sample, and eyepiece field stop all lie on conjugate image planes, this adjustment allows the illuminating rays to completely fill the eyepiece field of view, while minimizing the amount of extraneous light which must be blocked by the eyepiece field stop. Such extraneous light scatters inside the system and degrades contrast. Testing for and setting up Köhler illumination Microscopes using Köhler illumination must be routinely checked for correct alignment. The realignment procedure tests whether the correct optical components are in focus at the two sets of conjugate image planes; the light source image planes and the specimen image planes. Alignment of optical components on the specimen image plane is typically performed by first loading a test specimen and bringing it into focus by moving the objective or the specimen. The field diaphragm is then partially closed; the edges of the diaphragm should be in the same conjugate image planes as the specimen, therefore should appear in focus. The focus can be adjusted by raising or lowering the condenser lenses and diaphragm. Finally, the field diaphragm is reopened to just beyond the field of view. In order to test the alignment of components on the light source image plane, the eyepiece must be removed to allow observation of the intermediate image plane (the position of the eyepiece diaphragm) either directly or by using a phase telescope/Bertrand lens. The light source (e.g. the bulb filament) and the edges of the condenser diaphragm should appear in focus. Any optical components at the back focal plane of the objective (e.g. the phase ring for phase contrast microscopy) and at the condenser diaphragm (e.g. the annulus for phase contrast microscopy) should also appear in focus. See also Köhler integration Spatial filtering, the optical principle implemented in the Köhler illuminator's field diaphragm Polarized light microscopy Hoffman modulation contrast microscopy References External links A 6 step guide to Koehler Illumination by Scientifica Köhler illumination setup instructions Optical microscopy Lighting Nonimaging optics Laboratory equipment Microscopy