Sensors and methods for detecting objects

Abstract

Optoelectronic sensor (10) for detecting objects (20) in a monitoring area (18), which for receiving received light (22) from the monitoring area (18) in a wavelength range, comprises a light receiver (24) with a plurality of avalanche photodiode elements (24a), each of which can be biased with a voltage above a breakdown voltage and thus operated in a Geiger mode, a receiving optic (26, 28) with a first optical filter (28, 28a) tuned to the wavelength range, and a control and evaluation unit (30) configured to acquire information about the objects (20) from a received signal of the light receiver (24), wherein the wavelength range shifts during operation of the sensor (10) due to drifts within a tolerance wavelength range, wherein the first filter (28, 28a) has a passband,which is narrower than the tolerance wavelength range and wherein the receiving optics (26, 28) has at least one further optical filter (28a) whose passband is narrower than the tolerance wavelength range.

Description

[0001] The invention relates to a sensor and a method for detecting objects.

[0002] Optoelectronic sensors are available in a wide range, ranging from one-dimensional light barriers and light sensors to laser scanners and cameras. They capture a wide variety of optically detectable properties. Two common examples of applications are the presence detection of an object and the determination of the distance to an object. Distance sensors based on the time-of-flight principle measure the time of flight of a light signal, which corresponds to the distance via the speed of light. In a pulse-time-of-flight method, a short light pulse is emitted, and the time until a remission or reflection of the light pulse is received is measured. Alternatively, in a phase method, transmitted light is amplitude-modulated and a phase shift between the transmitted and received light is determined, whereby the phase shift is also a measure of the time of flight.

[0003] Important parameters for such sensors are long ranges and the ability to detect both highly and weakly remitted objects. The range between near and far objects, as well as the different remissions, from shiny surfaces to black velvet, is considerable. Therefore, there is a need for highly sensitive receiving systems that detect as few photons as possible and also cover very wide dynamic ranges of > 100 dB.

[0004] Avalanche photodiodes (APDs) are traditionally used to increase sensitivity in some optoelectronic sensors. The incident light triggers a controlled avalanche effect. This multiplies the charge carriers generated by the incident photons, creating a photocurrent that is proportional to the light reception intensity but significantly larger than that of a simple PIN diode.

[0005] Even greater sensitivity is achieved with avalanche photodiodes operated in so-called Geiger mode (SPAD, Single Photon Avalanche Diode). In this mode, the avalanche photodiode is biased above the breakdown voltage, so that even a single charge carrier released by a single photon can trigger an uncontrolled avalanche, which then recruits all available charge carriers due to the high field strength. After this, the avalanche stops (passive quenching) and is no longer available for detection for a certain dead time. Alternatively, it is also known to detect and extinguish the avalanche from the outside (active quenching). A SPAD thus counts individual events like a Geiger counter. SPADs are not only highly sensitive with amplification factors of up to 10 6 and more, but also relatively inexpensive. Furthermore, they can be integrated onto a circuit board with little effort.

[0006] High sensitivity is desired for triggering photons from the received light to be detected. However, even a minimal interference event, such as an extraneous light photon or dark noise, can generate the same maximum received signal as a useful light signal. One measure to achieve greater robustness is the joint evaluation of multiple SPADs. The number of extraneous light photons, and thus the number of interference events that only contribute to noise, can also be drastically reduced by placing a bandpass filter tuned to the wavelength range of the useful light in the receive path of the SPAD receiver.

[0007] The problem here is that the wavelength of the useful light is subject to drift. In the particularly common case of a dedicated light transmitter, its wavelength depends, among other things, on temperature. Such drifts mean that the wavelength no longer matches the filter, and the useful light can no longer pass through it sufficiently. For this reason, the passband of the bandpass filter must not be too narrow, which also limits the suppression of extraneous light.

