Method and apparatus for determining reflectance values indicative of reflectance of an object

Abstract

Embodiments of the present disclosure relate to methods and apparatuses for determining a reflectivity value indicative of a reflectivity of an object. A method for determining a reflectivity value indicative of a reflectivity of an object is provided. The method comprises performing a time-of-flight (ToF) measurement using a ToF sensor. A correlation function of the ToF measurement increases with distance over a measurement range of the ToF sensor such that an output value of the ToF sensor for the ToF measurement is independent of a distance between the ToF sensor object and the ToF sensor object. The method further comprises determining the reflectivity value based on the output value of the ToF sensor for the ToF measurement.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to reflectivity sensing. In particular, examples relate to methods and apparatuses for determining a reflectivity value indicative of a reflectivity of an object using a time-of-flight (ToF) sensor. BACKGROUND

[0002] Two-dimensional imaging using ToF cameras is used for object detection and classification (e.g. face recognition, production, smart surveillance, etc.).

[0003] If the scene is illuminated by a light source close to the light capturing part, the measured light intensity depends on the distance to the object according to the inverse square law of point light sources. This prevents measuring the actual reflectivity of the object, which is important information for object recognition. Moreover, objects close by reflect too much light, which can lead to saturation at the ToF camera. Furthermore, if the light source is located close to the light capturing part, objects close by (e.g. cover glass or organic light emitting diode, OLED, displays) can cause stray light to enter the light capturing part.

[0004] Therefore, there can be a need for improved reflectivity sensing using a ToF sensor. SUMMARY

[0005] This requirement can be met by the subject matter of the appended claims.

[0006] One example relates to a method for determining a reflectivity value indicative of a reflectivity of an object. The method comprises performing a ToF measurement using a ToF sensor. The correlation function of the ToF measurement increases with distance within a measurement range of the ToF sensor, such that an output value of the ToF sensor for the ToF measurement is independent of a distance between the ToF sensor and the object. The method further comprises determining the reflectivity value based on the output value of the ToF sensor for the ToF measurement.

[0007] Another example relates to an apparatus for determining a value indicative of a reflectivity of an object. The apparatus comprises a ToF sensor configured to perform a ToF measurement. The correlation function of the ToF measurement increases with distance within a measurement range of the ToF sensor, such that an output value of the ToF sensor for the ToF measurement is independent of a distance between the ToF sensor and the object. The apparatus further comprises processing circuitry configured to determine the reflectivity value based on the output value of the ToF sensor for the ToF measurement. BRIEF DESCRIPTION OF DRAWINGS

[0008] Some examples of apparatuses and / or methods will hereinafter be described by way of example only, and with reference to the drawings, in which

[0009] Figure 1 A flowchart illustrating an example of a method for determining a reflectivity value is shown;

[0010] Figure 2 An example of an apparatus for determining reflectivity values is shown; and

[0011] Figure 3 An example process of the correlation function of a ToF sensor, an output value of the ToF sensor, and the light intensity of light received at the ToF sensor versus distance is shown. DETAILED DESCRIPTION

[0012] Some examples are now described in greater detail by reference to the following drawings. Such other examples, however, are not limited to the features of the embodiments described in detail and can include modifications and equivalents of the features as well as additional and / or alternative features. Moreover, the terminology used herein to describe certain examples should not be limiting to other possible examples.

[0013] Throughout the description of the drawings, same or similar reference numbers refer to same or similar elements and / or features, which can be identical or modified from one another while providing the same or similar function, throughout the drawings. The thicknesses of lines, layers, and / or regions can also be exaggerated for clarity.

[0014] When two elements A and B are used in “or” combination, this is to be understood as disclosing all possible combinations, i.e. only A, only B, as well as A and B, unless otherwise explicitly defined in the individual case. As an alternative wording to the same combination, “at least one of A and B” or “A and / or B” can be used. The same applies to combinations of more than two elements.

