Set Up for Proper Koehler Illumination
  • Insert and center the recommended light source, covering it with a blue filter to to absorb excess yellow light from the tungsten light source.
  • Focus the specimen with a lower power objective (fig. 1).
  • With the specimen in focus, move the condenser with its rack and pinion focus on the sub-stage up toward the top of its range and, closing the field diaphragm, move it slightly up and down until a sharp image of the field iris diaphragm appears (fig. 2).
  • Center the circle of light from the field iris diaphragm using the condenser centering screws (fig. 3).
  • Open the field iris diaphragm until the dark edges are just outside the field of view, illuminating the entire field (fig. 4).
  • Remove one eyepiece and looking into the tube, adjust the iris diaphragm (aperture diaphragm) so that it covers 2/3 to 3/4 of the circular illuminated area that represents the objective aperture.
  • Replace the eyepiece. Proper Koehler illumination has been attained.