METHOD FOR DETERMINING THE DISTANCE AND REFLECTIVITY OF AN OBJECT SURFACE

Abstract

A method for determining a distance (d) and a reflectivity of an object surface (14) using a laser source (10) that emits light (12) at a certain power and using a detector (16) that detects a level of irradiance of light (18) reflected by or scattered back from the object surface (14) and that outputs a time-dependent voltage signal on the basis thereof comprises: setting (100, 110, 220, 230, 240) the laser source (10) so that the latter emits light (12) at a specified first value of power in at least one pulse, setting (100, 110) the detector (16) so that the latter emits outputs a first voltage signal with a specified second value for a gain factor on the basis of a level of irradiance of the detected reflected or back-scattered light (18), determining (120, 260) a first value for the distance of the object surface (14) from a measured light time-of-flight (ToF) assigned to the first voltage signal, adapting (130, 150 220) the first value of the power of the laser source (10) and/or the second value of the gain factor of the detector (16) on the basis of the determined first value for the distance (d), emitting (110, 240) light (12) again using the laser source (10) and detecting the reflected or back-scattered light (18) by the detector (16) and outputting a corresponding second voltage signal using the adapted first and/or second value, determining (120, 260) a second value for the distance (d) of the object surface from a measured light time-of-flight (ToF) assigned to the second voltage signal.

Claims

1. A method for determining a distance and reflectivity of an object surface (14) using a laser source (10) that emits light (12) having a power, and using a detector (16) that detects the light (18) reflected or backscattered from the object surface (14) and having an irradiance and depending thereon outputs a time-dependent voltage signal, comprising: setting (100, 110, 220, 230, 240) the laser source, such that the latter emits light having a predetermined first value of the power in at least one pulse, setting (100, 110) the detector, such that the latter outputs a first voltage signal having a predetermined second value for a gain factor depending on the irradiance of the reflected or backscattered light detected, determining (120, 260) a first absolute value for the distance of the object surface from a measured light time of flight assigned to the first voltage signal, adapting (130, 150, 220) the first value of the power of the laser source and/or the second value of the gain factor of the detector depending on the determined first absolute value for the distance, once again emitting (110, 240) light by means of the laser source (10) and detecting (110) the reflected or backscattered light (18) by means of the detector (16) and outputting (110) a corresponding second voltage signal using the adapted first and/or second value, determining (120, 260) a second absolute value for the distance (d) of the object surface (14) from a measured light time of flight (ToF) assigned to the second voltage signal.

2. The method as claimed in claim 1, wherein a silicon photomultiplier (SiPM) is provided as detector (16).

3. The method as claimed in claim 1 or 2, wherein a laser that operates in the near infrared spectral range, preferably in the range of wavelengths of 840 nm to 1550 nm, is provided as laser source (10).

4. The method as claimed in any of claims 1 to 3, wherein the steps of the method are carried out repeatedly for individual pixels in the context of a LiDAR application in the field of driver assistance systems or systems for autonomous driving for scanning various object surfaces (14) of surroundings of a vehicle for the computer-aided construction of a three-dimensional image of the surroundings.

5. The method as claimed in any of claims 1 to 4, furthermore comprising: predefining a first, upper voltage limit value (SiPM-MAX) for a voltage, below which value (39) for the detector (16) there is a substantially linear relationship between the irradiance of the incident light (18) and a voltage output as a consequence thereof, and above which value (38) the relationship is nonlinear and/or saturated, determining an amplitude (ampl) of the first output signal, comparing (140) the amplitude (ampl) with the voltage limit value (SiPM-MAX), wherein in the step of adapting (150) the first value of the power of the laser source (10) and/or the second value of the gain factor of the detector (16), the extent of the adaptation is carried out depending on the result of the comparison (140).

6. The method as claimed in claim 5, wherein if the amplitude (ampl) exceeds the voltage limit value (SiPM-MAX), the adaptation includes a decrease of the first and/or second value, such that in the subsequent step (110) the irradiance of the incident light (12) is reduced in the detector (16) and as a consequence thereof an amplitude (ampl) of the second voltage signal falls below the predefined first voltage limit value (SiPM-MAX).

7. The method as claimed in claim 6, wherein the decrease includes a reduction of the first and/or second value by 50% or more.

8. The method as claimed in any of claims 1 to 7, comprising predefining a second, lower voltage limit value (SiPM_MIN) for a voltage, which value ensures a predefined signal-to-noise ratio, preferably 2 dB or more, more preferably approximately 10 dB or more, for the detector (16), determining an amplitude of the first output signal, comparing the amplitude with the second voltage limit value (SiPM-MIN), wherein the step of adapting (150) the first value of the power of the laser source and/or the second value of the gain factor of the detector includes an increase of the first and/or second value, such that in the subsequent step (110) the irradiance of the incident light (12) is reduced in the detector (16) and as a consequence thereof an amplitude of the second voltage signal lies above the predefined second voltage limit value (SiPM-MIN).

