WO2024115519A1

WO2024115519A1 - Time-of-flight sensor and corresponding operating method

Info

Publication number: WO2024115519A1
Application number: PCT/EP2023/083427
Authority: WO
Inventors: Peter Trattler; Thomas Jessenig
Original assignee: Ams Osram AG
Current assignee: Ams Osram AG
Priority date: 2022-12-02
Filing date: 2023-11-28
Publication date: 2024-06-06
Anticipated expiration: 2025-06-02

Abstract

A simple and compact Time-of-Flight sensor with a reference channel shall be provided. To this end, a Time-of-Flight sensor (42) comprises a light source (12) and a light detector (24) arranged side by side in a housing (4), wherein the light detector (24) comprises a focal plane (66) with an array of photodetectors, and wherein in front of the housing (4) a transparent cover element (44) is arranged, said cover element (44) laterally extending over the light source (12), the light detector (24), and a region in between, such that at least some rays of light emitted by the light source (12) along a reference signal path (56) are reflected at a surface of the cover element (44) by way of a predominant single reflection and then are directed at a dedicated region, in particular a peripheral region (54), of the focal plane (66), wherein the Time-of-Flight sensor (42) comprises a control and evaluation unit (68) which uses the output of the photodetectors in the dedicated region, in particular the peripheral region (54), of the focal plane (66) as a reference for determining the emission timing of a light pulse emitted by the light source (12).

Description

TIME-OF-FLIGHT SENSOR AND CORRESPONDING OPERATING METHOD

DESCRIPTION

TECHNICAL FIELD

The invention relates to a Time-of-Flight sensor . It also relates to a corresponding method of operating such a sensor .

BACKGROUND

One technique for measuring three-dimensional depth is to sense the time it takes for light to travel from a light source to a reflective obj ect or target and back to a light detector . This travel time is called Time-of-Flight ( ToF) . In a Time-of-Flight sensor the light detector is typically arranged adj acent to the light source in a common housing . A direct Time-of-Flight ( dToF) sensor is configured such that the obj ect of interest may be detected directly . Usually, such a sensor comprises a light source capable of emitting ultra-short light pulses and a light detector capable of sensing a reflected portion of the respective pulses .

Due to the great velocity of light , a high time resolution of the measurement is required . For example , a distance of 1 mm corresponds to a travel time of 7 ps . Hence , even a small delay between the triggering of a light pulse and actual emission of said pulse can lead to unpredictable time of fsets of the measured values for the travel time and therefore to incorrect distance estimates . Even worse , the delay may vary with various environmental or internal parameters , like temperature . It is therefore desirable to include a reference channel within a ToF sensor which allows for measuring the onset or switch-on delay by measuring the "travel time" ( i . e . , the time interval between triggering a light pulse and sensing it at the light detector ) for a very short ( i . e . , negligible ) distance . This information can then be used to calibrate the sensor and to correct further measurements . SUMMARY

One obj ective of the present invention is to provide a simple and compact Time-of-Flight sensor with a reference channel . A corresponding method of operation which yields accurate distance values shall also be provided .

With respect to the device the obj ective is met by a Time-of- Flight sensor according to claim 1 . The corresponding method is speci fied in claim 12 .

Accordingly, there is a Time-of-Flight ( ToF) sensor comprising a light source and a light detector arranged side by side in a housing, wherein the light detector comprises a focal plane with an array of photodetectors , and wherein in front of the housing a transparent cover element is arranged, said cover element laterally extending over the light source , the light detector, and a region in between, such that at least some rays of light emitted by the light source along a reference signal path are reflected at a surface of the cover element by way of a single reflection and then are directed at a dedicated and preferably delimited region, in particular a peripheral region, of the focal plane .

This way, a reference channel for obtaining information on the pulse emission timing is established in a cost and si ze ef ficient way by using a crosstalk signal . The crosstalk signal is based on the light rays along the reference signal path which due to predominant single reflection at the cover element hit the light detector with relatively high magnitude or intensity in a dedicated region, i . e . , the reference region, which is delimited or can easily be separated from the measurement region . Therefore , the crosstalk is in most cases higher than a signal by target and/or ambient light , and it can be used as a zero reference for the measurement . As a result , high accuracy distance sensing is enabled . In a preferred embodiment the cover element is configured such that the single reflection is a total internal reflection at or within the cover element . This means that the crosstalk signal used for the reference channel has a high magnitude or intensity .

