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Part 1: Scanner types

This feature aims to help you choose the right scanner and to get the most out of it, whether you are an amateur on a tight budget or a professional needing high quality for reproduction and sale. It looks at the main types of scanner, explains the significance of resolution, bit depth and density range, and looks at hardware and software issues.

Which type of scanner do you need?

If you are buying a scanner, the first thing you need to decide is what you want it for. Scanners can be used to scan prints and other flat art work, and also to scan film negatives and transparencies. They are also used to scan text so it can then be converted into editable files using appropriate software. More specialised usages are also possible, with some scanners being used to record both two and three-dimensional objects.

Scanners run in price from under $50 to more than $50,000, and not surprisingly their quality and facilities differ. The top-end models are largely used in publishing or by companies offering expensive scanning services, and this feature will concentrate on those more likely to be of interest to photographers.

Fortunately, the quality produced by the best affordable scanners can be very high, and scans produced from 35mm negatives and slides from the best equipment aimed at advanced amateurs and professionals can pass as 'drum scanned.' If you have high quality large format negatives the differences are still clearly evident in large prints.

Flat bed Scanners

A flat bed scanner is designed for scanning prints and other flat artwork. Most have a glass bed or 'platen' on which the original is placed, on the top of a box containing a light source and photosensitive receiver, usually a CCD array or a CIS (Compact Image Sensor), a single row of photocells. Older scanners were often monochrome only, but now all are 'three colour' RGB devices using separate photocells to measure red, green and blue light.

The print is scanned in a number of parallel lines, with the photocell moving along each line to record the light reflected from the print. The light source may also move or it may be a strip across the width of the scanner. Some scanners use a LED light source that can rapidly switch from red to blue to green, allowing a single row array to read all three colours one after the other. Others use a white light source with RGB sensitive cells in the array.

At the end of each line, a stepper motor moves the light source and receiver a small step across the print to let the CCD to read the next line, continuing in this way until the end of the scan. The resolution of the scanner in one direction is thus controlled by the closeness of the steps across the print. In the other dimension, the resolution depends on the spacing of the photocells in the array.

You can read more about how flatbed and other scanners work in the links to simple technical features at 'Howstuffworks' and 'Extremetech' - see box at top right.

Resolution

These two resolution figures give what is known as the 'optical resolution' of the scanner, usually expressed as two figures in dots or pixels per inch. So a typical flatbed scanner may have an optical resolution of 1200x600. There is a small problem with unequal figures like this, in that we need equal horizontal and vertical resolution for most image files. The scanner software can either sample down the higher figure to give 600 ppi, or extrapolate the lower figure to produce 1200 ppi.

Although there is little practical advantage (and the disadvantage of doubling the file size) not surprisingly the marketing departments of scanner manufacturers prefer the second approach. Often they take this further, by providing a software option that will increase the apparent resolution, perhaps to 4800 ppi. Should you ever have a need to do this, there is software that will do a better job than the scanner software. So always ignore any figures higher than the optical resolution.

Resolution is seldom important if you are buying a scanner solely to scan prints. You will seldom if ever want to use more than 600dpi for this purpose, and most scanners provide this. You will want higher resolutions if the scanner includes an option for scanning transparencies (see the next part of this feature.)