CAD users wanting to buy a standalone wide format scanner for archiving, printing or raster to vector conversion of technical drawings and maps face a growing array of choices and confusing claims.
We wrote this article, first published in MCAD Magazine (16th September 2007), to help you make an informed choice about the large format scanner that's best for you.
There are two main types of wide format scanner technology: CCD (Charge Coupled Device) and CIS (Contact Image Sensor).
Vested interests on either side have white papers on "CCD versus CIS" explaining the superiority of their technology over the other. Depending on whose article you read the other technology is utterly unsuited to your requirements.
Nothing is further from the truth. CCD technology has been in production longest and has the greatest number of users worldwide. CIS technology is newer, more affordable, growing in popularity and, despite what its CCD critics say, capable of good results, especially sharp monochrome ones.
CCD scanners are agreed to have a wider colour gamut, (ability to scan a wider range of colours) and a higher dynamic range, (ability to capture smooth gradations of tone). Generally, this makes CCD better suited to demanding professional reprographics work. Both technologies are suitable for scanning monochrome or colour technical drawings but increasingly CIS scanners are more likely to do it at a price the average CAD user can justify.
There are many large format scanner suppliers but only a few genuine scanner manufacturers.
Contex (Denmark) and Graphtec (Japan) are the two major scanner manufacturers. Contex sell only CCD scanners while Graphtec sell only CIS ones. Both are sold under other brand names: Calcomp, HP, Oce, Vidar (Contex) and KIP (Graphtec).
Colortrac (UK) is the only manufacturer to produce both CCD and CIS scanners. Colortrac recognise "the individual merits of CIS and CCD sensor technology and .... provide products optimised for technical documents or graphic arts imaging applications". Colortrac CCD scanners are also sold under the Paradigm Imaging (USA) brand.
Image Access (Germany) offer the WideTEK 36, an OS independent, networked CCD scanner that includes a built-in PC. It is branded by Bowe Bell + Howell in the USA. Shapemakers (Australia) offer the Deskan, another unique device, a CCD scanner that scans an A0 drawing in seven A4 strips which are automatically stitched together.
Oce (Holland) and Xerox (USA) manufacture their own CCD scanners for use within their MFP (multifunction peripherals that combine a scanner and a printer) solutions. HP and KIP also offer MFP systems using Contex and Graphtec scanners respectively. For the purposes of this article we concentrate on standalone scanners only.
Contex, Colortrac and Graphtec all offer different standalone large format scanner models at different prices.
Within any manufacturer's range, models are largely differentiated on the basis of one of more of the following: colour capability (black and white only or colour), scan width, the thickness of the media you can scan, scan speed and interpolated resolution.
Contex differentiate on speed and interpolated resolution.
Colourtrac and Graphtec differentiate on speed, colour and media thickness.
Because many buyers, specifically government departments, buy on specification and price, not image quality, some manufacturers manipulate product descriptions and system specifications in order to create what appears on paper to be a competitive edge.
There are three types of resolution - optical, interpolated (often referred to as "enhanced" or "extended") and "real".
Optical resolution is the only resolution that means anything. It is the highest resolution that the scanner is physically capable of scanning at. Most technical drawings require to be scanned between 200 and 400 dpi optical. Most scanners today are 600 dpi optical devices with the exception of Contex scanners which are largely 508 dpi.
Interpolated resolution should be ignored as it adds no detail to a scan but adds enormously to the file size. Both Graphtec and Contex use it as a justification for their higher priced Plus models.
"Real" resolution is a Contex inspired measurement of image quality. Contex recently replaced optical resolution with a value of their own choosing which they call "real" resolution. Contex argue that their "real" resolution better indicates the overall scanned image quality that their products offer. Thus you will see Contex scanners with 508 dpi optical resolution described as having "600 dpi resolution".
Some Colortrac ads describe SmartLF Gx scanners as having 1200 dpi optical resolution. In fact SmartLF Gx scanners scan at 1200 x 600 dpi optical and save at 600 x 600 dpi, so they are more accurately described as having 600 dpi optical resolution.
No standard benchmark test exists for wide format scan speed. As most manufacturers rate scan speed differently any straightforward comparison of their claims will be misleading.
The lower the resolution the faster the scan, so the lower the resolution used for the published scan speed, the faster the scanner appears. Colortrac publish scan speed at 200 dpi. Contex publish speed at "400 dpi Turbo" which is effectively 200 dpi. Graphtec publish scan speed at 400 dpi. Image Access and Shapemakers publish scan speed at various resolutions.
Scan speed is also influenced by the host PC specifications, especially when scanning at high resolutions and in colour. As a rule, scanners scan faster than PCs can capture the data. This can make an expensive fast colour scanner as slow as an entry-level slow colour scanner for all practical purposes.
Note that the speed of the scanner is only a part of the overall time taken to process a scanned technical drawing.
48-bit colour capture
Contex and Colortrac both claim to offer 48-bit colour capture. This means that the scanner scans in over 281 trillion colours and then chooses "the best" 16.7 million colours (24-bit colour), which it saves. Graphtec offer 42-bit colour capture and Image Access 36-bit. Like Contex and Colortrac, both save 24-bit colour.
36-bit colour capture (68.7 billion colours) captures the entire range of colours that can be represented on the best quality scannable media (film transparency). Therefore the extra colours that can be captured by 42- or 48-bit capture offer no extra detail. It especially offers nothing to CAD users scanning monochrome technical drawings or drawings with only a few colours.
All scanners except those that use one camera, like the Contex Hawk-Eye, scan in strips and "stitch" them together. Despite manufacturers' claims, all have potential stitching issues and need to be calibrated from time-to-time to overcome this problem. We have seen stitching errors in scans made by scanners from all the major manufacturers.
Regardless of CCD or CIS technology, today's wide format scanners capture technical drawings with much the same scan quality. Paying more does not bring further benefits for most technical drawings. Any practical difference is in the scanning software.
Most scanners have TWAIN support which means they can be used with any software that supports TWAIN, such as Photoshop and Scan2CAD raster to vector conversion software. In addition to TWAIN support, scanners are usually supplied with free and/or paid-for proprietary software. There are three major productivity features to look for.
- The ability to change the scan settings AFTER scanning the image.
This means that the correct scan settings can be established without
having to rescan all or part of the drawing.
- The ability to VIEW the changes you make to the scan
settings in REAL TIME as you make them. This means that you don't
blindly set scanning values without knowing the effects they will have.
It avoids endless experimentation.
- A good range of zooming and panning tools so that it is quick and easy to view the image in detail to make sure it has been scanned to your satisfaction.
For cleaning dirty technical drawings it is vital that your software offers practical thresholding functions. Most scanning software offers thresholding but in some the implementation is so poor it renders the function awkward and or useless.
Finally, other features like auto paper sizing, image rotation, crop and deskew will add to your productivity.
The perception is that the higher the resolution the better the print quality. This is not strictly true. Many good prints are made from scans as low as 200 dpi optical. Printing at 600 dpi optical resolution creates a bigger file which takes longer to print.
CIS scanners do not capture colour as well as CCD ones, particularly artwork, photographs and maps. Colour calibration and special software will be needed if the colours in the scanned image are to match the printed ones correctly for reprographics purposes.
However, for the purpose of copying or printing scans of colour
technical drawings getting the exact colour match is unnecessary.
Technical draftsmen and CAD users do not need the fine colour
sophistication of CCD scanners.