Preferably, the cover element is a cover plate with a back surface facing the housing and a front surface facing away from the housing . Furthermore , it is preferred that the cover element is configured such that the single reflection happens at the front surface of the cover element .

The cover plate has a preferred thickness in the range of 0 . 3 mm but can be as thick as several millimeters . The distance of the cover plate to the frontside of the housing ( i . e . , the thickness of the optional air gap ) preferably is within the range 0-500 pm . This distance preferably is small in comparison to a typical target distance .

In an expedient embodiment guidance of the light rays achieved by the cover element is such that the rays along the reference signal path predominantly hit a peripheral region of the light detector, and preferably to a much lesser extent a central region of the focal plane . This has the advantage that the crosstalk signal does not noticeably af fect the actual measurement signal .

Preferably, the reference signal path corresponds to a peripheral portion of a field of illumination associated with the light source . Accordingly, the reference signal path corresponds to a peripheral portion of a field of view associated with the light detector .

In an advantageous implementation the photodetectors are single photon avalanche diodes ( SPADs ) . The light source advantageously is a vertical-cavity surface-emitting laser (VCSEL ) . The ToF sensor comprises a control and evaluation unit which uses the output of the photodetectors in the dedicated (peripheral ) region of the focal plane as a reference for determining the emission timing of a light pulse emitted by the light source .

Advantageously, the control and evaluation unit uses the thus-determined emission timing to determine an emission- corrected Time-of-Flight for light pulses incident on a central region of the focal plane after reflection at an external obj ect .

Preferably, the reference measurement for setting the timezero and the actual measurement for determining the distance of an external obj ect happen during one and the same measurement cycle , i . e . are based on the same light pulse emitted by the light source ( triggered by the same electrical signal ) .

Preferably, the ToF sensor is a direct ToF sensor .

In term of method, the invention relates to a method of operating a Time-of-Flight sensor, the Time-of-Flight sensor comprising a light source and a light detector arranged side by side in a housing, wherein the light detector comprises a focal plane with an array of photodetectors , and wherein in front of the housing a transparent cover element is arranged, said cover element laterally extending over the light source , the light detector, and a region in between, the method comprising the steps of directing at least some rays of light emitted by the light source along a reference signal path such that they are reflected at a surface of the cover element by way of a single reflection and then are directed at a dedicated region, in particular a peripheral region, of the focal plane .

In a preferred embodiment the output of the photodetectors in the dedicated (peripheral ) region is used as a reference for determining the emission timing of a light pulse emitted by the light source . The thus-determined emission timing is advantageously used to determine an emission-corrected Time- of-Flight for light pulses incident on a central region of the focal plane after reflection at an external obj ect .

In a preferred advancement of the main idea reference pixels and measurement pixels of the light detector are virtually adj usted to compensate for optical misalignment . This means that the type of the respective pixel is not hard-coded or hard-wired but can be set , preferably in software , such that the reference region and the measurement region on the focal plane can be shi fted to some extent to accommodate to the field of illumination . This concept can be used to compensate for manufacturing tolerances of the package .

What has been said with respect to the device may analogously be applied to the method, and therefore need not be repeated here . Device embodiments and details have a counterpart in the method and vice versa .

Typically, the external obj ect whose distance is to be measured is far away from the ToF sensor, in any case much farther than the distance between the cover element and the housing .

Typical fields of application for a ToF sensor according to the invention comprise :

• Augmented Reality (AR)

• Robot Obj ect Detection

• Digital Proj ector Angle to Wall Detection

• Camera Laser Detection Auto Focus ( LDAF)

• Gesture Based User Interface

• Presence Detection

These and other features , embodiments , obj ectives , and advantages of the invention will become apparent from the subsequent description . BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are discussed below with reference to the accompanying drawings .

