Kohler illumination on Zeiss IM microscope using Carl Zeiss 0.9 NA Swing Flip Top Condenser, Zeiss 475638 extension tube and Zeiss condenser diaphragm

My Zeiss IM microscope came set up for phase contrast. Although phase contrast is an amazing technique, the condenser was limited in its ability to be used to set up Kohler illumination.

Kohler illumination is a method of illumination of microscopic objects in which the image of the light source is focused on the substage condenser diaphragm and the diaphragm of the light source is focused in the same plane with the object to be observed; maximizes both the brightness and uniformity of the illuminated field (http://medical-dictionary.thefreedictionary.com/Kohler+illumination)

To achieve Kohler illumination with the Zeiss IM or IM35 microscopes, I needed to obtain one of Zeiss’ bright light condensers. The manual showed the microscope in use with a flip top condenser so I purchased one of these from ebay together with the extension tube and condenser diaphragm shown in the manual – had to wait a bit until one was available.

Success! I can now focus the diaphragm edge in the same plane as the image of the slide, improving illumination and contrast to the maximum available for the microscope…..at least in theory – and seems to work today when I tried it.


Carl Zeiss 0.9 NA Swing Flip Top Condenser (below). With this arrangement, I am able to open the diaphragm up to the full field of view:

Condenser mounted on microscope – both with flip top lens in and out (below):

Image of condenser diaphragm edge shown against the background slide – I have closed down the diaphragm somewhat to show the edge (below):

I also tried mounting one of my other Zeiss condensers. This one has bright field, phase and dark field options. It achieved focus although the condenser had to be much closer to the slider, but, even when fully open, the diaphragm could not illuminate the whole field evenly (below):

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