How to inspect a clear film label on a clear web
A clear label on clear film has almost no front reflection. Front light alone leaves an empty field. Add backlight, or run the web on a roller that makes contrast. Pinholes and missing print are clearest with backlight. Backlight is not for metallic ink or color.
A clear label on clear film has almost no front reflection. Front light alone leaves the camera looking at an empty field. Add backlight, or run the web against a roller that makes contrast. Pinholes and missing print are clearest with backlight. Backlight is not for judging metallic ink or color.
Where the contrast comes from
The camera needs the target and the background to differ. Clear ink on clear film looks clear on both sides from the front. Backlight from the other side of the film is blocked where there is ink and bright where there is none, so missing print and pinholes appear. A white or black idler can supply part of the contrast, but a light-through defect such as a pinhole is still more direct with backlight.
Once backlight is on, transmitted light changes color, and metallic ink goes dark or bright in a way that is not true. Color needs a separate front light, and it must not share the frame with the backlight.
How to set the light
| What you need to see | Light | Front light alone |
|---|---|---|
| Image and pinholes on clear film | Backlight | Often invisible |
| Color | Specified front light | Backlight interferes |
| Metallic-ink defects | Dark field | Backlight is not the first choice |
Check at a job change
- Take a clear sample with a known pinhole. Turn the backlight off and confirm it disappears, then on and confirm it returns.
- Confirm this frame is not being used to judge color difference.
- When the job changes to opaque paper, turn the backlight off so it does not shine through the sheet.