FIG . 1 shows a schematic sectional view of a conventional

Time-of-Flight sensor .

FIG . 2 illustrates a field of illumination and a field of view of a Time-of-Flight sensor .

FIG . 3 shows a schematic sectional view of a Time-of-Flight sensor according to the invention .

FIG . 4 shows a top view of a focal plane of a light detector used in a Time-of-Flight sensor according to FIG . 3 .

Similar elements are designated with the same reference numerals throughout the description .

DETAILED DESCRIPTION

The conventional Time-of-Flight sensor 2 shown in FIG . 1 in a simpli fied sectional view comprises a housing 4 with a first cavity and a second cavity which may be interconnected . The housing has a backside 6 (here shown as the bottom side ) and a frontside 8 ( shown as the upper side ) . The first cavity, also called emitter cavity 10 , holds a light source 12 capable of emitting light pulses . The respective light pulse is shaped by a beam shaping element 14 above the light source 12 and a suitably shaped emitter aperture 16 in the frontside 8 of the housing 4 to form a cone-like field of illumination 18 centered around an optical emitter axis 20 , as schematically illustrated in FIG . 2 ( there may be a narrow cone corresponding to a long-distance mode and a wider cone corresponding to a short-distance mode ) . Similarly, the second cavity, also called detector cavity 22 , holds a light detector 24 for detecting reflected light pulses in combination with incident background or ambient light from other sources . A beam shaping element above the light detector 24 and a suitably shaped detector aperture 28 in the frontside 8 of the housing define a cone-like field of view 30 for the light detector 24 around an optical detector axis 32 . The detector aperture 28 is adj acent to the emitter aperture 16 , and the optical detector axis 32 is parallel to the optical emitter axis 20 , as illustrated in FIG . 2 .

The light source 12 preferably is capable of emitting ultra- short light pulses with pulse duration in the range of picoseconds . The light source 12 may for example be a laser diode , in particular a vertical-cavity surface-emitting laser (VCSEL ) , preferably arranged on a substrate 34 , in particular a semiconductor waver or a die , which in the context of integrated circuits is a small block of semiconducting material . The light detector 24 preferably comprises an array of photo detectors , preferably capable of time-resolved light or photon measurement with a resolution in the range of picoseconds . In particular, the photo detectors may be reali zed as single photon avalanche diodes ( SPADs ) . For example , a plurality of SPADs may be arranged in chessboard or matrix-like configuration in a focal plane above a substrate 36 , in particular a semiconductor waver or a die . The substrate 34 which holds the light source 12 may be laterally extended to also hold the light detector 24 , as illustrated in FIG . 1 ( i . e . , in this case substrates 34 and 36 are one and the same ) .

A corresponding control and evaluation unit (not shown in the figures ) correlates the reflected light pulses incident on the light detector 24 with the light pulses emitted by the light source 12 , and via the known light velocity and the measured travel time calculates the distance of a reflecting obj ect . Since in practice there is a small but noticeable delay between triggering the light source 12 (which usually goes along with arming the light detector 24 ) and the actual light pulse emission, the zero point of the travel time measurement is not well established . This in turn can lead to erroneous distance measurements . In order to better establish the zero point of travel time by measurement , the ToF sensor 2 of FIG . 1 comprises a reference channel in which a portion of the emitted light pulse is branched of f from the main light cone and is guided via a very short reference signal path to the light detector 24 . The reference signal path 38 is short in comparison to a typical measurement signal path to a reflecting obj ect and back to the light detector 24 , such that for practical purposes the reference signal path 38 approximates a zero distance . Therefore , the moment when the reference signal is detected at the light detector 24 establishes in good approximation a true zero point (= start of the pulse ) for the travel time measurement with respect to the measurement signal . In other words , the travel time measurement and hence the distance measurement can be compensated for the onset delay of the light pulse emission relative to a triggering event by measuring the arrival time of the measurement signal relative to the arrival time of the reference signal . Alternatively, the onset delay can be obtained once in a calibration measurement and then be subtracted from measured raw (uncorrected) travel time values in order to correct them .

