Resolution

You’ve got a scanner already? And what were you looking at when choosing it? Besides the speed factor, many users guide themselves by the resolution parameter which is usually declared in the description of any scanner. The real optical resolution depends not only on the number of elements in the CCD array but also on the entire optical system. It is possible to practically check how well the scanner digitizes an original with numerous tiny details. Special patterns with alternating black and white lines are employed for such tests. The spatial frequency parameter of a pattern is determined by the number of line pairs per inch (lppi). A pattern looks like that:

Applied Image QA-69-P-RM test target

There are five patterns here with a differing spatial frequency: 30, 70, 95, 140 and 180lppi. I digitized them at the scanner’s maximum optical resolution (4800dpi in this case).

The CDF graphs of the two Perfections coincide almost perfectly. The deviation between them is really negligible. The test data confirm the accuracy and precision of my calculations at any rate.

Click to enlarge

Next I found the modulation transfer function for the five patterns – it is represented with sinusoidal functions in the diagram. To compare the tested scanner with some other models I took the min and max values and put them into the following formula:

So, we can compare the modulation transfer functions of the scanners. As you see, the Perfection 4990 is somewhat better than the others, but not too much. The three functions are close to each other, thus confirming that the scanners all belong to a class of devices with a high optical resolution.