Calibration device for a time-of-flight camera system

Abstract

Calibration device (50) for calibrating time-of-flight camera systems (1) comprising one illumination (12) and one time-of-flight camera (20), with - a light guidance system (52) which has several optical waveguides (58) of different lengths or is configured to selectively integrate an optical waveguide (58) from a set of optical waveguides (58) of different lengths, - a coupling device (54) for detecting light emitted by the illumination (10) of the time-of-flight camera system (1) and for coupling this light into the optical waveguides (58) or the optical waveguide (58) and - an illumination device (56) for illuminating a time-of-flight sensor (22) of the time-of-flight camera (20) via its camera optics (25) with the light from the optical fiber (58) or the optical fiber (58), wherein the illumination device (56) has a positioning device (60) for positioning the coupling areas (62) of the optical fiber (58) or the coupling area (62) of the optical fiber (58).

Description

[0001] The invention relates to a calibration device for calibrating time-of-flight (TOF) camera systems, each comprising an illumination element and a TOF camera. The invention further relates to the use of such a calibration device for calibrating corresponding TOF camera systems. The term "TOF camera system" encompasses not only systems that determine distances directly from the time of flight, but also, in particular, all TOF camera systems that derive time-of-flight information from the phase shift of emitted and received radiation. PMD cameras with photomixing detectors (PMDs), such as those described, inter alia, in DE 197 04 496 A1 and available, for example, from ifm electronic GmbH or pmdtechnologies ag as Frame-Grabber O3D or CamBoard pico flexx, are particularly suitable as TOF cameras.The PMD camera allows, in particular, a flexible arrangement of the light source and the detector, which can be arranged both in one housing and separately.

[0002] German publication DE 44 39 298 A1 describes a calibration device for a time-of-flight camera system for its spatial calibration. The calibration device comprises a light guide system with several optical fibers of different lengths, a coupling device for coupling light from an illumination module of the time-of-flight camera system into the light guide system, and a device that assigns output coupling areas to specific pixels of a receiver of the time-of-flight camera system configured as a time-of-flight camera.

[0003] US Patent 5825464 discloses a calibration device for a lidar system in which radiation emitted by the lidar system is captured by a hollow sphere and distributed across multiple optical fibers. Bragg gratings are arranged at the ends of the optical fibers, which have different lengths, to reflect the light back and simulate a target object for the lidar system.

[0004] DE 10 2009 047 303 disclosed a calibration device for a lidar sensor, comprising a reflector module with matrix-like arrangement of reflector elements. A diaphragm mask for selecting individual reflector elements is arranged in the beam path between the lidar sensor and the reflector module. In addition to the reflector elements, a light receiver is provided, which can emit light towards the lidar sensor with a time delay via a light transmitter.

[0005] The object of the invention is to provide a simple and efficient calibration device for calibrating time-of-flight camera systems, as well as a corresponding use of this device. Using such a calibration device, it is then possible to improve the accuracy of the distance measurement of a time-of-flight camera system.

[0006] The problem is advantageously solved by the calibration device according to the invention with the features of claim 1 and the use with the features of claim 10.

[0007] The calibration device according to the invention for calibrating time-of-flight camera systems, each comprising an illumination and a time-of-flight camera, comprises (i) a light guide system having several optical waveguides of different lengths or being configured to selectively integrate an optical waveguide from a set of optical waveguides of different lengths, (ii) a coupling device for detecting light emitted by the illumination of the time-of-flight camera system and for coupling this light into the optical waveguides or the optical waveguide, and (iii) an illumination device for illuminating a time-of-flight sensor of the time-of-flight camera via its camera optics with the light from the optical waveguides or the optical waveguide, wherein the illumination device has a positioning device for positioning the coupling areas of the optical waveguides or the coupling area of ​​the optical waveguide.Calibrating the time-of-flight camera systems is, in particular, a successive calibration of the individual time-of-flight camera systems. Preferably, the time-of-flight camera systems to be calibrated are fed one after the other, for example, during the manufacturing process of these systems, to a station with the calibration device according to the invention and calibrated there against a reference.

