Conventional laser surface scanners generally incorporate a rigid laser/camera assembly. This assembly is usually fixed in space and the object being scanned is rotated or translated on a mechanical platform. Alternatively, the object is fixed and the laser/camera assembly is moved mechanically around or along it. Usually the movement takes the form of a rotation about one axis (contained within the object) or translation in one direction (along the object). In either case, the conventional scanning process relies upon a predetermined knowledge of the geometrical relationships between the components of the scanning apparatus.
Examples of such fixed-axis laser scanners include WO 94/15173, U.S. Pat. No. 4,705,401, GB 2,240,623 A and U.S. Pat. No. 5,193,120.
Unless the object being scanned is a simple surface where every point on its surface is able to be `seen` by the scanning apparatus in the course of a complete scan, a fixed-axis scanner is unable to measure the complete surface of an object in one scan orientation. This represents a significant limitation when scanning complex objects (for example, an entire human head). Features such as overhangs and indentations are unlikely to be visible in situations where a scanning device follows a fixed predetermined path.
Further, a fixed scan pattern or geometry cannot take into account variations in object detail or areas on the object's surface which might be obscured by intervening features protruding from the surface of the object.
Schulz (WO 92/07233) provides, to some extent, for limited freedom of movement of a laser/camera assembly by tracking the position and orientation of a plurality of light sources fixed to the scanning assembly. The location of the light sources (and therefore the scanning assembly) in three-dimensional space is determined by means of fixed photoelectronic sensors arranged in a predetermined static relationship to the fixed object. The light sources are time multiplexed with the sensors and the spatial location is derived from the three locations of the light sources. Therefore all three light sources (fixed to the scanning assembly) must be visible to all fixed photoelectronic sensors. There will generally be situations where at least one light source is obscured by the object itself, the operator or by tilting the assembly. Thus the Schulz scanner would appear to offer advantages over a fixed geometry scanner in only certain applications.
A further significant limitation inherent in the Schulz scanning system is that the object must remain stationary with respect to the reference frame defined by the photoelectronic sensors during the scan. This can present difficulties, particularly in medical applications, due to the potential for movement of the part of the subject being scanned. Accordingly, it would be desirable to be able to scan an object which may be arbitrarily located and oriented in space where such arbitrariness is understood to include an object which may be moving, or to at least provide the public with a useful choice.
The present invention attempts to overcome the limitations and disadvantages inherent in the prior art by providing a means and method of scanning an arbitrarily located and oriented object with the scanner components being arbitrarily located and oriented in space.
The present invention also provides a scanning system which may operate in ambient light and has reduced susceptibility to optical interference in the process of imaging the scanned portion of the object.
The present invention also provides for a non-optical technique for determining the spatial locations and orientations of the components of the scanning system and object.