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A Prototype Optical Tracking System Investigation and Development - page 15 / 170

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Chapter

Introduction and Background

Laser scanners are used in a diversity of fields to produce 3D models of objects. Dur- ing the making of “The Lord of the Rings”, the FastSC N laser scanner was used to scan clay characters for Computer Graphics (CG) operators to animate [1]. By sweeping a laser beam produced by the scanner across the surface of an object, the 3D coordinates of sur- face points are determined and a model of that surface constructed. Laser scanners have been used in the medical [2, 3] and film industries [1, 4], and for reverse engineering [5], restoration [6], and the cataloging of artifacts [7, 8].

The FastSC N uses a Fastrak [9] electromagnetic tracking system provided by Polhemus Ltd. to determine the position and orientation (the ‘pose’) of the scanner. The intersection between a line of laser light emitted from the scanner head and the surface of an object is recorded by one or two cameras attached to the scanner. Using these intersections and knowledge of the scanner pose, the surface points can be calculated precisely.

R NZ Scanning Ltd. ( SL), a New Zealand company developed FastSC N and pro- duces this laser scanner in conjunction with Polhemus Ltd. Figure 1.1 shows the FastSC N Cobra(TM) [10] that is one of two models of laser scanner offered by the companies. In this figure, the operator is shown holding the scanner. The laser line is emitted from the top of the scanner and the camera is below the operator’s hand.

While accurate 3D models can be produced using this system, the Fastrak has limitations. Fluctuations in the magnetic field strength can lead to distortions in the models produced. Such fluctuations can be caused by ferromagnetic objects near the tracking system and interfering fields such as mains power fields [11]. This limits the materials that can be scanned accurately to non-ferrous objects. It would be of great benefit if a scanner could scan objects containing ferrous materials.

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