[0008] Calibrating time-of-flight camera systems requires measuring known light transit times. This calibration device generates these different transit times using a set of optical fibers of varying lengths. By cleverly arranging the optical fibers, all necessary measurements can be performed in a single, static setup.

[0009] According to a preferred embodiment of the invention, the positioning device for positioning the coupling areas or the coupling area is arranged in the near field N of the time-of-flight camera in an image plane projected blurred onto the time-of-flight sensor by the illumination device. In this way, the light from each optical waveguide with its characteristic length is supplied to a large number of pixels of the time-of-flight camera.

[0010] In particular, it is provided that the positioning device is designed as a holding plate with openings for receiving the coupling areas of the optical fibers.

[0011] In other words, a calibration device for a time-of-flight camera system is provided, comprising a coupling device for detecting light emitted by an illumination of the time-of-flight camera system and for coupling this light into a light guidance system, wherein the light guidance system has several optical waveguides of different lengths, with an illumination device (projection device) which has a projection plate with openings for receiving the coupling areas of the optical waveguides.

[0012] According to a further preferred embodiment of the invention, the illumination device has an imaging screen downstream of the positioning device. An intermediate image is created on this screen in a desired plane.

[0013] According to a further preferred embodiment of the invention, the illumination device comprises a diffusely scattering optical element (or a diffuser of a different design) downstream of the positioning device. This diffusely scattering optical element can be located directly downstream of the positioning device or via the interposition of the imaging screen.

[0014] According to a preferred embodiment of the invention, the illumination device includes optics downstream of the positioning device. These optics of the calibration device particularly complement the camera optics of the time-of-flight camera.

[0015] According to a further preferred embodiment of the invention, the calibration device has a selector device for selectively enabling or blocking the light of the individual optical waveguides.

[0016] Advantageously, the calibration device also includes a means for isolating the light path from the illumination source, via the coupling device, the light guide system, and the illumination device to the time-of-flight sensor, from external ambient light. This prevents disruptive external light interference. This means that the isolation mechanism is typically comprised of components from the coupling device, the light guide system, and the illumination device.

[0017] According to a preferred embodiment of the invention, the calibration device is arranged such that it and the respective camera system can be brought into the calibration position by simply placing the system onto the device or the device onto the system. This avoids unnecessary adjustment steps.

[0018] According to a further preferred embodiment of the invention, the calibration device has at least one centering mark by which the illumination can be aligned with the coupling device and the time-of-flight camera with the illumination device. In this way, unnecessary alignment and adjustment steps are avoided.

[0019] In the invention, a calibration device is used to calibrate time-of-flight camera systems, each comprising a light source and a time-of-flight camera. The aforementioned calibration device is selected for use. In other words, the invention relates to a method for calibrating time-of-flight camera systems, each comprising a light source and a time-of-flight camera, using the aforementioned calibration device. This calibration is also referred to as (appropriately) operating this device.

[0020] This approach has the advantage that different distances from the sensor can be recorded with a single measurement and calibrated based on the known time of flight of light.

[0021] In particular, the time-of-flight camera of the time-of-flight camera system to be calibrated and the illumination device are aligned to each other in such a way that the time-of-flight sensor of the time-of-flight camera detects the projections of the optical fibers and the coupling device is connected to the illumination of the time-of-flight camera system in such a way that no light from the illumination escapes into the outside space.

[0022] According to a preferred embodiment of the invention, the time-of-flight sensor is provided to be completely illuminated by the light from the optical waveguide / optical waveguide.

[0023] The invention will now be explained in more detail using exemplary embodiments and with reference to the drawings.

[0024] They show: Fig. 1 schematically a time-of-flight camera system, Fig. 2 a modulated integration of generated charge carriers, Fig. 3 a calibration device according to the invention with several optical waveguides according to a first embodiment of the invention, Fig. 4 a device with a single fiber in the near field according to a second embodiment of the invention, Fig. 5 a device with a single fiber and near-field diffuser according to a third embodiment of the invention, Fig. 6 a device with fiber selector and diffuser in the near field according to a fourth embodiment of the invention, Fig. 7 a device with fiber selector and lens according to a fifth embodiment of the invention.

[0025] In the following description of preferred embodiments, identical reference numerals denote identical or comparable components.