[0015] If singular forms are used, such as “a”, “an” and “the”, and it is not explicitly or implicitly defined that only a single element is to be used as mandatory, further examples can also be implemented using a plurality of elements for the same function. If a function is described below as being implemented using a plurality of elements, further examples can implement the same function using a single element or a single processing entity. It is further to be understood that the terms “include”, “including”, “comprise”, and / or “comprising” when used, describe the presence of the stated features, integers, steps, operations, processes, elements, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, processes, elements, components, and / or groups thereof.

[0016] Figure 1 A flowchart of an example of a method 100 for determining a reflectivity value indicative of a reflectivity of an object is shown. The method 100 will be further described below with reference to Figure 2 The method 100 is described, Figure 2An exemplary apparatus 200 for determining a reflectivity value indicative of a reflectivity of an object 201 is shown.

[0017] The apparatus 200 comprises a ToF sensor 210. The ToF sensor 210 comprises an illumination element 230 for emitting modulated light 202 towards a scene comprising the object 201 and a light capturing element 220 for capturing light 203 received from the scene.

[0018] The illumination element 230 generates the modulated light 202. The illumination element 230 can comprise any number of light sources. The illumination element 230 can for example comprise one or more light emitting diodes (LEDs) and / or one or more laser diodes (e.g. one or more vertical cavity surface emitting lasers VCSELs) which are excited based on an illumination signal.

[0019] The light capturing element 220 can comprise various components such as optical (e.g. one or more lenses) and electronic circuitry. In particular, the electronic circuitry comprises an image sensor comprising at least one light sensitive element or pixel (e.g. comprising a photon mixing device PMD or a charge coupled device CCD). The image sensor can for example comprise a plurality of light sensitive elements or pixels. The at least one light sensitive element or pixel is driven based on a reference signal.

[0020] The method 100 comprises performing 102 a ToF measurement using the ToF sensor 210. Parameters of the ToF sensor 210 are adjusted such that a correlation function (sensor response function) of the ToF sensor 210 for the ToF measurement increases (e.g. strictly monotonically) with distance within a measurement range of the ToF sensor 210 such that an output value of the ToF sensor 210 for the ToF measurement is independent of a distance between the ToF sensor 210 and the object 201.

[0021] The correlation function represents an expected distance dependent output of the ToF sensor 210 for the ToF measurement assuming that a light strength (intensity) of the light 203 received at the ToF sensor 210 during the ToF measurement is constant with distance within the measurement range of the ToF sensor 210. This is shown in Figure 3 . The abscissa of the correlation function represents the distance between the ToF sensor 210 and the object 201. In the example of Figure 3 , it is assumed that the distance of the entire abscissa is within the measurement range of the ToF sensor 210. The ordinate represents the output of the ToF sensor 210. Figure 3 An exemplary course 310 of the correlation function with distance is shown in . It can be seen from the exemplary course 310 that the correlation function for the ToF measurement increases with distance (i.e. the greater the distance between the ToF sensor 210 and the object 201, the greater the correlation function).

[0022] For the correlation function of the ToF measurement, it is assumed that the light intensity of the light 203 received at the ToF sensor 210 during the ToF measurement is constant. With reference to the example of Fig. 3, it is assumed that the light intensity of the light 203 reflected back to the light capturing element 220 by the object 201 is essentially constant, independent of the distance between the ToF sensor 210 and the object 201. Figure 2

[0023] However, the light intensity of the light 203 received by the light capturing element 220 is actually dependent on the distance between the ToF sensor 210 and the object 201. In particular, the light intensity of the light 203 received by the light capturing element 220 decreases with increasing distance between the ToF sensor 210 and the object 201. This is further illustrated in Fig. 4. Figure 3 Figure 3 An exemplary course 330 of the light intensity of the light 203 received at the ToF sensor 210 with respect to the distance is illustrated in Fig. 4. Figure 3 The ordinate of Fig. 4 further denotes the light intensity. As can be seen from the course 330, the light intensity decreases with the distance. For example, it can be assumed that the light intensity decreases according to the inverse square law. That is, the distance dependent light intensity of the light 203 received at the ToF sensor 210 can be assumed as follows:

[0024] (1)

[0025] denotes the light intensity of the light received at the ToF sensor 210, denotes the distance between the ToF sensor 210 and the object 201 reflecting the light 203 back to the ToF sensor 210.

