A Few More Details
The manufacturers emphasize the following features as selling points for their scanners.
Resolution
The first is resolution. This is always expressed in dpi (dots per inch) and indicates the number of dots that the scanner is capable of capturing in one square inch (2.54 cm on each side). It often takes the form 1200 x 2400 dpi. The smaller of the two figures refers to the sensitivity of the detector, and the second (the larger) refers to the number of movements that the optical system is capable of performing as it moves along an inch at a time.
This statement is misleading because what's written can't always be taken literally. The true working resolution is always a square of values. If you want to work at 1200 dpi, the system will do so by making 1200 upward movements, not 2400 as the specification might lead you to believe. The fact that a scanner is capable of performing 2400 upward movements is only relevant for the interpolated resolution of the scanner. A 1200 x 2400 scanner is likely to produce better results than a 1200 x 1200 dpi scanner if you work at an interpolated resolution of 2400 dpi. This has absolutely no effect when working at 72, 75, 300, 600, or 1200 dpi.
Depth Of Color
The detectors installed are of the RGB type. This means that each pixel is described by three factors, namely, its value in red, green, and blue. Each of them is generally 16 bit encoded, hence the statement in the specifications of a color depth of 16 x 3 = 48 bits. That's being slightly economical with the truth! A distinction should be made between the internal depth of color and the so-called external depth. If it's of the external type, this means that an image sent to the PC is actually 48 bit encoded. It's therefore up to you to convert it to a format that is capable of handling such a large amount of information, although, as you know, a JPEG, for instance, would not be capable of doing this. If it is internal, this means that the detector is sufficiently sensitive to work at 48 bits. But nothing is said about what is actually transmitted to the PC. The message might only be transmitted with 24 bit encoding, for example.
Nevertheless, this point affects the quality of the scan. Although it's said that CCD peripherals are generally considered to be more accurate than those based on CIS technology, all flatbed scanners lose a certain amount of sensitivity. The information lost en route is quite substantial. First, it occurs via the passage of light through the glass plate. Part of the light is reflected and passed back through the same glass plate. Once it has been reflected in the mirrors, the image passes through the lens. So, for the optical system alone, three steps are necessary and each of them adds to image deterioration. Then there is the decoding by components on 65,536 levels, which is really little more than an attractive sales pitch, and really due to the fact that manufacturers are so eager to offer more and more features that they have found themselves forced to economize on all components. Lens quality has suffered in particular, and the mechanical system has become more fragile, the classic problem being that the scanning carriage tends to stop moving. And so on. Everything becomes an excuse for saving money, as well a chance to reduce product quality in order to offer a greater number of more exciting features. That's why some of the optical 600 dpi models, such as Epson's Perfection 640, are far better than a number of 1200 dpi scanners.