[0008] For spectral analysis, filter arrangements with filter ranges tuned to different wavelength ranges are used. Such a filter is not used in an optoelectronic sensor for object detection or distance measurement, and would not directly help with regard to ambient light sensitivity: If the received light penetrates all filter ranges, the result corresponds to a filter with the combined passband of the filter ranges, with only a reduced transmission efficiency compared to a uniform filter with this passband. If the received light is limited to one filter range, the remaining filter ranges are unused and superfluous. The problem that the respective effective passband must be selected broadly to absorb wavelength drifts, and therefore ambient light is not sufficiently suppressed, remains in any case.

[0009] US Pat. No. 6,154,474 B1 discloses a control device for the optical wavelength of a laser diode in optical fiber communication. The temperature of the laser diode is adjusted via a Peltier element. The transmitted light is guided through an optical high-pass filter and an optical low-pass filter in a control loop, so that a control signal for the Peltier element is generated depending on the direction and extent of the wavelength deviation. An additional bandpass filter is intended to prevent an uncontrollable condition from occurring in the event of extreme changes in the wavelength into a range no longer covered by the high-pass and low-pass filters.

[0010] US Pat. No. 6,529,534 B1 describes another control circuit for the wavelength of a laser signal, designed to enable as many channels as possible in optical communication through stable wavelengths. It combines a control similar to that in US Pat. No. 6,154,474 B1 with a second control based on temperature measurement.

[0011] DE 10 2018 201 506 A1 discloses a LIDAR device with a particularly narrowband bandpass filter. The radiation source has a control system for its operating temperature to vary the emission wavelength depending on the radiation angle.

[0012] DE 10 2016 108 437 A1 deals with a multi-wavelength lidar array. To compensate for the different angles of incidence of the light beams emitted by different VCSEL elements on a bandpass filter, the emission wavelength can be individually selected.

[0013] DE 20 2018 002 044 U1 discloses an optical detection device with a time-of-flight measurement using SPADs. It also mentions a bandpass filter adapted to the emission band of the laser projector.

[0014] DE 10 2012 107 329 A1 presents a distance measuring system that has an optical reference path to adjust attenuation filters and thus enable normalization of the radiation detector with regard to gain and drift.

[0015] DE 10 2016 114 432 A1 describes a SPAD-based optoelectronic sensor for distance measurement in which the active SPAD areas are adapted to an elongated receiving light spot.

[0016] It is therefore an object of the invention to improve the measuring accuracy of a generic sensor.

[0017] This object is achieved by a sensor and a method for detecting objects according to claim 1 or 12 or 13, respectively. The sensor has a light receiver, a receiving optics with a first optical filter, and a control and evaluation unit. The light receiver is a SPAD light receiver with avalanche photodiode elements, each of which can be biased with a voltage above a breakdown voltage and thus operated in a Geiger mode. In order to detect useful or received light with a specified wavelength range, the filter is tuned to this wavelength range. The control and evaluation unit obtains measurement information about objects in the monitored area from the received signal of the light receiver, such as a distance to the object or the determination of whether an object is present or not, in particular with certain properties or at a certain position.

[0018] The specified wavelength range of the received light applies to specific environmental conditions and is therefore subject to drift, with temperature being a particularly influencing factor. The sensor is designed for specific environmental conditions, such as a specified temperature range. Therefore, a tolerance wavelength range can be specified that includes those wavelengths of the received light that occur due to drift during sensor operation.

[0019] The invention is based on the basic idea of ​​using an unusually narrow filter to suppress a particularly large amount of extraneous light. The passband is selected to be narrower than the tolerance wavelength range. Due to the particularly high selectivity, the sensor does justice to the special nature of the avalanche photodiode elements in Geiger mode with their strong reaction to extraneous light events. A conventional filter would cover the tolerance wavelength range, i.e. the passband of the filter would match the tolerance wavelength range or be larger. The passband of the first filter according to the invention is so narrow that, at first glance, the expected drifts shift the useful light at least partially into the blocking range, and this initially appears counterproductive. However, this effect is compensated for by the measures explained in various embodiments.

[0020] The invention has the advantage of significantly increasing the ratio of useful signal to ambient light. The particular sensitivity of SPADs to ambient light is compensated for. The improved signal-to-noise ratio can be used for more precise measurements, or it enables longer ranges or the use of lower-power and therefore more cost-effective light sources.

