EP4630845A1

EP4630845A1 - Circuitry, method, computer program and camera

Info

Publication number: EP4630845A1
Application number: EP23821604.8A
Authority: EP
Inventors: Simon GLINE; Fabian Krome; Nicolangelo LOPEZ; Gaetan Koers
Original assignee: Sony Depthsensing Solutions NV SA; Sony Semiconductor Solutions Corp
Current assignee: Sony Depthsensing Solutions NV SA; Sony Semiconductor Solutions Corp
Priority date: 2022-12-09
Filing date: 2023-12-08
Publication date: 2025-10-15
Also published as: WO2024121420A1

Abstract

The disclosure pertains to a circuitry for multi-illumination offset calibration for a time-of-flight module. The time-of-flight module includes a first illuminator, a second illuminator and a time- of-flight image sensor. The circuitry is configured to determine, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, and wherein the error image data are obtained by a set of time-of-flight measurements performed with the time-of-flight module; and apply the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

Description

CIRCUITRY, METHOD, COMPUTER PROGRAM AND CAMERA

TECHNICAL FIELD

The present disclosure generally pertains to a circuitry, a method, a computer program and a camera.

TECHNICAL BACKGROUND

It is generally known to perform time-of-flight (ToF) sensing. ToF sensing includes emitting a modulated light signal into a scene, receiving light from the scene with a plurality of pixels of a ToF image sensor and generating a depth map of the scene based on the received light. A distance associated with each pixel of the depth map is determined based on a time elapsed between emitting the modulated light signal and receiving a reflection of the modulated light signal from the scene by a corresponding pixel of the ToF image sensor.

It is further known to emit a modulated light signal for ToF sensing from a plurality of illuminators. There may be a temporal offset between modulated light signals emitted from the plurality of illuminators. Therefore, there is a need for calibrating an offset between illuminators.

Although there exist techniques for multi-illumination offset calibration, it is desirable to provide an improved circuitry and method for multi-illumination offset calibration.

SUMMARY

According to a first aspect, the disclosure provides a circuitry for multi-illumination offset calibration for a time-of-flight module, the time-of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the circuitry being configured to: determine, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and apply the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multiillumination offset calibration.

According to a second aspect, the disclosure provides a method for multi-illumination offset calibration for a time-of-flight module, the time-of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

According to a third aspect, the disclosure provides a computer program comprising program code causing, when being carried out on a computer, a computer to perform a method for multiillumination offset calibration for a time-of-flight module, the time-of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

According to a fourth aspect, the disclosure provides a camera, comprising: a time-of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor; and a delay unit configured for delaying an emission of a light signal by at least one of the first illuminator and the second illuminator according to a phase correction determined according to a method for multi-illumination offset calibration for the time-of-flight module, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

Further aspects are set forth in the dependent claims, the drawings and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are explained by way of example with respect to the accompanying drawings, in which:

Fig. 1 illustrates a ToF module with a plurality of illuminators with non-overlapping fields-of- illumination according to an embodiment; Fig. 2 illustrates a ToF module with a plurality of illuminators with overlapping fields-of- illumination according to an embodiment;

Fig. 3 illustrates a ToF module and a corresponding gradient error image according to an embodiment;

Fig. 4 illustrates examples of a bad correction and of a good correction by a gradient error correction;

Fig. 5 illustrates a circuit for multi-illumination offset calibration according to an embodiment;

Fig. 6 illustrates a method for multi-illumination offset calibration according to an embodiment;

Fig. 7 illustrates error image data in a multi-illumination offset calibration for an indirect time- of-flight module according to an embodiment; and

Fig. 8 illustrates an embodiment of a general-purpose computer.

DETAILED DESCRIPTION OF EMBODIMENTS

Before a detailed description of the embodiments under reference of Fig. 5 is given, general explanations are made.

As mentioned in the outset, time-of-flight (ToF) sensing includes emitting a modulated light signal into a scene, receiving light from the scene with a plurality of pixels of a ToF image sensor and generating a depth map of the scene based on the received light. A distance associated with each pixel of the depth map is determined based on a time elapsed between emitting the modulated light signal and receiving a reflection of the modulated light signal from the scene by a corresponding pixel of the ToF image sensor.

Further, a modulated light signal for ToF sensing may be emitted from a plurality of illuminators. There may be a temporal offset between modulated light signals emitted from the plurality of illuminators. Therefore, there is a need for calibrating an offset between illuminators.

In some instances, a time-of-flight (ToF) sensing solution includes an active light source and a ToF sensor. In some instances, a wide field-of-view (WFoV) sensing solution based on ToF requires several ToF sensors with dedicated active light sources.

However, WFoV ToF sensing solutions with a single ToF sensor have been developed that include several active light sources configured to illuminate a portion of a field-of-view (FoV) of the single ToF sensor (multi-illumination WFoV ToF module). Fig. 1 illustrates a ToF module 1 with a plurality of illuminators with non-overlapping fields-of- illumination (Fol) according to an embodiment. The ToF module 1 shown on the left side of Fig. 1 is an example of a WFoV ToF sensing solution that includes several active light sources.

The ToF module 1 includes a ToF image sensor 2, a lens 3, six illuminators 4 to 9 and a control unit 10. The ToF image sensor 2 includes a pixel array. Each pixel of the pixel array detects light from a scene. The ToF image sensor 2 is arranged behind the lens 3, as indicated by the dashed line of the ToF image sensor 2, such that the lens 3 focuses light from the scene onto the ToF image sensor 2.

Each illuminator 4 to 9 emits a modulated light signal into the scene such that reflections of the modulated light signals from the scene are detected by the ToF image sensor 2. Based on the modulation of the modulated light signals, a roundtrip time of the modulated light signals and, thus, a distance can be determined. Each illuminator 4 to 9 includes a laser driver (laser diode driver (LDD)) and a vertical -cavity surface-emitting laser (VCSEL) array. The laser driver controls an operation of the VCSEL array, e.g., switches the VCSEL array on and off, based on a control signal received from the control unit 10. The VCSEL array emits the modulated light signal including a plurality of laser dots into the scene.

The control unit 10 controls an operation of the ToF module 1. For example, the control unit 10 receives, from another device, a command to perform ToF sensing, controls the illuminators 4 to 9 to emit the modulated light signals, controls the ToF image sensor 2 to detect light, generates a depth map of the scene based on the light detected by the ToF image sensor 2, and transmits the generated depth map to the other device. The control unit 10 may further include a clock for emitting the modulated light signals synchronously from all illuminators 4 to 9 and synchronizing the detected light by the ToF image sensor 2 with the modulated light signals emitted by the illuminators 4 to 9.

In the embodiment of Fig. 1, the illuminators 4 to 9 are tilted in different directions such that fields-of-illumination (Fol) of the illuminators 4 to 9 do not overlap and, hence, the illuminators 4 to 9 illuminate different portions of the WFoV of the ToF image sensor 2.

In the embodiment of Fig. 1, the modulated light signals emitted by the illuminators 4 to 9 include light pulses with a predetermined duration that are emitted at a predetermined rate. On the right side of Fig. 1, a timing diagram of one light pulse 14 to 19 of the modulated light signal emitted simultaneously by each respective illuminator 4 to 9 is illustrated, as indicated by the dashed arrows. A rising (left) edge of each light pulse 14 to 19 indicates a time when an emission of each light pulse 14 to 19 starts, and a falling (right) edge of each light pulse 14 to 19 indicates a time when the emission of the respective light pulse 14 to 19 ends.

However, as shown in the timing diagram, the light pulses 14 to 19 are emitted with a time delay between each other. In the embodiment of Fig. 1, the light pulse 15 emitted by the illuminator 5 is emitted the fastest (i.e., first), and the light pulse 14 emitted by the illuminator 4 is emitted the slowest (i.e., last), as indicated by the corresponding dashed lines. The time delay between the light pulses 14 to 19 may, for example, be caused by variations between components of the ToF module 1, e.g., by different lengths and/or widths or by different capacities and/or resistances of circuitry or wiring. Such variations may be stochastic within a range and may not be controllable during manufacturing. For example, even if the ToF module 1 is designed such that a wiring that transmits the control signals from the control unit 10 to the laser drivers of the illuminators 4 to 9 has a same length for all illuminators 4 to 9, the variations (and the time delay) may occur and may not be correlated with a position of the illuminators 4 to 9 on the ToF module 1. In some embodiments, the time delay between the laser pulses 14 to 19 may be larger than a noise that occurs during operation.

Fig. 2 illustrates a ToF module 20 with a plurality of illuminators with overlapping Fol according to an embodiment. The ToF module 20 is another example of a multi-illumination WFoV ToF module. The ToF module 20 includes a ToF image sensor 21, a fisheye lens 22, a left illuminator 24, a right illuminator 25 and a control unit 26.

In general, the ToF module 20 is configured similarly to the ToF module 1 of Fig. 1. The ToF image sensor 21 receives, through the fisheye lens 22, light from a scene within a wide field-of- view (WFoV) 23 of the ToF module 20.

The left illuminator 24 includes a laser driver 24a and a VCSEL array 24b. The laser driver 24a controls an operation of the VCSEL array 24b according to a control signal from the control unit 26. The VCSEL array 24b emits a modulated light signal into its field-of-illumination (Fol) 24c.

