To make an accurate diagnosis, it is necessary to visualize samples as clearly as possible. Köhler illumination is a light microscopy where the user aligns the illuminating optics to produce a uniform background, maximize contrast, minimize artifacts, and reduce stray light. This video is a brief tutorial on how to set Köhler illumination on a compound microscope to get a brilliant image. Please note that the ability to adjust the condenser aperture or field aperture (diaphragm) may vary by manufacturer. Consult your manual for additional instructions.