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Lighting & imaging

AOI lighting cookbook

Lighting is often more important than the camera. Many inspection problems are actually lighting problems.

If I had to start my career again, I would spend more time learning lighting and less time reading camera specifications. Many defects are already present in the image. The problem is that nobody can see them.

The real question is usually "How do we increase contrast?" - not "How do we increase resolution?"

Four AOI lighting geometries: coaxial, ring, dome and low-angle
Four common lighting geometries. Each one reveals - and hides - different defects.

Coaxial lighting

  • Flat reflective surfaces, conductive traces, uniform illumination.
  • Excellent for revealing reflectivity differences.
  • Poor at revealing certain surface textures.

Ring lighting

  • General-purpose, fast to set up, flexible, inexpensive.
  • A great starting point.
  • Watch for directional shadows and reflection artifacts.

Dome lighting

  • Reflective parts and curved surfaces.
  • Diffuse, even illumination that hides texture you do not want.

UV lighting

  • Organic contamination, coatings, fluorescent materials.

IR lighting

  • Material penetration and print suppression.
Surface texture brought out by raking light
Surface texture that is invisible under flat lighting becomes obvious under the right angle.

The lesson

Many inspection problems are actually lighting problems. Solve the lighting and the algorithm often becomes trivial.