A CCD scanner boasts higher depth of field than its CIS counterpart. This is achieved by using an objective lens and a system of mirrors:
For your convenience, there is only one mirror in the picture,
although a typical scanner has at least three or four of them
CCD scanners are more widespread than CIS ones. This is obviously due to the fact that you usually buy a scanner not only to digitize text documents, but also to scan photographs and color images. If this is the case, the user would want to have a scanner with an accurate and realistic color reproduction, and a CCD scanner is much more reliable especially for reproducing color tones, lights and halftones than a CIS scanner. For your reference: standard CCD scanners have a color dispersion error of around ±20%, while CIS devices have an error of ±40%.
Schematic representation of a CIS sensor
CIS array consists of a line of LEDs that illumine the original, the self-focusing micro-lenses and the sensors themselves. The whole construction is quite compact, so this is a general rule that a contact-sensor scanner is always a little thinner than its CCD counterpart. This type of scanners also consumes little power and is highly mechanically stable. However, their functionality is somewhat limited as they don’t generally support slide-adapters and automatic document feeders.
Because of the technology peculiarities, CIS array provides a relatively low focal range. For example, CCD scanners have a focal range of ±30mm, while CIS ones of only ±3mm. In other words, if you place a thick book onto the bed of such a scanner, you will get a scan with a blurred band in the center, where the original doesn’t touch the glass. A CCD device would produce a sharp image, since it features a system of mirrors and a focusing lens. However, you have to “pay” for this better quality: the bulky mirror system doesn’t allow a CCD scanner to become as thin as its CIS analog. Note also that the optical system of a CCD scanner should meet strict requirements, so those rumors about some scanner models using “plastic mirrors” are quite groundless.