There are two type of sensor technology, Photomultiplier tubes (PMT) and Charge Coupled Devices (CCD) are used in scanners. They will be evaluated on their density range, signal-to-noise, resolution, and any extra benefits.
Density or dynamic range is the capability of the scanners' light detectors to capture detail in both highlights and shadows. It specifies the range of tones from lightest to darkest that a scanner can detect on a logarithmic scale of 0.0 to 4.0. 0.0 represents pure white and 4.0 represents pure black. If a scanner was capable of a 4.0 density range, it would have a 10000:1 ratio difference between the lightest and darkest intensities. If a scanner has a 3.0 density range, it would only have a 1000:1 ratio difference between the lightest and darkest intensities.
Generally, the human eye can detect 16 million different colours. A wide density range can be reproduced in a panoramic, continuous tone. Alternatively, a narrower density range at the lightest and darkest range will be cropped, and at the mid-point range will be compress. As the result, the human eye can be able to discern the colour have a jumble tone showing in short density range.
Noise is form from uncontrolled electric charge or vibration. When the sensor to receiving uncontrolled electric signal, those signal will combine with regular signal to product an image pixel was misinterpreted.
The resultant can lead the scanned image loss of sharpness, disproportionate colours and decrease of shadow intensities.