State-of-the-art of TOF range-imaging sensors

Abstract

The 3D information of a surveyed object or scene can be recorded with different types of sensors and measuring techniques. Contactless measuring techniques suitable to estimate the target distance exploit micro-, ultrasonic- or light-waves [1, 2]. However only the latter technique allows achieving good angular resolution performance, in a compact measuring setup, as required for a 3D imaging system [3]. In the common practice, the two ways to acquire an object's geometry are: (i) passive, by using multi-view image data or (ii) active, exploiting optical distance measurement techniques. The multi-view image acquisition method, coupled with the triangulation measurement principle, is already known and used for decades in the research community [4]. One of the advantages of the image approach with respect to other range measuring devices (such as LiDAR, acoustic or radar sensors) is the reachable high resolution and simultaneous acquisition of the surveyed area without energy emission or moving parts. Still, the major disadvantages are the correspondence problem, the processing time and the need of adequate illumination conditions and textured surfaces in the case of automatic matching procedures. Active optical measuring techniques using light-waves can be further classified in three main categories, namely: interferometry, triangulation and Time-Of-Flight (TOF) [5-7]. Triangulation techniques normally determines an unknown point within a triangle by means of a known optical basis and the related side angles pointing to the unknown point. This principle is used by active sensors based on structured illumination as well as by passive digital cameras. Continuous wave and pulse TOF techniques measure the time of flight or the phase shift of a modulated optical signal. These techniques usually apply incoherent optical signals. Typical examples of TOF are the optical rangefinder of total stations or classical LiDAR instruments (terrestrial or aerial) [8, 9]. In this latter case, actual laser scanners allow to acquire almost one million of points per second, thanks to fast scanning mechanisms. Their measurement range can vary to a great extent according to the instruments, varying between some decimeters up to some kilometers, with an accuracy ranging from less than one millimeter to some tens of centimeters respectively. Nevertheless, the main drawbacks of LiDAR instruments are their high costs and dimensions. Interferometry methods measure depths by means of the Time-Of-Flight techniques too. In this case, however, the phase of the optical wave itself is used. This requires coherent mixing and correlation of the wave-front reflected from the object with a reference wave-front. Many variants of the optical interferometry principle have been developed, such as multi-wavelength interferometry, holographic interferometry, speckle interferometry and white light interferometry. The high accuracy of the interferometry methods mainly depend on the coherence length of the light source: interferometry is not suitable for ranges greater than few centimeters since the method is based on the evaluation of very short optical wavelength. In the last few years a new generation of active sensors has been developed, which allows to acquire 3D point clouds without any scanning mechanism and from just one point of view at video frame rates. The working principle is the measurement of the TOF of an emitted signal by the device towards the object to be observed, with the advantage of simultaneously measuring the distance information for each pixel of the camera sensor. Many terms have been used in the literature to indicate such devices, normally called Time-Of-Flight (TOF) cameras, Range IMaging (RIM) cameras, 3D range imagers, range cameras or a combination of these terms. In the following sections and chapters the term TOF cameras will be prevalently employed, which is more related to the working principle of this technology. Such a technology is possible because of the miniaturization of the semiconductor technology and the evolvement of the CCD/CMOS processes that can be implemented independently for each pixel. Thus it is possible to acquire distance measurements for each pixel at high frame rate and with accuracies up to few centimeters. While TOF cameras based on the phase-shift measurement usually have a working range limited to ten/thirty meters, TOF cameras based on the direct TOF measurement can measure distances up to 1500 m. Moreover, TOF cameras are usually characterized by low resolution (no more than a few thousands of tens of pixels), small dimensions, costs that are an order of magnitude lower with respect to LiDAR instruments and a lower power consumption with respect to classical laser scanners. In contrast to multi-view image acquisitions, the depth accuracy is practically independent of textural appearance, but limited to about one centimeter in the best case. Recently a great alternative to TOF cameras came on the market: it is the line of sensors based on real-time pattern projection and triangulation technique which enable simultaneous acquisition of geometry and texture, at low-cost, high frame rate and with ranges up to 4-5 m. The most well know sensor of this family is the Microsoft Kinect [10, 11]. This book will not touch such devices as they are not based on the TOF measurement principle. In order to give an overview on the TOF cameras technology, this chapter will provide a quick introduction of the TOF cameras operation principle and a description of their main building blocks. Then, the main technologies available today for the realization of TOF detectors will be described and compared and finally some conclusions and future perspective will be given.