How to set a defect threshold without flooding the operator
The threshold must catch the prepared defect sample and must not alarm on the normal repeat of the signed sample. If one minute of alarms is more than a person can open, the operator will switch the system off. Clamp both ends. Do not only tighten.
The threshold has to catch the defect sample you prepared, and it must not alarm on the normal repeat of the signed sample. If alarms in one minute are more than a person can open, the threshold has no force: the operator will switch the system off. Clamp it from both ends with the defect sample and the signed sample. Do not only tighten.
Try both ends
Too loose, and the defect sample does not alarm: that is no inspection. Too tight, and paper grain, normal dot jitter, and a slight register move all become red boxes. After a few minutes the operator stops looking. Run the signed sample through the system. Red boxes should be close to none. Then run the smallest defect sample the customer specified. It must alarm. The range that satisfies both is the threshold.
If the signed sample is already a field of red, first check whether the region is drawn too large and includes the paper edge outside the cut, or whether the light is unstable. Widening the number again does not fix that.
How many alarms
| What you see | Meaning | Do first |
|---|---|---|
| Defect sample does not alarm | Too loose, or resolution is short | Tighten, or fix the image first |
| Signed sample has stable red boxes | Too tight, or the region is wrong | Change the region, then talk about the threshold |
| More per minute than anyone can open | The system will be switched off | Cut the useless region first |
Check before the job runs
- Prepare one defect you must catch and one signed sample.
- Run both and write down the alarm counts.
- Write the threshold on the ticket only when it alarms on the defect sample and not on the signed sample.
