Leica DM IRB manual Principle of LMC

Principle of LMC

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.

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.

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