[0026] Fig. Figure 1 shows a measurement situation for an optical distance measurement with a time-of-flight camera system 1, as is known, for example, from DE 197 04 496 A1.

[0027] The time-of-flight camera system 1 comprises a transmitter unit or illumination module 10 with an illumination 12 and an associated beam shaping optics 15, as well as a receiver unit or time-of-flight camera 20 with a camera optics (receiving optics) 25 and a time-of-flight sensor 22.

[0028] The time-of-flight sensor 22 has at least one time-of-flight pixel, preferably also a pixel array, and is in particular designed as a PMD sensor. The camera optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmitting unit 10 can, for example, be designed as a reflector or lens optics. In a very simple embodiment, optical elements can optionally be omitted on both the receiving and transmitting sides.

[0029] The measuring principle of this arrangement is essentially based on the fact that, starting from the phase shift of the emitted and received light, the travel time and thus the distance traveled by the received light can be determined. For this purpose, the light source 12 and the light travel time sensor 22 are jointly supplied with a specific modulation signal M0 with a base phase φ0 via a modulator 30. In the illustrated example, a phase shifter 35 is also provided between the modulator 30 and the light source 12, with which the base phase φ0 of the modulation signal M0 of the light source 12 is shifted by defined phase positions φ var can be shifted. For typical phase measurements, phase positions of φ are preferably used. var = 0°, 90°, 180°, 270° used.

[0030] Depending on the set modulation signal, the light source 12 sends an intensity-modulated signal S p1with the first phase position p1 or p1 = φ0 + φ var off. This signal S p1 or, in the case shown, the electromagnetic radiation is reflected by an object 40 and arrives with a corresponding phase shift Δφ(t) due to the distance traveled. L ) with a second phase position p2 = φ0 + φ var + Δφ(t L ) as a received signal S p2 on the light time-of-flight sensor 22. In the light time-of-flight sensor 22, the modulation signal M0 is combined with the received signal S p2 mixed, whereby the phase shift or the object distance d is determined from the resulting signal.

[0031] Infrared LEDs are preferably suitable as the illumination source or light source 12. Of course, other radiation sources in other frequency ranges are also conceivable, in particular light sources in the visible frequency range.

[0032] The basic principle of phase measurement is schematically shown in Fig. Figure 2 shows the time course of the modulation signal M0, which controls the lighting 12 and the light-time-of-flight sensor 22. The light reflected from object 40 arrives as the received signal S. p2 according to its light travel time t L phase-shifted Δφ(t L ) to the time-of-flight sensor 22. The time-of-flight sensor 22 collects the photonically generated charges q over several modulation periods in the phase of the modulation signal M0 in a first accumulation gate Ga and in a phase shifted by 180° M0 + 180° in a second accumulation gate Gb. From the ratio of the charges qa, qb collected in the first and second gates Ga, Gb, the phase shift Δφ(t) can be determined. L ) and thus determine a distance d of the object.

[0033] Calibrating time-of-flight camera systems requires measuring known light transit times. These different transit times are generated primarily by a fiber bundle containing multiple fibers of varying lengths. By strategically arranging the fibers, all necessary measurements can be performed in a single, static setup.

[0034] For offset measurement, each individual pixel must be illuminated with a known transit time. For distance- or intensity-dependent effects, different transit times and / or light intensities must be measured. Standard calibration concepts require considerable space for this. In this invention, all these parameters are realized using optical waveguides that diffusely illuminate the entire pixel matrix or sharply focus individual pixel areas in the near or far field of a camera. This makes it possible to realize a calibration apparatus with minimal dimensions.

[0035] Time-of-flight (TAF) cameras based on the PMD principle do not measure the time of flight directly, but rather by determining the phase of a modulated light signal. The phase is approximated from the camera's actual measurement data using known signal processing formulas. These theoretical formulas are based on ideal signal waveforms (optical and electrical), which are not always realized, especially at modulation frequencies of several megahertz. Therefore, for a more precise phase determination, it is necessary either to directly measure the actual generated signals or to determine the phase deviations using approximation. To determine these deviations, it is common practice to measure the TAF camera at several precisely adjustable distances from a reference object. From this, a correction function and / or a lookup table can be generated.This procedure is time-consuming and requires a lot of space, as the required distances are usually several meters.