In the example of FIG . 1 the reference signal path 38 is defined by the beam shaping element 14 above the light source 12 . In a simple embodiment this beam shaping element 14 may be a cover plate , but it may also have a more complex contour like a lens or multiple lens array (MLA) . The lower surface of the beam forming element 14 is partially reflective , and such is the surface of the substrate 34 which surrounds the light source 12 . In consequence , a reference light beam emitted by the light source 12 is guided sidewards by a number of reflections at the beam shaping element 14 and the substate 34 towards the light detector 24 . The light detector 24 laterally extends from the detector cavity 22 into the emitter cavity 10 . That is , the emitter cavity 10 holds a section of the light detector' s 24 array of photodetectors . In particular, a number of SPDAs located in the emitter cavity 10 act as reference SPADs or reference pixels 62 for determining the zero point of travel time , as described above , while the SPADs in the detector cavity 22 act as measurement detectors in the original sense of the ToF sensor 2 . In order to minimi ze crosstalk between the reference channel and the measurement channels , there is an optical barrier 40 arranged on the substrate 36 or die in the transitional area between the detector cavity 22 and the emitter cavity 10 .

This implementation is relatively elaborate as is requires a light detector 24 with a large lateral extension and an optical barrier 40 on the substrate 34 , 36 or die holding the light detector 24 and the light source 12 . The section of the light detector 24 directly beneath the optical barrier 40 cannot be used for measurement purposes .

FIG . 3 discloses an improved implementation of a ToF sensor 42 which does not have these drawbacks . Similar to the conventional implementation, the ToF sensor 42 comprises a housing 4 with an emitter cavity 10 and a detector cavity 22 which may be interconnected . The housing 4 has a backside 6 (here shown as the bottom side ) and a frontside 8 ( shown as the upper side ) . The emitter cavity 10 holds a light source 12 capable of emitting light pulses . The respective light pulse is shaped by an optional beam shaping element 14 above the light source 12 and a suitably shaped emitter aperture 16 in the frontside 8 of the housing 4 to form a cone-like field of illumination 18 centered around an optical emitter axis 20 , as schematically illustrated in FIG . 2 . Similarly, the detector cavity 22 holds a light detector 24 for detecting reflected light pulses in combination with incident background or ambient light from other sources . A suitably shaped detector aperture 28 in the frontside 8 of the housing 4 , in connection with a beam shaping element 26 , defines a cone-like field of view 30 for the light detector 24 around an optical detector axis 32 . The beam shaping element 26 above the light detector 24 focuses the incoming light beam onto the focal plane 66 of the light detector 24 . The detector aperture 28 is adj acent to the emitter aperture 16 , and the optical detector axis 32 is parallel to the optical emitter axis 20 , as illustrated in FIG . 2 .

Similar to the conventional case , the light source 12 preferably is capable of emitting ultra-short light pulses with pulse duration in the range of , e . g . , picoseconds . The light source 12 may for example be a laser diode , in particular a vertical-cavity surface-emitting laser (VCSEL ) , preferably arranged on a substrate 34 , in particular a semiconductor waver or a die . The light detector 24 preferably comprises an array of photo detectors , preferably capable of time-resolved light or photon measurement with a resolution in the range of picoseconds . In particular, the photo detectors may be reali zed as single photon avalanche diodes ( SPADs ) . For example , a plurality of SPADs may be arranged in chessboard or matrix-like configuration in a focal 66 plane above a substrate 36 , in particular a semiconductor waver or a die .

Also similar to the conventional case , a corresponding control and evaluation unit 68 ( only shown schematically in FIG . 3 with connecting lines to the light source 12 and the light detector 24 omitted) correlates the reflected light pulses incident on the light detector 24 with the light pulses emitted by the light source 12 , and via the known light velocity and the measured travel time calculates the distance of a reflecting obj ect .

