Virtually every digital image produced by digital cameras or scanners has noise constituents due to interference in the useful analog signal of the light-sensitive device 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 power of the grayscale sectors of the KODAK IT8.7/2 Q-60R2 target.
The delta SNR parameter is the total of the measurements.
As you see, the electronics of the CanoScan 8400F is superbly protected against interference.
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, as a result of a failure in the coordination of the digital timer and the carriage movement mechanism).
So, there can be various forms of correlated noise, and we can measure its amount in numbers. The amount of correlated noise produced by the CanoScan 8400F is compared with the results of some earlier tested models.