[0036] Similarly, offset values ​​must be determined for all pixels of a matrix (FPPN). This is done either using large, precisely aligned reference surfaces, or by direct, diffuse illumination of the entire pixel matrix with a known phase of the optical signal.

[0037] In Fig. Figure 3 shows an arrangement consisting of a time-of-flight camera system 1 and a calibration device 50 for calibrating such time-of-flight camera systems 1. The time-of-flight camera system 1 essentially corresponds to the one shown in Fig. The system 1 described in Section 1 comprises the illumination module 10, which includes the illumination 12, and the time-of-flight camera 20, which includes the time-of-flight sensor 22. The calibration device 50 has three main assemblies 52, 54, 56, namely (i) a light guide system 52, which includes several optical fibers (hereinafter referred to as light guides) 58 of different lengths, (ii) a coupling device 54 for detecting light emitted by the illumination 10 of the time-of-flight camera system 1 and for coupling this light into the light guide system 52, and (iii) an illumination device 56 for projecting the light from the light guide system 52 onto a time-of-flight sensor 22 of the time-of-flight camera 20, wherein the illumination device 56 includes a positioning device 60 for positioning the coupling areas 62 of the optical fibers 58.In the example shown, this positioning device 60 is a retaining plate 64 with openings for receiving the coupling areas 62 of the optical fibers 58. In the arrangement according to . Fig. 3 The modulated light from the illumination 12 of the time-of-flight camera system 1, such as a time-of-flight camera, is coupled into a fiber bundle of the light guide system comprising the optical waveguides 58, which consists of fibers (light guides 58) of different lengths. The individual fibers / light guides 58 project the light onto an imaging screen 66 or, alternatively, onto a diffusely scattering optical element 68 of the illumination device 56, which is imaged by the time-of-flight sensor 22 (of the time-of-flight camera 20). Each individual fiber (each individual light guide 58) generates its own light spot. In this way, different time-of-flights corresponding to distances of several meters can be measured in a compact system.

[0038] The in the Fig. The arrangements shown in figures 4 to 8 essentially correspond to the arrangement of the Fig. 3, so that the following mainly focuses on the differences to the one in Fig. The arrangement shown in section 3 will be addressed.

[0039] Fig. Figure 4 shows a variant with only one optical waveguide or fiber 58. The modulated light from the illumination 12 of a time-of-flight camera is coupled into an optical waveguide (single fiber 58 or fiber bundle with fibers 58 of the same length). The other end of the optical waveguide 58 is located in the near field N of the receiver (the time-of-flight camera) 20 of the time-of-flight camera system 1 and is therefore imaged out of focus on the receiver (sensor) 20. The calibration device 50 is configured to selectively integrate one optical waveguide 58 from a set of optical waveguides 58 of different lengths. This allows any desired time of flight to be calibrated in a compact device 50. Likewise, different light intensities can be set by selecting different optical waveguides / fibers 58 or by adding attenuating elements.

[0040] Fig. 5 shows a variant of the in Fig. The arrangement shown in section 4 uses only one optical waveguide or fiber. 58. To calibrate an entire pixel matrix, especially the offset values, it is possible to use the method described in Fig. Figure 5 shows how to position a diffuser element 68 between the fiber and the time-of-flight camera 20. The diffuser element is again a diffusely scattering optical element 68.

[0041] As an alternative to a single optical waveguide (single fiber or fiber bundle) 58 of defined length, a fiber bundle with optical waveguides / single fibers 58 of different lengths can be used. In the design according to Fig. A selector device 70, for example an optical switch, is inserted between the camera sensor and the output areas (ends) 62 of the optical fibers (individual fibers) 58. This device selectively enables or blocks the light from the individual optical fibers (fiber selector). The selector device functions like an aperture, allowing the individual optical fibers 58 to be selectively chosen. The light from the selected optical fiber 58 is directed onto the projection surface of the screen 66. The distance between the projection surface and the camera 20 can be arbitrarily selected to achieve either a blurred image in the near field N or a sharp image of the projection surface. All required light travel times can thus be measured sequentially.

