The part is carried from the part feeders on either side of the conveyor belt to the PCB, it is photographed from below. Its silhouette is inspected to see if it is damaged or missing (was not picked up), and the inevitable registration errors in pickup are measured and compensated for when the part is placed. For example, if the part was shifted 0.25 mm and rotated 10° when picked up, the pickup head will adjust the placement position to place the part in the correct location. Some machines have these optical systems on the robot arm and can carry out the optical calculations without losing time, thereby achieving a lower derating factor.