9. The method as claimed in any of claims 1 to 8, wherein provision is made of a function (V1 (d)) between the power of the laser source (10) and the distance (d) of the object surface (14) for a fixedly selected irradiance of the detector (16) in relation to the reflected and/or backscattered light (18), wherein the first value of the power predetermined for the adaptation (220) and/or the predetermined second value for the gain factor are/is ascertained with the argument of the first absolute value for the distance determined from the first voltage signal and the adaptation is carried out according to this function.

10. The method as claimed in claim 9, wherein before the step of the first setting (220) of the power of the laser and/or the gain factor of the detector, a start value (d.sub.0max) for the absolute value of the distance is predefined (210), and in a subsequent step (220), the power and/or the gain factor are/is ascertained from the predefined function, on the basis of which the laser source (10) and/or the detector (16) can subsequently be set.

11. The method as claimed in claim 9 or 10, wherein a lower power limit and an upper power limit are defined for the predefined function between the power of the laser source (10) and the distance (d) of the object surface (14), wherein for all distances (d<d.sub.0min) below the distance (d.sub.0min) assigned to the lower power limit, only the value of the lower power limit is returned and used, and wherein for all distances (d>d.sub.0max) above the distance (d.sub.0max) assigned to the upper power limit, only the value of the upper power limit is returned and used.

12. The method as claimed in claim 11, wherein the lower power limit is set in accordance with a minimum output power of the laser source, and/or the upper power limit is set either in accordance with a safety standard of the laser source or in accordance with a physical power limit of the laser source, depending on which value is lower.

13. The method as claimed in any of claims 1 to 12, wherein after the step of determining (120, 260) the second absolute value for the distance (d) of the object surface (14) from a measured light time of flight (ToF) assigned to the second voltage signal, a further step of determining (160) a reflectivity of the object surface (14) on the basis of the second voltage signal and the determined first and/or second value for the distance (d) is carried out.

14. The method as claimed in claim 13, wherein a second function (y.sub.act) is provided, which indicates a linearized response to an amplitude of the second voltage signal, having the form:
y.sub.act=−log(1−amp/c1).Math.c1/c2, wherein amp corresponds to the amplitude of the second voltage signal, and c1, c2 are coefficients determined from measurements by means of a mathematical fit, and a third function (y.sub.ref) is provided, which indicates a linearized reference response to an amplitude of the second voltage signal as a function of a distance of the object surface and a power of the laser source (10), having the form:
y.sub.ref=α(d).Math.x, wherein x corresponds to the power of the laser source and a is a linear gradient factor that is dependent on the distance (d) and is determined from measurements by means of a mathematical fit, wherein the linearized response (y.sub.act) is calculated from the amplitude of the second voltage signal determined by measurement, wherein the linearized reference variable (y.sub.ref) is calculated from the ascertained second value for the distance (d) and the set power of the laser source (10), and wherein the reflectivity is calculated from a quotient of the linearized response y.sub.act and the linearized reference variable y.sub.ref.

15. A device (1) for determining a distance (d) and reflectivity of an object surface (14), comprising: a laser source (10) that emits light (12) having a power, a detector (16) that detects the light (18) reflected or backscattered from the object surface (14) and having an irradiance and depending thereon outputs a time-dependent voltage signal, a control device (20) configured to carry out the method having the steps as claimed in any of claims 1 to 14.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

[0048] In the figures:

[0049] FIG. 1 shows a schematic block diagram of a device for determining a distance of an object surface which can be used to implement an exemplary embodiment of the method according to the invention;

[0050] FIG. 2 shows a schematic block diagram of a more specific device for determining a distance of an object surface which can be used to implement an exemplary embodiment of the method according to the invention;

[0051] FIG. 3 shows a block circuit diagram of an SiPM sensor;

[0052] FIG. 4 shows an equivalent circuit diagram of an SPAD microcell of an SiPM sensor;

[0053] FIG. 5 shows a diagram with a current-voltage characteristic curve of the microcell from FIG. 4 and a schematic illustration of the cyclic pass through the corresponding operating modes or phases;

[0054] FIG. 6 shows a schematic diagram illustrating the measurement of the light time of flight, wherein the strength of a signal Sn (e.g. voltage) of the pulse or the pulse response is plotted against time;

[0055] FIG. 7 shows a schematic comparison of the signal-to-noise ratio (SNR) plotted against the distance for SiPM and APD sensors;

[0056] FIG. 8 shows a schematic comparison of the pulse responses plotted against time between SiPM and APD sensors for six different power levels of the output signal in the laser source, the pulse responses between the sensor types being intentionally time-shifted relative to one another for the comparison;

[0057] FIG. 9 shows in a schematic diagram analogously to FIG. 6 for an SiPM sensor the boundary conditions to be complied with by means of laser safety standards, the laser power serving as a settable parameter;

[0058] FIG. 10 shows in a schematic diagram analogously to FIG. 6 for an SiPM sensor the boundary conditions to be complied with by means of laser safety standards, the gain factor or gain serving as a settable parameter;

