Oblique Illumination

Oblique Illumination is a lighting technique where the light source illuminates the specimen from the side, at an oblique angle, rather than from straight on. This off-axis illumination creates shadows and highlights, which impart a distinct three-dimensional appearance to the specimen under observation. From a theoretical standpoint, this occurs because when the specimen is obliquely lit, one of the diffraction sidebands tends to fall outside the acceptance angle of the objective and is not collected, whereas collection of the other diffraction sideband remains. This sideband suppression increases the resulting contrast in the final image (since the image is created by interference between the undiffracted zero-order light and the various diffraction orders within the collected sidebands). Because oblique illumination can be easily implemented on a typical light microscope at very little cost, it is very popular among amateur microscopists.