RESOLUTION:
Do the large format scanners with the highest advertised resolutions have the best resolving power?
(Note added on 10th April 2008: Since this article was written, the 508 dpi Contex Crystal has been replaced by the 600 dpi Contex HD4230. The 600 dpi Contex Puma has been renamed the Contex HD4230i.)
In the past, Scanners4CAD has taken both Colortrac and Contex to task for publishing what we believe to be misleading resolution information.
Colortrac claim that their Gx range of scanners have an optical resolution of 1200 dpi. In fact, Colortrac SmartLF Gx scanners scan at 1200 dpi x 600 dpi and save at 600 dpi x 600 dpi. Colortrac justify their claim on the basis that the CCD components they use are specified as 1200 dpi by the suppliers.
Contex describe their 508 dpi optical resolution scanners as having a "real" resolution of 600 dpi, and their 600 dpi optical resolution scanner (the Puma G600 iJET) as having a "real" resolution of 800 dpi. Contex justify their claims on the basis that they believe that their "real" resolution figures provide a more accurate description of the actual capabilities of their scanners than the optical resolution.
Scanners4CAD wanted to know whether the Colortrac SmartLF Gx 42 and Contex Puma G600 iJET really have better resolution than other scanners, and whether it matters anyway in the context of scanning a CAD drawing.
Over the last few months, we have been collecting scans of a standard T-10 resolution test chart made on a number of scanners. We decided to compare the scans.
The T-10 resolution chart
The T-10 resolution chart shows a series of labelled horizontal and vertical line sets (test elements):
The largest test element at top left is labelled 1.0. This means that there is one cycle (line plus space) per mm.
The next largest test element is labelled 1.1 - 1.1 cycles per mm.
The test elements continue all the way down to 18 - 18 cycles per mm!
The idea is to see at which point it becomes impossible to resolve individual lines.
On the printed T-10 resolution chart, we were just about able to visually resolve lines at 5 cycles per mm. Everything smaller looked like a grey blur.
The scans
We collected scans of the T-10 resolution chart made on the following scanners:
| Scanner | Technology | Optical Resolution | Advertised Resolution |
| Colortrac Cx 40 | CIS | 600 dpi | 600 dpi |
| Colortrac Gx 42 | CCD | Scans at 1200 x 600 dpi. Saves at 600 x 600 dpi. | 1200 dpi |
| Contex Crystal G600 | CCD | 508 dpi | 600 dpi |
| Contex Hawk-Eye G36 | CCD | 200 dpi | 200 dpi |
| Contex Puma G600 iJET | CCD | 600 dpi | 800 dpi |
| Graphtec CS600 | CIS | 600 dpi | 600 dpi |
| Graphtec SK200 | CIS | 600 dpi | 600 dpi |
| Image Access WideTEK 36 | CCD | Scans at 1200 x 600 dpi. Saves at 600 x 600 dpi. | 600 dpi |
All scans were:
- Made by a staff member of the manufacturer or distributor.
- Made at 600 dpi.
- Made in 24-bit colour so the results would not depend on the quality of the thresholding used to create a black and white scan.
- Saved in TIFF format so they would not be degraded by JPEG compression.
Visually resolvable lines
When scanning a CAD drawing, no point is served by being able to resolve lines that are closer together than you can see by eye.
As stated previously, the smallest test element on the T-10 resolution chart we were able to resolve by eye - just! - was 5 cycles per mm.
Below are the 5 cycle per mm test elements from each scanner, displayed in random order at 1 pixel on the scan = 1 pixel on the screen size.
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Only one scanner above cannot resolve the lines. The blurry scan was made by the Contex Hawk-Eye. The Hawk-Eye's optical resolution is just 200 dpi. This is less than half the resolution of the other scanners tested, so you would not expect it to perform as well as the other scanners in a resolution test.
ALL the scanners with 508 dpi or greater resolution were able to resolve the lines. In other words, if you can see it, they can scan it.
The resolution game
Now let's look at some test elements that are too small to resolve visually.
Below are the smallest test elements on the T-10 resolution chart from each scanner, displayed in random order at 1 pixel on the scan = 1 pixel on the screen size.
Click on the scan below that you think resolves the lines the best. How far down can you count five distinct lines?
When you click on the scan, we'll tell you which scanner it was made by.
Was it made by the Colortrac SmartLF Gx 42, advertised as having 1200 dpi resolution? Was it made by the Contex Puma G600 iJET, advertised as having 800 dpi resolution? Or was it made by another scanner? Click and find out ...
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Q&A: ASK AN EXPERT
Q: How do I scan damaged or torn drawings without them snagging?
A: The traditional solution is called sticky-tape. It provides a quick fix but is not an ideal solution.
A good alternative is to put the torn drawing into a plastic wallet also known as a document carrier sheet or a "skin". This provides a useful solution to the problem of scanning delicate or torn originals which cannot be taped prior to scanning.
Available to order with your scanner or separately these plastic scan wallets come in A1 and A0 sizes and are made from tough acetate or flexible polypropylene.
Scan quality is generally unaffected by the use of a plastic scan wallet, however despite the manufacturer's claims that they will resist scratching both polypropylene and acetate wallets will scratch with long use. This will ultimately have a negative impact on scan quality.
We're happy to answer questions! Click here to email us.
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