DE102013216434B3 - Time of flight camera system - Google Patents

Abstract

Time-of-flight camera and method for operating such a system, comprising the steps of: - emitting and receiving a modulated light (Sp1, Sp2), - demodulating the received modulated light (Sp2) with a modulation signal (M0), - the transmission and demodulation being carried out with three different phase positions (☐var) of the modulation signal are carried out - and wherein a first and second phase position are shifted by 180 ° to one another, and a third phase position is shifted by 90 ° to one of the first two phase positions, - determining a distance-dependent difference variable for each phase position, - Determination of a distance value from the determined difference quantities.

Description

The invention relates to a light transit time camera system and a method for operating such according to the preamble of the independent claims.

With the time of flight camera system, not only systems are to be included which determine distances directly from the light transit time, but in particular also all the time of flight or 3D TOF camera systems which acquire transit time information from the phase shift of an emitted and received radiation. In particular, PMD cameras with photonic mixer detectors (PMD) are suitable as the light transit time or 3D TOF cameras, as described, inter alia, in the applications EP 1 777 747 A1 . US Pat. No. 6,587,186 B2 and also DE 197 04 496 C2 described and, for example, by the company "ifm electronic GmbH" or "PMD Technologies GmbH" as a frame grabber O3D or as CamCube relate. In particular, the PMD camera allows a flexible arrangement of the light source and the detector, which can be arranged both in a housing and separately.

From the DE 197 04 496 C2 and also from the DE 10 2011 089 642 A1 Furthermore, the determination of a distance or a corresponding phase shift of the reflected light from an object is known. It is stated that at least three different phase relationships between transmitter and receiver are to be used to determine the relative phases, wherein preferably more than three phase positions should be used to reduce the noise. In particular, it is disclosed to selectively shift the transmitter modulation by 90 °, 180 ° or 270 ° in order to determine a phase shift and thus a distance from these four phase measurements via an arctan function.

Furthermore, the describes US 5,694,204 A a device for optical distance measurement and performs the phase measurement that are determined to determine the phase shift between transmitted and received signal three variables, namely: amplitude of the received signal, amplitude of the reference signal and phase shift between the received and reference signal. To determine the quantities, it is proposed to provide at least three subschedical phase positions for the modulation of a driver signal.

The object of the invention is to improve the distance measurement of a light transit time camera system.

The object is achieved by a method according to claim 1 or with a light transit time camera system according to claim 5.

• – Aussendung und Empfang eines modulierten Lichts,
• – Demodulation des empfangenen modulierten Lichts mit einem Modulationssignal,
• – wobei die Aussendung und die Demodulation mit drei unterschiedlichen Phasenlagen (φ_var) des Modulationssignals (M₀) durchgeführt werden
• – und wobei eine erste und zweite Phasenlage um 180° zueinander verschoben sind, und eine dritte Phasenlage um 90° zu einer der ersten beiden Phasenlagen verschoben ist,
• – und zu jeder Phasenlage eine entfernungsabhängige Differenzgröße ermittelt wird,
• – Ermittlung eines Entfernungswertes aus den ermittelten Differenzgrößen.

According to the invention, a method for operating a light transit time camera system is provided, with the steps:

• - transmission and reception of a modulated light,
• Demodulation of the received modulated light with a modulation signal,
• - Wherein the transmission and the demodulation with three different phase positions (φ _var ) of the modulation signal (M ₀ ) are performed
• - And wherein a first and second phase position are shifted by 180 ° to each other, and a third phase position is shifted by 90 ° to one of the first two phase positions,
• And a distance-dependent difference quantity is determined for each phase position,
• - Determination of a distance value from the determined difference sizes.

This procedure has the advantage that the time for performing a phase measurement can be reduced by 25%, whereby measurement deviations can be neglected when using a suitable calculation algorithm.

Starting from an algorithm for distance calculation on the basis of four phase positions, it is provided according to the invention to substitute one of the phase positions with the largest difference magnitude, and to perform the distance measurement and calculation with the remaining three phase positions.

This procedure shortens the measurement and calculation time and also makes it possible to adapt an analogue reference voltage, which is used to digitize the measured values, according to the saved and substituted differential quantity, thus expanding the dynamic range.

It is also advantageously provided for a preferred distance measuring interval to specify a phase substitution in such a way that the phase error within this distance interval is minimal.

