Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
lamp. Kohler illumination overcomes these problems and in principle requires only two
lenses, though the design of the microscope may make it necessary to include auxiliary
lenses, particularly when high and low N.A. objectives are used on the same turret.
Figure 12: Ray paths for critical illumination.
Figure 13 shows the optical arrangement for Kohler illumination. The source is imaged at
the condenser diaphragm by the lamp condenser, which becomes the field lens. This
change of name reflects the fact that the stage condenser in the object plane images the
diaphragm at this lens. Its size therefore controls the size of the illuminated object field,
hence the name of field diaphragm. Now the condenser diaphragm is in the back focal
plane of the condenser as noted previously, so each point on the source gives rise to an
image point at the condenser diaphragm; this causes a small parallel beam of light to pass
through the object field. The illumination of that field is therefore made up of a series of
parallel beams, inclined relative to each other and originating from different points of the
source. These produce a cone of light evenly illuminating the object field. Note that the
source and the condenser diaphragm are further imaged in the back focal plane of the
objective - the objective pupil - and again at the exit pupil of the eyepiece, where the eye
should be placed.
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