[0059] FIG. 11 shows in a flow diagram the schematic sequence of the method in accordance with the first exemplary embodiment;

[0060] FIG. 12 shows in a diagram for an SiPM sensor the relationship between the amplitude amp of a voltage signal respectively output and the overvoltage V.sub.OV of the detector or a variable x derived from the driver voltage of the laser source with a linear relationship;

[0061] FIG. 13 shows in a diagram the logarithm of the linear gradient, log(α), of the amplitude of the voltage signal relative to the overvoltage/driver voltage as a function of log (d), wherein d is the distance from laser source and detector, for the calculation of the reflectivity of the surface portion in accordance with the second exemplary embodiment;

[0062] FIG. 14 shows in a diagram a function V1 (d), which simplifies the calculation of the distance and represents the driver voltage of the laser source as a function of the distance d, wherein between d.sub.0min and d.sub.0max, by means of adaptation of the driver voltage V1 of the laser source, an irradiance in the detector, independently of the distance d, is always maintained approximately in the center of the dynamic region;

[0063] FIG. 15 shows in a diagram the irradiance Ir as a function of the distance d, wherein the target value in the center of the dynamic region lies between SiPM-MAX and SiPM-MIN and is 100 μW/m.sup.2, as long as the distance d falls between d.sub.0min and d.sub.0max;

[0064] FIG. 16 shows in a flow diagram the sequence of the method in accordance with a third exemplary embodiment.

PREFERRED EMBODIMENT(S) OF THE INVENTION

[0065] FIG. 1 shows, on the basis of a schematic block diagram, a device 1 for determining a distance of an object surface which can be used to implement an exemplary embodiment of the method according to the invention. It shows the basic set-up of a LiDAR device for distance determination by means of time of flight (ToF) measurement. A laser source 10 emits high-frequency pulses of monochromatic and coherent and sharply focused light 12 in the direction of an object surface 14, which reflects and/or backscatters said light. A detector 16 receives or detects the reflected and/or backscattered light 18. A central control device 20 (regularly an IC chip) of the device 1 is connected to the laser source 10 and the detector 16 via electronic lines with corresponding interfaces and coordinates the process of pulse generation and detection. In particular, the control device 20 can assign the relevant pulse signals and record the point in time in each case of the relevant pulse generation in the laser source 10 and the resultant pulse detection in the detector 16 and calculate the light time of flight 22 from the difference. With the speed of light being known, the distance d to the object surface 14 can immediately be determined therefrom (or from half the light time of flight, taking account of the outgoing path and return path).

[0066] In the exemplary embodiment of a device 1 as shown in FIG. 1, the detector 16 is a silicon photomultiplier (SiPM). The latter is distinguished by a low signal-to-noise ratio (SNR) and a high gain factor (gain), which moreover is linearly settable with the aid of the control device 20 by way of the control of the overvoltage, to be explained below. Likewise, the control device 20 can set inter alia the power of the laser radiation by way of the driver voltage of the laser source (just as, however, pulse duration and frequency and further parameters can also be set). The laser source 10 can comprise further optical elements—not shown here—such as lenses, diffusers, shutters, filters and mirrors, etc. The detector can likewise comprise further optical elements, in particular lenses, etc.

[0067] FIG. 2 shows a further, more specific exemplary embodiment of a device 1′, wherein the control device 20 is configured to carry out the steps of the below-described first exemplary embodiment of a method according to the invention. Identical reference signs designate features identical or similar to those in the first exemplary embodiment, a repetition of the detailed description being dispensed with.

[0068] The device 1′ concerns a LiDAR system for use in vehicles in order to support an ADAS system (advanced driver-assistance system), i.e. a driver assistance system. Here it is necessary not just simply to carry out a distance determination, but to generate a three-dimensional image of the complete or partial surroundings of the vehicle (not shown) in which the device is fitted, for example in order to evaluate obstacles or signs fitted in a stationary fashion, etc. The laser source 10 here comprises a laser diode that emits light 12 in a beam (as described above) in the near infrared (NIR) wavelength range (900 nm to 1550 nm). In order to scan the surroundings, a microelectromechanical component 28 (MEMS) having one or more micromirrors 30 adjustable at high frequency is provided, which micromirrors can deflect the light beam in a manner rotating or oscillating at high frequency about an individual axis under the control of the control device 20. The deflected laser beam (light 12) is guided through a diffuser 34, which expands the beam in a vertical direction (in the schematic illustration in FIG. 2 perpendicularly to the plane of the drawing and therefore only schematically indicated), such that the expanded beam of the light 12 is guided over the surroundings in a horizontal deflection direction 32. In the process its cross section sweeps over the respective surfaces 14, which reflect or backscatter the light substantially depending on the material and the surface constitution. In this case, the pulses of the laser diode are synchronized with the micromirror(s) 30.

