Before taking any measurements, the device needs to be calibrated. We need to know the relationship between the distance of 2 points on an image in pixels and this same distance in reality, in mm. In order to do this calibration the screen is replaced by the pepperpot plate and an image is taken. The center of the image in pixels must also be determined.
Figure
2: The pepperpot plate
The pepperpot evaluation program brings up a cursor if you press the first mouse button on the pepperpot image and the current cursor position is shown on the LCD display (bottom left in Figure 3). The screen is then put back and the distance between camera and screen stays fixed from then on.