Directional photodetector and optical sensor arrangement

Abstract

A directional photodetector comprises a photosensitive element and a light selector. The photosensitive element comprises a single-photon avalanche diode, SPAD, or an array of SPADs or SPAD array. The light selector is arranged on or above the photosensitive element, in particular on or above an active surface of the photosensitive element. The light selector is configured to restrict a field of view of the photosensitive element at least for light with a wavelength within a specified wavelength range. The light selector is configured to restrict the field of view by predominantly passing light with a direction of incidence within a range of passing directions of the light selector.

Claims

1. An optical sensor arrangement for time-of-flight measurement, the sensor arrangement comprising an optical emitter; and a measurement photodetector implemented as a directional photodetector and a reference photodetector implemented as a directional photodetector, each of the photodetectors comprising a photosensitive element comprising a single-photon avalanche diode (SPAD), or a SPAD array; and a light selector arranged on or above the photosensitive element and configured to restrict a field of view of the photosensitive element at least for light with a wavelength within a specified wavelength range by predominantly passing light with a direction of incidence within a range of passing directions of the light selector; wherein the measurement photodetector is arranged and configured to detect light entering the sensor arrangement through a measurement aperture in a housing of the sensor arrangement; the reference photodetector is arranged and configured to detect light emitted by the optical emitter and reflected by the housing; the range of passing directions of the light selector of the reference photodetector does not include an orthogonal direction with respect to an active surface of the reference photodetector; and the specified wavelength range corresponds to an emission spectrum of the emitter.

2. The optical sensor arrangement according to claim 1, wherein the light selector of the measurement photodetector comprises an interference filter with a passband, wherein a spectral position of the passband depends on an angle of incidence.

3. The optical sensor arrangement according to claim 2, wherein for orthogonal incidence, the interference filter has a maximum transmission value for light with a principal wavelength; and the range of passing directions of the light selector is at least partially defined by a shift between the principal wavelength and a characteristic wavelength of the specified wavelength range.

4. The optical sensor arrangement according to claim 1, wherein the light selector of the measurement photodetector comprises a stack of metal layers forming one or more channels through which light can pass.

5. The optical sensor arrangement according to claim 4, wherein each of the metal layers comprises metal structures and, to form the one or more channels, the metal structures of subsequent layers of the metal layers are arranged on top of each other; or are laterally shifted with respect to each other.

6. The optical sensor arrangement according to claim 1, wherein the range of passing directions of the light selector of the measurement photodetector includes an orthogonal direction with respect to an active surface of the measurement photodetector.

7. The optical sensor arrangement according to claim 1, wherein the light selector comprises an interference filter with a passband, wherein a spectral position of the passband depends on an angle of incidence; for orthogonal incidence, the interference filter has a maximum transmission value for light with a principal wavelength; the range of passing directions of the light selector is at least partially defined by a shift between the principal wavelength and a characteristic wavelength of the specified wavelength range; and the principle wavelength of the interference filter of the measurement photodetector matches the characteristic wavelength of the emission spectrum.

8. The optical sensor arrangement according to claim 1, wherein the light selector comprises a stack of metal layers forming one or more channels through which light can pass; each of the metal layers comprises metal structures; and to form the one or more channels, the metal structures of subsequent layers of the stack of metal layers are arranged on top of each other.

9. The optical sensor arrangement according to claim 1, wherein the range of passing directions of the light selector of the reference photodetector does not include a direction towards an emission aperture of the housing.

10. The optical sensor arrangement according to claim 1, wherein the light selector of the reference photodetector comprises an interference filter with a passband, wherein a spectral position of the passband depends on an angle of incidence; for orthogonal incidence, the interference filter has a maximum transmission value for light with a principal wavelength; the range of passing directions of the light selector is at least partially defined by a shift between the principal wavelength and a characteristic wavelength of the specified wavelength range; and the principle wavelength of the interference filter of the reference photodetector is shifted with respect to the characteristic wavelength of the emission spectrum.

11. The optical sensor arrangement according to claim 1, wherein the light selector of the reference photodetector comprises a stack of metal layers forming one or more channels through which light can pass; each of the metal layers comprises metal structures; and to form the one or more channels, the metal structures of subsequent layers of the metal layers of the reference photodetector are laterally shifted with respect to each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the disclosure is explained in detail with the aid of exemplary implementations by reference to the drawings. Components that are functionally identical or have an identical effect may be denoted by identical references. Identical components and/or components with identical effects may be described only with respect to the figure where they occur first and their description is not necessarily repeated in subsequent figures. All features and details of specific implementations may be combined with other implementations if not stated otherwise. In the drawings,

(2) FIG. 1 shows schematically an exemplary implementation of a directional photodetector according to the improved concept;

(3) FIG. 2 shows schematically an emission spectrum of a laser diode;

(4) FIG. 3 shows schematically the transmission of an interference filter to be used in an exemplary implementation of a directional photodetector according to the improved concept as a function of wavelength;

(5) FIG. 4 shows schematically the transmission of an interference filter to be used in an exemplary implementation of a directional photodetector according to the improved concept as a function of an incident angle;

(6) FIG. 5 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept;

(7) FIG. 6 shows schematically the transmission of an interference filter to be used in a further exemplary implementation of a directional photodetector according to the improved concept as a function of an incident angle;

(8) FIG. 7 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept;

(9) FIG. 8 shows schematically a stack of metal layers of a further exemplary implementation of a directional photodetector according to the improved concept;

(10) FIG. 9 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept;

(11) FIG. 10 shows schematically a stack of metal layers of a further exemplary implementation of a directional photodetector according to the improved concept;

(12) FIG. 11 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept;

(13) FIG. 12 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept; and

(14) FIG. 13 shows schematically an exemplary implementation of an optical sensor arrangement according to the improved concept.

DETAILED DESCRIPTION

(15) FIG. 1 shows schematically an exemplary implementation of a photodetector according to the improved concept. The directional photodetector comprises a semiconductor body S, for example a semiconductor substrate, and a photosensitive element P embedded in the semiconductor body S. The semiconductor body and the photosensitive element P may for example be part of a semiconductor die.

(16) The photosensitive element P comprises a SPAD or a SPAD array. Furthermore, the photodetector comprises a light selector LS. In the example of FIG. 1, the light selector LS comprises an interference filter F implemented as a band-pass filter with a passband having an angular dependent spectral position. The interference filter F is arranged on a top surface of the semiconductor die or semiconductor body S and is in particular arranged above or on an active surface of the photosensitive element P. A range of passing directions of the light selector LS is determined by the interference filter F.

(17) Optionally, the photodetector comprises one or more backend layers BL arranged between the photosensitive element P and the interference filter F. The backend layers BL may for example comprise insulator layers such as oxide layers.

(18) FIG. 2 shows schematically an emission spectrum of an optical emitter, for example a laser diode, in particular a VCSEL or VECSEL.

(19) The emission spectrum may have a maximum at a characteristic wavelength lc, which may for example lie in the infrared spectrum of electromagnetic radiation. For example, the characteristic wavelength lc may be 940 nm. The emission spectrum drops to zero or essentially to zero at a lower wavelength l1 and an upper wavelength l2 limiting the emission spectrum. A linewidth of the emission spectrum given by a difference between the lower and the upper wavelength l1, l2 may lie in the order of few nanometers. For example for a characteristic wavelength lc of 940 nm, the lower and the upper wavelength l1,l2 may be approximately 938 nm and 942 nm, respectively.

(20) FIG. 3 shows schematically the transmission of an interference filter F to be used in an exemplary implementation of a directional photodetector according to the improved concept as a function of wavelength. The interference filter F of FIG. 1 may for example have a transmission as schematically shown in FIG. 3.

(21) A solid line in FIG. 3 shows schematically the transmission of the interference filter F at orthogonal incidence that is for zero incidence angle measured from a normal to a main surface of the interference filter F. The transmission has a passband for orthogonal incidence PB, which is for example centered or approximately centered or has a peak at a principal wavelength lp. The transmission has for example a maximum transmission value for light with a wavelength corresponding to the principal wavelength lp for orthogonal incidence. The maximum transmission value may for example correspond to less than 100% transmission, for example to a value between 70% and 100% transmission, for example to a value of 80% or approximately 80% transmission.

(22) A dashed line in FIG. 3 shows schematically the transmission of the interference filter F for a nonzero incidence angle, for example for an incidence angle of 30° measured from the normal to the main surface of the interference filter F. The transmission has a passband for the nonzero incidence angle PB′. The passband for the nonzero incidence angle PB′ corresponds for example to the passband for orthogonal incidence PB shifted towards smaller wavelengths. This may be a consequence of a composition of the interference filter F comprising an alternating stack of electrically insulating layers with different indices of refraction.

