The Difference Between Brightfield and Barkfield

Brightfield (BF) illumination is the most common illumination technique for metallographic analysis. In incident BF, the light path comes from the light source, passes through the objective lens, is reflected off the surface of the specimen, returns through the objective, and finally reaches the eyepiece or camera for observation.

Flat surfaces produce a bright background due to reflection of a large amount of the incident light into the objective lens, while non-flat features, such as cracks, pores, etched grain boundaries or features with distinct reflectivity, such as precipitate and second phase inclusions on the surface appear darker as incident light is scattered and reflected at a variety of angles or even partially absorbed.

Darkfield (DF) is a less known but powerful illumination technique. The light path for DF illumination passes through an outer hollow ring of the objective, falls onto the specimen at a high angle of incidence, reflects off the surface, then passes through the interior of the objective lens, and finally reaches the eyepiece or camera.

This type of illumination causes flat surfaces to appear dark, as the vast majority of the light reflected at the high incident angle misses the interior of the objective lens. For samples having a flat surface with occasional non-flat features – cracks, pores, etched grain boundaries, etc. – the DF image shows a dark background with brighter areas corresponding to the non-flat features, which scatter more light into the objective.

Brightfield: Only direct light falls on the sample surface, where it is absorbed or reflected. The quality parameters of the image are brightness, resolution, contrast and field depth.   

Darkfield: Only refracted, diffracted or reflected light falls on the sample surface. Darkfield is suitable for all samples with structured surfaces and can also be used to visualize structures below the resolution limit. The surface structures appear bright on a dark background.