optical components to function: Collector lens and/or field lens Field diaphragm Condenser diaphragm Condenser lens These components lie in this order between the light source and the specimen and control the illumination of the specimen. The collector/field lenses act to collect light from the light source and focus it at the plane of the condenser diaphragm. The condenser lens acts to project this light, without focusing it, through the sample. This illumination scheme creates two sets of conjugate image planes, one with the light source image and one with the specimen. These two sets of image planes are found at the following points (see image for numbers and letters): Light source image planes (labeled with light green bar in image): Lamp filament (1) Condenser diaphragm (2) Back focal plane of the objective (3) The eyepoint (4) Specimen image planes: Field diaphragm (A) Specimen (B) Intermediate image plane (the eyepiece diaphragm) (C) The eye retina or camera sensor (D) Advantages The primary advantage of Köhler illumination is the uniform illumination of the sample. This reduces image artifacts and provides high sample contrast. Uniform illumination of the sample is also critical for advanced illumination techniques such as phase contrast and differential interference contrast microscopy. Adjusting the condenser diaphragm alters sample contrast. Furthermore, altering the size of the condenser diaphragm allows adjustment of sample depth of field by altering the effective numerical aperture of the microscope. The role of the condenser diaphragm is analogous to the aperture in photography although the condenser diaphragm of a microscope functions by controlling illumination of the specimen, while the aperture of a camera functions by controlling illumination of the detector. Altering the condenser diaphragm allows the amount of light entering the sample to be freely adjusted without altering the wavelengths of light present, in contrast to reducing power to the light source with critical illumination (which changes the color temperature of the lamp). This adjustment is always coupled to an alteration of the numerical aperture of the system, as stated above, and so adjustment of the illumination source intensity by other means is still necessary. By adjustment of the field diaphragm, the image of the field diaphragm aperture in the sample plane is set to a size slightly larger than the imaged region of the sample (which corresponds in turn to the portion of the sample image thrown into the eyepiece field stop). As the field diaphragm,