Likewise, the right illuminator 25 includes a laser driver 25a and a VCSEL array 25b. The laser driver 25a controls an operation of the VCSEL array 25b according to a control signal from the control unit 26. The VCSEL array 25b emits a modulated light signal into its Fol 25c.

The Fol 24c and 25c of the illuminators 24 and 25 are tilted such that the Fol 24c and 25c cover the whole WFoV 23 of the fisheye lens 22. A tilt angle at which the illuminators 24 and 25 are arranged results in a region where the Fol 24 and 25 overlap. Therefore, in contrast to the ToF module 1 of Fig. 1, a portion of the WFoV 23 of the ToF module 20 is illuminated by both illuminators 24 and 25.

The control unit 26 has a similar function as the control unit 10 of Fig. 1. A detailed description of the control unit 26 is therefore omitted. A control signal generated by the control unit 26 for controlling the illuminator 24 is transmitted to the laser driver 24a via pads 27a and 27b and wiring 27c, and a control signal generated by the control unit 26 for controlling the illuminator 25 is transmitted to the laser driver 25a via pads 28a and 28b and wiring 28c.

As mentioned, although, in some embodiments, a multi-illumination system (including a plurality of illuminators) is triggered by a control unit and/or a ToF image sensor (through one or more laser drivers), a time spread for light pulses emitted by each illuminator can be observed, as shown in more detail in Fig. 3 below.

Fig. 3 illustrates the ToF module 1 and a corresponding gradient error image 34 according to an embodiment. The ToF module 1 corresponds to the ToF module 1 of Fig. 1 with the six illuminators 4 to 9. The Fol 31 of the illuminator 6, the Fol 32 of the illuminator 5 and the Fol 33 of the illuminator 4 are exemplarily shown in Fig. 3.

The gradient error image 34 is an example of an error image and indicates a phase offset measured for light signals in ToF measurements with the ToF module 1. Each dot 35a, 36a, 37a in the gradient error image 34 corresponds to one light signal detected by one pixel of the ToF image sensor 2. The abscissa of the gradient error image 34 indicates a pixel row in the pixel array of the ToF image sensor 2. The ordinate of the gradient error image 34 indicates a phase offset (i.e., a time difference between emitting a light signal and detecting a reflection of the light signal). Dots 35a, 36a, 37a in a same column (i.e., above each other) of the gradient error image 34 correspond to different measurements of a same pixel of the ToF image sensor 2, i.e., to light signals detected by the pixel at different times. The gradient error image 34 exemplarily shows data of one pixel column (perpendicular to the pixel rows) in the pixel array of the ToF image sensor 2.

As mentioned above, the Fol of the illuminators 4 to 9 of the ToF module 1 do not overlap. Therefore, each pixel of the ToF image sensor 2 detects a light signal from one corresponding illuminator 4 to 9. Dots 35a in a region 35 of the gradient error image 34 correspond to a light signal emitted by the illuminator 6 of the ToF module 1 into the Fol 31, driven by a laser diode driver LDD1. Dots 36a in a region 36 of the gradient error image 34 correspond to a light signal emitted by the illuminator 5 of the ToF module 1 into the Fol 32, driven by a laser diode driver LDD2. Dots 37a in a region 37 of the gradient image 34 correspond to a light signal emitted by the illuminator 4 of the ToF module 1 into the Fol 33, driven by a laser diode driver LDD3.

As can be seen in the gradient error image 34, a phase offset detected by a same pixel can vary between measurements in some embodiments.

The gradient error image 34 also shows a difference of the average phase offset of the Fol 31 to 33 driven by the respective laser diode drivers LDD1 to LDD3. The difference of the phase offset of the Fol 31 to 33 results in a time delay of each respective illuminator 4 to 6, which, in some instances, cannot be calibrated with conventional correction techniques.

A conventional correction technique (gradient error correction) is based on a correction curve 38 extrapolated by fitting a polynomial curve to the error to correct, i.e., to the data points illustrated as dots 35a, 36a and 37a in the gradient error image 34.

Fig. 4 illustrates examples of a bad correction and of a good correction by a gradient error correction. Fig. 4 shows two gradient error images 41 and 43, which are formatted like the gradient error image 34 of Fig. 3.

The gradient error image 41 is an example of a bad correction. A curve 42 fitted by gradient error correction does not approximate the data points well. The curve 42 is only good (i.e., approximates an average phase offset) in a middle portion of a pixel row (at a middle pixel), but increases an error in an outer portion of the pixel row (outside of the middle pixel). The gradient error correction of the gradient error image 41 does thus not bring a benefit.

The gradient error image 43 is an example of a good correction. A curve 44 fitted by gradient error correction approximates the data points well in all portions of a pixel row. Thus, the gradient error correction of the gradient error image 43 brings a benefit and helps to decrease an error.

Accordingly, the curve 38 does not approximate the measured data points 35a, 36a and 37a of the gradient error image 34 of Fig. 3 sufficiently well. Hence, the conventional gradient error correction is not capable of correcting a phase difference between several illuminators of a multiillumination ToF module in some embodiments.

However, in some embodiments, a multi-illumination ToF system that counts several light sources (wherein each light source is driven by its own laser driver exclusively), such as the ToF module 1 or the ToF module 20, may have considerable time spread for each light pulse (light signal) emitted by any one of the light sources. In some instances, where Fol of multiple illuminators of a multi-illumination ToF system are arranged such that at least two Fol overlap in a FoV of a ToF image sensor of the multiillumination ToF system (and, thus, also in a region of a ToF image acquired by the ToF image sensor), a time delay between the multiple illuminators of the multi-illumination ToF system needs to be evaluated by testing them independently to prevent interference between the illuminators, which would render the evaluation very complex.

In some instances, performing multiple calibration acquisitions to evaluate each illuminator independently requires a very robust phase delay measurement method to not introduce additional errors.

Consequently, some embodiments of the present disclosure pertain to a circuitry for multiillumination offset calibration for a time-of-flight (ToF) module, wherein the ToF module includes a first illuminator, a second illuminator and a ToF image sensor, wherein the circuitry is configured to: determine, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, wherein the error image data are obtained by a set of ToF measurements performed with the ToF module; and apply the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multiillumination offset calibration.

The circuitry may include any entity capable of performing a multi-illumination offset calibration, e.g., capable of executing an algorithm for performing a multi-illumination offset calibration. For example, the circuitry may include a processor such as a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), a reduced instruction set computer (RISC) and/or a complex instruction set computer (CISC). The circuitry may further include a volatile and/or non-volatile storage that stores instructions to be executed by the processor and/or data relating to the multi-illumination offset calibration.

The circuitry may be provided separate from the ToF module, e.g., in a general-purpose computer, or may be provided within the ToF module or in a camera or mobile device that includes the ToF module.

For example, the ToF module may be configured to perform indirect time-of-flight (iToF) measurements. The ToF module may include further illuminators in addition to the first illuminator and the second illuminator. The illuminators of the ToF module may be configured as active light sources. Each illuminator of the ToF module may include, for emitting a light signal, an array of vertical -cavity surface-emitting lasers (VCSELs) and/or a light-emitting diode (LED). Each illuminator may emit the light signal into a respective field-of-illumination (Fol) associated with the illuminator.

The light signal may, for example, include an array of laser dots and/or may be spread over an area (e.g., solid angle) with a predetermined intensity distribution (e.g., Gaussian or homogeneous distribution). The light signal may be a modulated light signal, e.g., based on emitting light pulses (e.g., laser pulses) at a predetermined rate.

Each illuminator of the ToF module may further include a driver (e.g., laser driver / laser diode driver) that controls an operation of emitting a light signal by the illuminator based on a control signal. The ToF module may include a control unit that generates a control signal for the illuminators of the ToF module to emit a light signal into their respective Fol in a synchronized manner.

The ToF image sensor may include a pixel array with a plurality of pixels and detect reflections of the light signals emitted by the illuminators of the ToF module from a scene. For example, the ToF image sensor may have a wide field-of-view (WFoV) and the Fol of each illuminator of the ToF module may illuminate a respective portion of the WFoV of the ToF image sensor. The Fol of the illuminators may or may not overlap within the WFoV of the ToF image sensor. The ToF module may further include a lens that collects light from the WFoV of the ToF image sensor and focuses the collected light on the pixel array of the ToF image sensor. The ToF image sensor may generate ToF image data, for example, based on storing charges generated by light received by the ToF image sensor in an alternating one of a plurality of floating diffusions according to a modulation rate of the light signals emitted by the illuminators of the ToF module and generating the ToF image data based on a ratio between charges stored in the plurality of floating diffusions after a plurality of light pulses (e.g., laser pulses) emitted by the illuminators. The alternating of the plurality of floating diffusions for storing the generated charges may be based on the control signal generated by the control unit of the ToF module and may be synchronized with the modulated light signals emitted by the illuminators of the ToF module. Examples of the ToF module include the ToF module 1 of Fig. 1 and the ToF module 20 of Fig. 2.

