Mörkfältsspridning

Mörkfältsspridning, also known as dark field scattering, is an optical phenomenon observed when light scatters from an object or a medium such that the illuminating light is blocked, and only the scattered light is collected. This technique is often used in microscopy to visualize transparent or unstained specimens. By preventing direct illumination from reaching the observer or detector, the contrast of weakly scattering or refractive structures is greatly enhanced against a dark background.

The principle behind dark field scattering involves creating an optical setup where the illuminating beam is directed at an angle that misses the objective lens. Only light that is scattered by the specimen at different angles enters the objective and becomes visible. This scattered light originates from variations in the refractive index of the specimen, causing it to deviate from its original path. Objects that do not scatter light, such as the surrounding medium, remain dark.

Dark field microscopy is particularly useful for observing small, unstained biological samples like bacteria, spirochetes, and flagella, which would be difficult or impossible to see with standard bright field microscopy. It is also employed in materials science and the analysis of colloids and nanoparticles. The technique can reveal fine details and structures that might be obscured by the bright background of other methods. The intensity of the observed scattered light depends on the size, shape, and refractive index of the scattering particles.