How much resolution a label web actually needs
A defect must cover about 3 pixels. A 0.1 mm pinhole needs about 0.03 mm per pixel, about 850 dpi across. 200 dpi from a sales sheet shows coarse dirt, not this pinhole. Write the smallest defect first, then back-calculate. Do not pick the camera first.
To see a defect reliably it must cover about 3 pixels. A 0.1 mm pinhole therefore needs about 0.03 mm per pixel, about 850 dpi across the web. A sales sheet at 200 dpi can show coarse dirt; it will not show this pinhole. Write the smallest defect first, then back-calculate resolution. Do not pick the camera first.
Split the two directions
Cross-web resolution is set by camera pixels and web width. Web-direction resolution on a line-scan camera is set by line rate and speed. If speed rises and line rate stays put, the web direction goes coarse first. So “8 megapixels” does not say the web direction is enough. Calculate at the top speed you promised.
Fine type and pinholes do not ask for the same pixels. A 0.1 mm broken stroke and a 0.3 mm dirt spec cannot be signed off with one dpi. Take the smallest defect on the ticket and divide by 3.
Write the defect before the camera
| Smallest defect | Pixel size needed | Meaning |
|---|---|---|
| 0.3 mm dirt | 0.1 mm | About 250 dpi |
| 0.1 mm pinhole | 0.03 mm | About 850 dpi |
| No smallest defect written | Cannot choose | Write the defect, then buy |
Check before the job runs
- Write the smallest size you must catch.
- At top speed, calculate cross-web and web-direction separately.
- The coarser of the two must still be smaller than one third of the defect.
