40 PART II A Deeper Look into a Light Microscope
Illumination and the Microscope
Optical Train
The condenser with the aperture diaphragm and the illuminating beam path with
the luminous-field diaphragm are the critical elements in achieving Köhler illumination.
Image forming light rays passed through the specimen are captured by
the microscope objective and directed into the eyepieces. Throughout the optical
train of the microscope, illumination is directed and focused through a series
of diaphragms and lenses as it travels from the light source to illuminate the
specimen and then into the eyepieces. By closing or opening the condenser diaphragm,
you control the angle
of the light rays emerging from the condenser and
reaching the sample. Along with the numerical aperture of the objective, this setting
also determines the numerical aperture of the entire microscope system.
With the luminous-field diaphragm you determine which area and size of the
specimen is illuminated. The aperture diaphragm of the condenser is imaged into
the pupil of the objective and regulates the illumination of this pupil. The entire
optical system is designed in such a manner that aperture angles of the light cones
are correctly set together with the aperture diaphragm.
Condensers Help to Adjust the
Illumination
When properly adjusted, light from the condenser will fill the back focal plane of
the objective with image-forming light by projecting a cone of light to illuminate
the field of view. The condenser aperture diaphragm is responsible for controlling
the angle of the illuminating light cone and, consequently, the numerical aperture
of the condenser. Another reason for the existence of adjustable diaphragms and
filters in the microscope is that the illumination should often be readjusted after
each change of the objective. This is, in part, because the size of the observed object
field changes with every objective magnification. Example: an objective with
a low magnification (for example 4×) provides a large object field. If a switch is
made to the 40× objective, the diameter
of the imaged field of view shrinks by a
factor of 10.