Adjusting the Koehler illumination. KERN OBN 135

5.6 Adjusting the Koehler illumination

To make sure that perfect image results are achieved during microscopic observation, it is important that the direction of light of the microscope is optimised. If, as with the devices in the KERN OBN-13 series, the lighting can be set in accordance with Koehler, the result is homogenous illumination of the slide and avoidance of disruptive stray light.

The necessary control elements for this are:



Height-adjustable and centre-adjustable condenser with aperture diaphragm



Field diaphragm

When adjusting the Koehler lighting for the first time, you must first select the lowest possible objective magnification, so that you can carry out the following steps.

1. Use the condenser focus wheel to position the condenser directly below the specimen stage. Switch on the lighting and use the coarse and fine adjustment knob to bring the slide with the cover glass positioned facing upwards into focus.

2. Close the field diaphragm completely using its adjusting ring. When you look in the microscope a blurred image appears in the aperture. If the microscopic image is completely dark, the image for the field diaphragm is outside the field of view and must be brought into the field of view using the centring screws on the condenser.

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3. Adjust the height of the condenser until the image from the field diaphragm appears clearly in the field of view. For some microscopes there is a risk that you will lift the condenser up so high that it collides with the object holder.

Therefore care is needed when doing this.

4. Use the centring screws of the condenser holder to bring the image from the field diaphragm into the centre of the field of view.

5. Open the field diaphragm until it just disappears out of the field of view.

If necessary, simply re-centre using the centring screws on the condenser holder.

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6. Use the aperture diaphragm of the condenser to find the very best compromise between contrast and resolution for the microscopic image. The scale divisions on the condenser can be used as a guideline. Select in accordance with the objective being used.

The view in the tube without the eyepiece should look something like the illustration on the right.

The diameter of the aperture diaphragm which is then visible should make up approximately 2/3 of the pupil diameter.

If the eyepiece should be removed, for checking, then please make sure that no dirt or dust falls into the tube.

7. It is possible to alter the brightness of the bulb using the dimmer. The brightness is always controlled by the bulb brightness and not by the aperture diaphragm.

8. Possibly there is the need of re-adjusting the focus and x-y axis.

9. Observe the object.

If another magnification is selected afterwards, then the Koehler illumination does not have to be reset from scratch, only the aperture diaphragm and field diaphragm need to be adjusted as required.

As a result you can always check whether the condenser needs to be re-centred.

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