[0021] The first filter is preferably a narrowband bandpass filter. The passband can therefore be specified by a lower and upper limit, or alternatively, a center frequency, around which the passband is usually roughly symmetrical. A bandpass filter is particularly suitable for suppressing extraneous light by excluding light of both short and long wavelengths.

[0022] The receiving optics preferably have an additional optical filter whose passband is narrower than the tolerance wavelength range. Particularly preferably, the passbands of the filters as a whole cover the tolerance wavelength range. A plurality of additional optical filters can also be used, each with a passband narrower than the tolerance wavelength range, so that each individual passband remains even narrower while still covering the tolerance wavelength range overall. Overlapping of the passbands is possible.

[0023] Preferably, at least one avalanche photodiode element is assigned to each of the filters. The assigned avalanche photodiode element receives the portion of the received light that has just passed through this filter. In other words, the received light can be considered as divided into partial light beams, each passing through a specific filter onto a specific avalanche photodiode element or a specific group of avalanche photodiode elements. A group of avalanche photodiode elements assigned to the same filter is used for joint evaluation to compensate for any remaining extraneous light and dark events using summed signals or statistical analysis.

[0024] The control and evaluation unit is preferably configured to select the received signal of the at least one avalanche photodiode element assigned to the filter whose passband corresponds to the wavelength range of the received light under the current operating conditions. If the wavelength range of the received light shifts due to drift, one of the filters will still offer the appropriate narrow passband, and the at least one avalanche photodiode element assigned to this filter will receive a useful light signal without any extraneous light outside the wavelength range of the useful light. The other avalanche photodiode elements are located behind a filter with a passband not adapted to the wavelength range of the useful light and therefore contribute a large amount of, or even only, extraneous light. The control and evaluation unit therefore ignores their signal contribution.

[0025] The control and evaluation unit preferably selects the received signal of the at least one avalanche photodiode element based on its level or temperature information. The at least one avalanche photodiode element behind the filter currently matched to the wavelength of the useful light will generally deliver the strongest or at least one of the strongest signals. If a clear decision cannot be made based on the current levels of the respective avalanche photodiode elements assigned to a filter, a history can also be used. Wavelength drifts are usually not a sudden, but rather a slow and continuous process. Therefore, the at least one avalanche photodiode element selected so far is a good candidate even for the current situation. Otherwise, those avalanche photodiode elements whose filter is spectrally adjacent to the previously selected filter are particularly considered.Another possible criterion is the operating temperature, especially of a dedicated light transmitter, which significantly influences the wavelength shift. For example, a conversion table (LUT, lookup table) can be used to find the appropriate filter and thus at least one avalanche photodiode element for the currently measured temperature.

[0026] The first filter and subsequent filters preferably form a striped or checkered pattern. These simple patterns simplify filter manufacturing and create clearly separated areas on the light receiver, allowing the filter and avalanche photodiode elements to be aligned.

[0027] The sensor preferably has a light transmitter for transmitting light in the wavelength range into the monitored area. The light transmitter preferably generates light in a narrow wavelength range, which is then the wavelength range expected on the receiving side, although this can shift due to drift, as explained above.

[0028] The sensor preferably has a reference light receiver with an optical reference filter whose passband corresponds to that of the first filter, in order to obtain drift information indicating the extent to which the wavelength range of the transmitted light has shifted due to drift. This allows the control and evaluation unit to know the current wavelength range shifted by drift. Based on this, for example, the appropriate filter and, accordingly, at least one avalanche photodiode element providing the useful signal can be selected. The drift information is, in particular, a level.

[0029] The control and evaluation unit is preferably designed to adjust the wavelength range of the transmitted light into the passband based on the drift information. To do this, the wavelength range is shifted so that the level at the reference light receiver is maximized. This is not a simple temperature control that would merely maintain an operating temperature; rather, the wavelength is actually controlled, which also takes into account, for example, temperature shifts in the filters. The control of the transmitted light ultimately prevents drift and thus differs from the approaches explained so far, which start at the receiving end and react to the drift.

