Full-surface print defect inspection on printed webs
The printed web is scanned across its full width by line-scan cameras under a diffuse tunnel light. The image is aligned to the print repeat and compared with the variation range learned from good prints; ink spots and splashes are logged with their metre position and the web edge is flagged.
Every defect is written to the roll map with area, colour difference and metre position; the labeller applies a flag at the web edge. The map is passed to the rewinder control so the roll can be stopped at the defective metre.
- 01 Scan
- 02 Compare
- 03 Position log
- 04 Flag
The printed web runs under the tunnel light; every repeat is aligned to the reference.
- Position
- —
- Defect area
- —
- Colour difference
- —
Scenes are illustrative simulations; they are not customer references or performance guarantees.
Need
The print pattern is already full of dark and light areas, so a small ink spot disappears in it. The web stretches and wanders sideways, so a one-to-one comparison with a fixed template produces false defects at the edges of the print itself.
Solution approach
A diffuse tunnel light illuminates glossy ink and paper without reflections; line-scan cameras side by side cover the web width. Every print repeat is aligned on its reference points and compared pixel by pixel with the acceptable variation range learned from good prints. A region outside the range counts as a defect by its colour difference and area.
Process steps
- 01
Scan
The printed web runs under the tunnel light; every repeat is aligned to the reference.
- 02
Compare
The image is compared with the good-print model; a region outside the range is marked.
- 03
Position log
Area, colour difference and the metre position from the encoder are logged.
- 04
Flag
The labeller applies a flag at the web edge in line with the defect.
Related applications
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