Light time-of-flight camera

Abstract

Time-of-flight camera designed for distance measurement using a phase measurement principle, - with a lighting system for emitting modulated light, - and with a time-of-flight sensor, with multiple time-of-flight pixels, for receiving the emitted and reflected modulated light from a scene, - wherein the light time-of-flight sensor has two pixel classes (CW, UR), namely a CW and a UR pixel class, which can be synchronously controlled with different CW and UR modulation signals (CW-M, UR-M) and with different CW and UR modulation frequencies (f1, f2), - wherein the CW pixel class (CW) has at least two CW pixel groups (CW-M0, CW-M90) that are operated with different phase angles (M0, M90, M180, M270) of the CW modulation frequency (f1), and the UR pixel class (UR) has at least one first UR pixel group (UR-M0) that is operated with a first phase (M0) of the UR modulation frequency (f2), - where the CW modulation frequency (f1) is an integer multiple n of the UR modulation frequency (f2) with f1 = n * f2, so that during a UR period the CW periods repeat n times, - where n CW periods form a CW clock sequence, - wherein at least one CW clock sequence and at least one UR period are traversed in parallel during an integration interval, - and where the lighting is designed in such a way, that either only 1 light pulse or n - 1 light pulses are emitted synchronously with the CW modulation signal within a CW clock sequence, - wherein the light of the illumination is detected simultaneously by both pixel classes (CW, UR) in each integration interval, and a relative distance value is determined from the measurement of the CW pixel group and a uniqueness range is determined from the measurement of the UR pixel group, - whereby an absolute distance value is then determined from the relative distance value and the uniqueness range.

Description

[0001] The invention relates to a time-of-flight camera according to claim 1.

[0002] Time-of-flight cameras or time-of-flight camera systems refer in particular to all time-of-flight or 3D-TOF camera systems that derive time-of-flight information from the phase shift of emitted and received radiation. PMD cameras with photomixing detectors (PMDs), such as those described in DE 197 04 496 A1, are particularly suitable as time-of-flight or 3D-TOF cameras.

[0003] German patent DE 10 2012 204 512 A1 relates to a time-of-flight sensor for phase measurement of modulated light, comprising multiple time-of-flight pixels as photomixing detectors. These are connected to modulation drivers and at least one phase shifter. The sensor is designed to operate in at least two areas with different phase measurement modes. The first area is located in the center of the sensor, the second at the edge. This allows for flexible adjustment of the measurement times and modes, for example, to minimize motion artifacts and increase measurement accuracy. By using multiphase modes, several phase positions can be detected simultaneously during an exposure, which reduces the spatial resolution but also minimizes motion artifacts.The invention also includes a method for operating the sensor in which, during an integration time, all pixels in the first area are operated with the same phase position and groups of pixels with different phase positions are operated in the second area.

[0004] German patent DE 10 2022 102 992 A1 relates to a time-of-flight distance measurement system, in particular a time-of-flight camera system, comprising an illumination system for emitting light and a time-of-flight sensor for receiving and demodulating modulated light. A modulator generates the modulation signal for the illumination system and the sensor. In the distance measurement, the modulation signal is provided with a first modulation frequency as a pseudo-noise signal for selecting a distance range. In a control measurement, the modulation signal is provided as a common-mode signal with a second, lower modulation frequency. This allows for a comparison of the amplitudes from both measurements to validate the distance measurement. The system utilizes pseudo-random binary sequences (PN sequences) such as maximum sequences (MLS) due to their advantageous autocorrelation properties to enable accurate and noise-resistant measurements.However, when using PN sequences directly, unequal numbers of logical zeros and ones can lead to performance problems. Substituting the bits of the basic PN sequence with sub-bit sequences achieves a symmetrical charge carrier distribution, thus making MLS usable for modulating a PMD depth image sensor. The system offers advantages such as multi-target capability and a reduction in mixed phases and stray light. Measurement plausibility is ensured by an additional measurement at a lower frequency to prevent measurement errors.

[0005] US Patent 2020 / 0167942 A1 concerns a method for performing depth measurements with an image sensor. This involves performing continuous wave phase measurements and coded modulation measurements for at least one pixel. A mask value is determined from the coded modulation measurement and applied to the distance value calculated from the phase measurements to obtain a masked distance value without phase winding ambiguity. This technique improves the accuracy of depth measurements by eliminating ambiguities that can arise from phase windings. The method involves performing multiple phase measurements with varying phase differences and determining the mask value by comparing the coded modulation measurement to a threshold value. The mask value is then applied to the distance values ​​to obtain masked distance values.An image processing system comprises a sensor with multiple pixels that generate signals in response to received light, and a control circuit that manages the execution of measurements and the application of mask values. This technique can also be used to filter image data, thereby restricting the data to objects within a predefined distance range.