[0030] The light transmitter preferably has a temperature adjustment element with which the wavelength range of the transmitted light can be changed. The wavelength range of the light transmitter is thus shifted back against drifts detected by the reference light receiver via temperature adjustment. Examples of temperature adjustment elements include a Peltier element or an adjustment of the laser current.

[0031] The control and evaluation unit is preferably designed to determine the light propagation time of the transmitted light emitted by the light transmitter and received by the light receiver from the monitored area as received light. This determines the distance to the object as measurement information.

[0032] The method according to the invention can be further developed in a similar manner and thereby exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the subclaims following the independent claims.

[0033] The invention will be explained in more detail below with regard to further features and advantages, using exemplary embodiments and with reference to the accompanying drawings. The figures of the drawing show: Fig. 1 a schematic representation of an optoelectronic sensor with an optical multiple filter, the filter areas of which with very narrow, staggered passbands are assigned to different avalanche photodiode elements; Fig. 2 a schematic representation of an optoelectronic sensor with reference light receiver and temperature adjustment element for controlling the wavelength range of the transmitted light; Fig. 3 a schematic representation of a laser scanner using a filter as in Fig. 1; and Fig. 4 a schematic representation of a laser scanner that has a control as in Fig. 2 is used.

[0034] Fig. Figure 1 shows a simplified schematic block diagram of an optoelectronic sensor 10 in an embodiment as a single-beam light sensor. A light transmitter 12, preferably with a sharply defined wavelength range such as a laser light source, generates transmitted light 14, which is bundled or collimated by a transmitting optics 16, shown purely as an example as a single lens, and thus emitted into a monitoring area 18.

[0035] If the transmitted light 14 strikes an object 20 there, a portion is reflected and, superimposed with extraneous light components independent of the transmitted light 14, returns to the sensor 10 as received light 22. There, it is guided via a receiving optics to a light receiver 24. The receiving optics comprise, for example, beam-shaping elements, shown here for the sake of simplicity as a single lens 26, as well as an optical filter 28 whose passband is adapted to the wavelength range of the light transmitter 12 in a manner to be described in more detail below. The received signals of the light receiving elements 24a are read out by a control and evaluation unit 30 and evaluated there.

[0036] The light receiver 24 has a plurality of light-receiving elements 24a, which are designed as Geiger-mode avalanche photodiodes or SPADs. In principle, other light-receiving elements 24a are also conceivable, which could also be designed discretely instead of together in a line or matrix arrangement.

[0037] The filter 28 has at least two, generally n, and here purely by way of example, four filter regions 28a. The striped pattern shown is advantageous, but can also be varied, for example, as a checkered pattern or similar. The filter regions 28a differ from one another in their passbands. The passbands complement one another, possibly partially overlapping, to form a larger passband. For example, each filter region 28a is a bandpass filter with a narrow passband of at least approximately the same width, with the center frequencies increasing from filter region 28a to filter region 28a.

[0038] Under ideal conditions, a single filter region 28a could be tuned to the wavelength range of the light transmitter 12 with an extremely narrow band. However, since this wavelength range is subject to drift during operation, primarily due to temperature influences, the match would not be appropriate at many times. The result would be that the filter region 28a would block useful light contrary to its intended purpose. Therefore, a wider passband is conventionally selected, referred to here as the tolerance wavelength range, which also includes the drifting wavelength range.

[0039] However, the filter regions 28a according to the invention have a narrower passband than the tolerance wavelength range. In particular, the filter regions divide the tolerance wavelength range approximately evenly, so that each individual passband is as wide as 1 / n of the tolerance wavelength range. Then, during operation of the sensor 10, the wavelength range still shifts out of the passband of an individual filter region 28a, but simultaneously into the passband of another filter region 28a.

[0040] Thus, the filter 28 blocks the useful light in many places, namely the filter areas 28a that are currently not suitable due to drifts, but allows it to pass locally at least in one filter area 28a with sharp clipping of extraneous light.

