Noise appears in the image as a result of impurities in the useful analog signal of the light-sensitive sensor of the scanner. This only happens on the earliest phase when low-power electrical signals are transferred to the analog-to-digital converter – that’s where the signal is the most vulnerable. Such noise distortion may be random or correlated. The difference between the two phenomena allows dividing my testing of the noise into two steps.
The graph below shows the dependence of the random noise value on the reflecting capability of the areas of the Kodak IT8.7/2 Q-60R2 target. It’s clear that the signal level of the first part of the scale (white) will always be higher than that of the last part (black), while the deviation value may sometimes be random. The signal-to-noise ratio should be regarded as “the more – the better”. Thus, higher SNR values indicate that the scanner is tolerant to noise.
The graph shows the dependence of the random noise
value on the reflective capacity of the original
The next graphs allow estimating the value of relative color deviations of two scanners: EPSON GT-15000 and Perfection 4870. I used a scan of gray target as a test sample, digitized by both scanners with the same parameters. Ideally, the RGB values should equal each other, producing a straight line.
Color deviation of the EPSON GT-15000 scanner
Color deviation of the EPSON Perfection 4870 scanner
As we might have predicted, the Perfection 4870 shows much better results than the office scanner. Well, the two devices clearly have different purposes.
Unlike random noise, which shows itself as “snow” in the resulting scan, correlated noise is distinct and complies with a certain law. It can show up as stripes, regular raster patterns or diagonal, vertical and horizontal lines (in relation to the lines of the raster). Any form of correlated noise is easily recognizable visually.
The vertical form of correlated noise (Streak Noise) has its origin in the CCD technology itself. The scanner’s lamp has unstable light intensity. Added the electrical pickups of the photosensitive array, the image may acquire noise, correlated to the raster lines. The diagonal correlated noise may appear because of parasite influence on the useful signal, which is being transferred from the digital device to the analog (for example, as a result of the digital timer and the moving mechanism of the scan carriage getting out of sync). Such noise is called crosstalk. As you see, there are many noise-creating factors, and we can estimate them in numbers. Using out methods of testing SOHO scanners, we created the following table:
EPSON Perfection 4870
EPSON Perfection 3200
This table allows making judgments about the tolerance of the electronics of the scanner to noise. However strange, the EPSON Perfection 3200 showed the best results. The explanation of this fact may lie in differences in the electronic stuffing of the machines.