Scanning Guidelines

Simple Steps to Good Colour Scanning

Thanks to Vickie McLeod, Media Lab, Kierstead Hall

1. Start with good scans - GIGO.

2. Scan at the proper resolution. The proper resolution is determined by the end product. Where will this image be displayed? On a printer? On a monitor screen? On the Web? You only need to scan at the resolution required by your output.

If printing your file to a printer, the rule of thumb with colour and gray-scale images is that the image resolution should be 1 1/2 to 2 times the screen frequency of your printer (lpi). For example, a 600 dpi laserwriter printer has a line screen of 66 lpi, therefore you should scan at a resolution of between 100 to 130 for best results.

If your image will only be displayed on a monitor screen, you only need to scan at a resolution of 72 dpi, as most monitor screens will not show an appreciable difference between this resolution and a higher one.

Before you scan, you need to understand the difference between scanning resolution and image resolution. A scanner samples an image every 1/100th inch, 1/300th inch, or whatever you choose spi, (samples per inch). The marks that laser printers make are dots (hence, dpi, dots per inch). When you scan a 2 x 2 inch image at 100 spi (your scanning resolution), you end up with a 100-spi image. Place that image on a page, reduce it to 50%, and the image resolution increases to 200 spi, the same number of samples in half the space. The basic rule therefore is: reduce an image and you increase its resolution; enlarge the image and you reduce resolution.

Using lower-resolution scans can save scads of time and disk space. Double the resolution of a scan and file size increases by a factor of four. Triple the resolution and the file is nine times as large.

Image Resolution 8-bit Gray-Scale (or 256 Colour) Image 24-bit Colour Image
150 spi 439K 1.28 MB
225 spi 989K 2.89 MB
300 spi 1.72 MB 5.15 MB

3. Tonal Correction. Scanned images always require tonal correction - they have to be made brighter - before they will print well.

4. Sharpening. By running scans through a software sharening filter, you can produce sharp, crisp-looking images quite easily.

5. Adobe Website. For an in-depth description and explanation of scanning and scanner technology, visit the Adobe Website

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Jane Fritz,

Last modified January 1997