I dwelt upon the nature of digital noise in my earlier reviews. In brief, the signal-to-noise ratio which is measured in this test is indicative of the tolerance of the scanner’s electronics to various kinds of interference. The image noise can be random or correlated. I’ll deal with each type of noise independently.
The diagram below shows the dependence of the amount of random noise on the reflection power of the grayscale sectors of the KODAK IT8.7/2 Q-60R2 target.
The signal-to-noise ratio, which is the ratio of the median of a grayscale sector to the deviation, should be regarded as “bigger is better”. That is, the bigger the SNR, the higher the scanner’s noise tolerance is. The delta SNR parameter is the total of the measurements.
Correlated forms of noise are the most annoying – they show themselves as image “patterns” (horizontal or vertical stripes, usually). The predominance of such noise greatly reduces the signal-to-noise coefficient, which is calculated as the ratio of the median to the deviation. The calculated coefficients are put into the next diagram. Bigger values are better:
By the way, now that this section is all about noise, I’d want to acknowledge here that the Epson Perfection 4490 is very quiet at work.