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CCD or CIS: The Technology Decision

CCD Technology

Most high end large format scanners use CCD technology for their optical system. The Charge Coupled Device (CCD) converts light on its surface into electrical signals. Color filters for red, green and blue on three consecutive rows of CCD elements provide a very high color gamut, which is typical for CCD scanners and cannot be reached with CIS type scanners. The illumination system for a CCD scanner usually consists of two fluorescent lamps which have a significant warm up time and require frequent recalibration because their light output varies with temperature, time and sometimes even the mains voltage.

The fact that there is no lens or CCD available to capture 36” wide documents at high resolution all at once leads to another downside of CCD scanners, commonly referred to as “stitching”. The scanner has to be recalibrated on a regular basis because it consists of three or four independent CCD subsystems, each having their own lenses and mirrors. During this process, a high precision test target is scanned and the offset in the horizontal and vertical direction between the individual cameras is measured. These values are later used to “stitch” the three or four images from the individual CCD together.

Benefits of the CCD technology are:

Exceptionally large color gamut.
High resolution with no color artifacts due to real RGB pixels.
High depths of focus simplify document transport requirements.
Fast because of small chip size.

The disadvantages are:

Frequent calibration needed to compensate for the change in illumination.
Geometric distortion has to be calibrated regularly.
High quality optical components such as lenses are expensive.

CIS Technology

Entry level large format scanners use the CIS technology. The CCD is combined with a 1:1 Selfoc lens at a very close distance and an LED-based illumination system, all assembled into a compact module. These modules are quite inexpensive and are produced in very high quantities for the flatbed consumer scanner market. Some large format scanner vendors use four or five of these modules mounted side by side to achieve a scanning width of 36” or even more. Stitching effects and geometric distortion are far less significant because of the 1:1 optical system and the fixed distance between the individual modules. This is not true for scanners that have CIS modules staggered in the transport direction. Because of the large distance between the individual modules, which can be up to 1000 lines apart; stitching becomes dependant on the properties of the document and also requires a very stiff transport system.

LED illumination typically has no warm-up time but introduces some color artifacts because LED illumination systems typically consist of a red, a green and a blue LED that are each switched on for the duration of 1/3 of a scan line. This produces colored edges on black and white originals because each color image is taken from a slightly different position. The color gamut is also significantly lower because color filter technology, as used in CCD sensors, is much more advanced than the color balance possible with LEDs.

The depth of focus of CIS sensors is very small, usually a fraction of a millimeter. Therefore, it is mandatory to press the original document against a glass surface; resulting in all of the issues that one can easily imagine; dirt, dust and scratches all degrade the image quality and can harm the original.

Benefits of the CIS technology are:

If mounted side to side, very little geometric distortion.
Stable illumination system, little recalibration necessary.
Smaller and cheaper designs possible.

The disadvantages are:

The real color resolution is significantly lower than listed.
Reduced color gamut.
Limited depth of focus.

Image Access has evaluated both technologies in great detail. As a result we have designed a new scanner family which uses the high color fidelity, high speed CCD technology and overcomes its disadvantages compared to CIS.

Depending on the scanner model, three or four CCD cameras and their optical components such as lenses, mirrors, etc. are mounted into one ultra-stable camera housing to avoid stitching errors introduced through thermal expansion of the scanner and its mounting parts. The patented process of calibrating out the remaining stitching offsets before every single scan makes stitching a “non issue” for the user. There is no reference target or stitching procedure necessary, a great relief for many users.

Conclusion

There are heated arguments about CCD versa CIS technology in the large format scanner business. The same arguments are used for each of the two, depending upon the technology favored by the respective author or vendor. Many of the articles and advertisements we have read recently contained both misleading and incorrect statements. We read things like:

"CCD supports a wider color gamut/bit depth, capturing 48bit color compared to 24bit color for CIS, and this is why CCD is used in today´s digital cameras."

Even a non-technical user will instantly come to the conclusion that the A/D converter´s resolution has nothing to do with the source of the signal. Both technologies produce an analog output signal that will be digitized in a later stage through the A/D converter. This can have any resolution between 8 bit and 48 bit but is independent of the CCD/CIS sensor.

The last sentence in the above statement has the highest score in nonsense we have found yet. Why would anyone in the world even consider using a Contact Image Sensor with a maximum focal distance of less than one millimeter and a picture size of 1 by 2000 pixels for a digital camera? This logic remains the secret of its author.

We at Image Access do not want to get involved in this discussion because we are convinced that the customers purchasing decision will largely be influenced by many more important details than the "right technology".