Collimated coaxial illumination
When rays of light in a beam are parallel, the light is collimated. Using light from a collimated light source is especially useful for detecting shallow flaws and dents on flat, reflective objects.
The drawing shows the light from a point light source passing through a lens that creates a parallel light beam. Wherever the surface is perfectly flat, the light is reflected back through the lens, These areas appear bright in the image.
Any surface features such as sepressions or bumps reflect the light out of the field of view of the lens, and appear as dark areas on the image. The example with the dented coin cell shows this effect clearly.