[0041] On the other hand, the light receiver 24 is divided into a plurality of light receiving elements 24a, each of which receives only light from an associated filter region 28a. Thus, the light receiver 24 comprises as many groups with at least one light receiving element 24a as there are filter regions 28a, and each of these groups receives a strong wanted signal with little extraneous light, while the signal-to-noise ratio for the remaining groups is actually deteriorated due to the poorly adapted passband of the associated filter region 28a to the current drift.

[0042] The control and evaluation unit 30 then selects the group with the strong wanted signal and ignores the signals from the remaining groups in order to obtain a received signal with an overall reduced level but a significantly better signal-to-noise ratio. Given the high sensitivity in Geiger mode, the low level is easily tolerated. Selection can often be made simply by selecting the strongest level of all groups, at least as long as the transmitted light 14 is not dominated by extraneous light in neighboring spectral ranges, and in such a case, the measurement conditions for each sensor would be extremely unfavorable. However, the control and evaluation unit can also utilize a history, since the drifts do not cause random jumps in the wavelength range, but rather switch in a traceable manner across spectrally neighboring filter ranges 28a.

[0043] The received signal thus obtained can be evaluated in a variety of ways. In a preferred embodiment, the sensor 10 is a distance-measuring device. The control and evaluation unit 30 then determines the time it takes for the transmitted light 14 to reach the received light 22 and converts this into a distance based on the speed of light.

[0044] In a practical embodiment, the sensor 10 comprises further elements, in particular further electronics and various connections, which are not discussed here for the sake of simplicity. The separation into light receiver 24 and control and evaluation unit 30 in Fig. 1 is also conceivable in practice, but serves primarily for explanation. These elements are preferably at least partially integrated on a common chip, the surface of which is shared by the light-sensitive regions of the avalanche photodiodes and the circuits associated with individual or groups of avalanche photodiodes for their evaluation and control. All components can be compactly housed in a sensor 10 and, in particular, in a single housing.

[0045] The illustrated embodiment of the sensor 10 as a biaxial, one-dimensional scanner is only an example. On the one hand, the light transmitter 12 can also be arranged coaxially to the reception path, either by means of a beam splitter or by arranging the light transmitter 12 in the reception path. Furthermore, the monitoring area 18 can be extended to obtain a 2D or even 3D sensor. Thus, several single-beam systems can be combined to form a scanning light grid with several, usually parallel beams, which measures or monitors distances in each beam. Furthermore, scanning mechanisms with mechanical scanning movement can be considered, as will be explained below with reference to the Fig. 3 and Fig. 4, but also so-called solid-state scanners without moving mechanical parts, which instead use MEMS mirrors, optical phased arrays, acousto-optical modulators, electro-optical modulators, and the like to deflect the scanning beam. Another possibility is to ensure spatial resolution of the light receiver 24. For this purpose, the arrangement of light receiving elements 24a and filter areas 28a is essentially regarded as a macropixel, and this is multiplied in one direction to form a line or in two directions to form an area. Finally, it is also conceivable that the sensor 10 does not have its own light transmitter 12 at all, either because a cooperating light transmitter is mounted opposite, as in a light barrier, or because the received light 22 originates from some other spectrally limited light source.

[0046] Fig. 2 shows a schematic block diagram of a further embodiment of the optoelectronic sensor 10. The same reference numerals designate identical or corresponding features.

[0047] In contrast to the embodiment according to Fig. 1, the filter 28 now has only a single filter range, which, however, is still designed with a narrower band than the tolerance wavelength range. To compensate for drift, the wavelength range of the light transmitter 12 is regulated instead. For this purpose, a reference path with a reference filter 32 and a reference light receiver 34 is provided, into which a portion 14a of the transmitted light 14 is coupled. The reference filter 32 can have characteristics comparable to those of the filter 28, but in any case ensures that, if the wavelength of the transmitted light 14 drifts, the reference light receiver 34 detects corresponding drift information.

