How small a spot a line scan can truly see
A dirt spot a line scan can see steadily usually spans at least 3 pixels. Pixel size is the effective web width divided by the pixels across the web. At 330 mm and 4000 pixels, one pixel is about 0.08 mm, and a steady defect is about 0.25 mm. That is geometry, not the customer’s tolerance.
A dirt spot that a line scan can see steadily usually has to span at least 3 pixels. Pixel size equals the effective web width divided by the number of pixels across the web. At 330 mm and 4000 pixels, one pixel is about 0.08 mm, so a steady defect is about 0.25 mm. That is geometry. It is not the customer’s tolerance.
Why it is judged this way
A spot smaller than one pixel comes and goes. It cannot be written as “can be detected”. If the customer asks for a smaller defect, add pixels or shrink the field of view first. Do not turn the threshold down to pretend the spot is seen. Turning it down only makes the paper grain alarm.
What to compare
| What you see | Call it | Do not |
|---|---|---|
| The defect spans about 3 pixels or more | It has a chance of a steady alarm | Promise that smaller spots can be seen |
| It occupies only 1 pixel | It flickers. It is not a capability | Write it into the acceptance |
| The field gets wider and the pixels get larger | The smallest defect gets worse | Keep the old detection promise |
Check at a job change or a new roll
- From the web width and the pixel count, work out how many millimetres one pixel is.
- Turn 3 pixels into millimetres and write that in this machine’s capability note.
- If the customer wants smaller, change the resolution. Do not change the spoken promise.
