Close Range Active Techniques
When recording the surface geometry and texture of objects such as detailed carvings, architectural detail, or museum objects the Discovery Programme uses several approaches.
Laser Triangulation Scanners
One of the most widely used active acquisition methods is Laser Triangulation (LT). The method is based on an instrument that carries a laser source and an optical detector. The laser source emits light in the form of a spot, a line, or a pattern on the surface of the object while the optical detector captures the changes of the light pattern due to the surface’s morphology and enables the depth of the features to be calculated. Laser scanners are known for their high accuracy in geometry measurements and dense sampling. Most systems are able to capture both geometry and colour using the same composite laser beam while being unaffected by ambient lighting and shadows.
The Discovery Programme has utilised this approach initially when recording detailed objects, using the NextEngine 3D Laser Scanner which is a moderately costed LT scanner that is suited more for tabletop scanning of artefacts rather than use in the field as the device cannot be moved during the time it is scanning an object.
Structured Light Scanners
Structured Light (SL) – also known as fringe projection systems is another popular active acquisition method that is based on projecting a sequence of different alternated dark and bright stripes onto the surface of an object and extracting the 3D geometry by observing the change of each pattern. By examining the edges of each line in the pattern, the detail of the object’s surface is recorded. Some SL systems have the ability to capture surfaces dynamically by moving the instrument around an object as its being scanned, which enables larger objects to be documented. Systems also have the ability to capture 3D surfaces in real-time which enables onsite inspection of the scanning progress.
The Discovery Programme has several devices which utilise this approach which can be used across a range of scales and environments:
- Artec Eva Scanner – Handheld structured light scanner which is suitable for recording medium objects (0.3m – 2m) such as carved stones and architectural details. The instrument has the ability to be moved around an object but requires connection to a PC to enable scanning to take place which sometimes limits its use in more awkward locations. Models produced are accurate up to 0.1mm and of a resolution up to 0.2mm.
- Artec Space Spider Scanner – Handheld structured light scanner which is suitable for recording small objects (0.05 – 0.3m) such artefacts or specific carved details. Similarly to the Eva scanner, it can be moved around an object and requires a connected PC to carry out the scanning process. The resolution (0.05mm) and accuracy (0.1mm) of the models produced are twice as good as the Eva Scanner but have a much more limited scanning window and is, therefore, best suited to smaller objects.
- Artec Leo Scanner – The Artec Leo scanner records to a similar resolution and accuracy to the Eva scanner. However, it has the added benefit that processing is carried out onboard (no PC connection is required) making this scanner much more versatile and mobile in the field when recording objects which can be awkward and challanging difficult at times on archaeological sites.
As all these scanners rely on their own light source to record objects, they are highly suited for scanning in dark conditions such as portal tombs and have been used by The Discovery Programme to record megalithic art in Knowth and Newgrange. In contrast, strong sunlight and illumination are detrimental to the scanning process and therefore must be controlled through the use of a blackout tent or other shading means.