Polarimeters

The used polarizers should have a high degree of extinction. In practice, one often uses Nicol prisms or similar high-quality devices.

The optical mount of the analyzer must be made with precise fine mechanics and equipped with a fine scale to allow the user to accurately determine the angular orientation of the polarizer's axis.

It is preferable that the investigated sample is quite homogeneous, since otherwise one would not obtain a uniform amount of polarization rotation, and the measurement accuracy would be reduced.

The light source should not have a too large optical bandwidth because the degree of optical activity may vary within that bandwidth, and that would lead to some broadening of the observed intensity minimum and thus to a reduced accuracy of the measurement. Nevertheless, one does not need to use a laser as a quasi-monochromatic light source; instead, one may also use a light-emitting diode LED, for example, or a substantially more broadband light source (e.g. an incandescent lamp) in combination with an optical bandpass filter (e.g. an interference filter).

An improved method, based on an intervention of the Austrian physicist Ferdinand Franz Lippich in the late 19^th century, uses an additional optical element which introduces some rotation of the polarization direction for about half of the beam area after the polarizer. One can then not obtain complete darkness after the analyzer for any orientation. Instead, one adjusts the analyzer for equal brightness of both areas. Lippich's approach allows for more accurate measurements, essentially because the brightness difference depends linearly on the angular deviation, while the brightness in the simple polarimeter as described above depends on the square of the angular deviation from the minimum intensity point.

Some advanced polarimeters can perform automated measurements, where the analyzer is rotated and the device itself determines the analyzer orientation for minimum optical power transmission. Using a precise mechanical drive (with fine resolution and a high degree of reproducibility) and optimized software (e.g. utilizing data for multiple orientations, to which a curve can be fitted), one can not only obtain a substantially higher measurement accuracy than for manual measurements, but also a much reduced measurement time of e.g. one second instead of multiple seconds. Such an instrument may also be quite simple to handle and less prone to possible errors of the operator. The amount of optical activity, as calculated from the measured rotation angle and length of the sample, may directly appear on a digital display.

There are instruments where the analyzer is continuously rotating with a constant angular velocity, and the phase of the resulting output signal is used to derive the optical activity.

Instead of a rotatable analyzer, one may use a Faraday rotator in combination with a wire coil, mounted before the analyzer, which in that case does not need to be rotated. One may then determine the required electric current through the coil which produces a suitable amount of optical activity through the Faraday effect, such that minimum transmission through the analyzer is achieved. That method can be both quite accurate and fast.

Such a coil-driven Faraday rotator is a kind of polarization modulator. An additional small alternating current can modulate the polarization direction periodically, so that the null point can be found with phase-sensitive detection: at the null point, the signal component at the modulation frequency vanishes, and its sign on either side indicates the direction of the required correction. Other instruments, e.g. for measuring circular dichroism, use photoelastic modulators with lock-in detection.

The specific optical rotation of a substance is temperature-dependent. For many substances, the rotation changes significantly with temperature, necessitating precise temperature control during measurement to ensure accuracy and reproducibility. Modern digital polarimeters often feature built-in temperature control systems, such as Peltier elements, to maintain the sample at a standard temperature (typically 20 °C or 25 °C) or to compensate mathematically for known temperature coefficients.