In contrast to the previous example of FIG . 1 , the ToF sensor 42 of FIG . 3 comprises a transparent cover element 44 in front of the frontside 8 of the housing 4 . The cover element 44 preferably is a flat cover plate which may be aligned parallel to the frontside 8 of the housing 4 , with or without an air gap 46 , and which laterally extends to cover the emitter aperture 16 , the detector aperture 28 , and the region in between . The cover element 44 preferably comprises a flat back surface 48 facing the frontside 8 of the housing 4 and a flat front surface 50 facing away from the housing 4 . The front surface 50 borders on the ambient air or space. The back surface 48 is preferably parallel to the front surface 50. That is, the cover element 44 preferably has a constant thickness d. The cover element 44 may be made of glass or plastics or any other suitable transparent material.

Most of the emitted or reflected light will pass the cover element 44 without major attenuation or major lateral offset. Hence, the overall geometry of the field of illumination 18 and the field of view 30 illustrated in FIG. 2 will be largely unaffected by the cover element 44. However, the presence of the common cover element 44 for the emitter aperture 16 and the detector aperture 28 introduces a certain amount of crosstalk between the light source 12 and the light detector 24 due to internal light reflections within the cover element 44. This is illustrated schematically in FIG. 3 by a number of exemplary light rays emitted by the light source 12. Only the reflected rays are shown here. The much larger percentage of light rays passing through the cover element 44 are not depicted.

Those light rays emitted by the light source 12 which have a relatively small angle with respect to the optical emitter axis 20 (or a steep angle relative to the frontside 8 of the housing 4) may undergo multiple internal reflections at the boundary layers between the cover element 44 and the adjoining air or material layer, i.e., at the front surface 50 and the back surface 48, before they eventually reach the light detector 24, with a relatively small angle with respect to the optical detector axis 32. However, the related crosstalk is negligible, since for each of the multiple reflections the respective light ray is considerably attenuated. In fact, the attenuation is multiplicative for each reflection. If, for example, at each boundary layer 5 % of the light is reflected, then after three reflections the remaining light intensity is 0,05 x 0,05 x 0,05 = 0,00125 = 1,25 k». Such a small amount of crosstalk, even if the corresponding light rays reach a central portion of the light detector 24 , does not disturb operation of the ToF sensor 42 and does not af fect the derived distance values .

The situation is di f ferent for those light rays emitted by the light source 12 which have a relatively large angle with respect to the optical emitter axis 20 ( or a flat angle relative to the frontside 8 of the housing 4 ) . These light rays are generally located at a peripheral region of the field of illumination 18 . They may undergo j ust a single internal reflection at the front surface 50 of the cover element 44 , accompanied by a relatively small amount of attenuation, before they may enter the detector cavity 22 and may hit the light detector 24 , in fact with a relatively large angle with respect to the optical detector axis 32 ( or with a relatively flat angle relative to the detector surface or focal plane 66 ) . These incident light rays with a single internal reflection at a surface of the cover element 44 are generally located at a peripheral region of the field of view 30 and tend to hit the focal plane 66 in an outer region or peripheral region or edge region ( i . e . , grazing or flat-angle incidence ) .

The attenuation for such beams or rays may be particularly low i f the angle of incidence at the inner front surface 50 of the cover element 44 is smaller than a given materialspeci fic angle of total internal reflection . Total internal reflection is an optical phenomenon in which waves arriving at the interface (boundary) from one medium to another are not refracted into the second ( " external" ) medium, but ( almost ) completely reflected back into the first (" internal" ) medium .

The invention makes use of the grazing or flat-angle incidence of single-internally-reflected light rays in the outer regions of the light detector 24 in that these rays are used to establish a reference channel . Similar to the conventional approach of FIG . 1 , the reference channel is used to determine the start time of each light pulse of a ToF measurement . To this end, the light detector 24 is subdivided in terms of measurement or evaluation into a central region 52 and a peripheral region 54 ( see also FIG . 4 ) . The central region 52 is dimensioned and arranged such that it is relatively little or not af fected by the single internal reflections within the cover element 44 . This defines the actual measurement region of the light detector 24 . In contrast , the peripheral region 54 of the light detector 24 which is predominantly hit by those light rays with single internal reflections , in particular total internal reflections , at the cover element 44 defines a reference region which is used to establish a reference channel in the above-described sense .

