Laurent's half-shade polarimeter[edit]
When plane polarised light passes through some crystals,the velocity of left polarised light is different from that of the right polarised light thus the crystals are said to have two refractive indices i.e. double refracting
Biquartz polarimeter[edit]
In biquartz polarimeters, a biquartz plate is used. Biquartz plate consists of two semi circular plates of quartz each of thickness 3.75mm. One half consists of right-handed optically active quartz,while the other is left-handed optically active quartz.
Lippich polarimeter[edit]
Quartz-Wedge polarimeter[edit]
Manual[edit]
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.
Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.
Semi-automatic[edit]
Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.
Fully automatic[edit]
Fully automatic polarimeters are now available and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a second, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.
Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.
As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier Elements to actively control the temperature. Special techniques like a temperature controlled sample tube reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing.[7] Traditionally, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system allows accurate monitoring of the sample and filling conditions in the sample cell from inside the instrument. A telecentric camera gives a sharp image over the complete length of any sample cell placed within modern instruments. The online monitoring of the filling process ensures that no bubbles or particles obstruct the measurement. A picture can be saved together with the recorded data. Any temperature gradients, inhomogeneous sample distributions or air bubbles can immediately be recognized before measurement, so that potential errors caused by bubbles or particles are no longer an issue.