[0026] Therefore, the ToF sensor 210 is adjusted such that the correlation function of the ToF sensor 210 for the ToF measurement increases with the distance within the measurement range of the ToF sensor 210 allows to compensate for the decrease of the light intensity of the light 203 received at the ToF sensor 210. For example, the course 310 of the correlation function with respect to the distance can be adjusted to be inverse to the course 330 of the light intensity of the light 203 received at the ToF sensor 210 with respect to the distance. The distance dependent correlation function may, for example, be adjusted as follows:

[0027] (2)

[0028] As a result, the output value of the ToF sensor 210 for the ToF measurement is independent of the distance between the ToF sensor 210 and the object 201. This is further illustrated in Fig. 5, Figure 3 Figure 3 An exemplary course 320 of the output value of the ToF sensor 210 is illustrated in Fig. 5.​​​

[0029] The output value of the ToF sensor 210 is proportional to the reflectivity of the object 201, as the reflectivity of the object 201 determines how much light reaches the ToF sensor 210 during the ToF measurement. Thus, the output value of the ToF sensor 210 varies with the reflectivity of the object 201 - independent of the distance between the ToF sensor 210 and the object. Therefore, when using the correlation function for a ToF measurement as described above, the output value of the ToF sensor 210 allows characterizing the reflectivity of the object 201.

[0030] Referring again to Figure 1 , the method 100 further comprises determining 104 a reflectivity value indicative of the reflectivity of the object 201 based on the output value of the ToF sensor 210 for the ToF measurement. For example, determining 104 the reflectivity value can comprise applying at least one correction to the output value of the ToF sensor 210 for the ToF measurement. The output value of the ToF sensor 210 for the ToF measurement can for example be scaled and / or offset corrected to obtain the reflectivity value. Thus, systematic errors (e.g. noise) can be corrected.

[0031] The apparatus 200 comprises a processing circuitry 240 coupled to the respective configuration of the ToF sensor 210. For example, the processing circuitry 240 can be a single dedicated processor, a single shared processor, or multiple individual processors some or all of which are shared, digital signal processor (DSP) hardware, application specific integrated circuit (ASIC), or field programmable gate array (FPGA). The processing circuitry 240 can optionally be coupled to, for example, a read-only memory (ROM), a random access memory (RAM), and / or a non-volatile memory for storing software. The processing circuitry 240 is configured to determine a reflectivity value indicative of the reflectivity of the object 201 based on the output value of the ToF sensor 210 for the ToF measurement.

[0032] For example, the processing circuitry 240 can further output data indicative of the reflectivity value (e.g. a two-dimensional image).

[0033] The apparatus 200 can comprise further hardware - conventional and / or custom made.

[0034] In other words, a two-dimensional sensing method based on a ToF camera is proposed. The result of the sensing can be for example a two-dimensional image, where the pixels in the two-dimensional image respectively indicate the amount of light reflected by the object. As described above, this can be achieved by using a sensor response function that is the inverse of the function of the proximity received light intensity over distance. Since this makes the sensor output larger over distance, it compensates for the loss of signal strength. This results in a constant sensor output of the object independent of the distance.

[0035] As mentioned above, to perform 102 the ToF measurement, the method 100 comprises a) illuminating a scene comprising the object 201 with the modulated light 202 of the illumination element 230 based on an illumination signal, and b) driving the light capturing element 220 based on a reference signal. To adjust the correlation function of the ToF measurement such that it increases with distance within the measurement range of the ToF sensor 210, at least one of the illumination signal, the reference signal, and a time offset between the illumination signal and the reference signal can be varied during the ToF measurement.

[0036] For the ToF measurement, a coded modulation (CM) measurement as well as a continuous wave (CW) measurement can be used.

[0037] For example, if the CW measurement is used for the ToF measurement, each of the illumination signal and the reference signal exhibits a respective alternating series of high and low pulses with equal duration (length). Thus, the modulated light 202 is a series of light pulses with equal pulse length (duration) and equal pulse spacing. To adjust the correlation function of the ToF measurement such that it increases with distance within the measurement range of the ToF sensor 210, a time offset between the illumination signal and the reference signal can be varied during the CW measurement.

