Leica DM IRB manual Setting Koehler illumination, Fig.Ê 62Ê Ê

Setting Koehler illumination

Turn a 10x objective into the light path and focus the specimen.

–Engage the condenser disc into the “H” = Hellfeld = brightfield position if necessary.

–Close the field diaphragm.

–Adjust the height of the condenser until the edge of the field diaphragm is sharply in focus and also:

Centre the image of the field diaphragm in the middle of the field of view with the two cen- tring screws.

Open the field diaphragm until it just dis- appears from the field of view.

When objectives are changed, the condenser centration may have to be slightly adjusted with the knurled screws and the field dia- phragm reset.

The field diaphragm protects the specimen against unnecessary heat and keeps all light not required for imaging away from the specimen, thereby enhancing contrast.

Fig.Ê 61ÊKoehlerÊ illuminationÊ

a Field diaphragm closed, not focused, not centred, b Field diaphragm focused, but not centred, c Field diaphragm focused and centred, but diameter too small, d Diameter of field diaphragm = diameter of field of view (Koehler illumination)

Therefore it is only opened wide enough to just illuminate the observed or photographed object field. A change in magnification al- ways necessitates adjustment of the field diaphragm.

–Narrow the aperture diaphragm until you obtain the desired image contrast.

The aperture diaphragm determines the lateral resolution, field depth and contrast of the microscope image. The best contrast is obtained when the apertures of the objective and the condenser are roughly the same.

Fig.Ê 62Ê Ê

1 Binocular observation and phototube, 2 Tube clamp screw,

3 Objective nosepiece, sextuple, 4 Clamp screw for SLR/TV adapter, front port, 5 Transmitted light lamphousing, 6 Trans- mitted light illumination column, 7 Condenser holder, 8 Con- denser 0.53 S23 with disc, 9 Screw for opening lamphousing 105, 10 Lamphousing 105, 11 Adjustment wheel for tube lens 1x, 1.5x or Bertrand lens (B), 12 Beamsplitter switch rods, 13 Coarse and fine focus

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