Issue link: https://digital.copcomm.com/i/77744
Scanning n n n n much paint we would lose if we displayed it upright due to the movement of the wood or internal stresses, as the wood itself is made up of sections—it's not just one long piece." The Scanning Process As Garland explains, the anthropomorphic inner coffin is very large (larger than life-size), contains significant gilding, and is brightly painted from head to toe. "It is in relatively good shape," she says. "It has all these wonder- ful pigments, some we hadn't known existed in the Egyptian palette." The body portion of the coffin is covered with designs, prayers, and incantations to the gods of the dead, and hieroglyphs. Mean- while, the head portion of the lid contains gilding of extremely pure gold, along with ar- eas of solid paint (for the hair), a richly painted headdress, and dark eyes and brows. A beauti- ful piece of carved and painted art, this part of the lid would present issues during the scan process, since laser scans typically do not work well on reflective surfaces like this. Moreover, the delicate surface of the artifact could not be touched during the process, making the data acquisition even more difficult. Initially, the museum contacted Ryan Wood- ward of EpicScan, a service provider with whom Nelson-Atkins had contracted for the period room scan. He assessed the project requirements and obstacles, and realized that the scanner he was using—which is typically employed for digitizing extremely large objects, such as en- tire rooms or building facades—was not ideally suited for an object such as this. So he contacted Hexagon Metrology Services, the parent com- pany of Leica Geosystems, which manufactures his Epic Scan equipment. Not only did Wood- ward receive more appropriate hardware for the job, but also the services of Hexagon application engineer Rina Molari, who assisted him with this delicate documentation project. "The type of equipment Ryan was using was not sufficient for the accuracy and point density required for an object this size," says Molari. "We have other equipment for appli- cations when tighter tolerances are required, like in this instance." According to Woodward, the technology used for this type of scan occurs in a very small window—4 by 4 feet or smaller. However, the coffin was much larger (over 7 feet in length) and would have required multiple scans that then would have to be stitched together, thus increasing the possibility of introducing errors as the scans were fit together. As a solution, Woodward and Molari used a two-pronged approach: a high-precision Leica laser tracker with a mounted camera that uses photogrammetry to read orientation around the target, and a non-contact Leica T-Scan hand scanner. The laser tracker reads the posi- tion of the laser scanner; the camera on top of the tracker reads the orientation of the scanner in space. The scanner, meanwhile, captures all the point-cloud information. "We wanted very dense data spread out over a fairly large envelope," says Woodward. "With the laser tracker and T-Scan combina- tion, we could leverage the large measurement volume accuracy of a laser tracker and the point density of a scanner." Nevertheless, there were still a number of obstacles that had to be considered—specifi- cally the coffin's multi-colored, reflective surface that could add noise to the data and affect the scan's accuracy. According to Hexagon Metrol- ogy, the T-Scan is an optical device that is nearly impervious to varying light conditions. As a result, it can handle a wide range of black and shiny surfaces without the need for a surface- coating spray, which was obviously out of the question here. (The face of the coffin contains a great deal of matted black coloring for the hair and shiny black paint for the eyebrows, both of which are in close proximity to the gold.) In addition, Woodward had to take into account the limited space he had to perform the scan. The workable space around the cof- fin was close to 20 by 20 by 10 feet, which was within the 30-foot envelope placement of the T-Scan to the laser tracker. The team also used the Leica HDS 6000 as a supplemental measuring tool that would enable them to compare both sets of data and document the room where the coffin was housed. Molari and Woodward spent two and a half days digitizing the outer portion of the object— half of a day to set up and two days for scan- ning it in both a flat and upright position. The ultra-high-speed hand scanner gathers millions of points, with an accuracy of .00079 of an inch. The exterior of the coffin had mostly smooth contours, with some areas around the face and lower portion containing sharp angles and surface complexity. "The act of scanning is similar to using a paintbrush, except you are painting light onto the surface and making sure you have overlaps and proper coverage," explains Molari. "But scanning a delicate ob- ject without touching it is certainly a human challenge. You can't lay your hands on the ob- ject, nor brush against it." Once the digitizing was complete, Molari and Woodward had two resulting point clouds: 32.5 million points for the object in the hori- zontal position, and 18.8 million points for it in the vertical placement. After processing the data, Woodward imported it into Geomagic's Studio, where a model of each object was cre- ated so the surface of the coffin in each position could be compared graphically. "We created mesh data to generate an er- ror map, which is color-coded, showing dif- ferent gaps," says Woodward. "The analysis showed that the main body of the coffin was very stable, and there was no movement in the body. In the upright position, we saw a shift in the foot, which was expected." As he explains, that's because the feet are semi-detached and have dowels (not original), holding the section in place due to previous damage. Reviewing the Results Indeed, Garland was not surprised that the scans showed that there was movement at the foot region, indicated by gaps in the data. "We June/July 2012 37