# Köhler illumination in five steps

> Even, bright and sharp. Setting up Köhler illumination takes about a minute and makes a visible difference to every slide you look at.

Canonical: https://unilab-cno.pages.dev/blog/kohler-illumination-step-by-step/
Last updated: 2026-09-19

Technique · By the Unilab Microscopes team

Many users switch on a microscope, focus, and adjust brightness with the dimmer. The image is usable, but often uneven, a little soft, or with glare in the background. Köhler illumination is the standard way to set up the light path so the field is evenly lit and the objective can resolve as much detail as it is capable of.

The method is named after August Köhler, who described it in 1893. You need a microscope with a field diaphragm in the base and a condenser that can be focused and centred. Many research and pathology microscopes have both.

## Before you start

Place a stained slide on the stage. Select the 10x objective and focus on the specimen. Set the interpupillary distance and diopter if you use a binocular head.

## Step 1: Close the field diaphragm

The field diaphragm is the adjustable iris at the light exit in the base. Close it until it is small. You should see a bright polygon in the field of view, probably blurred and possibly off centre.

## Step 2: Focus the condenser

Raise or lower the condenser with its focus knob until the edges of the polygon look sharp. If you cannot get a sharp edge, check that the condenser is close to the top of its travel.

## Step 3: Centre the condenser

Use the two condenser centring screws to move the sharp polygon into the middle of the field.

## Step 4: Open the field diaphragm

Open the field diaphragm until its edges just disappear outside the field of view. Opening it much further only adds stray light and reduces contrast.

## Step 5: Set the aperture diaphragm

The aperture diaphragm is the iris on the condenser itself. It controls contrast and depth of field rather than brightness. A good starting point is to open it to about two thirds or three quarters of the objective's numerical aperture. In practice, close it until the image gains contrast without becoming grainy or dark, then stop.

Use the lamp dimmer, not the aperture diaphragm, to adjust brightness.

## When you change objectives

Steps 4 and 5 should be repeated when you change to a different objective, because each one has a different field size and numerical aperture. Steps 1 to 3 usually stay correct for the session.

## If your microscope has no field diaphragm

Many student and routine microscopes do not have one. You can still improve the image: keep the condenser near the top of its travel, use the condenser iris to balance contrast and detail, and adjust brightness only with the dimmer.

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