[0038] Alternatively, if the CM measurement is used for the ToF measurement, at least one of the illumination signal and the reference signal exhibits a respective alternating series of high and low pulses with different duration (length). For example, the modulated light 202 can be a series of light pulses with different pulse length (length) and / or different pulse spacing. Similarly, for the CM measurement, the reference signal can exhibit an alternating series of high and low pulses with different duration. In other examples, the modulated light 202 can be a series of light pulses with equal pulse length and equal pulse spacing, while the reference signal exhibits an alternating series of high and low pulses with different duration. To adjust the correlation function of the ToF measurement such that it increases with distance within the measurement range of the ToF sensor 210, for at least one of the illumination signal and the reference signal, the respective alternating series of high and low pulses can be varied during the CM measurement. Alternatively or additionally, a time offset between the illumination signal and the reference signal can be varied during the CM measurement.

[0039] By changing one or more of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal during the ToF measurement, the ToF sensor 210 can create a plurality of different auxiliary correlation functions. Irrespective of whether a CW measurement or a CM measurement is performed, the ToF sensor 210 temporarily exhibits the respective auxiliary correlation function for each variation of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal used during the ToF measurement. The resulting (overall) correlation function of the ToF measurement can be understood as a combination (e.g., a sum) of the different auxiliary correlation functions used during the ToF measurement. In other words, the resulting (effective) correlation function of the ToF sensor 210 used for the ToF measurement is a combination of the auxiliary correlation functions for the variations of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal used during the ToF measurement. For example, by switching the auxiliary correlation functions within the exposure time of the ToF measurement, a weighted sum of different auxiliary correlation functions (i.e., different sensor response functions) can be obtained as the correlation function of the ToF measurement. Thus, a custom-shaped correlation function of the ToF sensor 210 used for the ToF measurement can be obtained / adjusted.

[0040] For example, in order to obtain a correlation function of the ToF measurement that increases with distance within the measurement range of the ToF sensor 210, at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal can be changed such that the auxiliary correlation functions are offset relative to each other (e.g., shifted along the abscissa in Figure 3 For example, in order to create a correlation function that increases with distance within the measurement range of the ToF sensor 210, the auxiliary CM correlation function with one correlation peak can be continuously shifted during the exposure.

[0041] Alternatively, the speed of the shifting can be modulated such that different shifts of the auxiliary CM correlation function are differently weighted in the resulting correlation function. For example, during the CM measurement, for at least one of the illumination signal and the reference signal, the respective series of alternating high and low pulses can be changed with an increasing rate of variation. For example, if one of the illumination signal and the reference signal is changed by continuously selecting different codes of a plurality (pool) of codes used for generating the respective one of the illumination signal and the reference signal, the rate / frequency of the selection or update can be increased during the CM measurement. Similarly, the time offset between the illumination signal and the reference signal can be changed with an increasing rate of variation during the CM measurement.

[0042] By changing the rate of variation, the offset between the resulting auxiliary correlation functions can be modulated such that the auxiliary correlation functions are differently weighted in the overall correlation function used for the ToF measurement.

[0043] The course of the correlation function of the ToF measurement over distance within the measurement range can for example depend on an estimated course of the actual light intensity of the light 203 received at the ToF sensor 210 over distance. For example, the estimated course of the actual light intensity of the light 203 received at the ToF sensor 210 over distance can be acquired in a factory calibration. Thus, the correlation function of the ToF measurement can be determined (in advance) in the factory calibration. For example, the variations of the illumination signal, the reference signal and the time offset between the illumination signal and the reference signal used during the ToF measurement can be selected / adjusted based on the factory calibration.

