When front lighting turns a curved part into a mirror

When a curved surface is imaged directly from above with front lighting, the camera sees exactly what you'd expect it to see, not the part, but a bright reflection of the light staring back.

That was this week's problem: small round-bottomed features that needed to be checked as filled or empty. On-axis lighting turned every one of them into a mirror; the curvature scattered a specular hotspot straight back into the lens, wiping out the detail we actually needed.

The fix was flipping the geometry. Lighting from behind and below instead of in front, so the camera sees transmitted light rather than reflected glare.

Filled features show up as a bright, diffuse signal; empty ones stay dark. Clean, reliable contrast, with no clever image processing required to get there.

It's the kind of problem that looks like it needs software to solve but doesn't; getting the lighting geometry right did the whole job.