(23) In the implementation shown in FIG. 1, the principal wavelength lp for orthogonal incidence of the interference filter F matches for example the characteristic wavelength lc of the emission spectrum as shown for example in FIG. 2. A resulting transmission curve of the interference Filter F as a function of the incidence angle for light with a wavelength corresponding to the characteristic wavelength lc is shown in FIG. 4.

(24) In agreement with the passband for orthogonal incidence PB of FIG. 3, the transmission of the interference filter has the maximum value for zero incidence angle. For increasing incident angles, the transmission value drops due to the shift of the passband as described in FIG. 3. Thus, at larger incidence angles, for example at an incidence angle of ±45°, the transmission for light with a wavelength corresponding to the characteristic wavelength lc may be zero or approximately zero.

(25) Consequently, for light L with a wavelength within the emission spectrum, in particular a wavelength corresponding to the characteristic wavelength lc, the light selector LS of the photodetector of FIG. 1 passes predominantly light with zero or small incident angles and blocks light with large incident angles. The blocking of the light L is depicted in FIG. 1 schematically by crosses.

(26) FIG. 5 shows schematically a further exemplary implementation of a directional photodetector according to the improved concept. The photodetector of FIG. 5 is based on the photodetector of FIG. 1.

(27) The interference filter F of FIG. 5 may for example have a transmission as schematically shown in FIG. 3. However, the light selector LS′, in particular the interference filter F′, of the photodetector of FIG. 5 differs from the interference filter F of the photodetector of FIG. 1. In particular, the principal wavelength lp for orthogonal incidence is shifted with respect to the characteristic wavelength lc. For example, for a characteristic wavelength lc of 940 nm, the principal wavelength lp for orthogonal incidence of the interference filter F′ may be 960 nm.

(28) Consequently, as indicated by the shift of the passbands PB, PB′ in FIG. 3, the transmission value of the interference filter F′ for light with a wavelength corresponding to the characteristic wavelength lc has the maximum value for incidence under a nonzero incidence angle, for example an incidence angle of ±45°.

(29) A resulting transmission curve of the interference Filter F′ as a function of the incidence angle for light with a wavelength corresponding to the characteristic wavelength lc is shown in FIG. 6.

(30) In agreement with the passband for orthogonal incidence PB of FIG. 3, the transmission of the interference filter F′ is reduced or zero for zero incidence angle. On the other hand, for a nonzero incidence angle, for example an incidence angle of ±45°, the transmission of the interference filter F′ has the maximum transmission value.

(31) Consequently, for light L with a wavelength within the emission spectrum, in particular a wavelength corresponding to the characteristic wavelength lc, the light selector LS′ of the photodetector of FIG. 1 passes predominantly light with a nonzero incidence angle, for example at or around ±45°, and blocks light with zero incidence angle.

(32) As readily recognized by the skilled reader, the incident angles under which the transmission of the interference filter F′ becomes maximum for light with the characteristic wavelength lc may be tuned or adapted by adapting the interference filter F′. In particular, the shift between the principal wavelength lp and the characteristic wavelength lc defines said incident angles.

(33) FIG. 7 shows schematically a further exemplary implementation of a photodetector according to the improved concept. The photodetector of FIG. 7 is based on the photodetector of FIG. 1.

(34) The light selector LS of the photodetector of FIG. 7 may for example not comprise an interference filter but a stack of metal layers MS. The stack of metal layers MS comprises several metal layers M1, . . . , M6 stacked onto each other as depicted schematically also in FIG. 8. It is pointed put that the number of metal layers comprised by the stack MS is not limited to the four or six metal layers M1, . . . , M6 as shown in FIGS. 7 and 8 as examples.

(35) The stack of metal layers MS is for example comprised by or embedded in the backend layers BL. In particular, the metal layers M1, . . . , M6 may be embedded in the insulator layers of the backend layers BL. The metal layers M1, . . . , M6 may for example correspond to metallization layers according to a semiconductor manufacturing process, in particular a standard semiconductor process, for example a CMOS process.

(36) Each of the metal layers M1, . . . , M6 comprises metal structures STR, wherein the metal structures STR of two subsequent metal layers M1, . . . , M6 are arranged on top of other, in particular congruently on top of each other. The metal structures STR may for example be implemented as strips, wherein the strips of a given metal layer are oriented parallel to each other. Thereby, channels between the metal structures STR are formed, the channels being orthogonal to a stacking direction of the metal layers M1, . . . , M6.

