Colortrac Wide Format Scanner Glossary

CCD (Charge Coupled Device)

Large format scanners use one of two imaging systems - CCD (Charge Coupled Device) or CIS (Contact Image Sensor).

CCD (Charge Coupled Device)

CCD wide format scanners contain one or more CCDs. Each CCD senses light that is incident on its silicon surface. This surface is divided into square cells, each several microns across (e.g. 5.25 um). In a color wide format scanner, each CCD has three closely-spaced parallel lines of cells, each line being overlaid by a different color filter (red, green, blue). Some color CCD large format scanners including Colortrac SmartLF CCD scanners have four lines of cells (quadri-linear CCDs). In this case the fourth line has no filter and is used for monochrome scanning. This is called panchromatic monochrome.

Now consider the geometric requirements for a CCD wide format scanner. To produce a scanner with 600 dpi optical resolution, the scanner manufacturer needs to design an optical system which focuses light from a 1/600" square pixel at the document (0.04233 mm) onto a 5.25 um (0.00525 mm) square cell at the CCD silicon surface - a reduction of approximately 8:1. This is done using photographic enlarger lenses and, usually, mirrors to fold the optical path (which can be up to a meter / several feet long) into a small space.

This long optical pathway with its sensitive components means that quality CCD large format scanners cost more to manufacture than quality CIS large format scanners, tend to be heavier and less compact (they need to be built with more rigidity) and potentially require more calibration.

The use of lenses and mirrors also introduces some geometric distortion and limits on focus, thereby affecting geometric accuracy and resolving power - CCD wide format scanners produce less sharp scans than CIS scanners. The parts of a scan most likely to suffer geometric distortion are those right at the edge of a CCD's range. Colortrac use multiple CCDs across the scan width of their CCD large format scanners. This avoids the the need to work right to the edge of the lens, thus minimizing distortion. Colortrac also minimize distortion and focus effects by their use of DCUs.

The manufacturer of a CCD wide format scanner must also design an illumination system, generally using one or two fluorescent tubes. This white light is combined with the color filters within the CCD to determine the colors produced by the scanner. Although the color filters within the CCD are controlled by the CCD manufacturer, the scanner manufacturer is able to control the illumination system and is able to apply proprietary processing to the CCD output to optimize the scanner's color performance.

This allows CCD wide format scanners to capture a wider gamut and dynamic range than CIS scanners. This makes CCD scanners more suitable for photographic material or documents where color integrity and shadow detail is important.

Colortrac SmartLF Gx large format scanners use CCD technology.