Although an emission of light signals by the illuminators of the ToF module and a detection of light by the ToF image sensor may be based on the control signal generated by the control unit for synchronization, the illuminators may emit the light signals with a phase difference between the illuminators, as described above. The multi-illumination offset calibration may be concerned with compensating for the phase difference between the illuminators such that the ToF image sensor detects a same phase offset for light signals from all illuminators of the ToF module. For example, the phase offset may correspond to a difference between a ground truth (e.g., an expected phase of an incoming light signal) and a measured phase of an incoming light signal.

The error image data may include a plurality of data points. Each data point may represent a light signal (e.g., a laser pulse) detected by the ToF image sensor and may indicate a pixel in the pixel array (e.g., a row and/or column in the pixel array) that has detected the light signal and may indicate a phase offset (e.g., a time elapsed between emitting the light signal from an illuminator of the ToF module and detecting a reflection of the light signal by the ToF image sensor of the ToF module (modulo a modulation period of the modulated light signal)) of the received light signal. The error image data may, for example, correspond to a gradient error image such as the gradient error image 34 of Fig. 3. For example, the error image data may be represented in a storage of the circuitry as a list, as a table, as an image and/or as a three-dimensional (3D) model.

The set of ToF measurements for obtaining the error image data may be based on emitting modulated light signals (e.g., light pulses) with the first illuminator and the second illuminator into a scene, detecting reflections of the emitted light signals with the ToF image sensor and determining a phase offset of each detected light pulse of the light signals. The set of ToF measurements may be performed within a predetermined and/or known setup. For example, the set of ToF measurements may be performed with a predetermined (known) distance between the ToF module and/or each illuminator of the ToF module and a scene (e.g., a screen) reflecting the light signals from the illuminators of the ToF module. For example, for the set of ToF measurements, a distance traveled by a light signal from an illuminator of the ToF module to the scene and back to the ToF image sensor of the ToF module may be compared to a target ground truth for the respective illuminators of the ToF module. The target ground truth may be computed based on a geometry of the ToF module, on a geometry of the scene, and/or on a distance between the ToF module and the scene. The target ground truth may be used as a reference for comparing phase offsets of the illuminators. For example, the phase offsets of the illuminators may correspond to a deviation (error) between the ground truth and the measured distance, roundtrip time or received phase for the respective illuminators. The phase difference may correspond to a difference between the phase offsets of the respective illuminators.

Determining the phase difference based on the ground truth makes the calibration less dependent on mechanical alignment accuracies in some embodiments.

For ensuring the accuracy of the phase measurements, specific ToF errors may be calibrated prior to analyzing the phase offset due to a time delay difference. For example, a noise impact may be removed/reduced, other types of errors of the camera (e.g., of the ToF image sensor) may be removed/reduced and/or a controlled setup may be used to find the ground truth.

The phase correction for the first illuminator with respect to the second illuminator may correspond to a phase difference between a light signal emitted by the first illuminator and a light signal emitted by the second illuminator. For example, the phase correction may indicate a (positive or negative) delay that should be applied to the first illuminator or to the second illuminator in order to compensate for the phase difference between the first illuminator and the second illuminator. The phase difference may correspond to a difference between a point in time at which the first illuminator emits a light signal (e.g., a laser pulse) based on a control signal from the control unit and a point in time at which the second illuminator emits a light signal (e.g., a laser pulse) based on the control signal from the control unit. The phase difference may be determined based on a difference between phase offsets of light signals from the first illuminator and phase offsets of light signals from the second illuminator. For example, the phase difference may be determined based on a difference between an average phase offset of a plurality of light signals from the first illuminator and an average phase offset of a plurality of light signals from the second illuminator.

The reference illuminator may be an illuminator with respect to which the phase correction for the first illuminator is determined.

The applying of the determined phase correction may include any processing capable of compensating for the phase difference between the first illuminator and the second illuminator. The compensation for the phase difference between the first illuminator and the second illuminator may include reducing, minimizing or eliminating the phase difference between the first illuminator and the second illuminator such that the phase difference is reduced below a predefined threshold, e.g., that a difference between an average phase offset of a plurality of light signals from the first illuminator and an average phase offset of a plurality of light signals from the second illuminator is not larger than the predefined threshold. The predefined threshold may, for example, be based on a standard deviation of phase offsets detected for a plurality of light signals from the first illuminator and/or of light signals from the second illuminator. For example, the compensation may include providing the control signal generated by the control unit of the ToF module earlier to one of the first illuminator and the second illuminator than to the other one of the first illuminator and the second illuminator. For example, the compensation may include causing at least one of the first illuminator and the second illuminator to emit a light signal with a delay after receiving a control signal from the control unit for emitting a light signal. For example, the ToF module may include a delay register associated with at least one of the first illuminator and the second illuminator, and applying of the determined phase correction may include configuring the delay register to cause a delay in emitting a light signal from the associated first and/or second illuminator according to the determined correction.

Therefore, in some embodiments, after applying the determined phase correction, the phase difference between the first illuminator and the second illuminator may be compensated and an offset of the first illuminator and the second illuminator may be calibrated.

In some embodiments, the circuitry is further configured to determine, based on the error image data, whether the phase difference meets a predefined criterion; and determine and apply the phase correction in the case that the phase difference does not meet the predefined criterion.

For example, the circuitry may terminate the multi-illumination offset calibration if it determines that the phase difference meets the predefined criterion, and may proceed with determining and applying the phase correction if it determines that the phase difference does not meet the predefined criterion.

After determining and applying the phase correction, the circuitry may again determine whether the phase difference meets the predefined criterion and may or may not determine and apply the phase correction again based on whether the phase difference meets the predefined criterion after applying the previously determined phase correction.

Hence, by determining whether the phase difference meets the predefined criterion after applying the determined phase correction, the circuitry may determine whether applying of the determined phase correction has been successful (e.g., the phase difference between the first illuminator and the second illuminator has been compensated sufficiently by applying the phase correction) or has not been successful (e.g., an overcorrection or an undercorr ection of the phase difference between the first illuminator and the second illuminator has occurred).

For example, it may be difficult to anticipate a response of the delay register of the ToF module to a configuration according to the phase correction with sufficient precision. Therefore, an effect of applying the phase correction may be determined based on the error image data obtained after applying the determined phase correction.

For example, the predefined criterion may include a predefined threshold for a phase difference between the first illuminator and the second illuminator, and determining whether the phase difference meets the predefined threshold may include determining whether the phase difference is reduced below the predefined threshold. In some embodiments, the circuitry is further configured to perform a coarse determination of the phase correction until the phase difference meets a first subcriterion of the predefined criterion; and perform a fine determination of the phase correction until the phase difference meets a second subcriterion of the predefined criterion, wherein the second subcriterion provides a higher granularity than the first subcriterion.

For example, the circuitry may at first determine whether the phase difference meets the first subcriterion. If the phase difference meets the first subcriterion, the circuitry may determine that the phase difference meets the predefined criterion if the phase difference meets the second sub criterion.

For example, the delay register may include a coarse section for configuring the delay with a low granularity and a fine section for configuring the delay with a high granularity that is finer than the low granularity. For example, a granularity (e.g., step size) of the coarse section may be one order of magnitude lower than a granularity (e.g., step size) of the fine section, without limiting the disclosure to this relation between the low granularity and the high granularity.

For example, the circuitry may perform the coarse determination and configure the coarse section of the delay register according to a phase correction determined by the coarse determination as long as the phase difference between the first illuminator and the second illuminator does not meet the first subcriterion. If the phase difference meets the first subcriterion, the circuitry may perform the fine determination and configure the fine section of the delay register according to a phase correction determined by the fine determination until the phase difference meets the second subcriterion. If the phase difference meets the second criterion, the multi-illumination offset calibration may be successful and the circuitry may stop determining a phase correction.

In some embodiments, the first subcriterion is based on a first threshold for the phase difference and the second subcriterion is based on a second threshold for the phase difference, the second threshold being lower than the first threshold.

For example, the first threshold may be based on a granularity of the coarse section of the delay register and the second threshold may be based on a granularity of the fine section of the delay register. Thus, if a phase difference between the first illuminator and the second illuminator is determined to be lower than the first threshold, the circuitry may perform the fine determination instead of the coarse determination.

In some embodiments, the circuitry is further configured to iteratively determine and apply the phase correction until determining that the phase difference meets the predefined criterion. For example, the circuitry may, before and/or after each application of a determined phase correction, determine whether the phase difference meets the predefined criterion, and if the phase difference does not meet the predefined criterion, the circuitry may perform the determination and apply the phase correction once more.

The determination of the phase difference may be based on a previous determination of the phase difference such that the previously determined phase difference is corrected based on error image data that are obtained after applying the previously determined phase difference.

The circuitry may thus refine the phase correction each time the phase correction is determined such that the phase difference may converge to meeting (satisfying) the predefined criterion. In some embodiments, iteratively determining and applying the phase correction allows more accurate and/or more robust convergence of the phase difference to meeting the predefined criterion.

In some embodiments, the circuitry is further configured to determine, in the error image data, first data points that correspond to light signals emitted by the first illuminator and second data points that correspond to light signals emitted by the second illuminator.

For example, each data point of the first and second data points may correspond to a detection of a respective laser pulse by a respective pixel of the ToF image sensor.