(37) Due to the stack of metal layers, incoming light L impinging on the light selector LS may be partially blocked and partially passed. In particular, light hitting the metal structures

(38) STR may be reflected and/or absorbed by the metal structures STR and consequently blocked.

(39) On the other hand, at least a fraction of light with an incoming direction corresponding to an orientation of the channels may pass through the channels and reach the photosensitive element P. In particular, this may be the case for light with an incident angle being zero or close to zero in with respect to an incident plane orthogonal to the orientation of the strips of the metal structures STR.

(40) In implementations where the respective metal structures of all metal layers M1, . . . , M6 are implemented as strips arranged congruently on top of each other, the channels may pass light irrespective of its incident angle with respect to an incident plane parallel to the orientation of the strips of the metal structures STR.

(41) In alternative implementations, the metal structures STR, in particular the strips, of at least one of the metal layers M1, . . . , M6 are rotated with respect to the metal structures STR, in particular the strips, of at least one further of the metal layers M1, . . . , M6. Consequently, the channels may pass light with an incident angle being zero or close to zero with respect to an incident plane orthogonal to the orientation of the strips of the metal structures STR and with respect to an incident plane parallel to the orientation of the strips of the metal structures STR and block other light.

(42) In alternative implementations, the metal structures comprise rotationally or axially symmetric structures, for example regular polygons such as squares, rectangles, hexagons, octagons and so forth or circular or elliptical structures.

(43) The geometry of the metal structures STR, a lateral distance or lateral distances between metal structures of a given metal layer M1, . . . , M6 and/or a distance, in particular vertical distance, between the metal layers M1, . . . , M6 define the range of incident directions of the light selector LS.

(44) It is further pointed out that the distances between the metal structures STR of a given metal layer M1, . . . , M6 are not necessarily equal as they are in the example of FIGS. 7 and 8. Also the distances between subsequent metal layers M1, . . . , M6 are not necessarily equal for all metal layers M1, . . . , M6 as they are in the example of FIGS. 7 and 8. For example, the distances between the metal layers M1, . . . , M6 may be predetermined by the semiconductor or CMOS manufacturing process.

(45) FIG. 9 shows schematically a further exemplary implementation of a photodetector according to the improved concept. FIG. 10 shows schematically a corresponding stack of metal layers MS′. The photodetector of FIG. 9 is based on the photodetector of FIG. 7.

(46) In the light selector LS′ of the photodetector of FIG. 9, the metal structures STR′ of subsequent metal layers M1, . . . , M6′ are not are arranged on top of each other as in the implementation of FIG. 7 but are laterally shifted with respect to each other. In particular, the metal structures STR′ of the metal layers M1, . . . , M6′ are laterally gradually shifted with respect to a lowest layer M1′ of the metal layers M1, . . . , M6′, the lowest layer M1′ being arranged closest to the photosensitive element P. Therein, being “laterally gradually shifted” means for example that a lateral shift of the metal structures STR′ of one of the metal layers M2′, . . . , M6′ with respect to the metal structures STR′ of the lowest metal layer M1′ is greater the greater the vertical distance between said one of the metal layers M2′, . . . , M6′ is with respect to the lowest metal layer M1′.

(47) A direction of the shift defines an orientation of the channels. The orientation of the channels in turn define the range of passing directions of the light selector LS′.

(48) In the non-limiting example of FIGS. 9 and 10, the metal structures STR′ are for example implemented as strips. The strips are for example laterally shifted in a direction perpendicular to the orientation of the strips. Consequently, the resulting channels may pass light with incident directions corresponding to a nonzero incident angle with respect to an incident plane orthogonal to the orientation of the strips of the metal structures STR′.

(49) The nonzero angle may for example lie between 15° and 90° measured from the normal with respect to the active surface of the photosensitive element P. In particular, the nonzero angle may lie between 20° and 60°, for example between 20° and 30°. However, obviously any nonzero angle may be achieved by adapting the lateral shift between the metal structures STR′ of subsequent metal layers M1′, . . . , M6′.

(50) FIG. 11 shows schematically a further exemplary implementation of a photodetector according to the improved concept based on the implementations described with respect to FIG. 1.

(51) The photodetector of FIG. 11 further comprises, for example embedded in the backend layers BL, a stack of metal layers MS as described with respect to FIGS. 7 and 8.