[0048] The control and evaluation unit 30 then regulates the wavelength range of the light transmitter 12 based on the drift information of the reference light receiver 34 so that it remains within a specified band. For this purpose, the temperature of the light transmitter 12 is influenced using a temperature adjustment element 36. Two exemplary embodiments of the temperature adjustment element are a Peltier element and a current adjustment of the current through the light transmitter 12. Due to the control, the extremely narrow passband of the filter 28 is always close to the actual wavelength range of the transmitted light 14 and therefore allows at least almost all useful light to pass through, although extraneous light with very sharp, close spectral boundaries is cut off.

[0049] In a mixed form, the multi-part filter 28 with several filter areas 28a is made of Fig. 1 instead of the one-piece filter 28 in Fig. 2. This allows, for example, the transmission-side control to remain coarser, because the appropriate filter area 28a or the at least one avalanche photodiode element 24a assigned to it is also exclusively selected. Conversely, it is also conceivable to select even narrower passbands for the filter areas 28a, because the extent of the remaining drifts has become significantly lower due to the transmission-side control. Another conceivable mixed form uses only the reference path 14, 32, 34 of the Fig. 2 in Fig. 1, in order to use the signal of the reference light receiver for the selection of the filter region 28a or the associated at least one avalanche photodiode element.

[0050] Fig. Figure 3 shows a laser scanner as a further embodiment of the optoelectronic sensor 10. The transmitted light 14 and the received light 22 are periodically guided through the monitoring area 18 with the aid of a rotating mirror 38 in order to scan a plane. A further deflection unit 40 is provided in the transmission path in order to direct the transmitted light 14 onto the rotating mirror 38. The beam-shaping optics 16, 26 are omitted only for the sake of clarity and are certainly provided in practice. Otherwise, the measuring principle is the one that corresponds to the fictitious fixed rotational position. Fig. 1, and the measurement points obtained result in a point cloud corresponding to the scanned contour in the scanned plane.

[0051] Fig. 4 shows a laser scanner that is not like Fig. 3 the measuring principle of Fig. 1, but now the measuring principle of Fig. 2. For this purpose, the simple small mirror, which is Fig. 3 as a further deflection unit 40 directs the transmitted light 14 onto the rotating mirror 38, is now replaced by a splitter mirror. As a result, a part 14a of the transmitted light 14 is transmitted and reaches a specific sub-area of ​​the light receiver 24 directly via a further mirror 42 and through a specific sub-area of ​​the filter 28. As a result, the filter 28 and the light receiver 24 function in a dual function as a reference filter 32 and as a reference light receiver 34 through the aforementioned sub-areas. The reference path thus constructed can be as in Fig. 2 can be used to control the wavelength range of the light transmitter 12.

[0052] In the illustrated embodiment, some light-receiving elements 24a of the light receiver 24 are used as reference light receivers. It is advantageous to provide an optical barrier 44 between them to prevent optical crosstalk or self-glare. Alternatively, a separate reference light receiver 34 can be used, as can a separate reference filter 32.

[0053] Numerous different designs of laser scanners are possible, for example with a rotating polygon mirror wheel or a completely rotating measuring head with light transmitter 12 and / or light receiver 24. By additional movement in elevation or several scanning beams offset in elevation, a 3D scanner can also be realized.