The circuitry may determine the phase difference between the first illuminator and the second illuminator based on the first data points and the second data points. For example, the circuitry may determine the phase difference based on a difference between an average phase offset of the first data points and an average offset of the second data points.

In some embodiments, the determination of the first data points and the second data points is based on spatial separation of the first data points and the second data points.

For example, if the Fol of the first illuminator and the Fol of the second illuminator do not overlap within a portion of the FoV of the ToF image sensor, each pixel of a corresponding portion of the ToF image sensor may receive light signals only from one of the first illuminator and the second illuminator, but not from both.

Therefore, it may be possible to associate pixels of the ToF image sensor (and, correspondingly, light signals detected by such pixels) with either one of the first illuminator and the second illuminator, e.g., based on characteristics and a known arrangement of the first and second illuminator, of the ToF image sensor and/or of a lens that focuses light from the FoV of the ToF image sensor onto a pixel array of the ToF image sensor. Spatial separation of the first data points and the second data points may allow a faster determination of the first data points and the second data points because the first data points and the second data points may be acquired synchronously.

In some embodiments, the determination of the first data points and the second data points is based on temporal separation of the first data points and the second data points.

For example, the first illuminator may be operated for acquiring the first data points, and the second illuminator may be operated for acquiring the second data points only after the first data points have been acquired. Conversely, the second illuminator may be operated and the second data points acquired only before the first illuminator may be operated for acquiring the first data points.

Thus, the temporal separation may be based on operating the first illuminator and the second illuminator (and, accordingly, acquiring the respective first and second data points) at different times to avoid an interference between light signals from the first illuminator with light signals from the second illuminator.

Temporal separation of the first data points and of the second data points may allow distinguishing between the first data points and the second data points even in a case where a pixel of the ToF image sensor cannot exclusively be associated with one of the first illuminator and the second illuminator but may receive light signals from both the first illuminator and the second illuminator, e.g., due to an overlap of the Fol of the first illuminator and of the second illuminator.

In some embodiments, the circuitry is further configured to select the second illuminator as the reference illuminator based on the error image data.

For example, the second illuminator may have no special configuration for being selected as the reference illuminator, but a selection of the reference illuminator may be based on the error image data, for example, on a phase offset of the first data points and of the second data points, e.g., on an average phase offset of the first data points and on an average phase offset of the second data points. The selection of the reference illuminator may be based on a relation between the average phase offsets of the first data points and of the second data points. For example, the circuitry may select, as the reference illuminator, an illuminator of the ToF module whose corresponding data points in the error image data have an average phase offset that is lower or higher than average phase offsets of data points corresponding to any other illuminator of the ToF module in the error image data. For example, the circuitry may select, as the reference illuminator, an illuminator of the ToF module whose corresponding data points have an average phase offset that is closer to an average phase offset of all data points of the error image data than for any other illuminator of the ToF module.

Some embodiments pertain to a method for multi -illumination offset calibration for a ToF module, wherein the ToF module includes a first illuminator, a second illuminator and a ToF image sensor, wherein the method comprises determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, wherein the error image data are obtained by a set of ToF measurements performed with the ToF module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

For example, the method may be performed by the circuitry described above, and the method may be configured corresponding to the circuitry. Thus, any feature described above as a feature of the circuitry may correspond to a feature of the method.

Some embodiments pertain to a computer program comprising program code causing a computer to perform the method described above, when being carried out on a computer.

Some embodiments pertain to a camera that includes a ToF module that includes a first illuminator, a second illuminator and a ToF image sensor; and a delay unit configured for delaying an emission of a light signal by at least one of the first illuminator and the second illuminator according to a phase correction determined according to the method described above.

The delay unit may correspond to the delay register configured by the circuitry described above according to the determined phase correction. The delay unit may be included in the ToF module of the camera or may be provided in the camera separately from the ToF module.

For example, the method described above may be performed when manufacturing the ToF module or before, during or after assembling the camera, and the delay unit of the camera may be configured according to the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator. For example, a first, a second and possibly further ToF measurements may be performed with the ToF module, a respective ToF measurement result may be obtained, via a communication connection to the ToF module, by a circuitry that performs the method (e.g., the circuitry described above), and the delay unit may be configured with the phase correction determined by the circuitry when performing the method. For example, a manufacturer of the camera may write an indication of the determined phase correction into a storage (e.g., an electrically erasable programmable read-only memory (EEPROM) or the like) of the camera. The camera may further include a battery and/or a power input interface, and may supply the ToF module with electrical power necessary for an operation of the ToF module.

Some embodiments pertain to a non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method described above to be performed.

Some embodiments pertain to a camera that includes a ToF module that has a first illuminator, a second illuminator and a ToF image sensor; and the circuitry described above.

For example, the circuitry may perform a multi-illumination offset calibration of the ToF module as described above after the camera has been assembled. For example, the multi -illumination offset calibration of the ToF module of the camera by the circuitry may be performed by a manufacturer of the camera and/or by a user of the camera, e.g., when the camera is in a maintenance mode.

The methods as described herein are also implemented in some embodiments as a computer program causing a computer and/or a processor to perform the method, when being carried out on the computer and/or processor. In some embodiments, also a non-transitory computer- readable recording medium is provided that stores therein a computer program product, which, when executed by a processor, such as the processor described above, causes the methods described herein to be performed.

In some embodiments, an algorithm according to the disclosure allows correcting a time delay of each illuminator of a ToF module and accounting for over correction, wherein each laser driver instance may have different time delay characteristics.

Returning to Fig. 5, Fig. 5 illustrates a circuit 50 for multi-illumination offset calibration according to an embodiment. The circuit 50 is an example of a circuitry for multi-illumination offset calibration according to the disclosure. In the following, the ToF module 1 of Fig. 1 is used as an example of a ToF module for which the circuit 50 performs a multi-illumination offset calibration.

The circuit 50 includes a processor 51, a storage unit 52, an input/output (I/O) unit 53, an error image data obtaining unit 54, a separation unit 55, a reference selection unit 56, a criterion matching unit 57, a phase correction determining unit 58, a phase correction applying unit 59 and an iteration unit 60. The processor 51 controls an overall function of the circuit 50. The processor 51 obtains instructions from the storage unit 52 and executes the instructions. Executing the instructions includes reading data from the storage unit 52 and writing data into the storage unit 52.

The storage unit 52 includes a non-volatile portion in which the instructions for the processor 51 are stored and a volatile portion in which data are stored.

The I/O unit 53 receives data from a device that is provided separate from the circuit 50, e.g., from the ToF module 1, and transmits data to a device that is provided separate from the circuit 50, e.g., to the ToF module 1.

The error image data obtaining unit 54 obtains, via the I/O unit 53, error image data from the ToF module 1 and writes the obtained error image data into the storage unit 52. An example of the error image data is the gradient error image 34 of Fig. 3. The error image data are obtained by a set of ToF measurements performed with the ToF module 1, a noise impact is reduced, other types of errors of the ToF image sensor 2 are reduced and a controlled setup is used to find a ground truth for determining the phase differences indicated by the error image data. For iteratively performing a multi-illumination offset calibration, the error image data obtaining unit 54 obtains error image data in every iteration and replaces, in the storage unit 52, the error image data from the previous iteration with the error image data obtained in the current iteration as the latest error image data.

The separation unit 55 obtains the error image data from the storage unit 52 and determines, in the error image data, first data points that correspond to light signals emitted by a first illuminator of the ToF module 1 and second data points that correspond to light signals emitted by a second illuminator of the ToF module 1. The illuminator 5 of Fig. 3 is an example of the first illuminator and the dots 36a in the region 36 of Fig. 3 are an example of the first data points 36a. Likewise, the illuminator 6 of Fig. 3 is an example of the second illuminator, and the dots 35a in the region 35 of Fig. 3 are an example of the second data points 35a. The separation unit 55 writes an indication of the first data points 36a and of the second data points 35a into the storage unit 52.

The separation unit 55 includes a spatial separation unit 61 and a temporal separation unit 62.

The spatial separation unit 61 determines the first data points 36a and the second data points 35a based on spatial separation of the first data points 36a and the second data points 35a. The spatial separation is based on determining, as the first data points 36a, data points in the error image data that correspond to a light signal detected by a pixel of the ToF image sensor 2 that only receives light from a region of the FoV of the ToF image sensor 2 that is illuminated by the first illuminator 5 and by no other illuminator of the ToF module 1, and on determining, as the second data points 35a, data points in the error image data that correspond to a light signal detected by a pixel of the ToF image sensor 2 that only receives light from a region of the FoV of the ToF image sensor 2 that is illuminated by the second illuminator 6 and by no other illuminator of the ToF module 1. 1.e., the spatial separation unit 61 distinguishes between the first data points 36a and the second data points 35a based on non-overlapping portions of the Fol of the first illuminator 5 and the second illuminator 6.

