Adjusting your microscope for Koehler Illumination

Koehler illumination is a procedure for setting up and adjusting your microscope to achieve the best possible combination of contrast and resolution.

  1. Turn on the microscope illuminator and place the specimen slide you wish to examine on the microscope stage. Rotate the 10X (YELLOW) objective into position and focus on the image of your specimen.
  2. Move the sub-stage condenser up to its top position using the sub-stage condenser knob. See Figure 1.
  3. Check to make sure that both the field iris (in the microscope base) and the aperture iris (in the sub-stage condenser) are turned fully to the right (fully open).
  4. Rotate the filed iris control ring until the edge of the field iris is just visible at the outer edge of the field of view.
  5. Using the sub-stage condenser focus knob, raise and lower the sub-stage condenser until the edge of the field iris comes into sharp focus. Usually it is in the sharpest focus possible when the sub-stage condenser is all the way up.
  6. If the image of the field iris appears to be off-center in relation to your field of view, then it will be necessary to align the field iris so that it is centered in the field of view.

    Rotating the sub-stage condenser centering screws (one on each side of the sub-stage condenser. DO NOT TURN THE SMALLER SCREW IN THE CENTER FACING YOU) in either direction allows you to move the image of the field iris until it is concentric with the field of view.

  7. With the image of the field iris centered and focused as sharply as possible, open the field iris diaphragm until the image of the edge of the iris diaphragm is just outside of the field of view.
  8. Remove one of the eyepieces and look into the eyetube. Look at the light. You may need to reduce the intensity by rotating the light control. Close down the aperture iris using the aperture iris lever located on the side of the sub-stage condenser until approximately two thirds of he disc of light you are observing remains visible. Replace the eyepiece.