If a scratch is difficult to see, try changing lighting direction before increasing brightness. On reflective metal, the same surface can look bright or dark depending on how light returns to the camera.
One direction can miss a feature
Smooth surfaces reflect strongly in particular directions. Scratches and small uneven areas change that reflection direction. A suitable relationship between the lighting, surface and camera creates contrast.
Scratch orientation also affects appearance. A scratch visible under one direction may be inconspicuous under another. Avoid choosing conditions from only one sample and one lighting direction.
Compare high-angle, coaxial and lateral light
High-angle
Illuminate from above at an angle to compare flat and uneven areas.
Coaxial
Use reflections returning from flat surfaces along the camera axis.
Lateral
Use a low angle to emphasize scratches and step edges.
Capture each direction separately first to identify which reveals the feature. These are previously published OptTech examples. Different materials or surface treatments can produce different results under the same direction.
Combining light is more than adding brightness
Balancing several directions may reduce texture while retaining scratches. Increasing every direction can instead fill useful shadows and make scratches less visible.
The aim is contrast between the feature you want to retain and the texture you want to suppress, rather than the number of light sources.
The hybrid lighting article shows metal surface examples using three directions. Treat simultaneous illumination and software combination of separately acquired images as different methods.
Change one condition at a time and check acceptable samples too
- Prepare acceptable and scratched samples with matching position and field of view.
- Fix exposure and gain, then capture each lighting direction.
- Select useful directions and adjust their relative brightness.
- Check scratches in other orientations and texture on acceptable samples.
- Record lighting output, working distance and exposure settings.
If brightness differs substantially between directions, avoid saturation and underexposure when repeating the comparison, and record setting changes. See comparison conditions.
Control brightness and reproduce conditions
A controller that manages output independently for each direction supports this process. Our four-channel controller supports front-panel adjustment and external PC operation. Read the separate articles on line scan and area scan when choosing a camera.