[0044] Alternatively or additionally, the correlation function of the ToF measurement can be adapted / adjusted (e.g. to correct for errors such as distance dependent errors) on the fly. For example, the method 100 can comprise performing a plurality of ToF calibration measurements with the ToF sensor 210 to acquire calibration data indicating the actual light intensity of the light 203 received at the ToF sensor 210 for different distances between the ToF sensor 210 and the reference object 201. Thus, the calibration data is an estimate of the course of the actual light intensity of the light 203 received at the ToF sensor 210. Thus, the correlation function of the ToF measurement can be adjusted based on the calibration data. For example, the illumination signal, the reference signal and the variations of the time offset between the illumination signal and the reference signal used during the ToF measurement can be adapted (adjusted) based on the calibration data.

[0045] Furthermore, a drift of the operating parameters of the illumination element 230 can be taken into account to acquire the constant output value of the ToF sensor. For example, the generation of the illumination signal and the operation of the driver electronics in the illumination element 230 are temperature dependent. Thus, the method 100 can comprise measuring the temperature at the illumination element 230. The apparatus 200 can comprise one or more temperature sensors for measuring the temperature at the illumination element 230. Alternatively or additionally, the method 100 can comprise measuring the light intensity and / or the rise time of the modulated light 202 emitted by the illumination element 230. The apparatus 200 can comprise one or more light sensors (e.g. photodiodes) for measuring the light intensity and / or the rise time of the modulated light 202 emitted by the illumination element 230. Based on at least one of the temperature measured at the illumination element 230 and the measured light intensity and / or the rise time of the modulated light 202 emitted by the illumination element 230, the illumination signal, the reference signal and the time offset between the illumination signal and the reference signal can be varied to compensate for temperature dependent drifts in the operation of the illumination element 230.

[0046] According to examples, the reflectivity sensing described above can be used together with depth sensing to provide depth and reflectivity data (e.g., depth and reflectivity images). For example, the method 100 can further include performing one or more additional ToF measurements using the ToF sensor 210. Thus, a distance value indicative of a distance of the ToF sensor 210 to the object 201 can be determined based on an output of the ToF sensor 210 for the one or more additional ToF measurements. Further, data indicative of the reflectivity value and the distance value can be output. For example, one or more images indicative of the reflectivity value and the distance value can be output.

[0047] In other examples, the correlation function increases with distance within a measurement range of the ToF sensor 210, but does not match a hypothetical / estimated procedure of a light intensity of the light 203 received at the ToF sensor 210. In other words, the correlation function does not satisfy the mathematical expression (2) described above. Adjusting the correlation function can enable high dynamic range (HDR) imaging.

[0048] Examples as described herein can be summarized as follows:

[0049] Some examples relate to a method for determining a reflectivity value indicative of a reflectivity of an object. The method includes performing a ToF measurement using a ToF sensor. A correlation function of the ToF measurement increases with distance within a measurement range of the ToF sensor such that an output value of the ToF sensor for the ToF measurement is independent of a distance between the ToF sensor and the object. The method further includes determining the reflectivity value based on the output value of the ToF sensor for the ToF measurement.

[0050] According to some examples, the correlation function represents an expected distance-dependent output of the ToF sensor for the ToF measurement, given that a light intensity of the light received at the ToF sensor during the ToF measurement is constant with distance within a measurement range of the ToF sensor.

[0051] In some examples, determining the reflectivity value includes applying at least one correction to the output value of the ToF sensor for the ToF measurement.

[0052] According to some examples, performing the ToF measurement includes illuminating a scene including the object with modulated light based on an illumination signal, driving a light capturing element of the ToF sensor based on a reference signal, and varying at least one of the illumination signal, the reference signal, and a time offset between the illumination signal and the reference signal during the ToF measurement.

[0053] In some examples, each of the illumination signal and the reference signal exhibits a respective alternating series of high and low pulses having equal durations, wherein a time offset between the illumination signal and the reference signal is varied during the ToF measurement.

[0054] In alternative examples, at least one of the illumination signal and the reference signal exhibits a respective alternating series of high and low pulses having different durations, wherein the respective alternating series of high and low pulses is varied for at least one of the illumination signal and the reference signal during the ToF measurement.

[0055] According to some examples, the respective alternating series of high and low pulses is varied for at least one of the illumination signal and the reference signal during the ToF measurement at an increasing rate of variation.

[0056] In some examples, a time offset between the illumination signal and the reference signal is varied during the ToF measurement.