[0069] Part of the backscattered or reflected light 18 passes through a lens optical unit 26, which focusses the light onto a photodiode array 24, which comprises detectors 16 embodied as SiPM sensors in this exemplary embodiment, too, which are arranged vertically in series. The number of detectors 16 is chosen in accordance with the expansion of the beam (light 12). The detectors detect the light 18 assigned to them via the optical unit, from which, with application of the method, the control device 20 ascertains for each pixel a distance d and a value for the albedo (reflectivity). The pixels are defined in a vertical direction by the detectors 16 arranged in series in the photodiode array 24, and in a horizontal direction by discrete angular positions of the micromirror(s) for the relevant pulses. The image finally constituted can have a resolution of e.g. 256×84 pixels, or 0.25°×0.3° given an image field of 60° horizontally and 20° vertically. The ranges are more than 200 m for detecting pedestrians or more than 300 m for detecting other vehicles. The values indicated are merely by way of example and on no account restrict the scope of protection defined by the claims.

[0070] FIGS. 3 to 5 illustrate the function of the detector 16 embodied as an SiPM sensor such as is used for example in the devices 1, 1′ in FIG. 1 or 2. The subject matter of these three figures constitutes useful background knowledge. A more detailed explanation in this respect can also be found in “Introduction to silicon photomultipliers (SiPMs)”, White Paper by First Sensor, Version 03-12-15, downloaded from https://www.first -sensor.com/en/products/optical-sensors/detectors/silicon -photomultipliers-sipms/ on Nov. 7, 2018. FIG. 3 shows a block circuit diagram of such an SiPM sensor. In each microcell 36, an avalanche photodiode (APD), which here is operated in the Geiger mode, between the anode terminal (at V.sub.BIAS) and the cathode terminal (at S.sub.OUT) is connected in series with a quench resistance R.sub.Q. The avalanche photodiode operated in the Geiger mode is also referred to as SPAD (single photon avalanche diode). This series connection of each cell is in turn connected in parallel throughout among one another, that is to say that the avalanche-generated current and voltage pulses of all the microcells 36 are superposed in the same way at the output of the circuit of the detector 16. C.sub.D denotes a diode capacitance respectively present.

[0071] FIG. 4 shows an equivalent circuit diagram of an SPAD microcell 36 of an SiPM sensor. In this case, the SPAD diode is formed from a switch S arranged in series, a voltage source V.sub.BD and a series resistance R.sub.S of the semiconductor substrate formed e.g. from silicon. The diode capacitance C.sub.D is connected in parallel therewith. As is shown in FIG. 3, therewith externally the quench resistance R.sub.Q is connected in series vis-à-vis the terminals of the voltage source V.sub.BIAS. In this case, the quench resistance R.sub.Q is much greater than the series resistance R.sub.S. During operation, in a first phase, which is referred to as quiescent mode and in which no photons are incident in the active region of the microcell 36, the reverse voltage V.sub.BIAS is applied or built up with regard to the diode capacitance C.sub.D. Since the cell is operated in the Geiger mode, V.sub.BIAS is above the breakdown voltage V.sub.BD. The difference between the reverse voltage V.sub.BIAS and the breakdown voltage V.sub.BD is referred to as overvoltage V.sub.OV. In the state described, the switch S is open and substantially no current flows.

[0072] In the case of photon capture, in the equivalent circuit diagram the switch S closes and so the current pulse caused by the charge carrier avalanche generated results in a discharge of the diode capacitance C.sub.D via the series resistance R.sub.S, with the consequence that the voltage proceeding from V.sub.BIAS falls back to the breakdown voltage V.sub.BD. This is referred to as discharge mode (discharge phase). As described in the introduction, the quench resistance R.sub.Q then becomes apparent by virtue of the voltage across the diode being quenched, as a result of which the switch S opens again.

[0073] In the phase that follows, referred to as recharge mode (recovery phase), the diode capacitance C.sub.D is recharged via the quench resistance R.sub.Q, and so a new cycle begins. The sequence is illustrated schematically in FIG. 5, in which a current-voltage characteristic curve is plotted.

[0074] FIG. 6 shows in a schematic diagram the principle of time of flight determination. A pulse signal Sn produced in the laser source 10 or a pulse signal response “ampl” detected in the detector 16 is plotted against the time axis t, coordinated with one another in the control device 20 (see FIG. 1 or 2). Maxima can be ascertained for the output pulse in the laser source 10 and also for the pulse response in the detector 16, which maxima are used as time marks t.sub.MAX1 and t.sub.MAX2, respectively, for the measurement. The temporal difference between the two time marks t.sub.MAX1 and t.sub.MAX2yields the light time of flight ToF. It goes without saying that the respective pulse widths (i.e. pulse durations) impose a limit for the measurement, in particular for the resolution or accuracy of the distances d determined.

