Devices such as computer monitors, printers and data projectors can process digital images. They translate the dots or pixels that make up a digital image to the device’s specifications. The number and colours of the pixels determine the appearance of the digital photo. The pixels are the intersections of a fine grid, which is why these images are called raster images.
The quality of a raster image depends on the following factors:
1. Resolution. The pixel dimensions of a raster image are the total number of pixels in the horizontal and the vertical dimension. The granularity or resolution is expressed as the number of pixels per inch (2.54 centimetres). The resolution must be tailored to the details of the object being digitized, i.e. not too coarse and not too dense.
2. Dynamics. To what extent does the raster image contain all the colours of the original and how are these colours coded; what colour space is applied? Accurate colour reproduction requires calibration of the recording equipment by an expert.
3. Compression. Because raster images may consist of millions of pixels, compression techniques can be used to reduce the file size.
4. Documentation. Descriptive and technical/administrative metadata. This can either be included in the raster image or created separately (or both). Many digital cameras support the EXIF standard (https://en.wikipedia.org/wiki/Exif), which contains descriptions such as the time of recording and the camera settings.
5. File format. The selected file format should efficiently and effectively support the above features.
With respect to the archiving and sustainability of raster images it is essential that they can be reproduced in the future in accordance with the intentions of the depositor. Using TIFF, JPEG and PNG it can be reasonably assumed that they will be displayed without problems and that standard image processing software will be available to render the images.
Preferred formats
- JPEG (.jpg, .jpeg)
- TIFF (.tif, .tiff)
- PNG (.png)
- JPEG 2000 (.jp2)
- DICOM (.dcm)