[0057] According to some examples, the ToF sensor exhibits a respective auxiliary correlation function for each variation of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal used during the ToF measurement, wherein the correlation function of the ToF measurement is a combination of the auxiliary correlation functions, and wherein at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal is varied such that the auxiliary correlation functions are offset relative to each other within a measurement range of the ToF sensor.

[0058] In some examples, the modulated light is emitted by an illumination element of the ToF sensor, and the method further comprises: measuring a temperature at the illumination element; and / or measuring a light intensity and / or a rise time of the modulated light emitted by the illumination element; and varying at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal based on at least one of the temperature measured at the illumination element and the measured light intensity and / or rise time of the modulated light emitted by the illumination element.

[0059] According to some examples, a procedure of the correlation function of the ToF measurement with distance depends on an estimated procedure of an actual light intensity of light received at the ToF sensor with distance within a measurement range.

[0060] In some examples, wherein the correlation function of the ToF measurement is predetermined in a factory calibration.

[0061] According to some examples, the method further comprises: performing a plurality of ToF calibration measurements to obtain calibration data indicative of an actual light intensity of light received at the ToF sensor for different distances between the ToF sensor and a reference object; and adjusting the correlation function of the ToF measurement based on the calibration data.

[0062] In some examples, the method further comprises: performing one or more further ToF measurements using the ToF sensor; determining a distance value indicative of a distance to the object based on an output of the ToF sensor for the one or more further ToF measurements; and outputting data indicative of the reflectivity value and the distance value.

[0063] Other examples relate to an apparatus for determining a value indicative of a reflectivity of an object. The apparatus comprises a ToF sensor configured to perform a ToF measurement. A correlation function of the ToF measurement increases with distance within a measurement range of the ToF sensor, such that an output value of the ToF sensor for the ToF measurement is independent of a distance between the ToF sensor and the object. The apparatus further comprises a processing circuitry configured to determine the reflectivity value based on the output value of the ToF sensor for the ToF measurement.

[0064] Examples of the present disclosure can enable depth-independent intensity imaging using a ToF camera. Examples of the present disclosure introduce a ToF modulation pattern that can provide a two-dimensional image in which pixel values depend on object reflectivity - independent of distance. In other words, the ToF camera operates in a pattern in which the sensor output for an object is uniform within the measurement range. This can be useful for surveillance and face recognition, for example, as for these applications the two-dimensional image carries more information than a depth image. A reflectivity image contains even more information.

[0065] Aspects and features described in relation to a particular example in the previous examples can also be combined with one or more further examples, to replace the same or similar features of that further example, or additionally introduce those features into the further example.

[0066] It should also be understood that disclosure of a number of steps, procedures, operations or functions in the specification or claims disclosed in the description should not be construed as implying that these operations must necessarily be dependent on the order described, unless explicitly stated or necessary for technical reasons in the individual case. The foregoing description therefore does not limit the execution of the number of steps or functions to a certain order. Furthermore, in further examples, a single step, function, procedure or operation can comprise and / or be broken down into a number of sub-steps, functions, procedures or operations.

[0067] If certain aspects have been described with respect to a device or system, these aspects should also be understood as a description of a corresponding method. For example, a block, device or functional aspect of a device or system can correspond to a feature of a corresponding method, such as a method step. Aspects described with respect to a method should therefore also be understood as a description of corresponding blocks, corresponding elements, properties or functional features of a corresponding device or a corresponding system.

[0068] The appended claims are hereby incorporated into the detailed description, wherein each claim can stand on its own as a separate example. It should also be noted that while the claims may be drafted in the singular form, plural forms can be intended to encompass plural claims, unless otherwise indicated. Furthermore, the claims may, without intention to limit the scope thereof, be read to indicate one or a combination of elements included in the claims.

