## 1 Introduction  
### Stereo Vision Based 3D Sensors  
Stereo vision based 3D measurement setups are being applied in the industry for many years. Applications include, for example, reverse engineering, digitization of cultural heritage objects [1], medicine [2]. These 3D sensors triangulate 3D points from pixel correspondences between two cameras or between a camera and a projector. They can be classified into passive systems which work on images taken with ambient illumination only, and active systems with an illumination unit which projects patterns onto the measurement object. Active illumination stereo vision systems can be classified in three different ways:  
1. **Single-shot vs. multi-shot. ** Single-shot systems work on a single image (or image pair for stereo camera systems) and a fixed projection pattern. Multi-shot systems record a sequence of images (or sequence of image pairs). The projected pattern is different for each successively taken image/image pair. 2. **Single-camera vs. multi-camera. ** Systems which use a single camera find correspondences between the camera and the projector. Multi-camera systems use at least two cameras and find correspondences between them. 3. **Coded light vs. statistical patterns. ** Coded light systems project well-known patterns or pattern sequences onto the measurement object. An overview of coded light techniques can be found in Reference [3]. On the other hand, statistical pattern systems do not require well-known patterns but can work with quasi random patterns which do not need to be known to the reconstruction algorithm. The Microsoft Kinect sensor is an example of a single-shot, single-camera, coded light system [4]. Classical photogrammetry could be interpreted as a single-shot, multi-camera, statistical pattern system which utilizes the measurement object's texture as a pattern. One of the most popular 3D sensor concepts is phase-shift profilometry, also called digital fringe projection with phase-shifting or phase-shift interferometry [5]. It is a multi-shot, coded light technique [6], typically implemented with a single camera but sometimes extended with additional cameras [7]. The projection patterns are (periodic) sinusoidal fringes which are shifted by a fixed phase-offset with each successive projection [6]. Some systems project additional patterns for fringe-phase unwrapping [8]. Algorithms for phase-shift pattern processing are a field of high research activity [9; 10; 11]. A correspondence between camera and projector can be found by decoding the brightness values of a single camera pixel. The 3D reconstruction only works on a very specific pattern sequence. In other words, information about the actually projected pattern sequence is required as an input for the reconstruction algorithm. Therefore, phase-shift profilometry (and other coded light methods) need a very well controllable and calibrated projector [12]. In contrast, the statistical pattern method makes this requirement unnecessary. It is always implemented with multiple cameras (usually two). It can be implemented as a single-shot system [13] or as a multi-shot system which yields higher accuracies [14]. The main reason for using statistical patterns (instead of coded light) is the possibility to build simpler projectors. The projector requirements are lower compared to the coded light method: the patterns just need to have \"sufficient\" spatial and temporal variation but the system does not rely on a known pattern sequence. Projection methods include, for example, laser speckle projection [13; 15], projection of band limited random patterns [15; 16], and projection of aperiodic sinusoidal fringes [17; 18]. In the past years, we have developed several multi-shot, dual camera, statistical pattern sensor systems. This technique has proved to work well, especially for applications where commercial projectors are not available (e.g., extended spectral ranges, high camera frame rates of 10 kHz or higher).