Case analysis:
The above picture shows the coin, which has two sides to be tested, the character side and the pattern side. The detection content is the surface casting collapse, surface scratch and other defects. The surface is silver and smooth, with serious reflection, the content of the character surface is slightly simple, the pattern surface is complex, and the selection of the detection light source is difficult for high requirements .
Lighting evaluation:
It is not suitable to use ring light, bar light and other common lights because of serious reflection and unclear contrast. Consider coaxial light, which can eliminate reflection, and the light path is vertical, can improve the surface bump contrast. Also consider parallel coaxial light, which is more vertical than coaxial light. Next, we will compare and analyze the differences between coaxial and parallel coaxial light sources.
The following picture shows the effect of coaxial light irradiation: the surface is uniform as a whole, with obvious characters and scratches. But the contrast between the background and features is not good enough.
Coaxial light
Structure
Effective picture of character surface 
Effective picture of pattern surface
Parallel coaxial light 
Structure
Effective picture of character surface 
Effective picture of pattern surfaceSummary:
Coaxial lights are able to eliminate reflections, light path is vertical, suitable for surface inspection applications. Parallel coaxial light has better parallelism and contrast than coaxial light. It should be noted that the parallel coaxial light is only suitable for the detection of smooth surface, while the coaxial light is more suitable for more scenarios although the verticality of the light path is slightly worse.
