## 2 Materials and Methods
### Contrast Scenarios (S0x)  
Differences in contrast between the target contour and the surrounding background were expected to affect the results achieved by alternative combinations of image processing  
Figure 2: Schematic flowchart of the experimental procedure.  
parameters. Contrast scenarios used in this research were generated modifying the part geometry or the background colour, while keeping illumination conditions constant. The reason was that, in the case of commercial FBSs, the lighting source is mounted on the scanner head and its angle of incidence is fixed by design. Additionally, the user does not commonly have total control upon image capture parameters, such as light intensity or exposure time. In fact, most commercial FBS software only allow image tuning after raw images have been captured.  
In the case of extrusion additive manufactured (EAM) parts, geometry can affect contrast in three ways: firstly, the relative orientation of target contours with regard to the different sources of light present during scanning modifies the pattern of shadows and reflections; secondly, the distance between the target layer and the surrounding background (formed by all the previously-manufactured layers) also modifies the contrast pattern; thirdly, machine structures and part geometry can produce shadows and occlusions. To include contrast as a possible factor of influence, several considerations were made:  
* To avoid contour-to-light relative orientation introducing bias in the analysis, a circular contour was selected as the test target. Light is directional in FBSs and no additional source of ambient light was present, therefore each possible angular relationship between normal-to-contour orientations and light direction was included in the test. This arrangement resembles a worst-case scenario in terms of contrast uniformity.
* Test specimens were designed to avoid interferences caused by structures or features in the surroundings of the objective contour.
* Due to the characteristics of FBS illumination, contrast increases when the distance between the top layer and the surrounding background increases. In this research, the height (\(H\)) of the layer that contains the target contour with respect to the background layer was used to generate different contrast scenarios (\(S0x\)): \(S02\) (\(H=8\) mm), \(S03\) (\(H\) = 4 mm) and S04 (\(H=2\) mm).
* An additional reference scenario (\(S01\)), presenting minimum contrast-dependant issues within the experiment was obtained by painting the background surface in matte black.  
The circular contour selected as the test target was contained in the top layer of an inverted conical frustum, isolated in the middle of each test specimen (Figure 3). This design assured that no interference in the contrast pattern could be derived by nearby structures or geometrical features, whereas the FBS would be capable of digitising a neat view of the test target. The nominal diameter of the circumference (\(D_{N}\)) was set to 40 mm to properly apply the FBS local distortion adjustment (LDA) procedure described in [18]. A squared profile with chamered corners was selected for the external shape of the specimens to fulfil reference and alignment requisites. All specimens were manufactured in 2.85 mm white BCN3D(r) PLA, with a BCN3D Sigma EAM machine, using the standard (0.1 mm layer resolution) manufacturing profile, as given by material's supplier.  
Figure 3: Geometry and dimensions of test specimens (**left**) and schematic calculation of \(\overline{P_{i}^{*}C^{*}}\) (**right**).