Polarimetry – optical activities

Classical physics teaches that light consists of electromagnetic waves, which vibrations are transverse to the direction of propagation. Polarized light is light, which vibration pattern exhibits preference. If natural light passes through a so called polarizing filter, most of its other vibrating directions will be filtered out, leaving only one specific direction. If this light is oscillating then in only one direction, it is called “linearly polarized”.

There is a great number of organic and inorganic substances being optically active in their crystalline, liquid or dissolved state. That means, these substances are able to rotate the direction of oscillation of polarized light around a determined angle. The principle of the polarimeter is accomplished if a dissolved optically active substance is introduced between two polarizing filters, crossed at 90°. The intensity of the light on the detector behind the second polarizing filter varies as a function of the angular position of these two filters.

Optical rotation in general means that the polarization of the direction of light will be rotated for a certain angle when penetrating an optically active substance. A simple polarimeter contains at least the following parts:

Graphic explanation of polarimetryAt point zero, polarizer and analyzer are set in an angle of 90° towards each other, which means that no light reaches the detector (0% transmission). As soon as an optically active substance is introduced into the sample room, the transmission will be rising in consequence of the plane of polarization. To measure the angle of the rotation, the analyzer is rotated up to a point where the transmission of the detector is again at a minimum. Optical rotation is measured in degrees of angle; this kind of instruments are thus called circle polarimeters.

In order to determine the exact compensation position, SCHMIDT + HAENSCH uses faraday modulation as an electronic enhancement. Without mechanical transmission by a V-belt or gear being necessary, we apply a direct coupling of optical encoder and analyzer in all measuring instruments.

That is how a high precision throughout the entire measuring range is achieved. Additionally, these principles guarantee for short measuring times and no mechanic wear. Consequently, highest sensitivity and fastest compensation time over the entire measuring range is archieved. The continuous measurement is also allowing monitoring of muto-rotation.

Technical basics polarimetry (PDF)

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