[0075] FIG. 7 shows a schematic comparison between SiPM sensors (solid line) and conventional APD sensors (dashed line) with regard to the respective signal-to-noise ratio SNR plotted against the distance d over which light signal pulses were transmitted (via the object surfaces 14) to the detectors 16. The SNR level “min” indicates that range (below the relevant line) in which the quality of the signal ampl is no longer sufficient to determine a distance, or to determine a distance including assigned reflectivity. The SNR curve in the case of SiPM sensors from short distances clearly visibly has a pronounced saturation region 38, which has the effect that the SNR ratio in the case of APD sensors (with less pronounced saturation region 40 there) is significantly better in the case of these distances. Outside this saturation region 38, however, the SNR ratio of SiPM sensors, as has been found by the inventors, is clearly superior to that of the APD sensors, in particular toward large distances d, which is why this alone already makes it possible to extend the range for the distance determination (see arrow 42 in FIG. 7) .

[0076] FIG. 8 shows a schematic comparison of the pulse responses plotted against time between SiPM and APD sensors as detectors 16 for six different power levels of the output signal in the laser source 10 (9, 10, 25, 50, 88, 100% of the possible power), the pulse responses between the sensor types being intentionally time-shifted relative to one another for the comparison. The diagram illustrates the pronounced saturation at high radiation powers in the laser source 10 in the case of SiPM sensors as detectors 16. APD sensors, by contrast, exhibit a largely linear relationship. It is furthermore evident that for low radiation powers (or correspondingly for large distances) of the laser source 10, SiPM sensors exhibit a systematic time shift that can be of the order of magnitude of the typical pulse width.

[0077] In order then in view of the substantive matter shown in FIGS. 7 and 8, in the case of SiPM sensors, to obtain an improved signal-to-noise ratio for short and also for large distances, it is then possible, in accordance with the exemplary embodiment of the method according to the invention, depending on the distance d, to adapt substantially two parameters: the power of the laser source 10 and the gain factor (gain) of the detector 16 or the SiPM sensor.

[0078] FIGS. 9 and 10 show analogously to FIG. 6 for an SiPM sensor schematically the boundary conditions to be complied with here by virtue of laser safety standards. FIG. 9 illustrates for the parameter of the laser power the case of a large distance d, in the case of which the pulse response of the detector 16 proves to be very weak precisely because of this. The figure shows five power levels of the laser source and correspondingly five signal responses on the part of the detector 16. An increase—taking account of the large distance—of the amplitude of the signal Sn of the laser power is at odds with an upper power limit L.sub.MAX, which results from those safety standards (which are intended to protect the human eye, for example) and, in the schematic diagram in FIG. 9, represents an exclusion region 46 to be taken into account in accordance with the exemplary embodiment.

[0079] In the case of short distances d (not discernible from FIG. 9), by contrast, a decrease of the laser power is advantageous because the saturation region 38 is left. Since the inherently greater noise in the saturation region 38 (excess shot noise) is avoided here despite a decreased power and thus a decreased signal response, the signal-to-noise ratio SNR can still assume very satisfactory values as a result.

[0080] In accordance with the exemplary embodiment, therefore, on the basis of a distance that has already been determined, depending on the latter, an increase of the value of the laser power set can be carried out if the distance is large, or can be decreased if the distance is small. Developments of this exemplary embodiment provide for performing a dynamic real-time adaptation depending on the distance respectively determined. A corresponding exemplary embodiment is explained further below.

[0081] FIG. 10 equally illustrates the case of a large distance d for the parameter of the gain factor (gain). Here, too, the exclusion region 46 with regard to the laser power is present, but the parameter adaptation, i.e. an adaptation of the value for the gain factor, is effected on the part of the detector 16, rather than the laser source 10, such that the adaptation of the parameter is not restricted by this condition in any case, as shown schematically by FIG. 10.

[0082] In the case of large distances d, in accordance with this exemplary embodiment, provision is made for increasing the gain factor (gain) in order to improve the signal-to-noise ratio and in particular also in order to keep the SiPM sensor in its dynamic region (see below for more details in this respect). By contrast, in the case of small distances, provision is made for decreasing the gain factor—likewise in order to keep the SiPM sensor in the dynamic region.

[0083] FIG. 11 shows in a flow diagram the schematic sequence of the method in accordance with this exemplary embodiment. In a step 100, in a LiDAR device 1, 1′ as shown in FIG. 1 or 2, for example, a predetermined first value for the power of the laser source 10 and a predetermined second value for the gain factor (gain) of the detector 16 (SiPM sensor) are predefined (in the case of FIG. 2: the detectors 16 in the SiPM sensor array 24).

[0084] In a subsequent step 110, the laser source 10 and the detector 16 are set accordingly and a pulse is generated in the laser source 10, in the case of which pulse light having the predetermined first value of the power is emitted, wherein the detector 16, depending on the irradiance of the reflected or backscattered light detected, outputs a first voltage signal using the predetermined second value for the gain factor.

[0085] In a subsequent step 120, a (then first) distance determination is carried out, that is to say that a check is made to establish whether a distance determination is possible at all, and if so (Y in step 120), a first absolute value for the distance d of the object surface 14 is ascertained from a measured light time of flight ToF assigned to the first voltage signal.

