The ability to create an optical beam of a controlled or modulated state of polarisation is an important requirement in a range of optical instruments, such as polarimeters and ellipsometers. The authors describe a range of novel techniques by which this may be achieved. The schemes described are of an interferometric type, and are the macroscopic analogues of extrinsically induced circular birefringence. They are based on the use of fibre-optic or acousto-optic modulators. A pseudoheterodyne electronic signal processing scheme for the measurement of circular birefringence is described, and results of demonstration experiments for the measurement of optical activity and particle size by light scattering are presented.