The temporal separation unit 62 determines the first data points 36a and the second data points 35a based on temporal separation of the first data points 36a and the second data points 35a. The temporal separation is based on determining, as the first data points 36a, data points in the error image data that correspond to a light signal detected by the ToF image sensor 2 in response to emitting a light signal only from the first illuminator 5 and from no other illuminator of the ToF module 1, and on determining, as the second data points 35a, data points in the error image data that correspond to a light signal detected by the ToF image sensor 2 in response to emitting a light signal only from the second illuminator 6 and from no other illuminator of the ToF module 1. 1.e., the temporal separation unit 62 distinguishes between the first data points 36a and the second data points 35a based on driving the first illuminator 5 and the second illuminator 6 at mutually exclusive times.

The reference selection unit 56 selects the second illuminator 6 as the reference illuminator based on the error image data. The reference selection unit 56 obtains the error image data from the storage unit 52, determines an average phase offset of the first data points 36a and an average phase offset of the second data points 35a, determines, as the reference illuminator, the one of the first and second illuminator 5 and 6 that corresponds to the lower one of the determined average phase offsets, and writes an indication of the determined reference illuminator into the storage unit 52. In an exemplary case shown in Fig. 3, the second data points 35a have a lower average phase offset than the first data points 36a. Therefore, the reference selection unit 56 selects the second illuminator 6 as the reference illuminator.

The criterion matching unit 57 determines, based on the error image data, whether the phase difference between the first illuminator 5 and the second illuminator 6 meets a predefined criterion. The criterion matching unit 57 obtains, from the storage unit 52, the error image data obtained by the error image data obtaining unit 54 and the first and second data points 36a and 35a, determines the phase difference between the first illuminator 5 and the second illuminator 6 based on the error image data, and determines whether the phase difference meets the predefined criterion. The criterion matching unit 57 then writes to the storage unit 52 an indication of a criterion matching result that indicates whether the phase difference meets the predefined criterion.

In the case of Fig. 5, the delay register of the ToF module 1 includes a coarse section, which can be configured to delay an emission of a light signal with a low granularity, and a fine section, which can be configured to delay an emission of a light signal with a high granularity that is finer than the low granularity. Therefore, the criterion matching unit 57 is configured with a high threshold 63 and a low threshold 64. The high threshold 63 indicates whether a phase difference is lower than a step size of the coarse section (i.e., corresponds to the granularity of the coarse section), and the low threshold 64 indicates whether a phase difference is lower than a step size of the fine section (i.e., corresponds to the granularity of the fine section).

First, the criterion matching unit 57 determines whether the phase difference meets a first subcriterion of the predefined criterion by determining whether the phase difference is below the high threshold 63. If the phase difference exceeds the high threshold 63, the criterion matching unit 57 provides to the phase correction determining unit 58 and to the phase correction applying unit 59 a criterion matching result which indicates that the phase difference does not meet the first subcriterion. Otherwise, if the phase difference does not exceed the high threshold 63, the criterion matching unit 57 determines whether the phase difference meets a second subcriterion of the predefined criterion by determining whether the phase difference is below the low threshold 64. If the phase difference exceeds the low threshold 64 (but not the high threshold 63), the criterion matching unit 57 provides to the phase correction determining unit 58 and to the phase correction applying unit 59 a criterion matching result which indicates that the phase difference does not meet the second subcriterion (but meets the first sub criterion). Else, if the phase difference does not exceed the low threshold 64, either, the criterion matching unit 57 determines that the multi-illumination offset calibration has been successful, and the criterion matching unit 57 causes the circuit 50 to terminate the multi-illumination offset calibration such that no further determination and application of a phase correction is performed.

The phase correction determining unit 58 determines a phase correction for the first illuminator 5 with respect to the second illuminator 6, wherein the phase correction determining unit 58 uses the second illuminator 6 as reference illuminator. The phase correction determining unit 58 obtains from the storage unit 52 the error image data, the indication of the first and second data points 36a and 35a, the indication of the reference illuminator and the criterion matching result. The phase correction determining unit 58 determines, as the phase correction, a phase difference between the first illuminator 5 and the second illuminator 6 (i.e., a difference between the average phase offset of the first data points 36a and the average phase offset of the second data points 35a), and writes the phase correction into the storage unit 52.

As, in the case of Fig. 5, the delay register of the ToF module 1 includes the coarse section and the fine section, the phase correction determining unit 58 includes a coarse determination unit 65 and a fine determination unit 66. If the criterion matching result from the criterion matching unit 57 indicates that the phase difference exceeds the high threshold 63 and, thus, does not meet the first subcriterion, the coarse determination unit 65 determines, as the determined phase correction, a coarse phase correction in accordance with a step size of the coarse section of the delay register. If the criterion matching result from the criterion matching unit 57 indicates that the phase difference does not exceed the high threshold 63 and, thus, meets the first subcriterion, the fine determination unit 66 determines, as the determined phase correction, a fine phase correction in accordance with a step size of the fine section of the delay register.

The phase correction applying unit 59 applies the determined phase correction. The phase correction applying unit 59 obtains the determined phase correction from the storage unit 52 and configures, via the I/O unit 53, a delay register of the ToF module 1 according to the phase correction. Since, in the exemplary case described herein, the phase offset of the first illuminator 5 is higher than of the second illuminator 6, the phase correction applying unit 59 configures the delay register such that the first illuminator 5 emits a light signal delayed according to the phase correction in response to a control signal. Therefore, a phase difference between the first illuminator 5 and the second illuminator 6 is compensated by applying the phase correction, and a multi-illumination offset calibration is performed.

As, in the case of Fig. 5, the delay register of the ToF module 1 includes the coarse section and the fine section, the phase correction applying unit 59 configures the coarse section of the delay register with the determined coarse phase correction if the criterion matching result from the criterion matching unit 57 indicates that the phase difference exceeds the high threshold 63 and, thus, does not meet the first subcriterion, and the phase correction applying unit 59 configures the fine section of the delay register with the determined fine phase correction if the criterion matching result from the criterion matching unit 57 indicates that the phase difference does not exceed the high threshold 63 and, thus, meets the first subcriterion.

The iteration unit 60 then causes the circuit 50 to perform a further iteration of the multiillumination offset calibration, including an operation of the error image data obtaining unit 54, of the separation unit 55, of the reference selection unit 56, of the criterion matching unit 57, of the phase correction determining unit 58 and of the phase correction applying unit 59 as described above.

In the further iteration, the criterion matching unit 57 determines whether the previous iteration of the multi-illumination offset calibration has been successful or whether an overcorrection or an undercorr ection has occurred.

In the further iteration, the phase correction determining unit 58 determines a phase correction based on the latest error image data, and the phase correction applying unit 59 applies the latest determined phase correction.

Note that, in some embodiments, the circuit 50 includes only one of the spatial separation unit 61 and the temporal separation unit 62. For example, in an embodiment without an overlap between Fol of illuminators of a ToF module, temporal separation of data points may not be necessary, and the temporal separation unit 62 may be omitted. For example, in an embodiment with an overlap between Fol of illuminators of a ToF module, first and second data points may always be distinguished by temporal separation, and the spatial separation unit 61 may be omitted.

Note also that, in some embodiments, the reference selection unit 55 selects the reference illuminator only in a first iteration of the multi-illumination offset calibration such that the illuminator selected as the reference illuminator in the first iteration is also used as the reference illuminator in all further iterations, whereas, in some embodiments, the reference selection unit 55 selects the reference illuminator in every iteration of the multi -illumination offset calibration, e.g., based on the respective latest error image data, or selects the reference illuminator in some iterations based on a predetermined pattern.

Note also that the division of the delay register into the coarse section and the fine section in the embodiment of Fig. 5 is only provided for illustrative purposes, and that, in some embodiments, the delay register is divided into more than two sections with more than two different granularities, and that, in some embodiments, the delay register is not divided into further sections with different granularities but provides a single granularity only. Accordingly, in some embodiments, the criterion matching unit 57 is configured with further thresholds in addition to the high threshold 63 and the low threshold 64 and determines whether the phase difference meets further subcriteria, which may, for example, be based on the further thresholds, and the phase correction determining unit 58 includes further units in addition to the coarse determination unit 65 and the fine determination unit 66 for determining a phase correction based on the further thresholds. Accordingly, in some embodiments, the criterion matching unit 57 is not configured with the high threshold 63 and the low threshold 64 but, for example, with a single predefined threshold, and only determines whether the phase difference meets a predefined criterion that is not divided into subcriteria, and the phase correction determining unit 58 does not include the coarse determination unit 65 and the fine determination unit 66, but, for example, may determine a phase correction based on the single predefined threshold.

Note further that, although the circuit 50 has been described with reference to the ToF module 1, the circuit 50 can as well be applied to a multi-illumination offset calibration for the ToF module 20.

Fig. 6 illustrates a method 70 for multi-illumination offset calibration according to an embodiment. The method 70 is an example of a method for multi-illumination offset calibration according to the disclosure. In the following, the ToF module 1 of Fig. 1 is used as an example of a ToF module for which the method 70 performs a multi-illumination offset calibration. In the following description of the method 70, the method 70 is executed by the circuit 50 of Fig. 5 for illustration purposes.

