1.1 3D Data capture

In this research, both the Wilton and Petworth portraits were scanned using a tripod-mounted Konica Minolta VI900 Laser Scanner. The VI900 emits a Class 2 red laser (maximum output 30mW, 690nm) through a cylindrical lens horizontally onto the object. The reflected light from the object is received by the CCD sensor, and then converted by triangulation into distance information. This process is then repeated by scanning vertically on the object surface using a Galvano mirror to obtain a 3D dataset of the object. Each scan takes 2.5 seconds and can capture up to 300,000 individual three-dimensional points (Konica Minolta 2003).

The scan data were imported in to Konica Minolta's proprietary software Polygon Editing Tool (PET) v2.2 for further processing and visualisation. Each individual scan created a point cloud that was rendered in order to generate a quad polygon mesh representing the surface of the portrait. Overlapping points in adjacent scans were then used to register (or glue) the individual scans together into a 'family'. These were then merged to create a composite 3D representation of the artefact that could be rotated and inverted for study.

1.1.1 Petworth House

The elevated position of Statue 55, in the corner of the Little Dining Room at Petworth House, meant that only a partial scan could be undertaken, recording the facial area between the ears, the neck and the hairline, as areas behind and on top of the head could not be safely accessed. The scanner was positioned on an extended tripod approximately 0.8m from the portrait, and, using the middle lens with a focal length of 14mm, a field of view of approximately 0.4m x 0.3m was achieved. The scanner was then moved incrementally around the statue until a total of 19 overlapping scan point clouds were collected. Each scan had a laser point separation of ~0.4mm and a point accuracy of ±0.05mm, with the exception of two scans covering the extreme right of the face. Here, owing to other artefacts on display, the scanner was positioned further from the portrait, at a distance of 1.5m, resulting in an increased laser point separation of ~0.8mm. Of the 19 individual scans, five were selected to create the final 3D model in PET. The overlapping scans created duplicated vertices, so the final model was simplified to produce a dataset containing ~100,000 points. The unscanned section of the portrait was left unmodified.

1.1.2 Wilton House

The Wilton portrait bust was removed from its wall fixtures in the East Cloister and mounted on a rotating turntable. The scanner was positioned ~0.6m from the bust and 23 overlapping scans were collected while the turntable rotated incrementally. Each scan had a laser point separation of ~0.4mm and a point accuracy of ±0.05mm. Using PET, 18 scans were then used to create a fully registered 360° 3D model. The data were simplified by removing duplicated vertices (from where the original scans overlapped), which resulted in the model containing ~200,000 individual 3D points Three small holes were identified in the data where the laser beam had not been reflected back to the sensor. These were manually filled by joining the vertices at the hole edge together to create a 'watertight' model.