector 48. As explained in the introduction, with a biaxial setup of the laser scanner 10 as shown in Fig. 1, it must also be ensured that a shadowing object does not obscure the near zone of the receiving optics 24. Fig. 5 shows a receiving lens 50, which can be used in the receiving optics 24 or as the receiving optics 24 itself, and in which the signal from objects in the near field can no longer be completely obscured by individual small shadowing objects. This increases the tamper resistance, and it is no longer necessary for the transmission monitoring to perform this function of tamper detection. This allows for a further reduction in the number of test channels required. The lens 50 has a main part 52 that is convex or plano-convex. Overall, it is therefore a converging lens for focusing the reflected light beam 22. On the right side in Fig. 5, the lens is cut off to bring the transmitting and receiving paths closer together. This cut-off is not strictly necessary. Furthermore, the lens 50 features an extended, here divided into two near zones 54a-b. A near zone is a known concept and, as described in the introduction, serves to enhance the signal dynamics for short distances by having different focusing properties than the main part 52. The spatial separation into two or more near zones 54a-b additionally ensures that the signal in the near range can no longer be completely obscured by individual small objects. Therefore, improved robustness against localized contamination and tampering is achieved up to a defined maximum size of the interfering or tampering object. A split near zone 54a-b is just one example of distributing the near zone over a larger area. The described advantage can also be achieved if the near zone extends in another way over an area that cannot be obscured by small objects. Another example is therefore an annular near zone near the outer edge of the receiving lens 50. The embodiments described so far involve a moving or rotating test light reflector 48 and static test light emitters 42 or test light receivers 44. It is also conceivable to mount the test light emitters 42 and test light receivers 44 on a kind of balcony of the deflection unit 18 and thus allow them to rotate as well. In this case, a single pair of test light emitters 42 and test light receivers 44 is sufficient to scan the front window 38 at all azimuth angles during the rotational movement. These embodiments of a transmission measurement with a moving test light reflector 48 are intended to serve as a starting point for systematically listing how test channels with static or moving elements can be constructed. In one family of test light paths, the test light path is reflexively formed from a test light transmitter, a test light reflector, and a test light receiver. Each of these elements can be static or moving, resulting in a total of eight combinations. In four of these combinations, the test light transmitter moves relative to the test light receiver. This is not a practical configuration for a reflective test light path. Instead, the test light transmitter and test light receiver should either both be static or both move together so that they are always static relative to each other and often positioned next to each other. In two of the remaining combinations, the test light reflector moves along with the deflection unit. If the test light transmitter and test light receiver are static, this corresponds to the embodiments described in detail above with reference to the figures. When the test light transmitter and test light receiver move together, the embodiment described above, following the figure description, is created.In the remaining two combinations, the test light reflector is static. This corresponds, with a static test light transmitter and receiver, to the procedure described in the introductory DE 43 45 446 C2, and with a moving test light transmitter and receiver, to the procedure described in the introductory EP 2 237 065 A1. In a second configuration, test light paths are formed directly, without a test light reflector. The pair of test light transmitter and test light receiver is aligned directly through the windscreen, with one element located inside and the other outside the windscreen. If the windscreen has a base, the test light transmitter and test light receiver can both be positioned inside the windscreen by piercing it twice, with one element then located below the base. Keeping both elements static is a known and functional solution, but it requires a correspondingly large number of test channels. Keeping the outer element static and moving the inner element with the deflection unit is possible, but this requires controlling and powering a moving element.Furthermore, the test light transmitter should preferably be moved along with the test light, because otherwise the test light receiver faces outwards and thus also detects ambient light. In principle, the deflection unit could also have a kind of platform extending below the base of the windshield. In this way, the test light transmitter and receiver can be moved together, and a single test channel is sufficient. However, this platform presents a significant mechanical challenge in preventing adverse effects on the deflection unit due to imbalance. In a third configuration, test light paths are formed with the actual scan beam of light source 12. Light source 12 thus also functions as a test light source. In principle, all variants mentioned in the previous paragraphs are possible with this replacement, but there is a significant risk of crosstalk into the actual measurement channel in each case, since a small portion of the measurement beam would have to be diverted and directed to the test light receiver after passing through the windshield once or several times. This eliminates the possibility of selectively suppressing this extremely damaging optical echo from the near field, which is extremely detrimental to the measurement system.

Claims

Optoelectronic sensor (10), in particular a laser scanner, for detecting objects in a monitoring area (20), comprising: - a front glass (38), - a light emitter (12) for emitting a light beam (16), - a movable deflection unit (18) for periodically scanning the monitoring area (20) with the light beam (16), - a light receiver (26) for generating a received signal from the light beam (22) emitted by the objects, to which a receiving lens (24) is arranged, - at least one test light emitter (42), at least one test light receiver (44) and at least one test light reflector (48) which form a test light path (46a-b) through the front glass (38), - and an evaluation unit (32) which is configured to obtain information about the objects in the monitoring area (20) from the received signal and to detect impaired light transmission of the front glass (38) from a test light signal recognize,that the test light receiver (44) generates from test light emitted by the test light transmitter (42) and reflected at the test light reflector (48), characterized in that the receiving lens (24) has a near-range zone (54a-b) which is significantly larger than the occlusion by a standardized covering object. Sensor (10) according to claim 1, wherein the test light transmitter (42) and the test light receiver (44) are arranged not to move with the deflection unit (18). Sensor (10) according to claim 1 or 2, wherein several test light emitters (42) and test light receivers (44) are arranged distributed in a circumferential direction around the deflection unit (18). Sensor (10) according to one of the preceding claims, wherein the evaluation unit (32) is configured to obtain information about the position of the deflection unit (18) from the test light signal. Sensor (10) according to one of the preceding claims, wherein the test light reflector (48) has retroreflective properties. Sensor (10) according to one of the preceding claims, wherein the test light reflector (48) has several partial reflectors (48a-b) arranged one above the other and / or tilted relative to each other. Sensor (10) according to one of the preceding claims, wherein the front window (38) has the shape of a body of revolution and, in section to a central axis of the body of revolution, has an inwardly pointing contour, and wherein the test light transmitter (42) and test light receiver (44) are arranged in a region of the front window (38) where it has a small radius. Sensor (10) according to one of the preceding claims, wherein the front window (38) has a circumferential base (40). Sensor (10) according to one of the preceding claims, wherein light transmitter (12) and light receiver (26) are arranged side by side in a biaxial structure with optical axes parallel to each other. Sensor (10) according to one of the preceding claims, which is designed as a safety laser scanner, wherein the evaluation unit (32) is designed to compare the position of detected objects with protective fields and, in the event of detection of an impermissible intrusion into the protective field, to control a safe output (34) with a shutdown signal. Method for transmission monitoring of a front window (38) of an optoelectronic sensor (10), in particular a laser scanner, with a movable deflection unit (18) for periodic scanning with a light beam (16, 22), wherein test light is emitted on at least one test light path (46a-b) through the front window (38) from a test light transmitter (42), reflected at a test light reflector (48) and converted into a test light signal in a test light receiver (44), and impaired light transmission of the front window (38) is detected from the test light signal, characterized in that, with the aid of a receiving lens (24) with a near-area zone (54a-b) that is significantly larger than the occlusion by a standardized covering object, complete occlusion of received light from the near area by individual small objects is prevented.

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