It is specific to the way the light is projected onto a slide in the microscope equipped with darkfield condenser. The method allows distinguishing elements in the field of view to be visible that would normally be invisible, or poorly visible in other methods of lighting (such as bright field or phase contrast light.) The method is commonly used in evaluation of slides where the object and the background are of the same color.  (Such as red blood cells in live - red - blood or clear bacterial organisms in saline.) What cannot be seen in Brightfield, can be seen in darkfield. In darkfield objects appear in the form of glowing outlines on the black/dark background