[0006] German patent DE 10 2021 117 139 A1 discloses a method for operating a time-of-flight camera system comprising illumination and a time-of-flight sensor with time-of-flight pixels, which have integration nodes for accumulating photogenerated charges. Distance determination is achieved by combining a common-mode (CM) measurement with a continuous-wave (CW) measurement. During the CM measurement, the camera system is operated in at least two CM integration intervals, each with differently coded modulation signals, to provide a rough distance estimate by assigning the object to one of several distance sectors. This assignment is based on the signs of the detected charge differences. In addition, during the CW measurement, the camera system is operated in at least two CW integration intervals with different phase angles of a periodic and an uncoded CW modulation signal to enable precise distance measurement.A final distance value is ultimately determined from the combination of the charge differences of both measurements.

[0007] German patent application DE 10 2020 215 041 A1 discloses a LiDAR sensor system comprising a transmitter and a receiver. The transmitter is designed to emit multiple light transmission sequences, each sequence having a phase code applied by a phase modulator. An evaluation unit is designed to generate an evaluation signal for a multitude of predefined code shifts by multiplying a received light signal by the phase code shifted by the respective code shift. A Doppler frequency is determined from the generated evaluation signals, in particular their spectra. Based on this Doppler frequency and the corresponding code shift of the evaluation signal containing the Doppler frequency, the distance to an object is determined.

[0008] DE 10 2021 108 054 A1 discloses a time-of-flight camera with an illumination system for emitting modulated light and a sensor for receiving the reflected light. The sensor has pixels, each equipped with a light-sensitive area, a discard node, and two integration nodes. The camera is designed such that, within an integration interval, the charge carriers generated in the light-sensitive area are directed partly to the integration nodes and partly to the discard node. By controlling the nodes in a specific time, photogenerated charge carriers corresponding to a preferred distance range are primarily detected by the integration nodes, while charge carriers from a less preferred distance range are primarily directed to the discard node.The lighting can also be controlled so that less light is emitted for the less preferred distance range.

[0009] German patent DE 10 2021 113 743 A1 relates to a method and a device for time-of-flight (ToF) scene detection. A ToF sensor performs multiple ToF measurements with a first modulation frequency to obtain measured values. Each correlation function of these measurements is periodic and exhibits an increasing amplitude over the distance within the sensor's measuring range. This allows the distance to an object to be determined based on the measured values. The ToF sensor emits modulated light pulses into the scene and detects the reflected light pulses. The sensor's processing circuit determines the distance based on these measured values. To overcome the ambiguity of the distance measurement, additional ToF measurements can be performed with a second modulation frequency. The correlation functions of the second measurements are also periodic and have an increasing amplitude.The distance is determined by comparing the phase shifts of the first and second measurements. This method reduces glare and saturation problems with nearby objects and improves time-of-flight (ToF) detection. Modulation codes such as Barker codes or m-sequences are used to control the light pulses and reference signals.

[0010] The object of the invention is to simplify the determination of an unambiguous distance by a time-of-flight camera that operates according to the phase measurement principle.

[0011] The problem is solved by a time-of-flight camera according to claim 1.

[0012] The following is a more detailed explanation of some of the terms used: - Continuous-wave or CW measurement refers to amplitude-modulated continuous light emission, whereby a phase difference between the emitted and received modulated light is determined in a CW measurement. - Code modulation or CM measurement uses coded modulation or so-called pseudo-noise modulation instead of uniform modulation. The unambiguous range (UR) describes the maximum range for which a unique distance can be determined in a measurement based on the phase measurement principle. For distances greater than the wavelength of the modulation signal, the measurement is no longer unambiguous. The unambiguous range can be increased by performing measurements with several different modulation frequencies. - dToF: direct Time of Flight method, all distance measurement methods that directly determine a distance from the 'flight time' of emitted light - iToF: indirect Time of Flight method, all methods which, unlike dToF methods, determine light-time-dependent indirect parameters, such as a phase shift between a transmitted and received modulation signal, for distance determination.

[0013] Achieving the most accurate and long-range distance measurement possible with iToF systems presents a particular challenge, as the distance noise is linear, while the uniqueness range size is inversely proportional to the modulation frequency of a continuous-wave measurement.

[0014] This discrepancy is typically resolved by cleverly combining several consecutive individual measurements. For this purpose, a CW measurement is combined either with another CW measurement or a coded modulation (CM) measurement. In the first case, the uniqueness range of the combined measurement is increased by a factor of typically 4 to 8 by selecting the uniqueness ranges of the individual measurements according to the principle of the Chinese remainder theorem. In the second case, the position of the uniqueness range of the CW measurement is determined by the CM measurement, thus multiplying the size of the combined uniqueness range.

[0015] Both methods have different advantages and disadvantages. A common disadvantage of both methods is the need for additional measurements, which require additional measurement time, data readout, and electrical energy. In many cases, this is also associated with limitations regarding laser safety due to the additional active illumination of the measurement scene.

[0016] The purpose of the invention is to overcome this disadvantage.

[0017] The problem is solved by the inventive method by enabling simultaneous CW and CM measurement.

[0018] The effective uniqueness range of a continuous-wave measurement according to the iToF principle can thus be multiplied by additional code modulation measurements while maintaining the distance noise.

[0019] They show: Fig. 1 schematically a time-of-flight camera system, Fig. 2 a modulated integration of generated charge carriers, Fig. 3 a cross-section through a PMD time-of-flight pixel with a potential distribution, Fig. 4. Charge integration patterns depending on the phase shift and position, Fig. 5 a relation of the phase shift in an IQ diagram, Fig. 6 a time-of-flight sensor with three groups of time-of-flight pixels that can be operated with different modulation signals Fig. 7 a modulation scheme for a time-of-flight sensor according to Fig. 6, Fig. 8 a time-of-flight sensor with six groups of time-of-flight pixels that can be operated with different modulation signals Fig. 9 a modulation scheme for a time-of-flight sensor according to Fig. 8, Fig. 10 typical autocorrelation functions for an operation according to the invention, Fig. 11 a modified modulation scheme for a time-of-flight sensor according to Fig. 8, Fig. 12 Extension of the modulation scheme from Fig. 9 for an additional UR modulation.

[0020] Fig. Figure 1 shows a measurement situation for an optical distance measurement with a time-of-flight camera, as is known, for example, from DE 197 04 496 A1.

[0021] The time-of-flight camera system 1 comprises a transmitter unit or illumination module 10 with an illumination 12 and an associated beam shaping optics 15, as well as a receiver unit or time-of-flight camera 20 with a receiving optics 25 and a time-of-flight sensor 22.

[0022] The time-of-flight sensor 22 has at least one time-of-flight pixel, preferably also a pixel array, and is in particular designed as a PMD sensor. The receiving optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmitting unit 10 can, for example, be designed as a reflector or lens optics. In a very simple embodiment, optical elements can optionally be omitted on both the receiving and transmitting sides.

[0023] The measurement principle of this arrangement is essentially based on the fact that, starting from the phase shift of the emitted and received light, the travel time and thus the distance traveled by the received light can be determined. For this purpose, the light source 12 and the light travel time sensor 22 are jointly supplied with a specific modulation signal M0 with a base phase φ0 via a modulator 30. In the illustrated example, a phase shifter 35 is also provided between the modulator 30 and the light source 12, with which the base phase φ0 of the modulation signal M0 of the light source 12 is shifted by defined phase positions φ var can be shifted. For typical phase measurements, phase positions of φ are preferably used. var = 0°, 90°, 180°, 270° used.

[0024] Depending on the set modulation signal, the light source 12 sends an intensity-modulated signal S p1with the first phase position p1 or p1 = φ0 + φ var off. This signal S p1 or, in the case shown, the electromagnetic radiation is reflected by an object 40 and arrives with a corresponding phase shift Δφ(t) due to the distance traveled. L ) with a second phase position p2 = φ0 + φ var + Δφ(t L ) as a received signal S p2 on the light time-of-flight sensor 22. In the light time-of-flight sensor 22, the modulation signal M0 is combined with the received signal S p2 mixed, whereby the phase shift or the object distance d is determined from the resulting signal.

[0025] Furthermore, the system has a modulation control unit 27 which, depending on the measurement task at hand, adjusts the phase angle φ. var the modulation signal M0 is changed and / or the modulation frequency is set via a frequency oscillator 38.

[0026] Infrared LEDs are preferably suitable as the illumination source or light source 12. Of course, other radiation sources in other frequency ranges are also conceivable, in particular light sources in the visible frequency range.

