Testing Scanners

Testing a scanner is a complex task for which specialist target images are devised. A number of vendors offer target images, but they are expensive and are really designed for professional use.

However, there are methods that ordinary users can employ. Any material with fine detail, such as paper currency, will do. A banana is especially good if you want to test a scanner's depth of field.

Colour is a tricky one. One system (from which samples are shown in the illustration) is designed for Web and print publishing. Each colour sample is accompanied by a table of RGB, CYMK, HTML hex, and Hexachrome values. A problem with this kind of material is that over time the colours change. High-end graphics applications usually have some method of showing colour values at any point in an image, and enable the user to key in colour codes. HTML provides for specifying colour by a six-digit hex code.

What has to be kept in mind is that a scanner is just one part of a system. The colour is eventually displayed or printed by some other device, and if that device is unable to reproduce colours accurately it is not necessarily the fault of the scanner. Professional printers can control colour quite accurately if they are given the necessary information, and there is a fair degree of control available on good inkjet colour printers.

Monitors degrade from day one. It is a slow process that usually goes unnoticed until an objective model is used to measure on-screen colour reproduction. Of course, a Web author or publisher has no control over the equipment employed by down-line viewers, which is a good reason for restricting the number of colours used in Web images.

In the end, colour - and even resolution - are matters of subjective choice. It is common for (human) viewers to have different opinions of which is the better of a group of pictures of the same subject.

If detail and colour fidelity are important, then it is worth the trouble of testing the scanner, calibrating one's monitor, and deciding on how best to print output. Two objects, readily available, are paper currency and old postage stamps (the kind that were printed from engraved plates). There is a lot of fine detail and colour variation that make them useful target images. Of course, replicating the coin of the realm is frowned upon, particularly if one tries to spend it.

The currency test is an effective way of comparing the performance of different scanners. It shows up the fact that stated resolution does not necessarily equate with a scanner's capacity to capture detail.

Figure 2. A Pantone Internet
Color System swatch with
RGB, CYMK, HTML, and
Hexachrome colour values.
Useful for entering colour
information codes or visually
checking colour output.

Reprinted from the March 2002 issue of PC Update, the magazine of Melbourne PC User Group, Australia