Each mirror is slightly curved to focus the image it reflects onto a smaller surface. The last mirror reflects the image onto a lens . The lens focuses the image through a filter on the CCD array. Look carefully at the image above and you can see all three of the mirrors plus the lens assembly in this scan head. The filter and lens arrangement vary based on the scanner.
Some scanners use a three pass scanning method.
After the three passes are completed, the scanner software assembles the three filtered images into a single full-color image. Click on the green Scan button to see the scanning process. Most scanners today use the single pass method. The lens splits the image into three smaller versions of the original. The scanner combines the data from the three parts of the CCD array into a single full-color image.