Claims

[1] Optoelectronic sensor (10) for detecting objects (20) in a monitoring area (18), which comprises a light receiver (24) with a plurality of avalanche photodiode elements (24a) for receiving received light (22) from the monitoring area (18) in a wavelength range, each of which can be biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode, a receiving optics (26, 28) with a first optical filter (28, 28a) which is tuned to the wavelength range, and a control and evaluation unit (30) which is designed to detect information about the objects (20) from a received signal of the light receiver (24), wherein the wavelength range shifts during operation of the sensor (10) due to drifts in a tolerance wavelength range, wherein the first filter (28, 28a) has a pass band,which is narrower than the tolerance wavelength range and wherein the receiving optics (26, 28) has at least one further optical filter (28a) whose passband is narrower than the tolerance wavelength range., [2] Sensor (10) according to claim 1, wherein the first filter (28, 28a) is a narrowband bandpass filter. [3] Sensor (10) according to claim 1 or 2, wherein the passbands of the filters (28a) as a whole comprise the tolerance wavelength range. [4] Sensor (10) according to claim 3, wherein at least one avalanche photodiode element (24a) is assigned to each of the filters (28a). [5] Sensor (10) according to claim 4, wherein the control and evaluation unit (30) is designed to select the received signal of that at least one avalanche photodiode element (24a) which is assigned to the filter (28a) whose passband corresponds to the wavelength range of the received light (22) under the current operating conditions. [6] Sensor (10) according to claim 5, wherein the control and evaluation unit (30) selects received signals of the at least one avalanche photodiode element (24a) based on its level or temperature information. [7] Sensor (10) according to one of claims 3 to 6, wherein a first filter (28, 28a) and further filters (28a) form a striped pattern or a checkered pattern. [8] Optoelectronic sensor (10) for detecting objects (20) in a monitoring area (18), which comprises a light receiver (24) with a plurality of avalanche photodiode elements (24a) for receiving received light (22) from the monitoring area (18) in a wavelength range, each of which can be biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode, a receiving optics (26, 28) with a first optical filter (28, 28a) tuned to the wavelength range, and a control and evaluation unit (30) designed to detect information about the objects (20) from a received signal of the light receiver (24), wherein the wavelength range shifts during operation of the sensor (10) due to drifts in a tolerance wavelength range, wherein the first filter (28, 28a) has a passband,which is narrower than the tolerance wavelength range, and wherein the sensor (10) further comprises a light transmitter (12) for emitting transmitted light (14) in the wavelength range into the monitoring area (18), and a reference light receiver (34) with an optical reference filter (32) whose passband corresponds to that of the first filter (28, 28a) in order to obtain drift information as to the extent to which the wavelength range of the transmitted light (14) has shifted due to drifts. [9] Sensor (10) according to claim 8, wherein the control and evaluation unit (30) is designed to regulate the wavelength range of the transmitted light (12) into the passband based on the drift information. [10] Sensor (10) according to claim 8 or 9, wherein the light transmitter (12) has a temperature adjustment element (36) with which the wavelength range of the transmitted light (14) can be changed. [11] Sensor (10) according to one of the preceding claims, wherein the control and evaluation unit (30) is designed to determine a light propagation time of the transmitted light (14) emitted by the light transmitter (12) and received again by the light receiver (24) from the monitoring area (18) as received light (22). [12] Method for detecting objects (20) in a surveillance area (18), in which a light receiver (24) with a plurality of avalanche photodiode elements (24a), each biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode, receives received light (22) from the surveillance area (18) in a wavelength range through a receiving optics (26, 28) with a first optical filter (28, 28a) which is tuned to the wavelength range, wherein information about the objects (20) is detected by evaluation (30) of a received signal of the light receiver (24), and wherein the predetermined wavelength range shifts due to drifts in a tolerance wavelength range, wherein the first filter (28, 28a) only allows received light (22) to pass in a pass band which is narrower than the tolerance wavelength range, and wherein the receiving optics (26,28) has at least one further optical filter (28a) whose passband is narrower than the tolerance wavelength range., [13] A method for detecting objects (20) in a surveillance area (18), in which a light receiver (24) with a plurality of avalanche photodiode elements (24a), each biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode, receives received light (22) from the surveillance area (18) in a wavelength range through a receiving optics (26, 28) with a first optical filter (28, 28a) that is tuned to the wavelength range, wherein information about the objects (20) is detected by evaluating (30) a received signal of the light receiver (24), and wherein the predetermined wavelength range shifts due to drifts in a tolerance wavelength range, wherein the first filter (28, 28a) only allows received light (22) to pass in a pass band that is narrower than the tolerance wavelength range,and wherein a light transmitter (12) emits transmitted light (14) in the wavelength range into the monitoring area (18) and a reference light receiver (34) with an optical reference filter (32) whose passband corresponds to that of the first filter (28, 28a) obtains drift information as to the extent to which the wavelength range of the transmitted light (14) has shifted due to drift.

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