How to light a metallic ink without drowning the camera
Specular reflection from metallic ink fills the camera and defects disappear. Use dark field or a low angle so the mirror reflection misses the lens. Crossed polarizers can hold a hot spot down. Do not add exposure to see the metal; bright places saturate first.
Specular reflection from metallic ink fills the camera and the defects disappear. Use dark field or a low angle so the mirror reflection misses the lens and only scatter from the defect gets in. Crossed polarizers, one before the lamp and one before the lens, can also hold the hot spot down. Do not add exposure to “see” the metal. The bright places saturate first.
Bright does not mean you saw it
Silver and gold inks are small mirrors. If the lamp sits on the reflection path into the lens, the center of the frame is white, and pinholes and scratches sit inside that white. Move the lamp to a low angle. The mirror throws the light outside the lens, the metal goes dark, and the defect becomes light or dark enough to separate. That is dark field.
Crossed polarization is one polarizer in front of the lamp and one in front of the lens, axes at right angles. Specular reflection is suppressed, scatter remains. It fits when the hot spot is only part of the frame. If the whole frame is saturated, move the lamp first, then consider polarizers.
Where to put the lamp
| Frame | Do this |
|---|---|
| Large saturated white | Take the specular path off the lens, switch to dark field |
| Only a local hot spot | Add crossed polarizers or change the angle |
| Defect is clear in dark field, color is wrong | Judge color on another light, not in this frame |
Check before the job runs
- Use a metallic sample with a known pinhole. Is the pinhole still in the frame.
- If it is gone and the surround is white, cut exposure and change the angle. Do not add light.
- Write the lamp angle down. After a job change, do not reset it to the lighting used for ordinary paper.
