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Auto-synchronized Laser Spot Scanning Technology

The patented auto-synchronized spot scanning principle is the basis of two of the systems, the High Resolution 3D Color Laser Scanner and the Large Volume of View Scanner, also known as a Random Access Scanner.

The auto-synchronized scanning configuration offers a number of significant features that alleviate compromises between field of view, resolution and shadow effects. It allows very large fields of view, which enables full coverage of the scene without compromising the measurement uncertainty. A reduction of shadow effects is inherently achieved with the smaller triangulation angles used in the triangulation based scanning and, due to the small instantaneous field of view, the system is relatively immune to effects from ambient light.

Figure 1: The auto-synchronized scanning configuration. Similar to photography, the CCD detector is mounted on an angle to conform to the Scheimpflug condition to maximize depth of view.

Principle of operation:
The scanner configuration (Figure 1) differs from the usual spot triangulation of other systems in that the directions of projection of the laser beam and the optical axis of the detection system are rotated synchronously through a double-sided mirror. The scanner works by projecting a small, low power - object safe - laser spot from a laser source to the object via one side of the scanning mirror. The spot is then imaged on a charge-coupled device (CCD) detector via the opposite side of the scanning mirror. The spatial (x,y,z) coordinates of the spot on the object are obtained by triangulation. As such, the instantaneous field of view of the position detector follows the spot as it scans the scene. The focal length of the lens is therefore related to the desired depth of field or measurement range and not to the total lateral field of view.

Figure 2 illustrates the resolution limits of the auto-synchronized technology in relation to the size of the object to be scanned. In the X and Y directions (perpendicular to the laser projection), optical triangulation is limited by the diffraction of the laser beam. As shown in the diagram, the laser beam does not maintain collimation with distance. The smaller the beam, the larger the divergence produced by diffraction. The solid line shows the relationship between the X and Y axes and physical dimensions of the object. While diffraction imposes resolution constraints on the X and Y axes, in a well designed system, the Z axis resolution is limited by laser speckle as shown by the dotted line.

Figure 2: Physical limits of 3D optical measurements based on laser projection

Figure 2: Physical limits of 3D optical measurements based on laser projection. The solid line shows the relationship between the X and Y axes (direction perpendicular to the laser projection) and the physical dimensions of the object to be scanned. The dotted line is the uncertainty in Z imposed by laser speckle.