Opol's low build cost is made possible by the use of a phase-based measurement technique (instead of an angular one), which removes the need for any precision-engineered parts. Stripped to its essentials (Fig. , top), the polarimeter consists of two vertically polarised laser beams L1 and L2 that pass through the centre of a polariser P before striking two photodetectors D1 and D2. The polariser is made to rotate at a constant speed and the two detectors are read by separate analogue input pins of a microcontroller, with the polariser rotation causing the measured signals to vary in a sinusoidal fashion. Inserting an optically active sample in one of the beam-paths opens up a phase difference 𝜙 between the signals (Fig. , bottom), which is equal to twice the optical rotation angle qrot. Hence, by calculating the phase difference between the two signals, the optical rotation angle can be found.