(52) It is pointed out that the order of the stack of metal layers MS and the interference filter F may also be reversed in some implementations such that the interference filter F is arranged between the stack of metal layers MS and the photosensitive element P.

(53) FIG. 12 shows schematically a further exemplary implementation of a photodetector according to the improved concept based on the implementations described with respect to FIG. 5.

(54) The photodetector of FIG. 12 further comprises, for example embedded in the backend layers BL, a stack of metal layers MS′ as described with respect to FIGS. 9 and 10.

(55) The stack of metal layers MS′ and the interference filter F′ are adapted with respect to each other. In particular, light with a wavelength within the specified spectral range, in particular light having the characteristic wavelength lc, being able to pass the channels formed by the metal layers M1′, . . . , M6′ is at least partially passed by the interference filter F′.

(56) Due to the combination of the respective interference filter F, F′ and the respective stack of metal layers MS, MS′, the restriction of the field of view, in particular the definition of the range of passing directions, of the photodetector may be further improved in implementations as in FIGS. 11 and 12. In particular, the field of view may be defined in a more accurate and/or a more flexible way as recognized by the skilled reader from the explanations above. Furthermore, a more effective or more strict separation of incoming directions being passed and incoming directions being blocked by the light selector LS may be achieved.

(57) FIG. 13 shows schematically an exemplary implementation of an optical sensor arrangement for time-of-flight, TOF measurement, according to the improved concept.

(58) The sensor arrangement comprises a carrier C and a housing H. The housing H may be mounted on the carrier C. Alternatively, the housing H and the carrier C may be comprised by a single piece of material.

(59) The sensor arrangement further comprises an optical emitter E, for example a VCSEL, in particular an infrared emitter, arranged and mounted on the carrier C. The optical emitter E is configured to emit light LE, in particular in a pulsed manner, according to an emission spectrum, for example an emission spectrum as in FIG. 2, through an emission aperture AE of the housing H arranged above the emitter E.

(60) The sensor arrangement comprises a reference photodetector PR and a measurement photodetector PM arranged on the carrier C. The reference photodetector PR is optional. If applicable, the reference photodetector PR may be arranged closer to the emitter E than the measurement photodetector PM. In some implementations, the photodetector PM and the reference photodetector PR are comprised by a detector die D mounted on the carrier C.

(61) The sensor arrangement, comprises an optical barrier B separating an interior of the sensor arrangement into a first cavity C1 and a second cavity C2. The optical barrier may be comprised by or attached to the housing H. The emitter E and the reference photodetector PR are arranged in the first cavity C1 and the measurement photodetector PM is arranged in the second cavity C2.

(62) Optionally, the interior of the sensor arrangement may be filled with a casting material, for example comprising an epoxy and/or silicone material.

(63) The optical barrier B may be attached or directly attached to the detector die D. In such implementations, there may be no gap G between the optical barrier B and the detector die D. In other implementations, the optical barrier B may be not directly attached to the detector die D and/or there may be a gap G between the optical barrier B and the detector die D. Alternatively, in particular if the photodetector PM and the reference photodetector PR are not comprised by a single detector die D, there may be a gap G between the optical barrier B and the carrier C.

(64) The housing H has a measurement aperture AM arranged above the measurement photodetector PM through which light to be measured LM may enter the sensor arrangement and hit the measurement photodetector PM.

(65) The measurement photodetector PM may be implemented as a directional photodetector according to the improved concept, in particular as described with respect to one of FIG. 1,7 or 11.

(66) The range of passing directions of the measurement photodetector PM includes incoming directions corresponding to the light LM entering through the measurement aperture AM, in particular includes directions orthogonal to the active surface of the photosensitive element P of the measurement photodetector PM.

(67) In operation, the emitter E may emit the light LE within the emission spectrum. The emitted light LE may be at least partially reflected by an external object (not shown) whose distance with respect to the sensor arrangement is to be determined. The partially reflected light may enter the sensor arrangement as the light to be measured LM.

(68) Since the light LM entering through the measurement aperture AM may hit the measurement photodetector PM predominantly under orthogonal incidence or under a relatively small incident angle, it may be passed by the light selector LS of the measurement photodetector PM and detected by the photosensitive element P of the measurement photodetector PM.

