Fig. 1.5 Active coaxial time-of-flight range estimation by phase shift correlation. Figure reprinted from [29] with permission

face back into the sensor. Figure 1.5 gives a schematic view of an active coaxial range sensor.20 The traveling time delay between outgoing and reflected wave is then measured by phase correlation or direct run-time shuttering, as a direct measure of object distance. Classical examples of such devices are laser-based systems, such as the LIght Detection And Ranging (LIDAR) scanner for long-distance depth estimation. The environment is scanned by deflecting a laser with a rotating mirror and distances are measured pointwise, delivering 3D point clouds. Recently, camera-based receivers are utilized that avoid the need for coherent laser light but use inexpensive LED light sources instead. (Such light sources are also easier to make eye-safe.) Again, the time shift of the reflected light is measured, either by gating very short light pulses directly, or by phase correlation of the time shift between the emitted and reflected light of a modulated continuous LED light source. Such range cameras [44] are depth estimation devices that, in principle, may deliver dense and accurate depth maps in real-time and can be used for depth estimation of dynamic time-varying scenes [31, 32]. Active sensing devices will be discussed in more detail in Chap. 3 and in Chap. 9 in the context of remote sensing.

1.4.3 Passive Stereo Versus Active Stereo Imaging

What are the relative merits of passive and active stereo imaging systems? In summary, since the computational burden of passive correspondences is alleviated, it is generally easier to build active systems that can generate dense range images at high frames rates (e.g. 30 fps for the Kinect). Lack of surface features or sufficiently

20Figures are a preprint from the forthcoming Encyclopedia of Computer Vision [29].