What is Barcode Verification Aperture?
What does it mean and how do you select the right size?
When grading barcodes, the term “aperture” has little to do with the traditional photography term describing the shutter opening in a lens. Instead the term refers to a specific size round circle where a sampling of light reflected to the sensor is recorded. The circle or “aperture” is comprised of pixels in image-based verification or a focused laser beam in laser-based verification. For 2D bar code grading with imager-based systems the “aperture” is also used to smooth the surface of the bar code by blurring of the image. The size of the aperture is an important dynamic in the outcome of the bar code grade.
Because the size of the aperture is significant it is always reported as part of a formal grade. Any real bar code verifier will always report a formal grade. This grade is always reported in a format where a numeric grade value is followed by the aperture size and then the wavelength of light used and then angle of light where required. The formal grade would look something like this 3.0/08/660/45Q. The 3.0 would represent 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 will always be 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 bar code 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 will be more susceptible to defects or substrate texture. A larger aperture will be able to blur or even out defects or texture but will be 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 bar code 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 what is reported. When using ISO-TR29158 the user does not have the opportunity to select the aperture, it is automatically calculated.