(69) The measurement photodetector PM may generate a measurement signal based on the detected light. A control unit (not shown) of the sensor arrangement, which may for example be comprised by the detector die D, computes a stop time for the TOF measurement depending on the measurement signal. The control unit further computes the TOF depending on the stop time. A distance between the external object and the sensor arrangement may then be determined depending on the TOF, in particular may be directly proportional to the TOF.

(70) Furthermore, light corresponding to optical crosstalk light CT may reach the measurement photodetector PM. The crosstalk light CT may for example be emitted by the emitter E and leak from the first cavity C1 to the second cavity C2 through the gap G.

(71) Alternatively or in addition, the crosstalk light CT may reach the second cavity C2 via a cover (not shown), in particular an optically transparent or translucent cover, covering the housing H, the apertures AE, AM and/or the sensor arrangement. In particular, the crosstalk light CT may be emitted by the emitter E, reflected from and/or within the cover and thereby reach the second cavity C2.

(72) The crosstalk light CT or at least parts of the crosstalk light CT may hit the light selector LS of the measurement photodetector PM under an incident direction outside of the range of passing directions of the measurement photodetector PM. In particular, at least parts of the crosstalk light CT may hit the light selector LS not under orthogonal incidence and not with a small incidence angle.

(73) Consequently, the crosstalk light CT is at least partially blocked by the light selector LS and not detected by the photosensitive element P of the measurement photodetector PM. Thus, an accuracy for computing the stop time may be improved. Furthermore, a saturation of the SPAD or SPAD array of the measurement photodetector PM may be avoided.

(74) The reference photodetector PR may be implemented as a directional photodetector according to the improved concept, in particular as described with respect to one of FIG. 5, 9 or 12.

(75) The light emitted by the emitter E may be internally reflected by the housing H, the optical barrier B, the carrier C and/or the cover and subsequently hit the reference photodetector PR. In particular, the reflected light LR may hit the light selector LS′ of the reference photodetector PR under various incident directions.

(76) A portion of the reflected light LR hitting the light selector LS′ with an incident direction within the range of passing directions of the light selector LS′ may pass the light selector LS′ and be detected by the photosensitive element P of the reference photodetector PR.

(77) Another portion of the reflected light LR hitting the light selector LS′ with an incident direction outside the range of passing directions of the light selector LS′ may be blocked by the light selector LS′.

(78) For example, the range of passing directions of the light selector LS′ may include directions with a component pointing from the reference photodetector PR to the emitter E.

(79) Consequently, the amount of light being detected by the reference photodetector PR is reduced avoiding a saturation of the SPAD or SPAD array of the reference photodetector PR.

(80) In some implementations, the range of passing directions of the light selector LS′ of the reference photodetector PR does not include directions pointing from the reference photodetector PR to the emission aperture or does not include all such directions. Consequently, an amount of background or ambient light entering the sensor arrangement through the emission aperture AE and being detected by the reference photodetector PR may be reduced. This may further decrease the risk of saturation of the SPAD or SPAD array of the reference photodetector PR.

(81) The reference photodetector PR may generate a reference signal based on the detected light. The control unit computes a start time for the TOF measurement depending on the reference signal. The control unit further computes the TOF depending on the stop time and the start time. The TOF may for example correspond or correspond approximately to a time difference between the start and the stop time. This may be denoted as double-differential measurement principle.

(82) In some implementations, only one of the reference photodetector PR and the measurement photodetector PM is implemented as a directional photodetector according to the improved concept.

(83) The reference photodetector PR is optional. In implementations without the reference photodetector PR, the start time may be estimated or determined by a calibration of the sensor arrangement.

(84) By using the reference photodetector PR, higher time measurement precision may be achieved by applying the double-differential measurement principle. Subtracting the start and the stop time from each other may lead directly to the TOF and may cancel out systematic errors of the response time of a driver circuit, the emitter E or a read out circuitry.

(85) By a directional photodetector according to the improved concept, photons entering with an unwanted or undesired incident direction are blocked or filtered out. This is achieved by an angular dependent interference filter F, F′ and/or a stack of metal layers MS, MS′ forming channels with respective passing orientation for example in the manner of louvers. To enhance the effect, the interference filter F, F′ and the louvers or stacks of metal layers MS, MS′ may be stacked.

(86) In an optical sensor arrangement according to the improved concept, one or two directional photodetectors according to the improved concept are used for example to reduce the impact of background or ambient light on the reference photodetector PR and/or to reduce the impact of crosstalk from the emitter E on the measurement photodetector PR.