In particular, the preferred ranging interval is less than or equal to a quarter of the uniqueness range.

By this procedure, it is advantageously possible to specify a distance measurement interval in which the measurement errors are as small as possible.

Advantageously, a light transit time measuring system according to said method is configured, with a light transit time sensor having at least one receiving pixel and with a light source, with a modulator, which is connected to the light transit time sensor and the light source, and with an evaluation, starting to determine a distance of signals of the light transit time sensor in three different phase position and for carrying out the above-mentioned method is configured.

The invention will be explained in more detail by means of embodiments with reference to the drawings.

Show it:

1 schematically a light transit time camera system,

2 a modulated integration of generated charge carriers,

3 a cross section through a PMD light transit time sensor with potential distribution,

4 a time profile of the integration voltages at a light transit time pixel,

5 Characteristics of the charge integration depending on the phase shift and position,

6 a relation of the phase shift in an IQ-diagram,

7 a modulation course over four phase positions,

8th a distance determination over several measuring cycles with four phase positions,

9 a distance determination over several measuring cycles with three phases,

10 a distance determination accordingly 9 with varying phase angles,

11 to be digitized voltage signals of a measurement with four phase angles,

12 to be digitized voltage signals of a measurement with three phase angles.

In the following description of the preferred embodiments, like reference characters designate like or similar components.

1 shows a measurement situation for an optical distance measurement with a light time camera, as for example from the DE 197 04 496 is known.

The light transit time camera system 1 comprises a transmitting unit or a lighting module 10 with a lighting 12 and associated beam shaping optics 15 as well as a receiving unit or light runtime camera 20 with a receiving optics 25 and a light transit time sensor 22 ,

The light transit time sensor 22 has at least one time-of-flight pixel, preferably also a pixel array, and is designed in particular as a PMD sensor. The receiving optics 25 typically consists of improving the imaging characteristics of multiple optical elements. The beam shaping optics 15 the transmitting unit 10 may be formed for example as a reflector or lens optics. In a very simple embodiment, if necessary, optical elements can also be dispensed with both on the receiving side and on the transmitting side.

The measuring principle of this arrangement is essentially based on the fact that, based on the phase shift of the emitted and received light, the transit time and thus the distance covered by the received light can be determined. For this purpose, the light source 12 and the light transit time sensor 22 via a modulator 30 together with a certain modulation signal M ₀ with a base phase angle φ ₀ applied. In the example shown is also between the modulator 30 and the light source 12 a phase shifter 35 provided with the base phase φ _{0 of} the modulation signal M _{0 of} the light source 12 can be moved by defined phase positions φ _var . For typical phase measurements, phase positions of φ _var = 0 °, 90 °, 180 °, 270 ° are preferably used.

The light source transmits according to the set modulation signal 12 an intensity-modulated signal S _p1 with the first phase position p1 or p1 = φ ₀ + φ _var . This signal S _p1 or the electromagnetic radiation is in the illustrated case of an object 40 reflects and hits due to the distance traveled corresponding phase-shifted Δφ (t _L ) with a second phase position p2 = φ ₀ + φ _var + Δφ (t _L ) as a received signal S _p2 on the light transit time sensor 22 , In the time of flight sensor 22 the modulation signal M _{0 is} mixed with the received signal S _p2 , wherein the phase shift or the object distance d is determined from the resulting signal.

Furthermore, the system has a modulation control unit 27 on, the phase position φ _var the modulation signal M ₀ changed depending on the present measurement task and / or a frequency oscillator 38 sets the modulation frequency.

As illumination source or light source 12 are preferably infrared light emitting diodes. Of course, other sources of radiation in other frequency ranges are also conceivable; in particular, light sources in the visible frequency range are also possible.

The basic principle of phase measurement is schematically in 2 shown. The upper curve shows the time profile of the modulation signal M ₀ with the illumination 12 and the light transit time sensor 22 be controlled. The object 40 Reflected light impinges on the light transit time sensor as received signal S _{p2 in} accordance with its light transit time t _L phase-shifted Δφ (t _L ) 22 , The light transit time sensor 22 collects the photonically generated charges q over several modulation periods in the phase position of the modulation signal M ₀ in a first accumulation gate Ga and in a 180 ° shifted phase position M ₀ + 180 ° in a second accumulation Gb. From the ratio of the charges qa, qb collected in the first and second gate Ga, Gb, the phase shift Δφ (t _L ) and thus a distance d of the object can be determined.

