PolyWorks and the Mona Lisa - Innovmetric Software

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C A S E S T U D Y © National Research Council Canada Figure 1: Color map and color plot merged on the Mona Lisa model in PhotoShop®. Figure 2: Side view of the top half and the bottom half of the painting. Measuring the distorted shape Specialists from the C2RMF wanted to evaluate the distorsion of the poplar panel after 500 years of exposure to factors, such as humidity and temperature changes. Using the PolyWorks/ IMInspect module, the NRC-IIT specialists generated a color map, where each color corresponds to a coordinate, to illustrate the overall shape of the poplar panel. The red area of the color map shows a convex deviation of 12 millimeters. A color-coded elevation level contour map, similar to a topographical map, was also computed to better understand the overall shape of the object. The color contour plot was created by slicing the 3D model of the Mona Lisa using PolyWorks. The graduation between each contour line corresponds to a change in elevation of 0.50 mm with the red lines the highest or most elevated areas and the blue-green lines the lowest. Figure 1 shows the color map and the color plot merged over the 3D model of the painting using Photoshop. This illustrates that the panel has a longitudinal convex curvature due to warping which is most pronounced in the area of the left shoulder. PolyWorks also allowed NRC-IIT scientists to visualize the distortion of the panel in 3D by slicing the model with a plane and flipping the panel horizontally. Looking at the sliced portion of the panel clearly illustrates the convexity of the right section of the painting. Fortunately, this warp doesn't appear to threaten Mona Lisa's smile. 4
C A S E S T U D Y Figure 3: Back of the panel with horizontal crosspiece (top), back of the panel without horizontal crosspiece (bottom), color map showing the deviation between both models (middle). © National Research Council Canada Specialists from C2RMF also wanted to evaluate the effect of the horizontal crosspiece (wooden strut) located on the rear side of the panel, which was added to hold the structure. To answer this question, the NRC-IIT crew scanned the rear side of the panel with the horizontal crosspiece, and rescanned it later without the strut. The two resulting point clouds were then compared to each other in the PolyWorks/IMInspect module. Using a color map to illustrate the deviation, the NRC-IIT specialists precisely measured the bending on the left end side of the panel (when looking from the back) by 3 millimeters caused by that the horizontal crosspiece. The pressure exercised by the many re-framing of the painting has created stress in the paint layers resulting in some specific craquelure patterns and a 12 cm split on the top of the painting (see Figure 4). Fortunately, using finite element model analysis, the specialist concluded that this pressure, although it could still cause cracks and stress in the long term, does not exercise too much pressure on the 12 cm split itself which was a source of concerns for the curators. Figure 4: Shaded view of the painting reveals a 12 cm split, a contour line revealing a silk veil, and an area with previous restorations. 5
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