Virtually every digital image produced with digital cameras or scanners has noise constituents. This noise appears due to interference in the analog signal of the light-sensitive sensor before it reaches the analog-to-digital converter. Noise can be either random or correlated, so we have two test steps in this section.
The diagram below shows the dependence of the amount of random noise on the reflection capacity of the grayscale sectors of a KODAK IT8.7/2 Q-60R2 target. Obviously, the signal on the first (white) sector will be always higher than on the last (black) sector, while the deviation may be random. The signal-to-noise ratio should be regarded as “bigger is better”. High SNR means that the scanner is highly tolerant to interference.
The delta SNR parameter is the total of the measurements.
Unlike random noise which shows up as the “snow”, correlated forms of noise show themselves in the scanned image as regular patterns, stripes, and diagonal, horizontal and vertical lines (relative to the raster lines). Each form of correlated noise can be identified visually.
The vertical form of correlated noise – streak noise – is an inherent defect of the CCD technology. Raster-correlated noise can appear due to uneven light intensity of the scanner’s lamp as well as due to electrical interference of the light-sensitive array. The diagonal form of noise may appear as a result of interference in the useful signal that is transferred from a digital to an analog device (for example, if a failure occurs in the coordination of the digital timer and the carriage movement mechanism). This kind of interference is called crosstalk. As you see, there can be various forms of correlated noise, but we can measure its amount in numbers.
Basing on our methods of testing SOHO scanners, I measured the amount of correlated noise produced by the CanoScan 5200F and put the numbers into the following diagram, comparing them with those of earlier tested models.
This graph shows how well a scanner’s electronics is protected against various forms of correlated noise.