[0027] The basic principle of phase measurement is schematically represented in Fig. Figure 2 shows the time course of the modulation signal M0, which controls the lighting 12 and the light-time-of-flight sensor 22. The light reflected from object 40 arrives as the received signal S. p2 according to its light travel time t L phase-shifted Δφ(t L) to the time-of-flight sensor 22. The time-of-flight sensor 22 collects the photonically generated charges q over several modulation periods in the phase of the modulation signal M0 in a first accumulation gate Ga and in a phase shifted by 180° M0 + 180° in a second accumulation gate Gb. From the ratio of the charges qa, qb collected in the first and second gates Ga, Gb, the phase shift Δφ(t) can be determined. L ) and thus determine a distance d of the object.

[0028] Fig. Figure 3 shows a cross-section through a pixel of a photomixing detector such as that known from DE 197 04 496 A1. The modulation photogates Gam, G0, Gbm form the light-sensitive area of ​​a PMD pixel. Depending on the voltage applied to the modulation gates Gam, G0, Gbm, the photonically generated charges q are directed either to one or the other accumulation gate or integration node Ga, Gb. The integration nodes can be configured as gates or diodes.

[0029] Fig. Figure 3b shows a potential profile in which the charges q flow towards the first integration node Ga, while the potential according to Fig. 3c allows the charge q to flow towards the second integration node Gb. The potentials are set according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in the range of 1 to 100 MHz. With a modulation frequency of, for example, 1 MHz, the period is one microsecond, so the modulation potential changes accordingly every 500 nanoseconds.

[0030] In Fig. Figure 3a further shows a readout unit 400, which may optionally already be part of a PMD time-of-flight sensor designed as a CMOS. The integration nodes Ga, Gb, configured as capacitors or diodes, integrate the photonically generated charges over a multitude of modulation periods. The voltage then applied to the gates Ga, Gb can be tapped off, for example, via the readout unit 400, using a high-impedance connection, as is known. The integration times are preferably selected such that the time-of-flight sensor or the integration nodes and / or the light-sensitive areas do not reach saturation for the expected amount of light. The readout paths of the two integration nodes Ga, Gb can also be referred to as A and B channels.

[0031] Fig. 4a and Fig. Figure 4b shows the curves of the charge difference Δq = q a - q b / (q a + q b ) as a function of the phase shift Δφ(tL ) of the received light signal S p2 with different phase positions. The Fig. Figure 4a shows a curve for an unshifted modulation phase M0 with a phase position φ var = 0°.

[0032] Upon arrival of signal S p2 without phase shift, i.e., Δφ(t L ) = 0°, for example, when the transmitted signal S p1 The phases of the modulation M0 and of the received signal S are directed directly onto the sensor. p2 identical, so that all generated charge carriers are detected phase-synchronously at the first gate Ga and thus a maximum difference signal with Δq = 1 is present.

[0033] With increasing phase shift, the charge at the first accumulation gate Ga decreases and at the second accumulation gate Gb increases. For a phase shift of Δφ(t LAt 90°, the charge carriers qa and qb are equally distributed at both gates Ga and Gb, and the difference is therefore zero, and after a 180° phase shift, it is "-1". With a further increasing phase shift, the charge at the first gate Ga increases again, so that the charge difference ultimately rises again, reaching a maximum at 360° and 0°, respectively.

[0034] Mathematically, this is a correlation function of the received signal S. p2 with the modulating signal M0. q(τ)=∫0τSp2(t−τ)M0(t)dt

[0035] As previously shown, modulation with a square wave results in a triangular correlation function. Modulation with, for example, a sine wave would result in a cosine function.

[0036] How Fig. Figure 4a shows that a measurement of the phase with a phase position is only possible up to a phase shift Δφ(t L ) ≤ 180° unambiguously.

[0037] For maximum detection of the phase shift, the IQ (In-phase quadrature) method is known, in which two measurements are carried out with phase positions shifted by 90°, for example with the phase position φ. var = 0° and φ var = 90°. The result of a measurement with the phase angle φ var = 90° is in Fig. 4b shown.

[0038] The relationship between these two curves can be described in a known way, for example for sinusoidal curves in an IQ diagram according to... Fig. 5. As a first approximation, this representation can also be readily applied to the triangular functions shown.

