Coaxial illumination is a unique lighting method integrated within the optical path of the lens.
(It is effective for observing silicon wafers, plated surfaces, polished metals, etc., and mirror-like objects.)

 

 

The differences in the obtained images can be discerned when comparing coaxial illumination with bright-field illumination (such as ring illumination).

 

	Below is the image captured when photographing a test pattern chrome-deposited on a transparent glass plate (left photo).
(coaxial illumination)	(ring illumination)

Ring illumination provides a more natural appearance, but coaxial illumination offers higher contrast between the glass and pattern areas due to the chrome deposition’s high reflectivity. Depending on the inspection requirements, coaxial illumination can be advantageous. (In the case mentioned, I believe coaxial illumination would be easier for inspecting scratches or defects on the chrome deposition.)

<When coaxial illumination is effective>
It is primarily used when observing flat surfaces with specular reflection (mirror-like objects) or objects close to specular reflection. It enhances strong contrast due to differences in reflectivity.

 

– Plated metal surface
(coaxial illumination)	(ring illumination)
– Patterns on silicon wafers
(coaxial illumination)	(ring illumination)
– Electrodes on substrate (gold-plated section)
(coaxial illumination)	(ring illumination)

<Instances where coaxial illumination should not be used>
For highly diffuse objects (such as paper, wood, or resin with sandblasting), there is no difference in surface reflectivity (consistent appearance from all angles). Therefore, coaxial illumination would result in images without contrast. Additionally, due to the object’s complete diffuse (Lambertian) nature, hotspots occur in the image (where the center shines brightly).

– White paper (black text printing)
(coaxial illumination)	(ring illumination)