How to test a system with a known-defect sample
Test an inspection system with a sheet whose defects are known, each with position and type written down. The item passes only when every one of them is reported. A good-looking good sheet does not prove the system can catch defects.
Test an inspection system with a sheet whose defects are known, and write the position and the type of each defect. This item passes only when the system reports every one of them. A good-looking good sheet used as a demonstration does not prove the system can catch defects.
Why it is judged this way
The known sheet needs at least these: a white spot that breaks text, a fine line, register outside tolerance, and a barcode whose quiet zone is occupied. Write the positions on paper. If the system reports only two of them, do not write the acceptance as passed and then move the threshold until it just passes.
What to compare
| What you see | Call it | Do not |
|---|---|---|
| Every known defect is reported | This item passes | Sign while a known defect was missed |
| A good sheet stops the press many times for no stated reason | Record the false rejects and look elsewhere | Use it in place of the defect sheet |
| The defect is seen only after the threshold is moved | File the threshold with the sheet | Keep only the final pass |
Check at a job change or a new roll
- Number each intentional defect and write the alarm type it should raise.
- Run once, and tick any miss against the number.
- If the threshold was changed, run the same sheet again.
