Quality Checking

Whatever the method of generating and reproducing a bar code image there is always a danger that the bar code will not scan correctly. Bar code standards vary in their recommendations for controlling symbol quality.

For controlling dimensional tolerances, some standards recommend the use of a printability gauge which consists of a number of blocks containing a series of parallel bars. Each contains bar spaced at a different pitch from each other.

The user obtains a film of the printability gauge which is then reproduced on to a printing plate. When the gauge is printed the user looks for the block with the finest bars that has not filled in. This then indicates which symbology magnifications may be printed safely.

The Printability Gauge

Many users also use a scanning device known as a "verifier" which performs a quality check on the bar code image.

These devices range in price form those costing a few hundred dollars to those in excess of $30,000 and the facilities vary accordingly.

At the top of the range a verifier will measure each bar and space individually record the contrast, confirm the check digit, magnification and symbology and provide a hard-copy for keeping on file or sending to the customer as proof of quality. It will read from positive or negative film as well as paper.

A verification printout.

At the opposite end of the scale, simple devices will only check that the symbol reads and will not give an indication of quality. Care should be taken in the choice of any quality checking device. Particularly that the spectral range of the light source used in the device is compatible with that specified by the symbology specifications.

Unfortunately, the only true guarantee of a bar codes quality is for it to read on the stores scanner. However, printed tolerances of bar codes are set at a level which allows a good safety margin between the printed tolerances and the point at which a code will not consistently decode correctly through the supermarket scanner.