[0039] The phase angle can then be determined in a known manner using an arctan function: φ=arctanΔq(90°)Δq(0°)

[0040] To compensate for sensor asymmetry, for example, additional phase measurements shifted by 180° can be performed, so that the phase angle can be determined as follows. ϕ=arctan 2Δq(90°)−Δq(270°)Δ(0°)−Δq(180°)

[0041] From the propagation-related phase shift Δφ(t L For object distances d that are smaller than half the wavelength λ of the modulation frequency d ≤ λ / 2, a distance can be determined in a known manner. d=Δφ(tL)λ2π⋅12

[0042] For distances d > λ / 2, there is usually no way to measure the phase shift absolutely, so that the determined phase shift can no longer be uniquely assigned to a distance value.

[0043] The core idea of ​​the invention is to design a time-of-flight sensor in such a way that different time-of-flight pixels can be operated with different phase angles and modulation signals, in order to be able to operate, for example, a CW modulation and a coded modulation CM simultaneously.

[0044] The invention proposes to perform CW and CM measurements simultaneously without additional readout of the imager.

[0045] For this purpose, CW and CM measurements are performed in parallel on different pixels.

[0046] Fig. Figure 6 shows a possible configuration of a time-of-flight sensor according to the invention with two pixel classes that can be operated with different modulation frequencies, wherein groups of pixels with different modulation phase angles can be controlled within the pixel classes. In the illustrated example, a CW pixel class is provided, with CW pixels CW-M0 and CW-M90 that are operated with a modulation phase angle of 0° and 90°, respectively. Furthermore, an UR pixel class is provided, which in the illustrated example is operated only with a first phase angle of 0°. 0° and 90° here represent the real and imaginary parts of the phase measurement.

[0047] For reasons of symmetry, it is also advantageous to measure the complementary phase of the CW pixels and the orthogonal phase of the UR pixel to fully cover the desired depth range, i.e., for example, in a second measurement, the complementary phase shifted by 180° is measured for the CW pixels and the orthogonal phase shifted by 90° is measured for the UR pixels.

[0048] Alternatively, it may also be provided for, as in Fig. Figure 8 shows how to provide pixels for complementary and orthogonal phase positions on the light-time-of-flight sensor. In the example according to Fig. Therefore, 6 pixel groups are provided, namely four CW pixels CW-M0, CW-M90, CW-M180, CW-270 and two UR pixels UR-M0, UR-M90

[0049] Fig. Figure 7 shows a preferred modulation scheme for the illumination and a time-of-flight sensor or a respective time-of-flight pixel according to Fig. 6. In the case shown, the CW modulation signal of the CW pixels has a modulation frequency 4 times higher than the UR modulation signal, with the UR modulation signal having no special encoding.

[0050] Starting from a modulation frequency four times higher, the CW modulation signal completes four CW periods during one period of the UR modulation signal. These four CW periods constitute one CW clock sequence.

[0051] According to the invention, it is provided that the length of the CW and UR periods or CW and UR modulation frequencies is increased by an integer factor n. f to differentiate. With f1 = n * f2.

[0052] Accordingly, n CW periods are traversed during one UR period. The n CW periods form a CW clock sequence. The illumination is designed such that only one light pulse is emitted synchronously with the CW modulation signal during a CW clock sequence. Alternatively, it can also be provided that n-1 light pulses are emitted during the CW clock sequence. In both cases, the light pulses preferably have a duration of at least 25% of the CW period length.

[0053] In the example shown, only one light pulse is emitted within n=4 periods of the CW modulation signal or within one period of the UR modulation signal.

[0054] In a second measurement, the complementary and orthogonal phase positions are preferably measured. The complete set of measurements is in Fig. 9 shown, which are either in two measurements with the time-of-flight sensor according to Fig. 6 or in a measurement using a time-of-flight sensor according to Fig. 8 can be carried out.

[0055] In Fig. Figure 10 shows the resulting autocorrelation functions (ACFs) of the measurement according to the invention, and it is immediately apparent that the combination of the information enables an unambiguous distance determination across four CW uniqueness domains. For this purpose, the position determination within the uniqueness domain of the CW pixels is combined with the determination of the location of the uniqueness domain from the information of the UR pixels.

[0056] The UR pixels register the arrival of each emitted light pulse, and the position of the CW uniqueness range can be clearly determined from the light travel time.

[0057] However, a disadvantage compared to a conventional CW measurement is the optical duty cycle of the CW measurement, which is reduced by a factor of four, resulting in a lower amplitude of the correlation signal per integration time.

[0058] The lighting scheme according to Fig. 11 avoids the strong reduction of the optical signal by not emitting a single light pulse, but by omitting a single light pulse or by emitting n-1 light pulses.

