CIS scanners cannot compete with CCD ones as far as their optical resolution is concerned, either. Some models of CCD scanners for home and office use provide currently an optical resolution of 3200dpi, while CIS scanners offer no more than 1200dpi (if I am not mistaken). Still, it wouldn’t be prudent to discard CIS technology altogether: such scanners found their niche digitizing sheet originals rather than books. The fact that such scanners are fully power-supplied by the USB bus and ask for nothing more is very attractive for portable computers owners: they can digitize an original and transform it into a text file wherever they are, without having to seek for a wall outlet. This makes up for the drawbacks of the contact sensor. That’s why the question “Which scanner is better?” should be answered basing on your own particular needs.
A CCD matrix is the most important component of a scanner
A CCD matrix (array), like the one you see above, looks like a “big microcircuit” with a glass window – that’s where the light beam reflected from the original falls. The array works all the time until the scan head passes to the end of the glass. Note: the full distance for the head to travel along the Y-direction is referred to as sample rate or mechanical scanner resolution (we will talk about these two characteristics a little bit later). The array fully covers one horizontal line of the bed (raster line). After the line has been processed, the head moves on to the next line.
Side view of a CCD-matrix
Do you see the two ordinary screws in the snapshot above? When the scanner was assembled, these screws helped to align the matrix (note also the U-shaped slits in the PCB in the photo with a view from above) so that light reflected from the mirrors could be evenly distributed on the matrix surface. By the way, if one of the optical system elements is misaligned, you will get a “striped” image.
Parts of a CCD matrix magnified
(the macro-snapshot was taken with Canon EOS D60 digital camera)
You can see in the enlarged snapshot of the CCD matrix that it has its own RGB filter. The filter is the main component of the color separation system, which has always been the talk of the town, but which appears pretty complicated to understand. You usually get an explanation like “a standard flatbed scanner uses a source of light, a color separation system and a charge-coupled device (CCD) to collect optical information about the scanned object”. In fact, light can be divided into color constituents that are then focused on the matrix filters. The scanner objective lens is also very important for the proper work of the color separation system.
The scanner objective lens is not as big as it seems in the snapshot