[0086] If the distance determination is not possible (N in step 120), because the voltage signal assumes a signal-to-noise ratio SNR below a predefined minimum value, the parameters: power of the laser source 10 and/or the gain factor of the detector 16 are adapted, i.e. here increased, in a step 130.

[0087] By contrast, if the distance determination was possible and yields the first absolute value for the distance (Y in step 120), a further step 140 involves checking whether the first voltage signal output is in the saturation region 38, i.e. not in the dynamic region 39 (see FIG. 8). For this purpose, a first, upper voltage limit value for a voltage is predefined, below which value (dynamic region 39) for the detector 16 there is a substantially linear relationship between the irradiance of the incident light 18 and a voltage output as a consequence thereof, and above which value the relationship is nonlinear and/or saturated (saturation region 38). Furthermore, in this step, an amplitude of the first voltage signal output by the detector 16 is determined and is compared with the voltage limit value.

[0088] If the voltage limit value is exceeded (Y in step 140), then the method continues to step 150. In step 150, the parameters: power of the laser source 10 and/or the gain factor of the detector 16 are adapted, i.e. here: decreased.

[0089] In both cases, step 130, in which the distance d is too large to yield a usable voltage signal with a sufficient signal-to-noise ratio SNR, and steps 140, 150, in which the distance d is so small or the power of the radiation source is so high that the SiPM sensor operates in the saturation region 38, one or both parameters is or are adapted dynamically in order to start a second pass.

[0090] This is effected recursively returning to step 110, in which the laser source 10 and the detector 16 are again set accordingly or then adapted. That is to say that a pulse is generated again in the laser source 10, in the case of which pulse light having the now possibly adapted first value of the power is emitted, wherein the detector 16, depending on the irradiance of the reflected or backscattered light detected, then outputs a second voltage signal, which is ideally different than the first voltage signal, using the possibly adapted second value for the gain factor, which is then usable and is not in the saturation region 38.

[0091] Overall, therefore, in steps 130, 110 and 150, 110, respectively, the first value of the power of the laser source and/or the second value of the gain factor of the detector 16 are/is adapted (decreased or increased in accordance with the above explanations with reference to FIGS. 9 and 10) depending on the determined first absolute value for the distance. In the unusable case (step 130), the determined distance d is not set or is more than a limit value determined in advance, or in the case of the saturation region 38, the determined distance is less than a limit value determined in advance (as becomes evident from FIG. 7).

[0092] After once again emitting light by means of the laser source 10 and detecting the reflected or backscattered light by means of the detector 16 and outputting a corresponding second voltage signal using the adapted first and/or second value, it is possible, finally, after repeating steps 110, 120, 140, to determine a second absolute value for the distance of the object surface from a measured light time of flight ToF assigned to the second voltage signal.

[0093] Said second absolute value should undergo the corresponding checks in steps 120, 140 in each case with a positive result (Y), after which the method in accordance with this first exemplary embodiment advances to step 160. Here the reflectivity of the relevant object surface 14 is calculated from the second voltage signal. Since the case of the dynamic region 39 is present here, with the indications of the distance d, the first value of the radiation power of the laser source 10, the gain factor (gain) of the detector 16 and the amplitude of the second voltage signal—optionally with suitable calibration—this value for the albedo can be calculated in a processor-aided manner by means of the central control unit 20 in step 160.

[0094] In order to determine the distance and reflectivity of a next object surface, the method returns to step 100. In this way, the surroundings of the device can be scanned step by step and a three-dimensional image can be generated as a result. This image can be evaluated by means of object-detecting software in order for example to recognize specific objects, persons or traffic signs etc. and, if appropriate, to take measures.

[0095] A second exemplary embodiment is shown in FIGS. 12 and 13. The focus here is on determining the reflectivity with the distance already having been determined. FIG. 12 shows for an SiPM sensor the relationship between the amplitude of a voltage signal respectively output and the overvoltage V.sub.OV of the detector 16 or a variable x derived from the driver voltage of the laser source 10 with a linear relationship. The dots in the diagram each correspond to a measurement in an exemplary device 1 as shown in FIG. 1, for instance.

[0096] The relationship corresponds to a function

amp=c1.Math.(1−exp(−(c2.Math.x)/c1)),  (1)

which yields a very good fit, wherein the amplitude amp of the voltage signal is yielded by the SiPM sensor and x is related to the driver voltage V1 of the laser source 10 by:

x=(V1−2.5)/0.5.  (2)

[0097] The coefficients c1 and c2 are determined by the fit. In the very specific exemplary embodiment, the coefficients are c1=0.3015 and c2=0.004296. The fit is indicated by a solid line in FIG. 12. What is discernible very well is a relatively linear relationship up to an amplitude of approximately 0.18 V, which here is the dynamic region 39, wherein above 0.18 V there follows a nonlinear saturation region (up to the asymptotic limit value at approximately 0.55 V in this specific, non-limiting example).

