WO2013127973A1

WO2013127973A1 - Intersystem interference avoidance

Info

Publication number: WO2013127973A1
Application number: PCT/EP2013/054118
Authority: WO
Inventors: Norbert HERSCHBACH; Jean-Luc Kaiser
Original assignee: Iee International Electronics & Engineering S.A.
Priority date: 2012-03-01
Filing date: 2013-03-01
Publication date: 2013-09-06

Abstract

The invention generally relates to intersystem interference avoidance, in particular using pseudo random timing sequences in pulsed illumination for time-of-flight and structured light 3D imaging or possibly night vision equipment with active illumination.

Description

DESCRIPTION

INTERSYSTEM INTERFERENCE AVOIDANCE

Technical field

[0001 ] The invention relates to the field of time-of-flight and structured light 3D (three-dimensional) imaging. More specifically, the present invention generally relates to intersystem interference avoidance, in particular using pseudo random timing sequences in pulsed illumination for time-of-flight and structured light 3D imaging.

Background Art

[0002] Instruments capable of capturing three-dimensional information of a region of space and developing this information into a three-dimensional model represented in computer memory are now used in many areas. With their development further pushed by increasing calculation power of computers and inventions in the field of three-dimensional imaging, 3D-graphics and in optics, these devices have become more compact, more precise and less expensive. Hence, many fields of application for these devices have opened up. In diverse areas, not only where three- dimensional information acquisition is traditionally necessary, but also where 3D- information could be useful, these devices are becoming widespread. These areas range from machine vision in robotics, ubiquitous in industrial production, over modern driver assistance systems in the automotive area, to applications in arts and in consumer products, to name only modern gesture recognition features and popular 3D-games.

[0003] With the increasingly widespread use of three-dimensional imaging devices situations in which two or more devices work on overlapping regions of space are often unavoidable. Depending on whether the devices in question are based on a similar or identical principle of operation and design, they can interfere mutually with each other, which can reduce their performance more or less severely and in some cases lead to erroneous results. In the most benign case of interference, the effect of the illumination of a second device is an increase of the background light level on the detector of the first device. For instruments of both types time-of-flight and structured- light devices this means a decrease in signal-to-noise ratio and an increase in distance noise level. Also night vision equipment using pulsed infrared illumination can in events of interference cause a decrease in signal-to-noise ratio in time-of-flight and structured-light 3D imaging instruments. In the case of structured-light devices being disturbed by the illumination from another structured-light device, one can also expect an increase in calculation effort to discriminate algorithmically its own illumination structure from that of the disturbing system. For time-of-flight systems based on the phase measurement using a periodic modulation of the illumination, the interference with a similar system using the same modulation frequency can be disastrous to the extend that distance measurements deliver completely wrong results. These errors may pass unrecognized, which is why appropriate measures have to be taken to test whether an interference is occurring and to exclude the possibility that the system is producing erroneous data.

[0004] The problem of interference has been recognized to exist for some time, and several solutions have been proposed and applied in systems commercially available. Most solutions for time-of-flight camera systems rely on a diversification of the modulation frequency from one system to the other. In order to avoid customization of each system with respect to the modulation frequency, the strategy is to randomly change the frequency periodically during operation. The situation is slightly more complicated as typically several measurements have to be made at several different modulation frequencies and for each measurement several frames are taken during which the modulation frequency has to be essentially constant. This strategy is followed in EP 1 647 839 A2: from one measurement to next the value of the modulation frequency is picked at random from a set of possible predefined values within a fixed frequency band. [0005] In US 7,405,812 B1 a system is described which uses similar randomly chosen changes of the modulation frequency from one measurement to the next in combination with the addition of a random noise on phase and frequency of the modulation during illumination, which should further increase immunity of time-of- flight systems to interference. [0006] An altogether different approach is to not use a periodic modulation, but to apply random patterns, such as Markov-sequences in pulse modulation of the illumination as described by [Buttgen2007] and [MALI2010] (see list of references hereinbelow). General Description of the Invention

[0007] The idea applies to three-dimensional imaging devices, which use an active illumination in a pulsed way and are based either on the time-of-flight method or on the structured light principle of operation. The idea also applies to night vision equipment with active illumination. The idea consists in achieving a time-multiplexing for illumination and detection of the individual devices by using random sequences in the timing of the illumination pulses and detection periods of the individual device.

[0008] In addition, a strategy is proposed which consists in a periodic testing for possible interference at fixed or variable intervals and subsequent random change of the illumination and detection timing in case a possible interference is detected.

Brief Description of the Drawings

[0009] A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1 is a schematic illustration of a three-dimensional imaging system, and Fig. 2 visualizes schematically the random timing of the illumination pulse of duration Tj and the related detection period within the period T_f allocated to a measurement.

