Oblique or Anaxial Illumination
Achieving conditions necessary for oblique illumination, which has been employed to enhance specimen visibility since the dawn of microscopy, can be accomplished by a variety of techniques with a simple transmitted optical microscope. Perhaps the easiest methods are to offset a partially closed condenser iris diaphragm or the image of the light source. In former years, some microscopes were equipped with a condenser having a decenterable aperture iris diaphragm. The device was engineered to allow the entire iris to move off-center in a horizontal plane so that closing the circular diaphragm opening would result in moving the zeroth order to the periphery of the objective rear focal plane. In advanced models, the entire diaphragm was rotatable around the axis of the microscope so that oblique light could be directed toward the specimen from any azimuth to achieve the best desired effect for a given specimen.
Explore changes in microscope light paths and demonstrates events at the objective rear focal plane as illumination progresses from axial to highly oblique in this interactive tutorial.
Explore the effects of popular contrast modes on image contrast and the modulation transfer function of the modified microscope in this interactive tutorial.
Examine how variations in the refractive index of a specimen and its surrounding medium alter visibility in the microscope when utilizing oblique illumination techniques.