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REVIEW:

Contex SD4490 large format scanner

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Accuracy

Contex do not suggest or recommend that users calibrate SD Series scanners unless a problem occurs. However, we did calibrate the scanner as part of this review. The calibration is fully automated - all the user must do is feed in the supplied calibration sheet. It took about 20 minutes to run.

We scanned an A0 (E size) drawing in portrait. We measured the lengths of the lines on the drawing and compared them to the lengths of the scanned lines. We found the scanner's accuracy to be just within its published tolerance across the width of the scan.

We found a 2mm error along the length of the scan. All scanners will scan some media with a dimensional error along the scan. This is because different media types have greater or less friction which affects their rate of passage through the scanner. We were able to correct the error along the scan using the SD4490's Vertical Correction feature, although working out the correct correction required a pen, a calculator and some thinking!

Resolution

We scanned a standard T-10 resolution chart (see our large format scanner resolution article).

Five cycles per mm is the smallest test element on the T-10 chart where we are just about able to resolve individual lines by eye. Lines closer together than this cannot be resolved by the human eye and so are unlikely to be found on a CAD drawing.

Below is the 5 cycle per mm test element scanned by the SD4490 along with the same test element scanned by the Contex HD4230i and the Contex Hawk-Eye (now obsolete). The SD4490 is a CIS technology scanner with 1200 dpi optical resolution. The other scanners are CCD technology scanners with optical resolutions of 600 dpi and 200 dpi respectively.

The scans below were made at 600 dpi and are displayed at 1 pixel on the scan = 1 pixel on the screen size.

SD4490 HD4230i Hawk-Eye

The lines scanned by the SD4490 are easily resolvable and are visually sharper than the scans made by its lower resolution CCD technology cousins. In particular, note the difference between the SD4490 and the Hawk-Eye, the scanner which the SD series replaces at Contex's entry level price point!

Sharper resolution means smaller files and faster processing

One of our test drawings contains some close together lines. Below are scans of these lines made by the SD4490, the HD4230i and the Hawk-Eye at 400 dpi.

SD4490
1200 dpi optical resolution
Uses CIS technnology
HD4230i
600 dpi optical resolution
Uses CCD technnology
Hawk-Eye
200 dpi optical resolution
Uses CCD technnology
Below are scans of the same lines made at 200 dpi.
SD4490
1200 dpi optical resolution
Uses CIS technnology
HD4230i
600 dpi optical resolution
Uses CCD technnology
Hawk-Eye
200 dpi optical resolution
Uses CCD technnology

Of the three scanners, the SD4490 is the only one that can fully separate the lines at 200 dpi - its use of CIS technology and 1200 dpi optical resolution gives it a sharpness advantage even when scanning at low resolutions.

This means that you can scan at a lower resolution and still separate fine details, giving you smaller files and faster processing times.

If your intent is to convert the scan to vectors using raster to vector conversion software, this potential enhanced image quality makes practical, useful raster to vector conversions more likely.

Turbo mode

Like Graphtec, Contex include a "mode" in their scanner that allows it to scan faster but that compromises scan quality. Both Graphtec and Contex use these modes to measure the scan speeds that they publish in their sales literature.

Below is the 5 cycle per mm test element from the T-10 chart scanned by the SD4490 at 600 dpi in normal mode and in Turbo mode.

Normal Turbo

You can see that in normal mode, both the horizontal and the vertical lines are resolvable. In Turbo mode, the horizontal lines have blurred into each other.

How does this affect a real drawing?

Below is our test drawing again, scanned at 400 dpi using normal mode and Turbo mode.

Normal mode
Turbo mode

As you can see, normal mode separates the lines perfectly, but using Turbo mode some of the lines have joined and the line quality has deteriorated to leave spurs, speckles and holes.

This would degrade the quality of the results you would get if you were to convert the scan to vectors using raster to vector conversion software.

Conclusion

We would conclude that the SD4490's resolution is more than adequate for CAD. In particular, its ability to separate fine details at lower resolutions potentially allows users to create smaller files with faster processing times. However, we would strongly suggest that CAD users avoid using Turbo mode.

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