Several technologies enable us to record objects in 3-dimensions. The technique discussed here belongs under a wide umbrella called “3D Laser Scanning”.
In a nutshell, a low-powered laser is used to measure the position of a point on an object in 3-dimensions. As time progresses, many measurements are taken, building up a complex and highly accurate 3-dimensional map of the surface of the object.
To fully record a complex object, such as the megaliths at Stonehenge, the scanner is manually moved around the object to measure points from many different angles, similar to photographing a stone from different sides to achieve complete coverage.
Within the broader topic of 3D Laser Scanning, there are essentially two types of device which operate in subtly different ways and produce radically different results: Triangulating 3D Laser Scanners and Time of Flight 3D Laser Scanners.
These 3D scanners are so called due to their use of the principle of triangulation. That is, typically a thin stripe of laser light is projected across the surface of an object and is viewed by a digital camera. Because the positions of the camera and laser emitter are fixed and known, it is simple to compute the position of points along the laser stripe in 3-dimensions.
A single detail scan of St. Orland's Stone. 700-800AD. The blue line is 12cm long.
Triangulating laser scanners typically have a very high resolution and accuracy making them ideal for accurately recording fine details on sculptured stonework.
In addition, the high accuracy also enables us to directly measure changes in the surface of the stone either caused by decay, or perhaps even vandalism. The downside, such as it is, is that extremely large datasets can be generated.
Time-of-flight scanners operate on a different set of principles to triangulating scanners. A time-of-flight scanner simply shoots a laser pulse at the object and measures the time taken for the pulse to return to the scanner. Given that the speed of light is constant, the distance from the scanner to the surface of the object can be calculated quite easily. The scanner's motors move the laser emitter backwards and forwards across the object shooting a laser pulse out at regular time intervals. The 3-dimensional points are calculated as a combination of the horizontal and vertical angles of the motors plus the measured distance.
Govan Cross, Glasgow, scanned with a Callidus time-of-flight scanner. 10th/11th century.
The resolution and accuracy of time-of-flight scanners is quite limited, typically operating at +/- 6mm accuracy. With this level of accuracy, the intricate surfaces of sculptured stonework are reduced to a formless blob.
A scan of the Govan Cross using a Callidus time-of-flight scanner shows this. The raw data is shown on the left (1) and is completely indecipherable. The middle image (2) shows the data after one iteration of a smoothing algorithm and some features are now distinguishable. The image on the right (3) shows the data after two iterations of a smoothing algorithm and is considered to be the “best” recording of that stone with that type of scanner, albeit still fairly poor quality and unsuitable for recording purposes.
Time-of-flight scanners cannot be used for recording fine detail, such as the Stonehenge axes. For accurate recording, a triangulating scanner should be used. Technology changes quickly and often, but a high-accuracy and high-resolution system should always be used.
© 2003 Wessex Archaeology / Archaeoptics Ltd