In the Brinell hardness test for metals, a clear deformation (bulge) appears around the indent. This deformation is more pronounced on soft metals than hard metals.
This in turn means that the edges of the indent are difficult to identify and evaluate if light field illumination is used. Use of a ring light for dark field illumination makes the edges of the indent easier to identify and evaluate.
In the upper image, light field illumination was used. Due to the material behaviour and deformation, the edges of the indent are not clearly identifiable, which makes precise evaluation difficult.
In the lower image, dark field illumination was used. The edges of the indent are clearly visible and evaluation can be conducted precisely.
Consequently, use of dark field illumination is recommended for Brinell measurements on soft metals with heavy bulging or poor test surfaces.