Claims

1. A method (100) for determining a reflectance value of an indicator object, the method (100) comprising: (102) ToF measurement is performed using a time-of-flight (ToF) sensor, wherein the correlation function of the ToF measurement increases with distance within the measurement range of the ToF sensor, such that the output value of the ToF sensor used for the ToF measurement is independent of the distance between the ToF sensor and the object; as well as The reflectance value is determined (104) based on the output value of the ToF sensor used for the ToF measurement. The ToF measurement described in (102) includes: The scene including the object is illuminated with modulated light based on the illumination signal; The light-harvesting element of the ToF sensor is driven by a reference signal; and During the ToF measurement, at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal is changed.

2. The method (100) according to claim 1, wherein, assuming that the light intensity of light received at the ToF sensor during the ToF measurement is constant with distance within the measurement range of the ToF sensor, the correlation function represents the expected distance-correlation output of the ToF sensor for the ToF measurement.

3. The method (100) according to claim 1 or claim 2, wherein determining the reflectance value (104) comprises: At least one correction is applied to the output value of the ToF sensor used for the ToF measurement.

4. The method (100) of claim 1, wherein each of the illumination signal and the reference signal presents a corresponding alternating series of high and low pulses of equal duration, and wherein the time offset between the illumination signal and the reference signal varies during the ToF measurement.

5. The method (100) of claim 1, wherein at least one of the illumination signal and the reference signal exhibits a corresponding alternating series of high and low pulses with different durations, and wherein the corresponding alternating series of high and low pulses varies for at least one of the illumination signal and the reference signal during the ToF measurement.

6. The method (100) of claim 5, wherein the corresponding alternating series of high and low pulses during the ToF measurement varies at an increasing rate for at least one of the illumination signal and the reference signal.

7. The method (100) according to claim 5 or claim 6, wherein the time offset between the illumination signal and the reference signal varies during the ToF measurement.

8. The method (100) of claim 1, wherein the ToF sensor temporarily exhibits a corresponding auxiliary correlation function for each change in the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal used during the ToF measurement, wherein the correlation function of the ToF measurement is a combination of the auxiliary correlation functions, and wherein at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal is varied such that the auxiliary correlation functions are offset relative to each other within the measurement range of the ToF sensor.

9. The method (100) of claim 1, wherein the modulated light is emitted by the illumination element of the ToF sensor, and wherein the method (100) further comprises: Measure the temperature at the lighting element; and / or Measure the light intensity and / or rise time of the modulated light emitted by the illumination element; as well as The lighting signal, the reference signal, and the time offset between the lighting signal and the reference signal are altered based on at least one of the temperature measured at the lighting element and the measured light intensity and / or rise time of the modulated light emitted by the lighting element.

10. The method (100) of claim 1, wherein the correlation function of the ToF measurement with distance depends on the estimation process of the actual light intensity of the light received at the ToF sensor with distance over the measurement range.

11. The method (100) of claim 1, wherein the correlation function of the ToF measurement is predetermined in factory calibration.

12. The method (100) according to claim 10, further comprising: Perform multiple ToF calibration measurements to obtain calibration data indicating the actual light intensity received at the ToF sensor for different distances between the ToF sensor and the reference object; as well as The correlation function of the ToF measurement is adjusted based on the calibration data.

13. The method (100) according to claim 1, further comprising: One or more additional ToF measurements are performed using the ToF sensor; A distance value indicating the distance to the object is determined based on the output of the ToF sensor used for the one or more additional ToF measurements; as well as The output data indicates the reflectivity value and the distance value.

14. An apparatus (200) for determining a value of the reflectance of an indicating object (201), the apparatus comprising: A time-of-flight (ToF) sensor (210) is configured to perform ToF measurements, wherein the correlation function of the ToF measurements increases with distance within the measurement range of the ToF sensor (210), such that the output value of the ToF sensor (210) used for the ToF measurements is independent of the distance between the ToF sensor (210) and the object (201); as well as The processing circuit (240) is configured to determine the reflectance value based on the output value of the ToF sensor (210) used for the ToF measurement. The time-of-flight ToF sensor (210) is further configured as follows: The scene including the object is illuminated with modulated light based on the illumination signal; The light-harvesting element of the ToF sensor is driven by a reference signal; as well as During the ToF measurement, at least one of the illumination signal, the reference signal, and the time offset between the illumination signal and the reference signal is changed.

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