What lighting angle shows a hickey that diffuse light hides
Low-angle grazing light shows a white-ring hickey that looks flat under diffuse light, because the missing-ink edge casts a shadow. Diffuse light is for color, not this shallow defect. 100% inspection with only front diffuse lamps will miss them. Add a low-angle channel; do not raise brightness instead of angle.
Low-angle grazing light shows a white-ring hickey that looks flat under diffuse light, because the missing-ink edge casts a shadow. Diffuse light is for color, not for this shallow surface defect. A 100% inspection with only front diffuse lamps will miss shallow hickeys. Add a low-angle channel for hickeys. Do not raise brightness in place of angle.
Why front light washes it out
A hickey is a shallow missing-ink patch or a ring of squeezed ink. The height difference is small. Light from the front lights both sides the same, so the camera sees little contrast. Light that skims at a low angle lengthens the edge shadow, and the white ring appears. Turning the diffuse lamp up brightens both sides together; the difference stays small.
Color measurement, though, dislikes that low angle because it emphasizes gloss. Use diffuse or 45° for color, grazing for hickeys. Do not merge both into one lamp and make both jobs mediocre.
How to set the lamps
| What you need to see | Light | Do not |
|---|---|---|
| White rings, crush marks, shallow scratches | Low angle | Only brighten the diffuse lamp |
| Solid color | Diffuse or the specified angle | Judge color under grazing only |
| Both | Two channels, can be time-shared | One compromise angle |
Check before the job runs
- Take a known shallow hickey and turn on only the diffuse lamp. See if it is in the image.
- If not, add low angle. Do not first loosen the threshold.
- Confirm the color channel is not being lit by the low-angle lamp at the same time.