Like in the conventional implementation of FIG . 1 , in the reference channel a portion of the emitted light pulse is branched of f from the main light cone and is guided via a very short reference signal path 56 to the light detector 24 . The reference signal path 56 is short in comparison to a typical measurement signal path to a reflecting obj ect and back to the light detector 24 , such that for practical purposes the reference signal 56 path approximates a zero distance . Therefore , the moment when the reference signal is detected at the light detector 24 establishes in good approximation a true zero point (= start of the pulse ) for the travel time measurement with respect to the measurement signal . In other words , the travel time measurement and hence the distance measurement can be compensated for the onset delay of the light pulse emission relative to a triggering event by measuring the arrival time of the measurement signal relative to the arrive time of the reference signal . Alternatively, the onset delay can be obtained once in a calibration measurement and then be subtracted from measured raw (uncorrected) travel time values in order to correct them .

That is , in the improved ToF sensor 42 of FIG . 3 the predominant crosstalk is not suppressed or eliminated but rather actively used to establish a reference channel . This is possible to the spatial focus or accumulation of the crosstalk signals in a peripheral region 54 of the light detector 24 .

In the example of FIG . 3 the reference region of the light detector lies at the left-hand edge of the light detector' s 24 focal plane 66 , i . e . , at that edge which faces away from the light source 12 and has the largest amount of incident single internal reflections . In other embodiments di f ferent regions may be used as reference regions .

In the example of FIG . 4 the light detector 24 comprises a focal plane 66 ( shown from above ) with a rectangular array of photodetectors , in particular SPADs . Each SPAD corresponds to a pixel . An inner rectangular area holds a plurality of measurement SPADs or measurement pixels 60 . A rectangular frame ( or alternatively a border strip ) holds a plurality of reference SPADs or reference pixels 62 .

Preferably, the distinction between reference pixels 62 and measurement pixels 60 is not hard-wired or hard-coded, but the type of each or at least some of the pixels can be set in an evaluation routine of an associated evaluation unit (not shown) . Therefore , the measurement region or zone and/or the reference region or zone of the light detector 24 may be adj usted, e . g . , to fit an actual illumination profile and/or for an optical misalignment correction . For example , an of fset of the optical center 64 in Y-direction of the focal plane 66 , as indicated in FIG . 4 by the double-headed arrow can be compensated for by shi fting the inner rectangle of measurement pixels 60 upwards in the same direction - at the expense of the upper border strip of reference pixels 62 which will get narrower and lose some pixels . As long as there are some redundant pixels remaining, this is usually not a problem .

Returning to FIG . 1 and 3 it is clearly visible that the lateral extension of light detector of the improved ToF sensor 42 can be smaller than in the conventional case of ToF sensor 2 . No optical barrier on the light detector 24 or the related die is required in the improved version .

In summary, in one embodiment of the invention a time-of- flight sensor 2 comprises a light emitter or light source 12 and a light detector 24 in front of which a common cover element 44 ( e . g . glass or transparent plastics ) is arranged . In addition to a main portion of light rays that penetrate the cover element 44 and are reflected by an external obj ect further away, there are (highly-attenuated and therefore negligible ) multiple internal reflections and (non- negligible ) single internal reflections ( along reference signal path 56 ) which normally are considered as undesired cross-talk . According to the invention, the single internal reflections are deliberately used and directed to a peripheral region 54 of pixels of the multi-pixel light detector 24 where upon detection they create a time- zero for the time-of- f light measurement of the main portion of light , the reflected part of which is directed to a central portion 52 of the light detector 24 . Hence , an accurate time-of- flight measurement is possible , which does not depend on a delay between the ( electrical ) triggering of a light pulse and actual emission of said pulse at the light emitter 12 .