At S71, the error image data obtaining unit 54 of Fig. 5 obtains error image data from the ToF module 1 via the I/O unit 53 and writes the obtained error image data into the storage unit 52. An example of the error image data is the gradient error image 34 of Fig. 3. The error image data are obtained by a set of ToF measurements performed with the ToF module 1, a noise impact is reduced, other types of errors of the ToF image sensor 2 are reduced and a controlled setup is used to find a ground truth for determining the phase differences indicated by the error image data. For iteratively performing a multi -illumination offset calibration, the obtaining of error image data at S71 obtains error image data in every iteration and replaces, in the storage unit 52, the error image data from the previous iteration with the error image data obtained in the current iteration as the latest error image data.

At S72, the separation unit 55 of Fig. 5 obtains the error image data from the storage unit 52 and determines, in the error image data, first data points that correspond to light signals emitted by a first illuminator of the ToF module 1 and second data points that correspond to light signals emitted by a second illuminator of the ToF module 1. The illuminator 5 of Fig. 3 is an example of the first illuminator and the dots 36a in the region 36 of Fig. 3 are an example of the first data points 36a. Likewise, the illuminator 6 of Fig. 3 is an example of the second illuminator, and the dots 35a in the region 35 of Fig. 3 are an example of the second data points 35a. The separation at S72 writes an indication of the first data points 36a and of the second data points 35a into the storage unit 52.

The separation at S72 includes a spatial separation at S73 and a temporal separation at S74. At S73, the spatial separation unit 61 of Fig. 5 determines the first data points 36a and the second data points 35a based on spatial separation of the first data points 36a and the second data points 35a. The spatial separation at S73 is based on determining, as the first data points 36a, data points in the error image data that correspond to a light signal detected by a pixel of the ToF image sensor 2 that only receives light from a region of the FoV of the ToF image sensor 2 that is illuminated by the first illuminator 5 and by no other illuminator of the ToF module 1, and on determining, as the second data points 35a, data points in the error image data that correspond to a light signal detected by a pixel of the ToF image sensor 2 that only receives light from a region of the FoV of the ToF image sensor 2 that is illuminated by the second illuminator 6 and by no other illuminator of the ToF module 1. 1.e., the spatial separation at S73 distinguishes between the first data points 36a and the second data points 35a based on non-overlapping portions of the Fol of the first illuminator 5 and the second illuminator 6.

At S74, the temporal separation unit 62 of Fig. 5 determines the first data points 36a and the second data points 35a based on temporal separation of the first data points 36a and the second data points 35a. The temporal separation at S74 is based on determining, as the first data points 36a, data points in the error image data that correspond to a light signal detected by the ToF image sensor 2 in response to emitting a light signal only from the first illuminator 5 and from no other illuminator of the ToF module 1, and on determining, as the second data points 35a, data points in the error image data that correspond to a light signal detected by the ToF image sensor 2 in response to emitting a light signal only from the second illuminator 6 and from no other illuminator of the ToF module 1. 1.e., the temporal separation at S74 distinguishes between the first data points 36a and the second data points 35a based on driving the first illuminator 5 and the second illuminator 6 at mutually exclusive times.

At S75, the reference selection unit 56 of Fig. 5 selects the second illuminator 6 as the reference illuminator based on the error image data. The reference selection at S75 obtains the error image data from the storage unit 52, determines an average phase offset of the first data points 36a and an average phase offset of the second data points 35a, determines, as the reference illuminator, the one of the first and second illuminator 5 and 6 that corresponds to the lower one of the determined average phase offsets, and writes an indication of the reference illuminator into the storage unit 52. In an exemplary case shown in Fig. 3, the second data points 35a have a lower average phase offset than the first data points 36a. Therefore, the reference selection at S75 selects the second illuminator 6 as the reference illuminator. At S76, the criterion matching unit 57 of Fig. 5 determines, based on the error image data, whether the phase difference between the first illuminator 5 and the second illuminator 6 meets a predefined criterion. The criterion matching at S76 obtains, from the storage unit 52, the error image data obtained at S71 and the first and second data points 36a and 35a, determines the phase difference between the first illuminator 5 and the second illuminator 6 based on the error image data, and determines whether the phase difference meets the predefined criterion. The criterion matching at S76 then writes to the storage unit 52 an indication of a criterion matching result that indicates whether the phase difference meets the predefined criterion.

In the case of Fig. 6, the delay register of the ToF module 1 includes a coarse section, which can be configured to delay an emission of a light signal with a low granularity, and a fine section, which can be configured to delay an emission of a light signal with a high granularity that is finer than the low granularity. Therefore, the criterion matching at S76 is configured with a high threshold 77 and a low threshold 78. The high threshold 77 corresponds to the high threshold 63 of Fig. 5 and indicates whether a phase difference is lower than a step size of the coarse section (i.e., corresponds to the granularity of the coarse section), and the low threshold 78 corresponds to the low threshold 64 of Fig. 5 and indicates whether a phase difference is lower than a step size of the fine section (i.e., corresponds to the granularity of the fine section).

First, the criterion matching at S76 determines whether the phase difference meets a first subcriterion of the predefined criterion by determining whether the phase difference is below the high threshold 77. If the phase difference exceeds the high threshold 77, the criterion matching at S76 provides to the phase correction determination at S79 a criterion matching result which indicates that the phase difference does not meet the first subcriterion. Otherwise, if the phase difference does not exceed the high threshold 77, the criterion matching at S76 determines whether the phase difference meets a second subcriterion of the predefined criterion by determining whether the phase difference is below the low threshold 78. If the phase difference exceeds the low threshold 78 (but not the high threshold 77), the criterion matching at S76 provides to the phase correction determination at S79 a criterion matching result which indicates that the phase difference does not meet the second subcriterion (but meets the first sub criterion). Else, if the phase difference does not exceed the low threshold 78, either, the criterion matching at S76 determines that the multi-illumination offset calibration has been successful, and the criterion matching at S76 causes the method 70 to terminate the multi-illumination offset calibration such that no further determination and application of a phase correction is performed. At S79, the phase correction determining unit 58 of Fig. 5 determines a phase correction for the first illuminator 5 with respect to the second illuminator 6, wherein the determining of the phase correction at S79 uses the second illuminator 6 as reference illuminator. The determining of the phase correction at S79 obtains from the storage unit 52 the error image data, the indication of the first and second data points 36a and 35a, the indication of the reference illuminator and the criterion matching result. The determining of the phase correction at S79 determines, as the phase correction, a phase difference between the first illuminator 5 and the second illuminator 6 (i.e., a difference between the average phase offset of the first data points 36a and the average phase offset of the second data points 35a), and writes the phase correction into the storage unit 52.

As, in the case of Fig. 6, the delay register of the ToF module 1 includes the coarse section and the fine section, the phase correction determination at S79 includes a coarse determination at S80, performed by the coarse determination unit 65 of Fig. 5, and a fine determination at S81, performed by the fine determination unit 66 of Fig. 5. If the criterion matching result from the criterion matching at S76 indicates that the phase difference exceeds the high threshold 63 and, thus, does not meet the first subcriterion, the coarse determination at S80 determines, as the determined phase correction, a coarse phase correction in accordance with a step size of the coarse section of the delay register. If the criterion matching result from the criterion matching at S76 indicates that the phase difference does not exceed the high threshold 63 and, thus, meets the first subcriterion, the fine determination at S81 determines, as the determined phase correction, a fine phase correction in accordance with a step size of the fine section of the delay register.

At S82, the phase correction applying unit 59 of Fig. 5 applies the determined phase correction. The applying of the phase correction at S82 obtains the determined phase correction from the storage unit 52 and configures, via the I/O unit 53, a delay register of the ToF module 1 according to the phase correction. Since, in the exemplary case described herein, the phase offset of the first illuminator 5 is higher than of the second illuminator 6, the applying of the phase correction at S82 configures the delay register such that the first illuminator 5 emits a light signal delayed according to the phase correction in response to a control signal. Therefore, a phase difference between the first illuminator 5 and the second illuminator 6 is compensated by applying the phase correction at S82, and a multi-illumination offset calibration is performed.

As, in the case of Fig. 6, the delay register of the ToF module 1 includes the coarse section and the fine section, the phase correction application at S82 configures the coarse section of the delay register with the determined coarse phase correction if the criterion matching result from the criterion matching at S76 indicates that the phase difference exceeds the high threshold 77 and, thus, does not meet the first subcriterion, and the phase correction application at S82 configures the fine section of the delay register with the determined fine phase correction if the criterion matching result from the criterion matching at S76 indicates that the phase difference does not exceed the high threshold 77 and, thus, meets the first subcriterion.

At S83, the iteration unit 60 of Fig. 6 then causes the method 70 to perform a further iteration of the multi -illumination offset calibration, including the obtaining of error image data at S71, the separation at S72, the reference selection at S75, the criterion matching at S76, the phase correction determination at S79 and the phase correction application at S82 as described above.

In the further iteration, the criterion matching at S76 determines whether the previous iteration of the multi-illumination offset calibration has been successful or whether an overcorrection or an undercorrection has occurred.

In the further iteration, the determining of a phase correction at S79 determines a phase correction based on the latest error image data, and the application of the phase correction at S82 applies the latest determined phase correction.

