When discussing 100% print inspection systems it is often important to understand how they work so that you can determine what you need. The specification of defect detection performance is important.
Pattern recognition is 1st step in all print inspection systems
Take the image of the girl. No matter how advanced a print inspection solution is when comparing images the system must allow for some color to color register. To do this, the algorithm must auto- detect the edges. The above is a common technique to achieve this.
Print register plays a part in your vision system
All printing process involve the application of layers of ink. Depending on the method, sheet or web, gravure or offset, there is always the task of print registration. The 'D' below shows an image with a degree of Miss-register. However no printing process is perfect and a print inspection system must allow for a certain amount of register. It does so by detecting the edges as show above, and then building a threshold image from those edges.
The three images below illustrate the effect. The edges on the print are detected. The last image shows that these edges are increased. This is the image that is then used as a threshold.
What does this mean for you?
When a printed image is compared to a golden template, the differences are then passed through a threshold image as above. Only the differences that are greater than the threshold are considered.
The results are:
- The more the register tolerance, the less sensitive the inspection system.
- The heavier the copy, the less sensitive the inspection system.
- The lower the resolution, the larger the threshold steps, the less sensitive the inspection system.
Resolution makes a difference
If a 100% print inspection system has a resolution of 0.2mm or 200um, this means that every step in the threshold on all edges of the print is 0.2mm. So if they allow two steps then 0.4mm on every edge is masked from inspection. This is key when you are specifying a system. If you have a defect, it is only considered once it exceeds this threshold.
What does this mean?
- If you want to detect 0.5mm defects. Let's assume the resolution is 0.25mm. Your acceptable register is 0.2 mm. The mask must be at least 0.25mm around the edges, else you will get false defects.
- Normally inspection systems need at least two pixels for detection. This means that the minimum defect near an edge will be 0.25mm + 0.5mm or 0.75mm.
Print inspection is a complex application. There are many variables. The algorithm itself is important. There are golden templates, repeat to repeat, dynamic templates and many other variations.
However the following are fundamentals.
- The register tolerance of your process.
- The resolution of the scanning system in your vision system.
- The algorithm being used.
OneBoxVision are industrial automation experts for web and sheet manufacturers. The founders have a combined experience of over 50 years in print inspection and have developed a suite of solutions for flexo, offset, gravure and screen printing. Download our latest whitepaper on Print Inspection Requirements and do not hesitate to contact us if you would like more information.