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.
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 , whereas the second one is a demonstrator developed by CSEM within the European project ARTTS . 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.
|Fig.2: Pictures of 3D TOF cameras. a) industrial-grade 3D TOF camera SR4000 as commercialized today by Mesa Imaging ; b) miniaturized USB-powered 3D TOF camera as developed within the EU-funded ARTTS project by the CSEM .|
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.