Koehler Illumination: When to Adjust the Field and Aperture Diaphragms

Plenty of people use a microscope for years without ever touching the ring under the condenser. Washed-out images, low contrast and a bright halo around the field are usually traced back to that ring. It is Koehler illumination, invented well over a century ago and still the default on every serious microscope.

1. The idea behind Koehler

The older scheme, critical illumination, simply projected an image of the bulb onto the specimen. But a filament is not a uniform emitter, so its shape landed on the sample and nothing looked clean.

Koehler took a different route and added two lenses. The first images the filament onto the aperture diaphragm of the condenser, and the second images that diaphragm onto the specimen plane. The filament's shape is scrambled, leaving a uniformly bright field. Better still, the diaphragm becomes a knob you can turn: adjusting it changes the angle and extent of illumination while the filament stays put.

2. Two diaphragms, two jobs

The field diaphragm, at the base of the stand or on the condenser mount, controls how much area is lit. Closing it until it just covers the field blocks stray light from entering the imaging path, and contrast improves immediately. Left wide open, light bounces around inside the tube and the image picks up a grey veil.

The aperture diaphragm on the condenser controls the angle of illumination. It sets how wide the illuminating cone is, which is equivalent to changing the numerical aperture of the illumination. Open it wide and resolution rises but contrast falls and glare increases. Close it and contrast improves, but resolution drops and diffraction fringes and moire patterns appear.

3. Six steps to get it right

First, focus on the specimen at 10X and close the field diaphragm all the way. A polygonal patch appears in the field: that is the diaphragm itself.

Second, raise or lower the condenser until the edges of that polygon are sharpest. Blurred edges mean the condenser is out of focus, so the diaphragm is not where it should be and everything after this is guesswork.

Third, open the field diaphragm until the polygon's edge just clears the field of view. That is the correct position.

Fourth, pull out one eyepiece and look down the tube at the back focal plane of the objective. A bright disc appears: the exit pupil.

Fifth, open and close the aperture diaphragm while watching that disc, and settle at roughly seven to eight tenths of its diameter. The working rule of 70 to 80 percent costs almost nothing in resolution while gaining a visible amount of contrast.

Sixth, when you change objectives, reset the field diaphragm, since each magnification has a different field size. The aperture diaphragm normally stays put, though you can open it to 90 percent for high-resolution photography.

4. Common mistakes

Leaving the field diaphragm wide open all the time. This is the most frequent error. People assume "widest is brightest", when in fact stray light is eating the contrast. Set it to just cover the field.

Using the aperture diaphragm as a brightness control. Dimming by closing it or brightening by opening it is wrong. Brightness belongs to lamp voltage or camera exposure; the aperture diaphragm balances resolution against contrast.

Closing the aperture with phase contrast objectives. The phase ring already defines the light path, and the aperture should be opened, otherwise the phase effect is destroyed.

5. Three techniques are exceptions

In fluorescence, excitation arrives through the objective and the condenser below plays no part, so neither diaphragm matters and the condenser can often be swung out entirely.

Darkfield requires a dedicated darkfield condenser that forms a hollow cone of light so only oblique rays enter the objective while the central beam is blocked. An ordinary brightfield condenser cannot produce true darkfield.

DIC adds polarizers and prisms to the light path, and the aperture diaphragm should be fully open, because DIC depends on the phase difference between two coherent beams and restricting the aperture weakens the interference directly.

6. A quick self-check

Before you press the shutter, do three things: close the field diaphragm to just cover the field, set the aperture to about three quarters, and check the condenser height. After those three steps most washed-out photographs become crisp.

If it still looks flat, the diaphragms are not the problem. Check whether the objective is dirty, the coverslip is the right thickness, or the lamp has aged.