Everybody that sold scanners offered three basic models: 1 camera, 2 camera and 3 camera scanners. The more cameras that each offered, the higher their price. What confused me was the resolution. From my knowledge of printers and plotters, I knew that the higher the resolution, the better it was, but what confused me was that the firms selling one camera scanners weren't quoting the same resolutions, the two camera scanners weren't offering the same and the resolutions offered on the three camera scanners differed by as much as almost 600 dpi. How could one firm offering a 3 camera scanner claim a 416 dpi resolution, and the next one claim a 1,000 dpi resolution ?

I needed to find out true resolution on the scanners before I could decide. I asked each firm the number of elements in each CCD. Everybody was using a 5,000 element CCD. Aha, now it was easy. A simple calculation would give me the true resolution of each scanner:

1 Camera Scanner = 5,000 elements / 36 inches = 138 dpi

2 Camera Scanner = 10,000 elements / 36 inches = 277 dpi

3 Camera Scanner = 15,000 elements / 36 inches = 416 dpi

So, I went back to everybody’s booth the next day with my figures, and everybody came clean, everybody admitted that their true resolutions were as explained above. But what about the claim of a 300 dpi scanner with only one CCD camera ? I was explained that the resolution in one direction is determined by the number of elements in the CCD array. The resolution in the direction of paper travel is determined by the stepping of the motor (i.e. the amount of the motor turn). While the resolution may only be 138 dpi, by turning the motor significantly less, the manufacturer is able to claim a 300 dpi resolution. The resolution would be 138x300 dpi, and repetitious software interpolation is used to create a 300x300 dpi image file. Without creating a much better image quality, I end up with a file size more than 4 times as large. No thanks ! If I need to get a 300 dpi resolution, I’d rather get a 2 CCD camera.

My next confusion was why the viewing software did not record 10,000 elements for the 2 CCD scanner and 15,000 elements for the 3 CCD scanner. Going from booth to booth, I didn’t always get the most straight forward answers, but every once in a while I would stumble across a kind soul that would set me straight. I learned that a 2 CCD camera wouldn’t really provide 10,000 pixels because of electronic stitching and divergence problems. Huh ? Multiple CCD cameras are prone to alignment problems, for which an overlap between the two cameras is used for correction. This is called electronic stitching. This process not only slows down the scanning, but the overlap causes a decrease in true image resolution. Many CCD camera based scanners disregard the periphery pixels because of optical divergence. This results in a further reduction in image resolution. So a 2 CCD scanner may only have 7,500 to 8,500 pixels.


Okay, so I understand what is the true resolution, compared to what is advertised. But what do I really need ? I decided to go to the professionals. I called 17 service houses across the country, and they all recommended a 200 dpi scan. Most explained that the cost justification for scanning involves the cost of storage memory. The higher the resolution, the larger the file size and a higher storage cost. The larger file size also increases the access time, thereby reducing productivity. Since most press clipping services use 200 dpi to scan newspapers prior to running it through OCR software, certainly it should be sufficient for most engineering drawings. But, some drawings might require a higher resolution to deal with very fine elements, and 400 dpi scanners seem to be the upper industry limits. Besides, If one were to vectorize a 400 dpi scanned raster image, the file size is relatively the same when compared to a 200 dpi image. The file size is only a factor if one is archiving the document.

WIDECOM offers a refreshing change to the above: By Suneet S. Tuli
No CCD cameras, no electronic stitching and no divergence. Simply the world’s fastest large document scanner, at a price almost 30% lower than its closest competitor.WideCom’s revolutionary Single Line Contact Scanner forms a 36" wide module with 14,400 pixels resulting in a true 400 dpi resolution(Interpolated at 800 dpi). By using a 36" wide single line fiber optic selfoc array, an individual lense is allocated for each pixel -- voila ! no divergence problem.

WideCom’s SLC936 scanner provides image resolution competitive with 3 CCD camera scanners at a fraction of the cost, and a significantly faster speed