[0098] This second exemplary embodiment then provides for performing a linearization of the curve shown. For this purpose, equation (1) for the nonlinear amplitude response amp is transformed according to the linearized amplitude response y.sub.act:

y.sub.act=x=−log(1−amp/c1).Math.c1/c2,  (3)

[0099] The measured amplitudes of the measurements for various (known) distances with differing laser power (in accordance with x) can then be inserted in equation (3) and yield in each case straight lines having a gradient a dependent on the distance d:

y.sub.ref=α(d).Math.x  (4)

[0100] In order to determine α(d), it is possible once again to use a fit, for example, wherein a transformation into the logarithmic scale was carried out here as well:

log(α)=k1.Math.log(d).sup.2+k2.Math.log(d)+k3  (5)

[0101] The exemplary fit is only up to the 2.sup.nd order, but without limitation could also be of a higher order. FIG. 13 shows however for the measurement points that the fit is sufficient. In FIG. 13, log(α) is plotted as a function of log (d), wherein d is the distance. The coefficients ascertained for this specific example read: k1=0.118; k2=−2.438; k=4.305.

[0102] The following arises for the thus empirical, linearized reference amplitude:

y.sub.refexp(k1.Math.log(d)2+k2.Math.log(d)+k3).Math.x.  (6)

[0103] In the second exemplary embodiment, therefore, the linearized reference amplitude y.sub.ref can be immediately calculated as a reference value from equation (6), given a distance d determined by light time of flight measurement. On the other hand, the actual, linearized amplitude response y.sub.act can be directly measured or determined anew. Since the distance d is the same in both cases (after all, the distance is obtained from the same voltage signal), a difference between the two variables y.sub.act and y.sub.ref is based exclusively on a difference in the underlying reflectivity or albedo. The albedo can be calculated from the quotient of y.sub.act and y.sub.ref:

albendo=albedo.sub.ref.Math.(y.sub.act/y.sub.ref).sup.1/2,  (7)

wherein albedo.sub.ref is the albedo of a reference material used to carry out the fit. Ideally the reference material is a material with a particularly high albedo, for example aluminum with albedo.sub.ref=0.88. Conversely, however, it is also possible to use other materials with a lower albedo as reference, such as e.g. steel with albedo.sub.ref=0.68 or titanium with albedo.sub.ref=0.34, etc. These method steps allow a particularly efficient and fast calculation of the reflectivity, which is necessary to achieve rapid updating of the detected surroundings.

[0104] The steps of the second exemplary embodiment can be carried out in the context of step 160 of the first exemplary embodiment, or else in the context of step 290 of the third exemplary embodiment described below:

[0105] In this regard, reference is made to FIGS. 14-16. FIG. 16 shows in a flow diagram the sequence of the method in accordance with the third exemplary embodiment. After the start 200 of the method sequence, firstly a maximum distance d.sub.max (as start value) is predefined in step 210. Afterward, in step 220, a driver voltage V1 (d) is sought for this start value, the laser source 10 being operated with said driver voltage in order to emit light having a power that is sufficient to operate the detector 16 in the dynamic region 39.

[0106] In this example, too, for this purpose a function V1 (d) simplifying the calculation is again predefined, this function being shown in FIG. 14. The aim here is, by adapting the driver voltage V1 of the laser source 10, to maintain an irradiance in the detector 16 always approximately in the center of the dynamic region 39 independently of the distance d. The dynamic region 39 is determined by an upper limit value and a lower limit value for the irradiance, which respectively correspond to an upper and lower voltage limit value for the voltage output. The upper limit value is determined by the incipient saturation as described. The lower limit value is determined by a minimum signal-to-noise ratio SNR that is usable and permissible for the detection, which is defined here as 10 dB. In one specific example, the lower limit value SiPM-MIN is approximately 2 μW/m.sup.2 and the upper limit value SiPM-MAX is approximately 200 μW/m.sup.2, such that the dynamic region still constitutes 20 dB.

[0107] A fit becomes necessary since although the power of the laser source 10 is proportional to the square of the distance d, and although the driver voltage is also related linearly to the power, a laser power is brought about only starting from a certain start value. A 3.sup.rd order polynomial fit has proved itself here:

V(P(d))=e.Math.P(d).sup.3+f.Math.P(d).sup.2+g.Math.P(d)+h.  (8)

[0108] In the specific example, the coefficients were determined as follows: e=0.000793; f=−0.005521; g=2.276; h=0.8674. The curve, which is substantially parabolic nevertheless, is shown in FIG. 14. In the case of the distance d.sub.0min, a minimum possible power for the laser source 10 is present, in the specific example 0.2 W. For distances d below this limit value, step 220 of the method in this specific third exemplary embodiment always returns the same value of magnitude 1.5 volts for the driver voltage V1. Likewise, for all distances d above a limit value d.sub.0max, which corresponds to a maximum power—predefined in accordance with the safety standards—of 25 W in this example, only the value of 66.5 volts is returned for the driver voltage V1. In the example, exactly this upper limit value d.sub.0max was taken as start value. The (first) value of the power of the laser source 10 is then set in step 220. The gain factor or gain is not varied in this exemplary embodiment.

