DLSoft Application Note 0609

Choosing a resolution for online images of small barcodes

When you create barcode images for others to print from their web browser you probably 
do not have knowledge of the resolution of the printer that will print the images. 
The printer resolution is significant because the bars in a barcode should be a whole 
number of printer dots wide - a printer cannot print a bar 1.5 dots wide!

The solution is to choose a resolution that will print all bars with a whole number of dots.

For example, if a barcode image is created at a resolution of 300 dpi it should print correctly 
on a 300 dpi printer. It will also print correctly on a 600 dpi printer - each bar will contain 
twice as many dots across the width, but that does not matter. Similarly the same image will 
be fine for any printer with a resolution that is a whole number multiple of 300.

In practice we have found that 300 dpi barcodes are usually printed acceptably on 720 dpi 
printers (a resolution commonly found for inkjets).

What is less likely to work so well is printing a image created at 600 dpi on a 300 dpi printer 
- because any bar that is not a even number of dots wide will be printed either 1 dot too small 
or 1 dot too wide. So the advice must be - aim for the lowest resolution likely to be used.

In some cases it may be that you need to allow for older thermal printers that have resolutions 
of 200 dpi. A 300 dpi image will not print well on a 200 dpi printer, and a 200 dpi image will 
not print well on a 300 dpi printer.

The obvious solution is to create a 100 dpi image - which will print fine on just about 
everthing. However, there is a catch.

100 dpi corresponds to a dot pitch of roughly 10 Mils (thousandths of an inch). The thinnest bar 
width (the X dimension) must be a whole number of dots, so that limits the X dimension to 10, 20, 
30, Mils etc. For comparison a barcode with other resolutions has a much wider ranger of options:

100 dpi X = (10), 20, 30, etc.
200 dpi	X = 10, 15, 20, 25, 30, etc.
300 dpi X = 10, 13, 16, 19, 22, 25, etc.
600 dpi X = 6.5, 8, 9.5, 11, 12.5, 15, etc.

[The nominal target for GS1 retail barcodes is X = 13 Mils and only values between 10.4 and 
26 Mils are acceptable.]

The large the X dimension the greater the width of the complete barcode, but a wide barcode that 
will scan is generally a better option than a narrow barcode that will not!

It is also worth noting that using vector images does not make the problem easier - even if the 
browser can display/print vector images - because the vector image will be translated into a 
bitmap for printing on most printer technologies.

dLSoft
Jan. 2009