To summarise, Opol is a high-performance laser polarimeter that uses a simple phase-based measurement procedure to avoid the need for costly optomechanical parts. The instrument uses separate reference and object beams derived from a common laser source. The reference beam passes through a fixed polariser and a rotating polariser, while the object beam passes through another fixed polariser, an optical cell, and the same rotating polariser. The rotating polariser induces a sinusoidal variation in the transmitted beam intensities, with the phase difference between the two intensities depending on the relative orientation of the fixed polarisers and the optical activity of the sample under test. The rotation angle is determined by two successive measurements of the phase difference, with and without an optically active sample in the path of the object beam.