3 shows a cross section through a pixel of a photonic mixer as it is for example from the DE 197 04 496 C2 is known. The modulation photogates Gam, G0, Gbm form the light-sensitive area of a PMD pixel. In accordance with 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 designed as a gate or as a diode.

3b shows a potential curve in which the charges q in the direction of the first integration accounts Ga tapped off, while the potential according to 3c the charge q flows in the direction of the second integration node Gb. The potentials are specified according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in a range of 1 to 100 MHz. At a modulation frequency of, for example, 1 MHz results in a period of one microsecond, so that the modulation potential changes accordingly every 500 nanoseconds.

In 3a is also a readout unit 400 which, if appropriate, may already be part of a CMOS PMD light transit time sensor. The integration nodes Ga, Gb designed as capacitors or diodes integrate the photonically generated charges over a large number of modulation periods. In a known manner, the voltage applied to the gates Ga, Gb, for example via the readout unit 400 be tapped high impedance. The integration times are preferably to be selected such that the light transit time sensor or the integration nodes and / or the light-sensitive areas do not saturate for the expected amount of light.

In 4 is a typical time course of the voltage applied to the integration node Ga, Gb during a phase measurement voltage U _a , U _b . Starting from a positive reset voltage U _DRS applied after a reset to the integration node, the voltage drops due to the accumulated voltage Photoelectrons at both integration nodes Ga, Gb. In accordance with the phase shift Δφ (t _L ) of the received signal, the voltages at the integration nodes Ga, Gb drop off to different degrees. At the end of the integration time t _int , the voltage U _a , U _b applied to the integration nodes Ga, Gb is read out. The voltage difference ΔU of the two voltages U _a , U _b corresponds in a known manner to the difference Δq of the charge q accumulated at the integration nodes Ga, Gb. The integration time t _int is preferably dimensioned such that no integration node Ga, Gb reaches its saturation potential U _S in a conventional exposure. For greater signal strengths, a so-called SBI circuit for signal compensation can also be provided. Such circuits are for example from the DE 10 2004 016 626 A1 or DE 10 2005 056 774 A1 known.

5a and 5b show curves of the normalized charge difference Δq = q _a - q _b / (q _a + q _b ) as a function of the phase shift Δφ (t _L ) of the received light signal S _p2 with different phase angles. The 5a shows a curve for a non-shifted modulation phase M ₀ with a phase angle φ _var = 0 ° When the signal S _p2 without phase shift so Δφ (t _L ) = 0 °, for example, when the transmission signal S _{p1 is directed} directly to the sensor, the phases of the modulation M ₀ and of the received signal S _{p2 are} identical, so that all generated charge carriers are detected in phase synchronization at the first integration node Ga and thus a maximum difference signal with Δq = 1 is applied.

As the phase shift increases, the charge accumulated at the first integration node Ga decreases and at the second integration node Gb increases. With a phase shift of Δφ (t _L ) = 90 °, the charge carriers qa, qb are equally distributed at both integration nodes Ga, Gb and the charge difference is thus zero and after 180 ° phase shift "-1". With further increasing phase shift, the charge at the first gate Ga increases again, so that as a result the charge difference increases again in order then to reach a maximum again at 360 ° or 0 °.

Mathematically, this is a correlation function of the received signal S _p2 with the modulating signal M ₀ .

In the case of a modulation with a square-wave signal, as already described, a triangular function results as the correlation function. For a modulation with, for example, a sine signal, the result would be a cosine function.

As 5a shows, a phase phase measurement is unique only up to a phase shift Δφ (t _L ) ≤ 180 °.

For maximum detection of the phase shift of the IQ (in-phase quadrature) method is known for example in which two measurements are performed with shifted by 90 ° phase positions, that is, 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 5b shown.

The relationship between these two curves can be determined in a known manner, for example for sinusoidal waveforms in an IQ diagram. 6 represent. In a first approximation, this representation is readily applicable to the triangular functions shown.

