What is polarimetry?

Measuring optical activities through polarimetry

Classical physics teaches that light consists of electromagnetic waves whose spatial oscillations are transverse to the direction of propagation. When natural light passes through a so-called polarizing filter, specific oscillation directions are filtered out, leaving only one spatial component. The light emerging from the filter is then referred to as “linearly polarized light”.

A great number of organic and inorganic substances are optically active in their crystalline, liquid, or dissolved state. These substances rotate the direction of polarized light by a specific angle. The principle of the polarimeter is achieved when a dissolved optically active substance is introduced between two polarizing filters that cross at an angle of 90°. The intensity of the light on the detector behind the second polarizing filter changes depending on the angular position of these two filters.

Optical rotation generally means that the polarization of the light direction will be rotated by a certain angle when penetrating an optically active substance. A simple polarimeter contains at least the following parts: Light source, polarising filter, sample tube, polarising filter, detector.

At point zero, the 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 angular degrees; therefore, these types of instruments are called circular polarimeters.

To determine the exact compensation position, SCHMIDT + HAENSCH uses Faraday modulation as an electronic enhancement. Without mechanical transmission by V-belts or gears, we use a direct coupling of optical encoder and evaluation unit in all measuring instruments.

This ensures high precision over the entire measuring range In addition, these principles guarantee short measuring times and no mechanical wear. Thus, highest sensitivity and fastest compensation time is achieved over the entire measuring range. The continuous measurement also allows monitoring of mutarotation.

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