Description of Preferred Embodiments

[0010] A typical setup of a three-dimensional imaging system is schematically drawn in Fig. 1 . A first important element is the illumination source 1 , which is for example a laser or LED based light source of dedicated design enabling the technique used. For structured light 3D-imaging, the source projects a particularly patterned light field into space, and for time-of-flight imaging an intensity modulated beam of light is emitted. A second essential element is the detector 2, which can be for example a CCD or CMOS type standard camera sensor chip in the case of a structured light system, whereas for time-of-flight systems it consists of a more specialized image sensor chip based on CCD/CMOS technology allowing detection and simultaneous lock-in like demodulation. The control and processing unit 3 drives the illumination source and the detector activities including the timing of these components and processes the acquired data to generate a computer representation of the imaged three- dimensional scene. To these components which are state-of-the-art of three- dimensional imaging systems, a device generating random data 4 is added to derive random values for timing of the illumination pulses and detection intervals. It can be for example a truly random number generator based on some physical process, or a more standard algorithmic pseudo random number generator or a feedback shift register based generator or also an algorithmic quasi random number generator. The general expression random is used in the following to describe more generally data directly obtained or derived from the output of the generators just mentioned. Alternatively, the random data could also be generated within the processing unit 3 using for example a standard pseudo-random number generator algorithm. Furthermore, as an option, it is possible to add an auxiliary photo-detector 5 with an optical system to collect light from the illuminated area. With the illumination switched off, the auxiliary photo-detector can serve to detect light from the illumination by other systems, which could potentially cause interference problems.

[001 1 ] Figure 2 visualizes schematically the random timing of the illumination pulse of duration T_i and the related detection period within the period T_f allocated to a measurement.

[0012] The timing is either periodically changed at fixed or variable intervals or only after an interference with another system was detected.

[0013] Periodic detection of foreign illumination at fixed or variable intervals is achieved by taking and evaluating one frame with the illumination turned off.

[0014] A separate detection of the presence of a foreign illumination, its frequency, its timing, using the auxiliary photo-detector with a light collecting lens is also possible.

[0015] The invention works for systems using a pulsed mode active illumination and detection. Pulsed mode active illumination schemes do not only allow for the incorporation of the present invention, they also have a number of other advantages. Apart from offering more flexibility, for example, to perform measurements at different modulation frequencies in time-of-flight systems, the main advantage is with respect to the performance under conditions of bright background illumination of the scene. Using shorter periods of active illumination with a light power increased inversely proportional to the duration of the illumination pulse, the contrast with respect to the background light contribution is dramatically increased, while keeping the average emitted light power at the same level as might be required, for example, by power consumption limitations or eye safety regulation. If a potentially interfering time-of- flight system uses longer illumination periods and lower light power in conjunction with longer integration time for the detection, the effect of interference is similarly reduced as the effect of other background light. Hence, it is desirable to use illumination periods of small duration T_i. Using randomly chosen timings for the illumination and detection period reduces the probability for an interference with a similar instrument to Tj divided by the duration between two periodic measurements T_f, when measurements are done for example at a fixed frame rate 1/T_f . Although by just making periodically a random change in the timing the probability for an interference event can be reduced by decreasing T_i/T_f, an occurrence of interference results in the acquisition of data which is difficult to interpret or leads to wrong results. Therefore depending on the application it might be preferable or necessary to periodically test for possible events of interference with another system in order to detect and avoid errors and to possibly prevent the interference by subsequently making a random change in the timing.

[0016] The proposed solution to detect and to avoid interference with other 3D- sensor devices also minimizes loss in signal-to-noise ratio by avoiding the increase in background light level produced by the other devices' illumination. This is a valuable feature which is not provided by other methods, for example all solutions proposed in patents EP 1 647 839 A2 and US 7,405,812 B1 as well as in literature [Buttgen2007] and [MALI2010].

References:

US 7,405,812 B1 : Cyrus Bamji (Canesta Inc.) : Method and System to avoid Inter-System Interference for Phase-Based Time-of-Flight Systems, May 2007.

EP 1 647 839 A2 : Helmut Riedel, Jorg Hilgenstock, Thorsten Ringbeck, Robert Lange (AUDI AG): Verfahren und Entfernungsmessvorrichtung zum Bestimmen der Entfernung zwischen einem Objekt und der Vorrichtung, Oct. 2005.

[Buttgen2007] B. Buttgen, M'H.-A. E. Mechat, F. Lustenberger, P. Seitz: "Pseudonoise Optical Modulation for Real-Time 3D Imaging with Minimum Interference", IEEE Trans. Circuits Syst., vol. 54, pp. 2109-21 19, Oct. 2007. - [MALI2010] El Mechat M'Hamed-Ali , Statistical Range Estimation for Optical Time-of-Flight 3D Imaging, PhD thesis ETH Zurich, 2010.

Claims

1 . Method of intersystem interference avoidance by introduction of random timings for the illumination pulse and detection period of time-of-flight and structured-light type three-dimensional imagers or of night vision equipment with active illumination.

2. Method as claimed in claim 1 , comprising making a random change in the timing of illumination pulse and detection period after one or a certain amount of measurements.

3. Method as claimed in claim 1 or 2, comprising testing, preferably periodically at fixed or variable intervals, for possible interference with the illumination from another 3D-sensor system by making a measurement with the illumination switched off.

4. Method as claimed in claim 3, wherein said testing is achieved using either an auxiliary photo-detector or the same detector that is used to acquire the three- dimensional data.

5. Method as claimed in claim 3 or 4, comprising making a random change of timing of the illumination pulse and the detection period based on the result of the interference test.

6. Device, e.g. 3D imager, configured to carry out the method as claimed in any one of claims 1 to 5.