How to Set Up Koehler Microscope Illumination Properly

There are two key areas on a compound biological microscope with Koehler illumination that need to be adjusted in order to set up the Koehler illumination properly: field diaphragm and aperture diaphragm.

The microscope's aperture diaphragm can be found on the condenser of the microscope and it is typically a lever that is adjusted, or a built-in slider on the condenser.

This is the RB40 microscope, the arrow points to the aperture diaphragm.

The aperture diaphragm on the microscope will usually have numbers on it that correlate to the numerical aperture (N.A.) on your objective lenses.

Aperture diaphragm on the microscope condenser.

Setting up Koehler Illumination

1. Start by adjusting the field diaphragm (on the bottom of the microscope where the light is). Focus on your image and open the field diaphragm all the way up so lots of light shines through.
2. Close the aperture diaphragm (on the condenser) down so you can see the edges of it. Is this dark circle centered in your field of view? If not, use the small centering screws on either side of the condenser to move the circle into the center. 
3. Open the aperture diaphragm back up until the image contrast is sharp and the edges of the aperture diaphragm are crisp and clear.

This video using the Motic BA410 microscope is a great tutorial to watch if you are uncertain of the steps to take when setting up Koehler illumination on your microscope.

You can learn more about Koehler illumination and setup here.