Note that, in some embodiments, the method 70 includes only one of the spatial separation at S73 and the temporal separation at S74. For example, in an embodiment without an overlap between Fol of illuminators of a ToF module, temporal separation of data points may not be necessary, and the temporal separation at S74 may be omitted. For example, in an embodiment with an overlap between Fol of illuminators of a ToF module, first and second data points may always be distinguished by temporal separation, and the spatial separation at S73 may be omitted.

Note also that, in some embodiments, the reference selection at S75 selects the reference illuminator only in a first iteration of the multi-illumination offset calibration such that the illuminator selected as the reference illuminator in the first iteration is also used as the reference illuminator in all further iterations, whereas, in some embodiments, the reference selection at S75 selects the reference illuminator in every iteration of the multi -illumination offset calibration, e.g., based on the respective latest error image data, or selects the reference illuminator in some iterations based on a predetermined pattern.

Note also that the division of the delay register into the coarse section and the fine section in the embodiment of Fig. 6 is only provided for illustrative purposes, and that, in some embodiments, the delay register is divided into more than two sections with more than two different granularities, and that, in some embodiments, the delay register is not divided into further sections with different granularities but provides a single granularity only. Accordingly, in some embodiments, the criterion matching at S76 is further thresholds in addition to the high threshold 77 and the low threshold 78 and determines whether the phase difference meets further subcriteria, which may, for example, be based on the further thresholds, and the phase correction determination at S79 includes further determinations in addition to the coarse determination at S80 and the fine determination at S81 for determining a phase correction based on the further thresholds. Accordingly, in some embodiments, the criterion matching at S76 is not based on the high threshold 77 and the low threshold 78 but, for example, with a single predefined threshold, and only determines whether the phase difference meets a predefined criterion that is not divided into subcriteria, and the phase correction determination at S79 does not include the coarse determination at S80 and the fine determination at S81, but, for example, may determine a phase correction based on the single predefined threshold.

Note further that, although the method 70 has been described with reference to the ToF module 1, the method 70 can as well be applied to a multi -illumination offset calibration for the ToF module 20.

Fig. 7 illustrates error image data 90, 93 and 97 in a multi-illumination offset calibration for an iToF module according to an embodiment. The error image data 90, 93 and 97 are formatted like the gradient error image data 34 of Fig. 3.

The error image data 90 corresponds to error image data obtained at S71 of the method 70 of Fig. 6 by the error image data obtaining unit 54 of Fig. 5 during a first iteration of a multiillumination offset calibration. A box 91 indicates second data points determined at S72 of the method 70 of Fig. 6 by the separation unit 55 of Fig. 5, which correspond to a reference illuminator that has been determined at S75 of Fig. 6 by the reference selection unit 56 of Fig. 5. A fitted curve 92 does not approximate the data points well and a phase difference between illuminators does not meet the predefined criterion.

The error image data 93 corresponds to error image data obtained at S71 by the error image data obtaining unit 54 in a further iteration after applying a phase correction at S82 by the phase correction applying unit 59. A box 94 indicates the second data points that correspond to the reference illuminator, and a box 95 indicates first data points that correspond to one or more other illuminators. A fitted curve 96 does not approximate the data points well, and the second data points, which had a lowest average phase offset in the error image data 90, have a highest average phase offset in the error image data 93. This means that an overcorrection has occurred. Therefore, the iteration unit 60 determines at S83 that the determination and application of a phase correction should be performed again.

The error image data 97 corresponds to error image data obtained at S71 by the error image data obtaining unit 54 in a further iteration after the second determination and application of a phase correction. In the error image data 97, a phase difference between all illuminators meets the predefined criterion and a fitted curve approximates all data points well. Therefore, the iteration unit 60 determines at S83 that the multi-illumination offset calibration has been successful and that no further iteration of determining and applying a phase correction should be performed.

As illustrated in Fig. 7, in some embodiments, a precision of a phase correction improves at each iteration of determining and applying the phase correction.

Note that, in some embodiments, more than two iterations are performed and that, in some embodiments, a phase difference between illuminators meets the predefined criterion after (or even before) a first iteration of determining and applying the phase correction such that only (or not even) the first iteration of determining and applying the phase correction is performed.

Fig. 8 illustrates an embodiment of a general -purpose computer 150. The computer 150 can be implemented such that it can basically function as any type of stationary device or mobile device. For example, a stationary device may include a server computer, a desktop computer, a workstation, a thin client or the like. For example, a mobile device may include a camera, a smartphone, smart glasses, a head-mounted display, a smartwatch, a mobile phone, a mobile tablet, a notebook, a terminal device or the like. The computer 150 has components 151 to 161, which can form a circuitry, such as any one of the ToF image sensor 2, the ToF image sensor 21, control unit 10, the control unit 26, the circuit 50 and/or any unit included in the circuit 50, as described herein. The computer 150 (or any one of its components 151 to 161) may be configured to perform the method 70.

Embodiments which use software, firmware, programs or the like for performing the methods as described herein can be installed on computer 150, which is then configured to be suitable for the concrete embodiment.

The computer 130 has a CPU 151 (Central Processing Unit), which can execute various types of procedures and methods as described herein, for example, in accordance with programs stored in a read-only memory (ROM) 152, stored in a storage 157 and loaded into a random-access memory (RAM) 153, stored on a medium 160 which can be inserted in a respective drive 159, etc. The CPU 151, the ROM 152 and the RAM 153 are connected with a bus 161, which in turn is connected to an input/output interface 154. The number of CPUs, memories and storages is only exemplary, and the skilled person will appreciate that the computer 150 can be adapted and configured accordingly for meeting specific requirements which arise, when it functions as a base station or as user equipment (end terminal).

At the input/output interface 154, several components are connected: an input 155, an output 156, the storage 157, a communication interface 158 and the drive 159, into which a medium 160 (compact disc, digital video disc, compact flash memory, or the like) can be inserted.

The input 155 can be a pointer device (mouse, graphic table, or the like), a keyboard, a microphone, a camera, a touchscreen, an eye-tracking unit etc.

The output 156 can have a display (liquid crystal display, cathode ray tube display, light emittance diode display, etc.; e.g., included in a touchscreen), loudspeakers, etc.

The storage 157 can have a hard disk, a solid-state drive, a flash drive and the like.

The communication interface 158 can be adapted to communicate, for example, via a local area network (LAN), wireless local area network (WLAN), mobile telecommunications system (GSM, UMTS, LTE, NR etc.), Bluetooth, near-field communication (NFC), infrared, Universal Serial Bus (USB), serial port (RS-232), parallel port (IEEE 1284), etc.

It should be noted that the description above only pertains to an example configuration of the computer 150. Alternative configurations may be implemented with additional or other sensors, storage devices, interfaces or the like. For example, the communication interface 158 may support other radio access technologies than the mentioned UMTS, LTE and NR.

In the following, general remarks are made with respect to the present disclosure.

In some embodiments, the light signals represented by the error image data have to be separated. For example, each illuminator may be isolated, so that a depth measurement can be achieved independent from the other illuminators. In case of a ToF module according to an embodiment, the illuminators do not overlap so that a region-of-interest (ROI) can be defined in each Fol (spatial separation). In other embodiments, each illuminator has to be evaluated while all other illuminators of a ToF module are deactivated (temporal separation).

In some embodiments, after separating the (data points corresponding to the) illuminators, one reference illuminator is chosen so that the other illuminator(s) can be adjusted towards the reference illuminator. For example, an illuminator with the lowest delay (average phase offset) may be selected as reference illuminator. Then, a phase difference or time delay with respect to a reference (e.g., the reference illuminator) may be computed. To get a true delay, the ToF measurements may have to be calibrated for their respective isolated errors. In an embodiment, calibrating the ToF measurements for their respective isolated errors includes gradient error correction (GEC), cyclic error correction (CEC) and temperature correction. Depending on the system, this calibration may be more or less complex.

Then, for applying a phase correction, delay registers for each illuminator except the reference illuminator may be configured according to the phase correction. However, in some embodiments, also a delay register for the reference illuminator may be configured according to the phase correction. For example, a laser diode driver (LDD) of each illuminator may include a delay register that causes a configurable delay between receiving, by the LDD, a control signal for emitting a light signal and driving a VCSEL array of the illuminator to emit the light signal.

In some embodiments, separation of illuminators in a context of calibration may impact a degree of correction that can be expected from a first iteration. Therefore, the process may be updated and repeated. The smaller a configured delay, the smaller an inherited impact of different calibrations may be. For example, in some embodiments, there may be a global gradient to be corrected, which may not easily be found if using only one illuminator or having different time delays, thus each update to the time delay calibration may improve the GEC when considering the illuminators working together. CEC may have inherently some remaining error, and the closer the time calibration, the smaller the impact of the inaccuracies may be.

In some embodiments, the disclosure allows to have a good calibration for a multi-illumination ToF system with a relatively simple calibration process that includes a feedback loop for detecting overcorr ection and/or undercorrection of a phase difference between illuminators.

Possible applications are not limited to iToF and could be extended to single-photon avalanche diode (SPAD) based types of detector or even stereo imaging WFoV systems.

