Scan2CAD raster to vector conversion software

EXPLANATION:

Large format scanner accuracy and calibration

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Dimensional accuracy

Stitching errors cause dimensional inaccuracies, however dimensional inaccuracies can also occur for other reasons.

Dimensional accuracy across the scan

CCD scanners potentially suffer from dimensional accuracy issues across the scan because the lens and mirror system they use introduces geometric distortion that needs to be compensated for.

For example, Contex's literature makes much of what Contex call "spherical lens error". This means that pixels scanned at the edge of a camera's range tend to be longer than those at the centre of its range. To counteract spherical lens error, Contex have developed a technology they call Accuracy Lens Enhancement (ALE). When Contex's "all in one" maintenance software is run, scanned measurements are calculated against actual measurements to provide a correction profile for each camera. This is then applied automatically to every scan.

Colortrac use five cameras across the image width instead of three or four to minimise the effects of spherical lens error. Using more cameras across the scan width avoids the need to work right to the edge of the lens. Image Access state that their WideTEK scanners use a very high quality lens with almost no spherical error, and the marks etched into the scanner glass that the WideTEK uses for stitching are additionally used for maintaining dimensional accuracy.

Dimensional accuracy along the scan

Dimensional accuracy along the scan is controlled by the scanner's motor speed. However, different media may pass through the scanner at different rates. This means that dimensional accuracy may not be correct for all documents on all media.

In addition, wear on the transport rollers may change their diameter over time, affecting the speed of the scanner and therefore its accuracy.

The software supplied with Contex and Graphtec scanners allows changes to be made to the speed of the scanner's motor. If a document is scanned and the dimensional accuracy along the scan is slightly out, the scanner's speed can be changed until the accuracy is exact.

Image Access scanners aim to achieve consistent speed through their scanners regardless of media type, however they do include a facility for fine-tuning the speed of the scanner to compensate for variations in the diameter of the transport rollers over time. Colortrac also allow the scanner motor speed to be adjusted from within ther maintenance software.

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