Dynamic Range Scanners

Dynamic range is somewhat similar to bit-depth in that it measures how wide a range of tones the scanner can record. It is a function of the scanner’s analogue-to-digital converter – along with the purity of the illuminating light and coloured filters and any system noise.

Dynamic range is measured on scale from 0.0 (perfect white) to 4.0 (perfect black), and the single number given for a particular scanner tells how much of that range the unit can distinguish. Most colour flatbeds have difficulty perceiving the subtle differences between the dark and light colours at either end of the range, and tend to have a dynamic range of about 2.4. That’s fairly limited, but it’s usually sufficient for projects where perfect colour isn’t a concern. For greater dynamic range, the next step up is a top-quality colour flatbed scanner with extra bit-depth and improved optics. These high-end units are usually capable of a dynamic range between 2.8 and 3.2, and are well-suited to more demanding tasks like standard colour prepress. For the ultimate in dynamic range, the only alternative is a drum scanner. These units frequently have a dynamic range of 3.0 to 3.8, and deliver all the colour quality one could ask of a desktop scanner. Although they are overkill for most projects, drum scanners do offer high quality in exchange for their high price.

In theory, a 24-bit scanner offers an 8-bit range (256 levels) for each primary colour – the difference between 256 levels is commonly accepted to be indiscernible to the human eye. Unfortunately, a few of the least significant bits are lost in noise, while any post-scanning tonal corrections reduce the range still further. That’s why it’s best to make any brightness and colour corrections in one go from the scanner driver before making the final scan itself. More expensive scanners with 30- or 36-bit depths have a much wider range to start with, offering better detail in the shadow and highlight areas, allowing you to make tonal corrections and still end up with a decent 24-bits at the end. A 30-bit scanner collects 10-bits of data each for the red, green and blue colour components while 36-bit scanners collect 12-bits for each. The scanner driver allows the operator to control which 24 of those 30 or 36 bits are kept and which ones are discarded – this adjustment being made by changing the Gamma Curve, accessed through the TWAIN driver’s Tonal Adjustment control.