Transmitted light microscopy is the general term used for any type of microscopy where the light is transmitted from a source on the opposite side of the specimen from the objective. Usually the light is passed through a condenser to focus it on the specimen to get very high illumination. Ater the light passes through the specimen, the image of the specimen goes through the objective lens and to the oculars where the enlarged image is viewed.
Transmitted light microscopic techniques were the first ones developed as the microscope was being developed.
The microscopic techniques requiring a transmitted light path include brightfield, darkfield, Zernicke phase (or just phase) and differential interference contrast (or Nomarski) optics. Other not as commonly used transmitted light techniques include Hoffman modulation, Varel optics, and polarization optics.
In order to get a usable image in the microscope, the specimen must be properly illuminated. The light path of the microscope must be properly set up for each optical method and the components used for image generation. The condenser was invented to concentrate the light on the specimen in order to obtain a bright enough image to be useful. Different ways of setting up the light path were worked out. But the best setup for proper specimen illumination and image generation is known as Köhler illumination after the man who invented it. It is used for most of the optical setups listed above.
Please CLICK HERE for a discussion of how to set up a microscope for Köhler illumination.