dark field illumination
dark ground illumination

Using light microscopy treponemes are only visible using Dark field illumination.

His patents include projection methods and illumination for kinematographs, microscope applications, and light and dark field illumination, among others.

Dark field illumination uses a carefully aligned light source to minimize the quantity of directly transmitted (unscattered) light entering the image plane, collecting only the light scattered by the sample.

These rely on white light, typically optimized at approximately 538 nM and use incident bright and dark field as well as transmitted bright and dark field illumination to see pin holes, edge defects and many forms of contamination and substrate defects.

'Rheinberg illumination' is a special variant of dark field illumination in which transparent, colored filters are inserted just before the Condenser (microscope) so that light rays at high aperture are differently colored than those at low aperture (i.e. the background to the specimen may be blue while the object appears self-luminous yellow).