Note that the ToF module 1 and/or the ToF module 20 may include a delay unit (e.g., a delay register) configured for delaying an emission of a light signal by at least one illuminator of the ToF module 1 or 20 according to a phase correction determined by the circuit 50 and/or according to the method 70. Thus, the ToF module 1 and/or the ToF module 20 may represent a camera that includes a ToF module including a first illuminator, a second illuminator and a ToF image sensor; and a delay unit configured for delaying an emission of a light signal by at least one of the first illuminator and the second illuminator according to a phase correction determined by the circuit 50 and/or according to the method 70. Note also that the circuitry 50 may be included in the control unit 10 of the ToF module 1 or in the control unit 26 of the ToF module 20, for example. Thus, the ToF module 1 and/or the ToF module 20 may represent a camera that includes a ToF module including a first illuminator, a second illuminator and a ToF image sensor; and the circuit 50.

It should be recognized that the embodiments describe methods with an exemplary ordering of method steps. The specific ordering of method steps is however given for illustrative purposes only and should not be construed as binding. For example, the ordering of S73 and S74 in the embodiment of Fig. 6 may be exchanged. For example, S76 in the embodiment of Fig. 6 may be performed after S79 or after S82. Furthermore, S80 and S81 in the embodiment of Fig. 6 may be performed during S82 instead of S79. Other changes of the ordering of method steps may be apparent to the skilled person.

Please note that the division of the circuit 50 into units 51 to 64 is only made for illustration purposes and that the present disclosure is not limited to any specific division of functions in specific units. For example, the units 65 and 66 in the embodiment of Fig. 5 may be included in the unit 59 instead of 58. For example, any one of the units 52 to 64 may be included in the processor 51 of the circuit 50. For instance, the circuit 50 could be implemented by a respective programmed processor, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) and the like.

All units and entities described in this specification and claimed in the appended claims can, if not stated otherwise, be implemented as integrated circuit logic, for example on a chip, and functionality provided by such units and entities can, if not stated otherwise, be implemented by software.

In so far as the embodiments of the disclosure described above are implemented, at least in part, using software-controlled data processing apparatus, it will be appreciated that a computer program providing such software control and a transmission, storage or other medium by which such a computer program is provided are envisaged as aspects of the present disclosure.

Note that the present technology can also be configured as described below.

(1) A circuitry for multi -illumination offset calibration for a time-of-flight module, the time- of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the circuitry being configured to: determine, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and apply the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

(2) The circuitry of (1), further configured to: determine, based on the error image data, whether the phase difference meets a predefined criterion; and determine and apply the phase correction in the case that the phase difference does not meet the predefined criterion.

(3) The circuitry of (2), further configured to perform a coarse determination of the phase correction until the phase difference meets a first subcriterion of the predefined criterion; and perform a fine determination of the phase correction until the phase difference meets a second subcriterion of the predefined criterion, the second subcriterion providing a higher granularity than the first subcriterion.

(4) The circuitry of (3), wherein the first subcriterion is based on a first threshold for the phase difference and the second subcriterion is based on a second threshold for the phase difference, the second threshold being lower than the first threshold.

(5) The circuitry of any one of (2) to (4), further configured to iteratively determine and apply the phase correction until determining that the phase difference meets the predefined criterion.

(6) The circuitry of any one of (1) to (5), further configured to determine, in the error image data, first data points that correspond to light signals emitted by the first illuminator and second data points that correspond to light signals emitted by the second illuminator.

(7) The circuitry of (6), wherein the determining of the first data points and the second data points is based on spatial separation of the first data points and the second data points.

(8) The circuitry of (6) or (7), wherein the determining of the first data points and the second data points is based on temporal separation of the first data points and the second data points. (9) The circuitry of any one of (1) to (8), further configured to select the second illuminator as the reference illuminator based on the error image data.

(10) A method for multi -illumination offset calibration for a time-of-flight module, the time- of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

(11) The method of (10), further comprising, determining, based on the error image data, whether the phase difference meets a predefined criterion; and determining and applying the phase correction in the case that the phase difference does not meet the predefined criterion.

(12) The method of (11), further comprising performing a coarse determination of the phase correction until the phase difference meets a first subcriterion of the predefined criterion; and performing a fine determination of the phase correction until the phase difference meets a second subcriterion of the predefined criterion, the second subcriterion providing a higher granularity than the first subcriterion.

(13) The method of (12), wherein the first subcriterion is based on a first threshold for the phase difference and the second subcriterion is based on a second threshold for the phase difference, the second threshold being lower than the first threshold.

(14) The method of any one of (11) to (13), further comprising iteratively determining and applying the phase correction until determining that the phase difference meets the predefined criterion.

(15) The method of any one of (10) to (14), further comprising determining, in the error image data, first data points that correspond to light signals emitted by the first illuminator and second data points that correspond to light signals emitted by the second illuminator.

(16) The method of (15), wherein the determining of the first data points and the second data points is based on spatial separation of the first data points and the second data points.

(17) The method of (15) or (16), wherein the determining of the first data points and the second data points is based on temporal separation of the first data points and the second data points.

(18) The method of any one of (10) to (17), further comprising selecting the second illuminator as the reference illuminator based on the error image data.

(19) A computer program comprising program code causing a computer to perform the method according to any one of (10) to (18), when being carried out on a computer.

(20) A camera, comprising: a time-of-flight module including a first illuminator, a second illuminator and a time-of- flight image sensor; and a delay unit configured for delaying an emission of a light signal by at least one of the first illuminator and the second illuminator according to a phase correction determined according to the method of any one of (10) to (18).

(21) A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to any one of (10) to (18) to be performed.

(22) A camera, comprising: a time-of-flight module including a first illuminator, a second illuminator and a time-of- flight image sensor; and the circuitry of any one of (1) to (9).

Claims

1. A circuitry for multi -illumination offset calibration for a time-of-flight module, the time- of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the circuitry being configured to: determine, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and apply the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

2. The circuitry of claim 1, further configured to determine, based on the error image data, whether the phase difference meets a predefined criterion; and determine and apply the phase correction in the case that the phase difference does not meet the predefined criterion.

3. The circuitry of claim 2, further configured to perform a coarse determination of the phase correction until the phase difference meets a first subcriterion of the predefined criterion; and perform a fine determination of the phase correction until the phase difference meets a second subcriterion of the predefined criterion, the second subcriterion providing a higher granularity than the first subcriterion.

4. The circuitry of claim 3, wherein the first subcriterion is based on a first threshold for the phase difference and the second subcriterion is based on a second threshold for the phase difference, the second threshold being lower than the first threshold.

5. The circuitry of claim 2, further configured to iteratively determine and apply the phase correction until determining that the phase difference meets the predefined criterion.

6. The circuitry of claim 1, further configured to determine, in the error image data, first data points that correspond to light signals emitted by the first illuminator and second data points that correspond to light signals emitted by the second illuminator.

7. The circuitry of claim 6, wherein the determining of the first data points and the second data points is based on spatial separation of the first data points and the second data points.

8. The circuitry of claim 6, wherein the determining of the first data points and the second data points is based on temporal separation of the first data points and the second data points.

9. The circuitry of claim 1, further configured to select the second illuminator as the reference illuminator based on the error image data.

10. A method for multi -illumination offset calibration for a time-of-flight module, the time- of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

11. The method of claim 10, further comprising, determining, based on the error image data, whether the phase difference meets a predefined criterion; and determining and applying the phase correction in the case that the phase difference does not meet the predefined criterion.

12. The method of claim 11, further comprising performing a coarse determination of the phase correction until the phase difference meets a first subcriterion of the predefined criterion; and performing a fine determination of the phase correction until the phase difference meets a second subcriterion of the predefined criterion, the second subcriterion providing a higher granularity than the first subcriterion.

13. The method of claim 12, wherein the first subcriterion is based on a first threshold for the phase difference and the second subcriterion is based on a second threshold for the phase difference, the second threshold being lower than the first threshold.

14. The method of claim 11, further comprising iteratively determining and applying the phase correction until determining that the phase difference meets the predefined criterion.

15. The method of claim 10, further comprising determining, in the error image data, first data points that correspond to light signals emitted by the first illuminator and second data points that correspond to light signals emitted by the second illuminator.

16. The method of claim 15, wherein the determining of the first data points and the second data points is based on spatial separation of the first data points and the second data points.

17. The method of claim 15, wherein the determining of the first data points and the second data points is based on temporal separation of the first data points and the second data points.

18. The method of claim 10, further comprising selecting the second illuminator as the reference illuminator based on the error image data.

19. A computer program comprising program code causing, when being carried out on a computer, a computer to perform a method for multi-illumination offset calibration for a time-of- flight module, the time-of-flight module including a first illuminator, a second illuminator and a time-of-flight image sensor, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.

20. A camera, comprising: a time-of-flight module including a first illuminator, a second illuminator and a time-of- flight image sensor; and a delay unit configured for delaying an emission of a light signal by at least one of the first illuminator and the second illuminator according to a phase correction determined according to a method for multi-illumination offset calibration for the time-of-flight module, the method comprising: determining, based on error image data, a phase correction for the first illuminator with respect to the second illuminator, wherein the second illuminator is used as a reference illuminator, the error image data being obtained by a set of time-of-flight measurements performed with the time-of-flight module; and applying the determined phase correction for compensating for a phase difference between the first illuminator and the second illuminator, thereby performing a multi-illumination offset calibration.