The process for 2D Datamatrix codes involves grading the code, often according to the ISO/IEC 15415 2011 Standard. This Standard covers measuring, evaluating and grading a symbol’s characteristics within 2D and stacked symbols that have been printed, laser etched or created by dot peen. Various physical attributes of the code are scored individually and combined into an overall print-quality grade. The codes are usually verified by a bench top verification system on samples from the total print run. 100% inspection and verification is much more desirable but is only possible if the codes can be read in-line, immediately after printing. Because of the speed of the line this can be hugely challenging.
A great example of this is the very high-speed printing and verification of a GS1 DataMatrix barcode on webbing moving at 750 inches per minute on a US production line. The in-line verification system developed for this application eliminates motion blur in the image by combining short camera exposure times with high-speed, high-intensity strobing of the LED illumination source using a Gardasoft lighting controller. This has the effect of ‘freezing’ the motion to allow high quality images to be acquired. The high intensity is achieved by using the lighting controller to ‘overdrive’ the LED to give greater than 100% output in short pulses.
In this application, the camera exposure was fixed at 250 μs and the LED array was strobed for 1 ms at 500%. The gain of the machine vision sensor was set to 1, then increased incrementally until the Symbol Contrast (SC) measured by the sensor matched the SC recorded using a Calibrated Conformance Standard Test Card. Setting the acceptance criteria on the in-line verifier one letter grade higher than that of the QC bench top verifier provides a safety margin for print quality degradation caused by subsequent handling and packaging.
The high intensity pulses produced by the lighting controller ensure that the measurements can’t be affected by any ambient lighting in the printing location, and the extremely short pulses eliminate any distortion in the image from vibrations on the web. Finally, the current to the LED can be adjusted to allow for any natural degradation in LED output intensity over time.