[0109] The laser source 10 is triggered in step 230 and, as a consequence thereof, a light pulse is generated in step 240. The light 18 reflected or backscattered from the object surface is received or detected by the detector 16 in step 250. In step 260. the first value for the distance, designated here as D, can be determined from the (first) voltage signal obtained. Step 270 involves checking whether d=D, i.e. whether the first value for the distance is equal to the distance predefined as start value. If this is not the case (N in step 270), the program sequence branches back to step 220. A new driver voltage V1 is sought here in accordance with the function in FIG. 14 or equation (8).

[0110] As is shown in FIG. 15, the parabolic portion shown in FIG. 14 corresponds to a flat portion of the irradiance Ir as a function of the distance. The target value in the center of the dynamic region between SiPM-MAX and SiPM-MIN is 100 μW/m.sup.2. As long as the distance d is between d.sub.0min and d.sub.0max, this median value in the dynamic region 39 is maintained again and again. The distances d.sub.0min and d.sub.0max are 3 and 16 m in the example. Above d.sub.0max, a fixed value of V1 (d) is returned, and so the irradiance Ir decreases, but evidently still up to a range of approximately 115 m is sufficient to lie above the lower limit SiPM-MIN, that is to say in the dynamic region 39.

[0111] On the other hand, correspondingly below d.sub.0min likewise only constant values for V1 (d) are returned. The latter only up to approximately 2 m result in irradiances that lie below the upper limit SiPM-MAX of the dynamic region.

[0112] The discrepancy between d and D in step 270 thus arises if the actual distance in FIG. 15 is less than d.sub.0max and V1 (d.sub.0max) is returned at the start of the program. The dashed curve in FIG. 15 reproduces this value for Vl. In the subsequent second pass, then the correct value for the driver voltage V1 is found and, following therefrom, the correct distance d=D is found in step 270.

[0113] Then as described above in the second exemplary embodiment, the amplitude amp is determined in step 280 (Y in step 270) and the albedo value is calculated in step 290. Step 300 involves checking whether further pixels are to be detected and, if that is applicable (Y in step 300), the program branches back to step 210. Otherwise, the program sequence ends (step 310).

[0114] It should be noted that in this third exemplary embodiment the laser power is not adapted only when the upper voltage limit or upper limit SiPM-MAX of the dynamic region 39 is exceeded, but rather is already adapted if any change at all vis-à-vis the preset distance is established.

LIST OF REFERENCE SIGNS:

[0115] 1, 1′ Device [0116] 10 Laser source [0117] 12 Laser light beam [0118] 14 Object surface [0119] 16 Detector, silicon photomultiplier (SiPM sensor) [0120] 18 Reflected or backscattered light [0121] 20 Central control device [0122] 22 Light time of flight (ToF) [0123] 24 Detector array (SiPMs) [0124] 26 Lens optical unit [0125] 28 MEMS [0126] 30 Micromirror [0127] 32 Direction of rotation of the micromirror/light beam [0128] 34 Diffuser [0129] 36 Microcell (SiPM) [0130] 38 Saturation region (SiPM) [0131] 40 Saturation region (APD) (comparative example) [0132] 42 Increase of range by SiPM [0133] 44 Decrease of the radiation power owing to safety standard [0134] 46 Exclusion region [0135] C.sub.D Diode capacitance [0136] L.sub.MAX Maximum permissible radiation power (laser) [0137] R.sub.Q Quench resistance [0138] R.sub.S Series resistance (Si substrate) [0139] S Switch (equivalent circuit diagram) [0140] 100 Predefining a first value for the power of the laser source and a second value for the gain of the detector [0141] 110 Setting the laser source and the detector, generating a pulse in the laser source, and detecting the pulse in the detector in accordance with the predefined values for outputting a voltage signal [0142] 120 Carrying out a distance determination or checking whether a distance determination is possible [0143] 130 Adapting the values for power of the laser source and/or gain of the detector [0144] 140 Checking whether the first voltage signal output is in the saturation region, i.e. not in the dynamic region [0145] 150 Adapting the values for power of the laser source and/or gain of the detector [0146] 160 Calculating the reflectivity of the relevant object surface from the second voltage signal [0147] 200 Start of the method sequence, providing the laser source, detector and object surface [0148] 210 Predefining a start value for the distance d (e.g. max. distance d.sub.max) [0149] 220 Seeking a value for the driver voltage V1 (d) with which the laser source is operated (adapting and/or setting a value for the power of the laser source) [0150] 230 Triggering the laser source [0151] 240 Generating a light pulse [0152] 250 Detecting the light reflected or backscattered from the object surface by means of the detector and outputting a voltage signal [0153] 260 Determining the distance D from the voltage signal output [0154] 270 Checking whether d=D (i.e. whether the determined distance is equal to the distance predefined as start value) [0155] 280 Determining the amplitude ampl [0156] 290 Calculating the albedo value [0157] 300 Checking whether further pixels are to be detected [0158] 310 End of the method sequence