UKOLN Good Practice Guide for Developers of Cultural Heritage Web Services

Image Quality


This section was originally published as QA Focus Briefing papers.


A digitised image requires careful preparation before it is suitable for distribution. This document describes a workflow for improving the quality of scanned images by correcting faults and avoiding common errors. It also offers advice 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.

Preparing your Master Image

The sequence in which modifications are made will have a significant contribution to the quality of the final image. Although conformance to a strict sequence is not always necessary, inconsistencies may be introduced if the order varies dramatically between images. The Technical Advisory Service for Images (TASI) recommends the following order:

  1. Does the image require rotation or cropping?
    In many circumstances, the digitiser will not require the entire image. Cropping an image to a specific size, shape or orientation will reduce the time required for the computer to manipulate the image and prioritise errors to those considered important.
  2. Are shades and colours difficult to distinguish?
    Scanners and digital cameras often group colours into a specific density range. This makes it difficult to differentiate shades of the same colour. Use the Histogram function with Photoshop (or other software) and adjust the different levels to best use the range of available tones.
  3. Is the colour balance accurate in comparison to the original?
    Some colours may change when digitised, e.g. bright orange may change to pink. Adjust the colour balance by modifying the Red, Green & Blue settings. Decreasing one colour increases its opposite.
  4. Are there faults or artefacts on the image?
    Visual checks should be performed on each image, or a selection of images, to identify faults, such as dust specks or scratches on the image.

Once you are satisfied with the results, the master image should be saved in a lossless image format - RGB Baseline TIFF Rev 6 or PNG are acceptable for this purpose.

Improving Image Quality

Subsequent improvements by resizing or sharpening the image should be performed on a derivative.

  1. Store work-in-progress images in a lossless format
    Digitisers often get into the habit of making modifications to a derivative image saved in a 'lossy' format, i.e. a format that simplifies detail to reduce file size. This is considered bad practice, will reduce quality and cause compression 'artefacts' to appear over subsequent edits. When repeatedly altering an image it is advisable to save the image in a lossless format (e.g. TIFF, PNG) until the image is ready for dissemination. Once all changes have been made it can be output in a lossy format.
  2. Filter the image
    Digitised images often appear 'noisy' or contain dust and scratches. Professional graphic manipulation (Photoshop, PaintShop Pro, etc.) possesses graphic processors that can be useful in removing these effects. Common filters include 'Despeckle' that subtly blurs an image to reduce the amount of 'noise' in an image and 'median' that blends the brightness of pixels and discards pixels that are radically different from adjacent pixels.
  3. Remove distracting effect
    If you are funded to digitise printed works, moiré (pronounced more-ray) effects may be a problem. Magazine or newspaper illustrations that print an image as thousands of small coloured dots produce a noticeable repeating pattern when scanned. Blur effects, such as the Gaussian blur, are an effective method of reducing noticeable moiré effects, however these also reduce image quality. Resizing the image is also an effective strategy that forces the image-processing tool to re-interpolate colours, which will soften the image slightly. Although these effects will degrade image to an extent, the results are often better than a moiré.

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

Comments On This Document

This section will be used to provide notes on the section, including details of any changes.

April 2006
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