Scan Resolution

Resolution relates to the fineness of detail that a scanner can achieve, and is usually measures in dots per inch (dpi). The more dots per inch a scanner can resolve, the more detail the resulting image will have. The typical resolution of an inexpensive desktop scanner in the late 1990s was 300 x 300.

A typical flatbed scanner has a CCD element for each pixel, so for a desktop scanner claiming a horizontal optical resolution of 600dpi (dots per inch) – alternatively referred to as 600ppi (pixels per inch) – and a maximum document width of 8.5in there’ll be an array of 5,100 CCD elements in what’s known as the scan head.

The scan head is mounted on a transport which is moved across the target object. Although the process may appear to be a continuous movement, the head moves a fraction of an inch at a time, taking a reading between each movement. In the case of a flatbed scanner, the head is driven by a stepper motor, a device which turns a predefined amount and no more, each time an electrical pulse is fed.

The number of physical elements in a CCD array determines the x-direction sampling rate, and the number of stops per inch determines the y-direction sampling rate. Although these are conveniently referred to as a scanner’s resolution, the term is not strictly accurate. The resolution is the scanner’s ability to determine detail in an object and is defined by the quality of electronics, optics, filters and motor control, as well as the sampling rate.

The actual scan head, though capable of reading a raster line 8.5in wide, will be much smaller than that, typically around 4in wide. The reflected light is presented to the scan head through a lens, and the quality of the optics can have a greater effect on the resolution of the scan than the sampling rate. High resolution optics in a 400dpi scanner is likely to produce better results than a 600dpi device with poor optics.

By late 1998 the physical limit as to how many CCD elements could be placed side by side in one inch stood at 600. It is, however, possible for the apparent resolution to be increased using a technique known as interpolation, which under software or hardware control guesses intermediate values and inserts them between the real ones. Some scanners do this much more effectively than others.

The determination of the appropriate resolution at which to scan an image is important both in terms of achieving the desired quality and in terms of the efficiency of the scanning process itself. Since modern advertising has conditioned us to think that more is always better, it is not difficult to understand why many users have a tendency to scan at too high a resolution. The key point is that scan resolution should always be determined by the capability of the output device. If the chosen scan resolution does not properly match the given output device, there will be adverse consequences. If it is lower than the output resolution, the display or printing process will interpolate the extra pixels that are needed and the final result will lose detail and sharpness. If, on the other hand, the chosen scan resolution is higher than the output resolution, the display or printing process will discard the extra pixels. The result will look fine, but the same quality result could have been achieved with lower resolution and a smaller image file.

The following discusses rules for determining an appropriate scan resolution for each of:

  • a glossy magazine
  • an inkjet printer, and
  • a computer monitor

Printed images use a technique called halftoning to reproduce different levels of colour. In magazines an ordered halftone is used, where regular dots of differing sizes produce the varying levels of colour. The typesetters used in offset lithography – the technology used for printing glossy magazines – are capable of printing at 133 lines per inch. This technology is not quite the same as laser or inkjet printer technology and the general rule here is for layout artists to scan at 1.5 times the printing resolution – an equivalent of 200dpi.

Most inkjet printers use dithering, where the dots are scattered across the area of each pixel. This produces better looking results at lower resolutions. The use of halftoning means that the number of pixels per inch the printer can reproduce is lower than its stated dpi resolution.

When scanning for output on a printer, the scan resolution should match the intended output resolution as closely as possible, taking into account the relative sizes of the original and the reproduction. If these are to be the same, no adjustments are necessary. If, however, the scanned image is to be printed at a different size – either larger or smaller than the original – the scan resolution needs to be adjusted accordingly.

This is best illustrated by an example. Suppose the intention is to print the scan of a 1×1.5in stamp at a size of 2x3in on an inkjet printer that has a print resolution of 600dpi. If the stamp were to be scanned at a 600dpi resolution, the scanned image would have 600 pixels vertically (1in times 600dpi) and 900 pixels horizontally (1.5in times 600dpi). Enlarging the image to the intended print size of 2x3in reduces the effective resolution to 300dpi – the 900 horizontal pixels will be spread out over 3in (900 divided by 3 equals 300), and the same is true in the vertical dimension. This is only half of the printer’s resolution and the output quality will be less than optimal. For the best quality final printed output – which actually uses the 600dpi resolution the inkjet is capable of – the image should be scanned at 1200dpi.

A similar adjustment needs to be made if the output size is smaller than the original. Suppose the intention is to scan a 4x5in cover that is to be displayed on a Web page at half size, 2×2.5in. Computer monitors typically have a resolution of 72 or 90dpi. Scanning the cover at 72dpi would produce an image with 288×360 pixels. Reducing this to half the size would still give an image with a vertical resolution of 144dpi, twice that necessary. In this example, therefore, the original image could be scanned at 36dpi with no loss of quality in the resultant displayed image.

The rules used in these examples can be summarised by the following formula:

SR = (DR x DW) / OW, where

SR = ideal scanning resolution, in dpi

DR = resolution of final display device, in dpi

DW = width at which the image will be printed or displayed, in inches

OW = width of the original being scanned, in inches