The division of the emitted light pulse into a measurement portion and a reference portion with separate detection of the response in the central region of the light detector on the one hand and that of the peripheral region on the other, whereby the reference portion defines a time zero , is preferably carried out in each measurement cycle , i . e . for each emitted light pulse . Hence , the time zero is adj usted ( and the distance measurement corrected) every time anew . LIST OF REFERENCE SIGNS

ToF sensor 2 housing 4 backside 6 frontside 8 emitter cavity 10 light source 12 beam shaping element 14 emitter aperture 16 field of illumination 18 optical emitter axis 20 detector cavity 22 light detector 24 beam shaping element 26 detector aperture 28 field of view 30 optical detector axis 32 substrate 34 substrate 36 reference signal path 38 optical barrier 40

ToF sensor 42 cover element 44 air gap 46 back surface 48 front surface 50 central region 52 peripheral region 54 reference signal path 56 measurement pixel 60 reference pixel 62 optical center 64 focal plane 66 control and evaluation unit 68

Claims

1. Time-of-Flight sensor (42) comprising a light source (12) and a light detector (24) arranged side by side in a housing (4) , wherein the light detector (24) comprises a focal plane (66) with an array of photodetectors, and wherein in front of the housing (4) a transparent cover element (44) is arranged, said cover element (44) laterally extending over the light source (12) , the light detector (24) , and a region in between, such that at least some rays of light emitted by the light source (12) along a reference signal path (56) are reflected at a surface of the cover element (44) by way of a predominant single reflection and then are directed at a dedicated region, in particular a peripheral region (54) , of the focal plane (66) , wherein the Time-of-Flight sensor (42) comprises a control and evaluation unit (68) which uses the output of the photodetectors in the dedicated region, in particular the peripheral region (54) , of the focal plane (66) as a reference for determining the emission timing of a light pulse emitted by the light source (12) .

2. Time-of-Flight sensor (42) according to claim 1, wherein the cover element (44) is configured such that the single reflection reaches the focal plane (44) .

3. Time-of-Flight sensor (42) according to claim 1 or 2, wherein the cover element (44) is a cover plate with a back surface (48) facing the housing (4) and a front surface (50) facing away from the housing (4) .

4. Time-of-Flight sensor (42) according to claim 3, wherein the cover element (44) is configured such that the single reflection happens at the front surface (50) of the cover element ( 44 ) .

5. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein guidance of the light rays achieved by the cover element (44) is such that the rays along the reference signal path (56) predominantly hit the peripheral region (54) , and preferably to a much lesser extent a central region (52) of the focal plane (66) .

6. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the reference signal path (56) corresponds to a peripheral portion of a field of illumination (18) associated with the Time-of-Flight sensor (42) .

7. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the reference signal path (56) corresponds to a peripheral portion of a field of view (30) associated with Time-of-Flight sensor (42) .

8. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the photodetectors are single photon avalanche diodes (SPADs) .

9. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the light source (12) is a vertical-cavity surface-emitting laser (VCSEL) .

10. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the cover element (44) is made of glass or plastics.

11. Time-of-Flight sensor (42) according to any one of the preceding claims, wherein the control and evaluation unit

(68) uses the thus-determined emission timing to determine an emission-corrected Time-of-Flight for light pulses incident on a central region (52) of the focal plane (66) after reflection at an external object.

12. Method of operating a Time-of-Flight sensor (42) , the Time-of-Flight sensor (42) comprising a light source (12) and a light detector (24) arranged side by side in a housing (4) , wherein the light detector (24) comprises a focal plane (66) with an array of photodetectors, and wherein in front of the housing (4) a transparent cover element (44) is arranged, said cover element (44) laterally extending over the light source (12) , the light detector (24) , and a region in between, the method comprising the steps of directing at least some rays of light emitted by the light source (12) along a reference signal path (56) such that they are reflected at a surface of the cover element (44) by way of a predominantly single reflection and then are directed at a dedicated region, in particular a peripheral region (54) , of the focal plane (66) , wherein the output of the photodetectors in the dedicated region, in particular the peripheral region (54) , is used as a reference for determining the emission timing of a light pulse emitted by the light source (12) .

13. Method according to claim 12, wherein the thus- determined emission timing is used to determine an emission- corrected Time-of-Flight for light pulses incident on a central region (52) of the focal plane (66) after reflection at an external object.

14. Method according to any one of the claims 12 to 13, wherein reference pixels and/or measurement pixels of the light detector (24) are virtually adjusted to compensate for optical misalignment.