Document Scanning

Raster images - the scanner output

The pattern of dots representing the document is known as a raster image. This is the electronic output from the scanner. Raster images can be visualized by imagining a fine grid of squares being laid over a document. The size of the grid squares is a measure of the resolution of the scan. The size of the squares is normally in the range between inch and inch, but resolution is usually expressed in dots per inch, or dpi. In this way a 200 dpi image is made up of a grid of inch squares.

The effect of this digitising of the image can be seen in displayed above. Some fine detail can be lost in making the raster pattern, depending on the resolution of the image. Higher resolutions are needed for more demanding applications where fine line detail is involved, or the shape needs to be preserved as well as possible without these pixelation or jaggie effects being too noticeable.

All raster scanners use this grid principle for the digitization of the original image. The data for each pixel depends on the type of image being recorded, monochrome, greyscale or colour.