[0059] In this scheme, the location of the CW uniqueness region is detected by the absence of a pulse. Consequently, the amplitude of the CW pixel signal is improved, while the noise level of the UR pixels is increased by the additional noise of the received light.

[0060] However, since only a simple A / B analysis is necessary for UR measurement, this does not represent a significant limitation. For the sake of simplicity, in Fig. 11. Only one CW modulation signal, CW-M0, was recorded as an example; of course, the other phase positions will be recorded accordingly and included in the evaluation.

[0061] One possible implementation of the modulation scheme from Fig. For example, 11 pixels on a pixel array can combine 3 different pixels, which are read out in 2 frames: • one CW pixel for CW measurement with 0° and 180° phase difference • one CW pixel for CW measurement with 90° and 270° phase difference • one UR pixel for A- and B-CM measurement (0°, 90°)

[0062] Depending on the use case and the resolution of the iToF module, it can be advantageous to distribute a small number of UR pixels across the imager and assume the same UR area for the surrounding areas. An adaptive selection of the density and position of the UR pixels, depending on the scene being viewed, is also conceivable.

[0063] The scheme from Fig. The combined uniqueness range can be increased by adding further UR pixels with a lower modulation frequency. With each additional UR measurement, the uniqueness range can thus be doubled.

[0064] Fig. Figure 12 shows such an extension, in which 3 UR signals are used to distinguish 8 CW uniqueness ranges.

[0065] The described scheme can easily be adapted in various ways for CW measurements with a different number of CW phase measurements. For example, for a 3-tap measurement with 3 CM measurements, the following implementation in 3 frames is possible: • one CW pixel for CW measurement with 0°, 120° and 240° phase difference • one UR pixel for A, B and C CM measurement

[0066] Furthermore, UR modulation can be implemented as so-called coded modulation, particularly in the form of a Gray code.

Claims

[1] Time-of-flight camera designed for distance measurement using a phase measurement principle, - with a lighting system for emitting modulated light, - and with a time-of-flight sensor, with multiple time-of-flight pixels, for receiving the emitted and reflected modulated light from a scene, - wherein the light time-of-flight sensor has two pixel classes (CW, UR), namely a CW and a UR pixel class, which can be synchronously controlled with different CW and UR modulation signals (CW-M, UR-M) and with different CW and UR modulation frequencies (f1, f2), - wherein the CW pixel class (CW) has at least two CW pixel groups (CW-M0, CW-M90) that are operated with different phase angles (M0, M90, M180, M270) of the CW modulation frequency (f1), and the UR pixel class (UR) has at least one first UR pixel group (UR-M0) that is operated with a first phase (M0) of the UR modulation frequency (f2), - where the CW modulation frequency (f1) is an integer multiple n of the UR modulation frequency (f2) with f1 = n * f2, so that during a UR period the CW periods repeat n times, - where n CW periods form a CW clock sequence, - wherein at least one CW clock sequence and at least one UR period are traversed in parallel during an integration interval, - and where the lighting is designed in such a way, that either only 1 light pulse or n - 1 light pulses are emitted synchronously with the CW modulation signal within a CW clock sequence, - wherein the light of the illumination is detected simultaneously by both pixel classes (CW, UR) in each integration interval, and a relative distance value is determined from the measurement of the CW pixel group and a uniqueness range is determined from the measurement of the UR pixel group, - whereby an absolute distance value is then determined from the relative distance value and the uniqueness range. [2] Time-of-flight camera according to claim 1, wherein the CW pixel class (CW) has four CW pixel groups with different phase angles of the CW modulation frequency (f1), and wherein the UR pixel class (UR) has two UR pixel groups with different phase angles of the UR modulation frequency (f2). [3] Time-of-flight camera according to claim 1, where the light time-of-flight sensor has two CW pixel groups and only one UR pixel group, wherein the time-of-flight camera is designed such that in a first measurement phase the first CW pixel group (CW-M0, CW-M90) is operated with 0° phase, the second CW pixel group with 90° phase and the one UR pixel group (UR-M0) with 0° phase, wherein in a second measurement phase the first and second CW pixel groups (CW-M0, CW-M90) are operated with the complementary phase of 180° and 270° and the first UR pixel group (UR-M0) with the orthogonal phase of 90°. [4] Time-of-flight camera according to one of the preceding claims, in which, in addition to the first UR pixel group, one or more further UR pixel groups are provided which can be operated with modulation signals of lower frequency or longer wavelength in relation to the first UR modulation signal to increase the uniqueness range.

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