Methods of Acquisition

Laser Ranging Systems

Laser ranging works on the principle that the surface of the object reflects laser light back towards a receiver which then measures the time (or phase difference) between transmission and reception in order to calculate the depth.
Most laser rangefinders:

• Work at long distances (greater than )
• Consequently their depth resolution is inadequate for detailed vision tasks.
• Shorter range systems exist but still have an inadequate depth resolution (1cm at best) for most practical industrial vision purposes.

Structured Light Methods
Basic idea:

• Project patterns of light (grids, stripes, elliptical patterns etc.) onto an object.
• Surface shapes are then deduced from the distortions of the patterns that are produced on Object's Surface.
• Knowing relevant camera and projector geometry, depth can be inferred by triangulation.

• Many methods have been developed using this approach.
• Major advantage -- simple to use.
• Low spatial resolution -- patterns become sparser with distance.
• Some close range (4cm) sensors exist with good depth resolution (around 0.05mm) but have very narrow field of view and close range of operation.

Moire Fringe Methods
The essence of the method is that a grating is projected onto an object and an image is formed in the plane of some reference grating as shown in Fig. 6.
The image then interferes with the reference grating to form Moire fringe contour patterns which appear as dark and light stripes, as demonstrated by Fig. 7. Analysis of the patterns then gives accurate descriptions of changes in depth and hence shape.