Standard coaxial illumination
This technique is used for illuminating reflective surfaces. A beam splitter or a semi-transparent mirror is used to divert the light from a light source that is fixed on the side (coaxial), so that it is projected almost parallel to the optical axes of the camera onto the object.
The technique of coaxial illumination is ideally suited to plain reflective, 'mirror-like' objects with no (or very little) profile or any surfaces which have diffuse backgrounds. Examples include PCB inspection, reflective labels, polished silicon wafers or print inspection.