Science and Technology - For Industry

Time-of-flight cameras

Besides interferometry and stereovision, time-of-flight (TOF) technology is a common technique to acquire real-time 3D images. Such a TOF camera is equipped with a modulated light source and a sensor with demodulating gates to sample the phase delay of the reflected light, fig.1. The camera system has to be capable to convert the analog samples into digital values, to store and to process these values to compute the distance. The distance resolution, i.e. the standard deviation of the measured distance, is mainly determined by the shot noise. The shot noise is increasing with decreasing signal amplitude and decreasing modulation frequency.

Principle of the time-of-flight measurement.

Fig. 1: Principle of the time-of-flight measurement.

Various companies and institutes are developing 3D TOF cameras. Figure 2 shows two examples, the first one corresponding to the SR4000 camera of MESA [1], whereas the second one is a demonstrator developed by CSEM within the European project ARTTS [2]. The SR4000 camera provides stable distance information in a robust, reliable hardware package with unique distance measurement capabilities. It features a lateral resolution of 176 x 144 pixels (QCIF), a distance accuracy of +/-1cm and a repeatability of better than 5mm at a range of up to 2 meters. The power consumption is below 10W. The ARTTS camera in turn demonstrates the possibility to further miniaturize 3D TOF cameras to a size of below 4 x 4 x 4 cm3.  It features a record-breaking low-power consumption of only 2.5W and is entirely powered through its USB 2.0 interface. The complete camera is built around two dedicated integrated circuits, the 3D TOF image sensor and the digital controller chip which provides all control and readout circuitry including AD converters.

industrial-grade 3D TOF camera
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miniaturized USB-powered 3D TOF camera
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Fig.2:  Pictures of 3D TOF cameras. a) industrial-grade 3D TOF camera SR4000 as commercialized today by Mesa Imaging [1]; b) miniaturized USB-powered 3D TOF camera as developed within the EU-funded ARTTS project by the CSEM [2].

TOF cameras enable the generation of true three-dimensional images in real-time. In such 3D images and 3D video sequences, objects are easily and reliably localized, making this technology very interesting for an increasing number of applications in robotics, machine vision, surveillance or gaming. Still a challenge are outdoor applications in full sunlight like in traffic or space applications. Sunlight appears as noise on the sensor and can reduce the SNR significantly. In the project ProViScout, CSEM will develop a TOF camera, which will deliver reliable distance maps even in very bright environments.

[1]    www.mesa-imaging.ch
[2]    http://www.artts.eu; 6th Framework Programme of the European Commission contract number FP6-IST-34107.
 

 

Ultra-miniaturized omniview cameras

Conventional camera systems provide an image from a limited viewing angle. In many applications such as security, surveillance, automotive, robotics, autonomous navigation or domotics it is desirable to “see” in all directions simultaneously. Omnidirectional cameras offer a horizontal field of view of 360°.  Catadioptric sensors , which are consisting of a camera, a camera lens and rotationally symmetrical mirror above the lens, capture panoramic images instantaneously in one frame. However, conventional catadioptric system designs require mirror diameters and optical path lengths of several centimeters, since the mirror is not integrated into the lens system of the camera. For robotics and space applications, for example, such systems can be too heavy or bulky.
CSEMs novel design combines the mirror and the lens functionality:  For packaging reasons and to ensure a high scale of miniaturization, it has been of advantage to integrate the mirror into the first lens of the optics, fig.1.

Schematic of the miniaturized catadioptrical design
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Fig.1: (a) Schematic of the miniaturized catadioptrical design, (b) omniview camera with mirror lens on the sensor.

In the European project muFly [1], the rigorous miniaturization resulted in a catadioptrical  system that has been so small and lightweight that it could  be used as a key navigation aid for an autonomous flying micro-robot, fig.2. The camera is used for active laser triangulation in multiple directions. The optical design has distorted the beam path to increase the distance resolution by a factor of 2. Further, the omniview camera has been equipped with a custom CMOS image sensor with a polar pixel field. Due to the pixel geometry a simple transfer has been used to reconstruct an undistorted panorama.

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Fig.2: Flying micro robot prototype of the EU muFly project with the integrated omniview camera and laser sources for active triangulation.

In the European project ProVisG [2], the omniview design has been adapted to a stereovision omniview camera for space applications. Here, the lens design has focused on a low distortion design to be smaller than 2 % along the complete image radius.

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Fig.3: (a) Top-down mounted omniview cameras for stereovision, (b) test rover with mounted omniview stereovision camera.

[1]     http://www.mufly.ethz.ch/index: 6th Framework Programme of the European Commission contract number FP6-IST-034120.
[2]     http://www.ProVisG.eu: 7th Framework Programme of the European Commission contract number FP7-SPACE- 218814