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The principle
Leica modulation contrast (LMC) is based on the
principle of Hoffmann modulation contrast.
This imaging technique is particularly suitable
for unstained, colourless objects with little
image contrast.
Such objects change the phase of the light
when it passes through them.
The conversion of these phase gradients into
differences in amplitude results in a three-di-
mensional image similar to that of differential
interference contrast.
To realise this technique, a light slit diaphragm
and an objective with integrated modulator are
required. The modulator is a filter built into the
rear focal plane which divides it into three
zones, a dark zone, a grey zone and a light zone.
Principle of LMC
The light coming from the light slit diaphragm is
diffracted at the object into different directions,
depending on the object’s refractive index
gradient, so that some of the rays have to pass
through the light zone of the modulator and
some through the dark zone. The non-diffracted
direct light passes through the grey zone and
produces the grey background of the entire field
of view. Most of the rays diffracted at the object
pass through the light zone and produce the
image.
If the condenser is set at the “brightfield”
position and the specimen is removed, the dark
and the grey zone can be seen at the edge of the
field of view. The image of the slit diaphragm is
in the light zone. To adjust, the light slit
diaphragm is rotated until the bright stripe of the
slit image covers the grey stripe of the modu-
lator.