The phase angle can then be determined in a known manner via an arctan function or arctan2 function: φ = Δφ (t _L ) = arctan Δq (90 °) / Δq (0 °)

Due to the linear relationship between charge and voltage, the phase angle can also be determined by the voltage differences: φ = Δφ (t _L ) = arctan ΔU (90 °) / ΔU (0 °)

In order to compensate, for example, asymmetries of the sensor, additional phase measurements shifted by 180 ° can be performed so that, as a result, the phase angle can be determined as follows. φ = Δφ (t _L ) = arctan Δq (90 °) - Δq (270 °) / Δq (0 °) - Δq (180 °)

Or shortened formulated:

Where the indices indicate the respective phase position of the differences a _i , with α ₁ = Δq (0 °) etc.

From the phase shift φ or Δφ (t _L ) can be for object distances d, which are smaller than half the wavelength λ of the modulation frequency d ≤ λ / 2, determine a distance in a known manner. d = Δφ (t _L ) λ / 2π × 1/2

In 7 is a complete set of distance measurement with four phase angles of 0 °, 90 °, 180 ° and 270 °. In the illustrated case, charge carriers are integrated in each case via modulation periods and in each phase position a value corresponding to the charge difference a ₁ , a ₂ , a ₃ , a _{4 is} read, from which a phase shift and a corresponding distance value can be determined as already described.

8th shows two consecutive measurement measurement windows or frames i, i + 1. In each frame i, a phase shift Δφ (t _L ) or distance d is determined from charge Δq or voltage differences ΔU determined in each phase position. The respective phase positions are shown for simplicity only with an envelope of the modulation signal.

To shorten the measuring time and thus a higher temporal resolution of the distance values, it is provided according to the invention to reduce the number of phase measurements by utilizing a measurement redundancy.

As already stated, the four phase measurements are used primarily to compensate for asymmetries of the real circuit.

In an ideal symmetric case applies Δq (90 °) = -Δq (270 °) ⇔ α ₂ = -α ₄ Δq (0 °) = -Δq (180 °) ⇔ α ₁ = -α ₃

The above relationship can thus be described in the symmetrical case as follows:

In a real measurement, however, measurements on different phase angles show differences: Δasym = Δq (0 °) + Δq (180 °) = Δq (90 °) + Δq (270 °) Δasym = α ₁ + α ₃ = α ₂ + α ₄

These asymmetry differences Δasym for the 0 ° / 180 ° and the 90 ° / 270 ° phase measurements are essentially the same, so that the distance determination according to the invention can be reduced by the measurement of a phase position:

Of course, not only the fourth, but each phase can be substituted.

9 shows a measurement according to the invention with three phase angles whose overall measurement time is shortened by the measurement duration of the fourth phase position. The measuring time is thus opposite to the in 8th shown four-phase measurement by 25% shorter.

In another in 10 In the embodiment shown, it is conceivable that a different phase, in this case 90 °, is substituted in a subsequent distance measurement. Also, it does not depend on the order of the phase measurements.

Furthermore, the phase angles do not have to be forced to begin with the phase angle 0 °. For example, phase positions shifted by a fixed angle are conceivable, for example 30 °, 210 °, 120 °, 300 °. General φvar = φvar + δφ with δφ ε [0 °, 360 °].

This concept of equidistant phase measurement opens up new possibilities for increasing the frame rate and / or reducing energy consumption, while the signal-to-noise ratio remains almost unchanged.

The autocorrelation function itself is over-determined by the four measurements. With the aid of the redundancy α ₁ + α ₃ = α ₂ + α ₄ , a measurement can be dispensed with. This can significantly increase the frame rate and / or reduce power consumption.

In addition to a high precession of the distance measurement, speed (framerate) and energy consumption are increasingly important for applications. Main energy consumers in the camera systems are the light sources that have to provide a minimum power in order to guarantee an acceptable signal-to-noise ratio. The frame rate is limited by the exposure time and the readout time. The procedure according to the invention reduces the energy consumption by 25% or, alternatively, the frame rate can be increased by 25%.

Another important advantage of this method is that the dynamic range of the camera can be extended by shifting the analogue reference point. Assuming that the phase which has the strongest negative signal is always eliminated, the reference point can be shifted down without any disadvantages. The other three phases are up to 50% more dynamic range available

In 11 For example, the voltage differences .DELTA.U for a measurement with four phase angles are shown. The digitization scope is specified with 0-4 V. To represent all charge or voltage differences, starting from the analog voltage reference AGND, which is set to 2 volts, +/- 2 volts are available.

