I dwelt upon the nature of digital noise in some earlier articles. In brief, the signal-to-noise ratio 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 independently.
The graphs below show 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. The delta SNR parameter is the total of the measurements.
The higher is the delta SNR value, the better is the protection of the scanner against EMI
As you see, the delta SNR of the Perfection 3490 is far lower than that of the 3170. I also want to offer you the results of the Perfection 2480 and 2580 models for comparison:
So, the hardware components of the latest scanner models from EPSON, especially of the inexpensive ones, seem to be poorly protected against interference.
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: