Guehring, Jens 
  
2 PHYSICAL SETUP FOR DENSE 3-D SURFACE ACQUISITION 
2. Classification of Triangulation based Sensors 
Dense surface acquisition is one of the most challenging tasks in computer vision. Active research in the last two 
decades led to a variety of high speed and high precision sensors including stereo camera systems, laser range finders 
and stripe projection systems. 
The class of triangulation based sensors observes the object from at least two different angles. In order to obtain three- 
dimensional measurements, point correspondences have to be established, allowing 3-D shape to be reconstructed in a 
way that is analogous to the way the human eye works. 
The family of triangulating sensors can be further subdivided in active and passive triangulation systems. Active 
triangulation systems illuminate the scene rather than relying on natural or uncontrolled lighting. 
A stereo camera is the prime example of passive optical triangulation. For stereo vision, two or more cameras are used 
to view a scene. Determining the correspondences between left and right view by means of image matching, however, is 
a slow process. For faithful 3-D reconstruction of objects, passive stereo vision techniques depend heavily on 
cooperative surfaces, mainly on the presence of surface texture. The fact that most industrial parts lack this feature 
reduces its usefulness in an industrial context. 
However, since the underlying measurement principle is strictly photogrammetric, those methods enjoy all the 
advantages of photogrammetric systems, like the possibility for multiple observations (geometrically constrained 
matching), self-calibration, robustness and self diagnosis. 
To overcome the need for cooperative surfaces and to speed up the evaluation steps, active triangulation systems project 
specific light patterns onto the object. The light patterns are distorted by the object surface. These distorted patterns are 
observed by at least one camera and then used to reconstruct the objects surface. 
Some of the most widely used active triangulation techniques are: 
e Light dot range finders: A single laser point is projected onto the surface and observed by a camera. If the position 
and orientation of the light source are known, a single 3-D point can be computed by intersection. For dense surface 
measurement, the light-dot must scan the surface. 
e Light stripe range finders: A single line, usually a laser beam, spread by a cylindrical lens, is projected onto the 
objects surface. The resulting contour is detected by a camera and used to precisely calculate a three dimensional 
profile. If the light-stripe sweeps across the object, dense surface data is obtained. Due to their robustness and 
simplicity, light stripe range finders are very popular. A positioning device is needed to sweep the light stripe across 
the surface. The overall accuracy is affected by the accuracy of the positioning device. 
e LCD shutter devices: A light projector shining through a computer controlled LCD screen is used to illuminate a 
surface in much the same way as in light-stripe range finder systems. The LCD effectively allows the surface to be 
scanned without needing to move the object. LCD shutter devices are suitable for measuring stationary objects, and 
are generally faster than light stripe systems for this type of task. Depth of field issues, however, make LCD devices 
less robust than light stripe devices. 
e Moiré devices: A set of fringe patterns is projected using an interference technique. The contours of the fringes are 
used to reconstruct the object’s surface. Moiré devices are appropriate for the precise acquisition of smooth surfaces 
with few discontinuities. 
2.2 Sensor Architecture 
We use a LCD type projector (ABW LCD 640 Cross, Wolf (1996)) for our experiments . The line pattern is generated 
by switching lines on a two-dimensional LCD illuminated from behind. This type of projector has the advantage that 
there are no moving parts. 
While normal LCD stripe projectors use two glass plates with conducting stripes aligned precisely (Figure 1 (b)), a 
cross-pattern projector has one of the glass plates turned by 90 degrees (Figure 1 (c)). Since all stripes can be switched 
individually, arbitrary vertical and horizontal stripe patterns can be generated (albeit no arbitrary 2D patterns can be 
generated, since the 2D pattern always results from a XOR of the two line patterns). 
  
328 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000.