UKOLN AHDS Producing And Improving The Quality Of Digitised Images


To produce high-quality digital images you should follow certain rules to ensure that the image quality is sufficient for the purpose. This document presents guidance on digitising and improving image quality when producing a project Web site.

Choose Suitable Source Material

Quality scans start with quality originals - high-contrast photos and crisp B&W line art will produce the best-printed results. Muddy photos and light-coloured line art can be compensated for, but the results will never be as good as with high-quality originals. The use of bad photos, damaged drawings, or tear sheets - pages that have been torn from books, brochures, and magazines - will have a detrimental effect upon the resultant digital copy. If multiple copies of a single image exist, it is advisable to choose the one that has the highest quality.

Scan at a Suitable Resolution

It is often difficult to improve scan quality at a later stage. It is therefore wise to scan the source according to consistent, pre-defined specifications. Criteria should be based upon the type of material being scanned and the intended use. Table 1 indicates the minimum quality that projects should choose:

Use Type Dots Per Inch (dpi)
Professional Text 200
Graphics 600
Non-professional Text 150
Graphics 300

Table 1: Guidelines To Scanning Source Documents

Since most scans require subsequent processing, (e.g. rotate an image to align it correctly) that will degrade image quality, it is advisable to work at a higher resolution and resize the scans later.

Once the image has been scanned and saved to in an appropriate file format, measures should be taken to improve the image quality.

Straighten Images

For best results, an image should lay with its sides parallel to the edge of the scanner glass. Although it is possible to straighten images that have been incorrectly digitised, it may introduce unnecessary distortion of the digital image.

Sharpen the Image

To reduce the amount of subtle blur (or 'fuzziness') and improve visual quality, processing tools may be used to sharpen, smooth, improve the contrast level or perform gamma correction. Most professional image editing software contains filters that perform this function automatically.

Correct Obvious Faults

Scanned images are often affected by many problems. Software tools can be used to remove the most common faults:

Be careful you do not apply the same effect twice. This can create unusual effects that distract the observer when viewer the picture.

Further Information