[0042] Instead of a diffusely scattering optical element 66 or other diffuser, according to Fig.7, as well as an additional lens or other optics 72 of the calibration device 50, which leads to a blurred image.

[0043] Lenses typically have self-adhesive protective films applied, which lead to blurred images. Instead of a diffuser or other diffusely scattering optical element 68 in the calibration setup, this protective film on a camera lens can also be used as a scattering object to enable calibration of the entire matrix. The protective film and its optical properties are precisely specified for this purpose. Reference sign 1 Light time-of-flight camera system 10 Lighting modules 12 Lighting 15 Beam shaping optics 20 receivers, time-of-flight camera 22 Light time-of-flight sensor 25 Camera optics 30 Modulator 35 Phase shifters, lighting phase shifters 40 objects 50 Calibration device 52 Light guidance system 54 Coupling device 56 Illumination device 58 optical fibers 60 Positioning device 62 Output area (optical fiber) 64 Mounting plate 66 Screen, imaging 68 optical element, diffusely scattering 70 Selector setup 72 Optics φ, Δφ(t L ) runtime-related phase shift φ var Phase position φ0 basic phase M0 modulation signal p1, p2 first and second phase Sp1 transmit signal with first phase SP2 Receive signal with second phase Ga, Gb Integration node d object distance q charge N Near field of the time-of-flight camera

Claims

[1] Calibration device (50) for calibrating time-of-flight camera systems (1) comprising a light source (12) and a time-of-flight camera (20), with - a light guidance system (52) which has several optical waveguides (58) of different lengths or is configured to selectively integrate an optical waveguide (58) from a set of optical waveguides (58) of different lengths, - a coupling device (54) for detecting light emitted by the illumination (10) of the time-of-flight camera system (1) and for coupling this light into the optical waveguides (58) or the optical waveguide (58) and - an illumination device (56) for illuminating a time-of-flight sensor (22) of the time-of-flight camera (20) via its camera optics (25) with the light from the optical fiber (58) or the optical fiber (58), wherein the illumination device (56) has a positioning device (60) for positioning the coupling areas (62) of the optical fiber (58) or the coupling area (62) of the optical fiber (58). [2] Calibration device according to claim 1, wherein the positioning device (60) is configured to position the coupling areas (62) or the coupling area (62) in an image plane projected indistinctly onto the time-of-flight sensor (22) by the illumination device (56) in the near field N of the time-of-flight camera (20). [3] Calibration device according to claim 1 or 2, wherein the positioning device (60) is designed as a holding plate (64) with openings for receiving the coupling areas (62) of the optical waveguides (58). [4] Calibration device according to one of claims 1 to 3, wherein the illumination device (56) has an imaging screen (66) downstream of the positioning device (60). [5] Calibration device according to one of claims 1 to 4, wherein the illumination device (56) has a diffusely scattering optical element (68) downstream of the positioning device (60). [6] Calibration device according to one of claims 1 to 5, wherein the illumination device (56) has optics (72) downstream of the positioning device (60). [7] Calibration device according to any one of claims 1 to 6, comprising a selector device (70) for selectively enabling or blocking the light of the individual optical waveguides (58). [8] Calibration device according to any one of claims 1 to 7, configured such that the calibration device (50) and the respective time-of-flight camera system (1) can be brought into calibration position by simply placing the system (1) on the device (50) or the device (50) on the system (1). [9] Calibration device according to one of claims 1 to 8, comprising at least one centering mark by which the illumination (12) can be aligned with the coupling device (54) and the time-of-flight camera (20) with the illumination device (56). [10] Use of a calibration device (50) according to one of the preceding claims for calibrating time-of-flight camera systems (1) comprising a light source (12) and a time-of-flight camera (20). [11] Use according to claim 10, in which the time-of-flight camera (20) of the time-of-flight camera system (1) to be calibrated and the illumination device (56) are aligned to each other so that the time-of-flight sensor (22) of the time-of-flight camera (20) detects the projections of the optical waveguides (58), The coupling device (54) is connected to the illumination (10) of the time-of-flight camera system (1) in such a way that no light from the illumination (10) escapes into the outside space. [12] Use according to claim 10 or 11, wherein the time-of-flight sensor (22) is completely illuminated by the light from the optical waveguide(s) (58).

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