When a scanner converts something into digital form, it looks at the image pixel by pixel and records what it sees. That part of the process is simple enough, but different scanners record different amounts of information about each pixel. How much information a given scanner records is measured by its bit-depth.
The simplest kind of scanner only records black and white, and is sometimes known as a 1-bit scanner because each bit can only express two values, on and off. In order to see the many tones in between black and white, a scanner needs to be at least 4-bit (for up to 16 tones) or 8-bit (for up to 256 tones). The higher the scanner’s bit-depth, the more accurately it can describe what it sees when it looks at a given pixel. This, in turn, makes for a higher quality scan.
Most modern colour scanners are at least 24-bit, meaning that they collect 8 bits of information about each of the primary scanning colours: red, blue, and green. A 24-bit unit can theoretically capture over 16 million different colours, though in practice the number is usually quite smaller. This is near-photographic quality, and is therefore commonly referred to as true colour scanning.
Recently, an increasing number of manufacturers are offering 30-bit and 36-bit scanners capable, so they claim, of producing better quality colour images. Whilst the fact is that very few graphics software packages are capable of handling images with more than 24 bits, the additional bits are still worth having. When a software program opens a 30-bit or 36-bit image, it can use the extra data to correct for noise in the scanning process and other problems that hurt the quality of the scan. With more inherent information present in 30- and 36-bit scanned images, users can more precisely alter image detail and RGB luminance values via the scanner’s driver software.
The alternative route to improved quality – simply by building better 24-bit scanners – that provide cleaner data, less affected by random noise in the lower-order bits – is actually much less economic. This approach would require highly precise optics for focusing reflected light to the CCDs (charge-coupled devices) that capture the data; minimal-distortion glass free from internal contaminants; CCDs that capture light and convert it to electrical signals with high accuracy; and a smoothly-moving scanning head with little vibration. Such a device would cost considerably more than one that used lower-cost and lower-quality components with slightly more expensive ADCs, capable of capturing 10-bits and 12-bits of data each – for the red, green and blue colour components respectively – rather than just 8.