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The main requirement is for each black or white bar in a barcode to be clearly visible on the image. All barcodes have at least 2 sizes of bar and the image needs to be of high enough quality to show this. For example, a Code 39 barcode consists of 9 bars per character, of which 3 must be wide and 6 must be narrow. The wide bars should be around 2.5 times the size of the narrow bars – but for simplicity we will say 2 times the size. This means that each character needs at least 12 pixels in an image. This assumes that each pixel in the image lies either entirely in a black bar or entirely in a white bar. In practise, this isn’t possible and so the resolution should allow around 24 pixels per character.

So, to encode a string of 10 characters (not including the start and stop *) you will need at around 288 pixels. If the barcode is 1 inch wide on the paper then you will need to scan at a resolution of around 300 DPI. If the barcode is 2 inches wide then you will need to scan at a resolution of around 200 DPI.

Some types of barcode have more than 2 widths of bar and a higher resolution still will be needed to ensure that the different widths can be distinguished on the image.

As for barcode height – if you have a very wide barcode then you need to ensure that it is high enough to cope with any skewing that takes place when the image is scanned.

In summary – the success of a project can depend on specifying the correct barcode size and image resolution.