What backlight inspection is, and which defects need it
Backlight shines from the back of the web so places that pass light go bright. Pinholes, missing print on film, and holes in opaque ink need it. Scratches that do not pass light, colour, and surface dirt stay on front light. Do not switch the front lamps off because backlight is fitted.
Backlight shines from the back of the web so places that pass light go bright. Pinholes, missing print on film, and small holes in opaque ink need it. A scratch that does not pass light, a wrong colour, and surface dirt are invisible to backlight. Those still use front light. Do not switch the front lamps off because backlight is fitted.
What the bright spot is
The web itself blocks the light, so a small hole becomes a bright spot on a dark ground, easier to separate than a pale grey spot under front light. A pinhole in opaque ink on clear film is the typical backlight target. If the paper is too thick for light to pass, backlight has no signal. Do not force that channel onto thick board.
A scratch that does not pass light stays as dark as its surround in backlight. Hue is pulled by the light coming through. Those two stay on front light or a spectro. If both lights are on together, the bright spot is washed out by the front light. Shoot them in separate frames.
Which channel
| Defect | Backlight | Front light |
|---|---|---|
| Light-through pinhole, missing print on film | Needed | Often not enough |
| Scratch or dirt that does not pass light | Invisible | Needed |
| Colour | Not used to judge | Needed, or use a spectro |
Check at a job change
- Take a known pinhole, turn on only the backlight, and confirm it is a bright spot.
- Take a surface scratch and confirm the front light is still on and can see it.
- If backlight has no signal on thick paper, do not write that channel into this job’s inspection list.
