What is Barcode Verification Aperture

Because the size of the aperture is significant, it is reported as part of a formal grade. Barcode verifiers report a formal grade. This grade is reported in a format where a numeric grade value is followed by the aperture size, then the wavelength of light used, and finally, the angle of light where required. The formal grade looks something like this 3.0/08/660/45Q. The 3.0 represents a B grade and the 08 represents an aperture of .008” using 660nm light and at an incidence of 45-degrees from four sides. Aperture is reported as thousands of an inch in the formal grade whether a metric system is used or not.

For 1D barcode grading a sampling is taken horizontally across all bars and spaces. The light reflected to the sensor from the aperture is used to create a profile called the Scan Reflectance Profile (SRP). The entire 1D grade is based on this profile. Changing the aperture size can change the amount of light reflected and thus change the profile. Smaller apertures will see defects as being bigger, bigger apertures lessen the effect of defects. Smaller apertures will not see print growth as a big factor where larger apertures will.

For 2D barcode grading a grid is created using clock tracks or alignment patterns. The grid intersections intersect in the centers of the individual modules of the code. The apertures are placed in the intersecting points. If the size of the aperture is changed, the effect can be significant based on ink spread, substrate texture, and so forth. While a smaller aperture may not be affected by ink spread, it is more susceptible to defects or substrate texture. A larger aperture is able to blur or even out defects or texture, but is more vulnerable to ink spread. The ink spread, or other marking issues, are most apparent in the reflectivity of a cell surrounded by cells of the opposite color.

So, what size aperture should an operator use? Aperture size guidelines are spelled out in the grading standards, but even there the recommendation is that the application dictate aperture size. For example, the UPC or EAN codes we all see daily on the products we purchase, the GS1 organization dictates the aperture size. Tables in their General Specification tell the user what size the codes can be printed at and the minimum quality specification. See the table below and notice the formal grade used as a method of dictating what’s acceptable C grade or better (1.5) using a 6mil (.006”) aperture and 660nm light.

Without an Application Standard for guidelines, general rules for 1D codes would be based on the x-dimension.

• Codes with x-dimensions 4-7mil should use a 3mil aperture
• Codes with x-dimension 8-13mil should use a 5mil aperture
• Codes with x-dimensions 14-25mil should use a 10mil aperture
• Codes with x-dimensions bigger than 25mil should use a 20mil aperture

For 2D codes, the ISO-15415 specification says the aperture should be 80% of the x-dimension. It goes on to say that if in an enterprise multiple size codes are used, it is okay to use an aperture equal to 80% of the smallest codes x-dimension.

A note about aperture when using the ISO-TR29158 grading standard: This grading method uses a variety of apertures to decode the barcode and then apertures of both 50% and 80% of the x-dimension are used to grade the code. The symbol must decode using exactly one or more of these apertures in order to pass the “Decode” grade parameter and then the better of the two grades is reported. When using ISO-TR29158, the user does not have the opportunity to select the aperture, it is automatically calculated.