For the procedure according to the invention, one of the phase measurements is particularly advantageously substituted by a high difference signal. So in this example, the 90 ° or 270 ° phase measurement. This results from the fundamental consideration that small differential signals are more sensitive to phase changes than large differential signals. By substituting one of the larger difference signals, the phase-sensitive signals remain in the calculation and ensure higher measurement accuracy.

Another advantage is in 12 shown. By substituting one of the phase positions with a very high voltage difference, in this case the 270 ° phase position, a larger voltage range can be made available for the remaining phases. In the example shown, a voltage range of .DELTA.U = 3 V instead of .DELTA.U = 2 V in the four-phase measurement is now available for the 90.degree. Phase position.

This can be achieved in particular if the desired working range is less than or equal to one quarter of the uniqueness range, which is often the case for near-field applications. Typically, the analog reference voltage (AGND) is in the middle of the conversion range to guarantee a symmetric maximum dynamic range. In the asymmetric 3-phase case without the last phase, the dynamics can be significantly increased as shown.

13 shows a curve of a phase error as a function of the phase shift Δφ (t _L ) of the received modulated signal or the distance traveled by the light or object distance d. Due to the periodicity, the signal is unique only up to 360 ° or up to the uniqueness range d _EB . Of the Uniqueness range d _EB essentially corresponds to the wavelength of the modulation signal or, if the object distance is half the wavelength, when the object distance is considered purely.

The phase error is generally calculated via the error propagation law

With da _i as angle error and dφ as total phase error.

For a measurement with four phase positions, a phase error of, for example, results for the so-called cos autocorrelation function dφ (4P) = 2 / Ada

And for three phases

In the 13 and 15 the phase errors are normalized to dφ / da and the simplicity factor 2 / A is set to one. Like the formulas and the 13 can be seen, the phase error dφ (4P) for the four-phase measurement over all phase shift Δφ (t _L ) is constant, while the phase error dφ (3P) for the three-phase measurement varies substantially sinusoidally.

The three-phase error is minimal when the substituted phase has a large signal. This behavior is exemplary in 14 represented in which the differential voltages .DELTA.U for the minima a, c and maxima b, d are shown. If there is no phase shift of the signal, the differential voltages are as in 14 (a) shown for the 0 ° - and complementary 180 ° phase position maximum and the "orthogonal" 90 ° and 270 ° phase position minimal or ideally zero. In a simplified view it can be assumed that the minimum differential voltages have a higher information content over the phase positions than the larger differential voltages. By substituting one of the larger differential voltages, the information content decreases insignificantly insofar as it is also reflected in the error propagation consideration.

In the illustrated examples according to the 13 . 14 and 15 was consistently substituted for the 180 ° phase. Due to this consistent substitution, there are phase shifts in which the substitution has a favorable or less favorable effect.

Starting from the unshifted signal according to 14 (a) With increasing phase shift, the charges distribute equally to both channels, so that the difference voltage to the 0 ° and 180 ° phase position, as in 14 (b) shown, add to zero. The orthogonal 90 ° and 270 ° phase positions, however, have a maximum differential voltage.

As described above, the low differential voltages include the largest phase information. By substituting the 180 ° phase position, the system "loses" phase information. This behavior is reflected in 13 by a maximum phase error E _max at 90 ° phase shift.

With increasing phase shift, the differential voltages change their sign in the result, however, the error consideration remains the same. At 180 ° phase shift of the phase error in the point (c) is then again minimal and at 270 ° in the point (d) maximum, and then at 360 ° again to approach the situation according to point (a).

The in 13 The course shown represents the basically existing fluctuation of the phase error. If the phase errors are already small in their absolute magnitude, then the fluctuations additionally impressed by the 3-phase measurement can generally be neglected. Depending on the application or the desired accuracy, however, phase error tolerances E _{tol can be} specified.

For example, it may be provided to tolerate half the maximum phase error, as E _tol = E _max / 2. The consequence of the choice of this limit range is that the distance values d have only partwise a tolerated phase error E _tol , namely in the intervals [0, 45], [135, 225] and [315, 360]. In a simple embodiment, it can be provided to use the light transit time sensor only for the interval [0.45] or from zero to 1/8 of the uniqueness range d _EB .

However, this area may be as in 15 shown by, for example, the phase of the illumination is shifted by 45 °. Due to the symmetry of the phase error is thus the effective measuring range of 1/8 to _^1/4 of the unambiguity area d _EB doubled.

