Surface defects inspection is an important step to in-line detect coating defects such as voids, streaks, clumps, bubbles, scratches etc. in continuous web coating and similar processes. It provides evidences for process optimization, root cause analysis, as well as quality control/assurance.
Defects inspection system commonly has two different types: transmission type and reflection type, which can be schematically shown in the following figure. This link has more information about lighting and incident light angle for different inspection modes.
The concept is straightforward, while one important concept is the resolution which determines the capability of a system to detect the minimum size of defects. Since the web is moving in MD direction, the resolution in MD and CD are different.
The CD resolution is determined by inspection width of a CCD camera and its pixel resolution, as shown in the following equation. For example, for a web of 570 mm and a monitoring 4K CCD camera has 4096 pixels, the resolution or minimum size of the defects in CD which can be detected is 570,000/4096=139 um. To improve resolution, better camera can be used, e.g. 16K (reference), and multiple cameras can be used, while each camera only monitor a narrow section.
For the MD resolution, it is determined by coating line speed, camera demodulation frequency, pixel numbers, or line/s (demodulation frequency divided by pixel number per line), as the following equation:
For example, in the case of line speed of 100m/min (1667mm/s); 4K camera, demudulation frequency of 80MHz, the resolution will be:
Or a camera’s line scan frequency, lines/s, is given in spec, e.g state of art line scan frequency can be 200K Hz, or 200K line/s. In the above example, the minimum defect size the camera can detect will be 1667 (mm/s)/200K (1/s)=8.3um. (reference)
Reference: aimcal.org; NDC; Vision263; Ametek; Edmund