## 2 Materials and Methods
#### Experimental Procedure  
The investigation of the two handheld scanners commenced with the preparation of the test part and the calibration of the two scanners. The positioning targets were applied to the surface of the car door as shown in Figure 4a. The distance between each target was maintained in the range of 2-10 cm. The larger distances were preferably located on the flat surfaces, while the shorter distances were preferably located on the curved surfaces. The calibration plate shown in Figure 4b was used to calibrate the scanners. Notice that the room ambient light does not have an effect on the measurement results for both scanners (Courtesy: Creaform). Therefore, the effect of luminosity (surrounding light or brightness) was assumed negligible. Moreover, the same luminosity was maintained during the scanning using the two different scanners. To further validate this assumption, experiments were carried out under different intensities of room ambient light. The procedure to compute the accuracy that can be achieved with each scanner can be realized in Figure 5. Initially, a CAD model was obtained using the CMM for the real car door part. This CAD model was referred as \"reference CAD model\". Subsequently, the scanned models acquired through laser scanner, as well as the white light scanner, were evaluated with reference CAD model using 3D comparison in Geomagics (Geomagics 2014, 3D systems, Rock Hill, SC, USA). Figure 4: (**a**) Car door with positioning targets; (**b**) Calibration of the scanner. Figure 5: Methodology adopted to compute the accuracy of two scanners. CAD: Computer-Aided Design. A medium-size bridge-type CMM (Accura, Zeiss, Oberkochen, Germany) with an accuracy of (1.6 + L/333) \(\upmu\)m, which complied with DIN EN ISO 10360-2:2001, was employed in this investigation as shown in Figure 6. A continuous passive scanning touch probe system was utilized to collect the data points on the car door. Since the accuracy of point coordinates is dependent upon scanning parameters therefore, it was very important to select the appropriate scanning parameters. The scanning parameters adopted to obtain the data points for reference CAD model were: scanning speed--5 mm/s, distance between lines--0.3 mm, distance between points--0.0025 mm, and distribution of points--uniform. Design of experiment was employed to design and conduct the experiments, as well as to analyze the performance of the two scanners. The first step was the identification of controlled scanning parameters for both the scanners. From a literature survey and the manufacturers' working guides, initial trial experiments factors were identified for the handheld laser scanner, and two controlling factors were found for the white light scanner as shown in Table 2. When using the white light scanner, the light power was adjusted automatically depending on the surface, and was thus not selected as a controlled scanning factor. Three levels were selected for each factor based on the initial trial experiments. The resolution defines the ability of the scanner to capture the smallest possible detail in the specified axis; it is measured in mm. The shutter time is the time taken by the camera (of the scanner) to capture each image; it is measured in ms. Similarly, the laser power defines the power of the emitting laser from the 3D scanner and is determined by percentage. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Laser Scanner**} \\ \hline
**Factors** & **Level 1** & **Level 2** & **Level 3** \\ \hline Resolution (mm) & 0.75 & 1.5 & 3 \\ Shutter speed (ms) & 3 & 6 & 9 \\ Laser power (\%) & 50 & 75 & 100 \\ \hline \multicolumn{4}{c}{**White Light Scanner**} \\ \hline
**Factors** & **Level 1** & **Level 2** & **Level 3** \\ \hline Resolution (mm) & 0.5 & 1.5 & 3 \\ Shutter speed (ms) & 2 & 4 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scanning factors with their levels. Figure 6: Bridge-type CMM. CMM: Coordinate Measuring Machine.