Of course, the tolerances and intervals can be changed depending on the application. In particular, the absolute measuring ranges can be predetermined by selecting a suitable modulation frequency or its wavelength.

Instead of constantly shifting the phase of the illumination by 45 ° and thereafter performing measurements in the phase positions 0 °, 90 °, 180 ° and 270 °, according to the invention a phase is substituted, it is equally possible the measurements at phase angles of 45 °, 135 °, 225 ° and 315 °.

Thus, it is fundamentally possible to find an optimum phase position for a distance value within a distance interval, in which phase measurement error E is minimal or the voltage difference ΔU is maximum. For the measurement with three phases, either this phase position or the shifted by 180 ° complementary phase position is substituted. The other two phase angles are +/- 90 ° around the optimum phase angle. The optimum phase position for the mean distance value in the range interval is preferably determined.

According to the above example, a distance interval of [0, d _EB / 4] or measured in phase shift of [0 °, 90 °] is given. Preferably, the phase position for the average value is thus optimized here d _EB / 8 or 45 °. For this point, the phase error for the phase positions 45 ° and the complementary phase position 225 ° is minimal and the voltage difference .DELTA.U maximum. When the 45 ° phase has been substituted, the three phase measurements at 225 ° and 225 ° +/- 90 ° = 315 ° u. 135 ° performed. In the case of substitution of the 225 ° phase position, the same phase positions result due to the periodicity for the orthogonal phases namely 45 ° +/- 90 ° = 135 ° and 315 °.

Furthermore, it is conceivable to exchange the phase position to be substituted as soon as the voltage difference .DELTA.U is no longer minimal. If this is determined, for example, the subsequent measurement could be carried out with a substitution of one of the phase positions shifted by 90 °. So according to the example 15 could be substituted at a phase shift of 100 °, the phase angle with 135 °.

LIST OF REFERENCE NUMBERS

Time of flight camera system

10

lighting module

12

lighting

20

Receiver, light time camera

22

Transit Time Sensor

27

evaluation

30

modulator

35

Phase shifter, lighting phase shifter

38

Modulation controller

400

evaluation

φ, Δφ (t _L )

term-related phase shift

φ _var

phasing

φ ₀

base phase

M ₀

modulation signal

p1

first phase

p2

second phase

Sp1

Transmission signal with first phase

sp2

Received signal with second phase

Ga, Gb

integration node

Ua, Ub

Voltages at the integration node

.DELTA.U

voltage difference

.DELTA.Q

charge difference

a _i

Charge / voltage differences to different phase angles

Δasym

asymmetry difference

d

subject Distance

AGND

Analog Voltage Reference

Claims (5)

Method for operating a light transit time camera system ( 1 ), Comprising the steps of: - (transmission and reception of a modulated light S _p1, S _p2), - demodulating the received modulated light (S _p2) with a modulation signal (M _0), - wherein the sending and the demodulation with three different phase positions (φ _var ) of the modulation signal (M ₀ ) are performed - and wherein a first and second phase position (φ _var ) are shifted by 180 ° to each other, and a third phase position (φ _var ) is shifted by 90 ° to one of the first two phase positions Determining a distance-dependent difference quantity (Δq, ΔU) for each phase position (φ _var ), determining a distance value Δφ (t _L ), d) from the determined difference values Δq, ΔU, characterized in that in an algorithm for Distance calculation is based on four phase positions, one of the phase positions with the largest difference size (.DELTA.q, .DELTA.U) is substituted, and the distance measurement and calculation with the remaining three Pha senlagen takes place. Method according to Claim 1, in which an analog reference voltage (AGND) is adapted to the substituted differential quantity (Δq, ΔU). Method according to one of the preceding claims, in which, for a preferred distance measuring interval, a phase substitution is set in such a way that the phase error within this distance interval is minimal. The method of claim 3, wherein the preferred range-finding interval is less than or equal to a quarter of the unambiguity range (d _EB ). Time of flight camera system, with a light transit time sensor ( 22 ), which has at least one receiving pixel and with a light source ( 12 ), with a modulator ( 30 ), which with the light transit time sensor ( 22 ) and the light source ( 12 ) is connected to an evaluation unit ( 27 ) used to determine a distance based on signals of the light transit time sensor ( 22 ) in three different phase position (φ _var ) and for the implementation of one of the methods mentioned in claims 1 to 4 is configured.

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