A ring laser gyroscope having counter-rotating beams rotating around a closed path. A laser gain section is provided for generating the counter-rotating beams. A means for generating and applying a magnetic field to a Faraday substance which substance is located within the optical path of the counter-rotating beams provides a selected bias to the optical beams. Means for combining and detecting a portion of the counter-rotating beams outside of the closed path yields a first signal which contains inertial rotation rate and Faraday bias information. Means for detecting selected components of the laser beams outside of the closed path yields a second signal that contains Faraday bias information but does not contain rotation rate information. A servo means responsive to the first electrical signal generates a control signal which signal is applied to the means for generating a magnetic field to create an operating condition that is held constant to thereby eliminate scale factor nonlinearities and stray field errors in the detected gyroscope rates. A means for correcting the frequency of the first signal by an amount determined by the frequency of the second signal is provided to yield a rotation rate devoid of Faraday bias or scale factor nonlinearity errors.

A unit for measuring optical properties. The unit includes the first through sixth reflecting surfaces, a reflecting surface rotating device and an object moving device. In order to accurately measure transmittance, reflectance and phase change in transmittance or reflection of optical elements, especially transmittance of a thick object, the first through the fourth reflecting surfaces M1-M4 are arranged so that an optical path formed by the reflecting surfaces M1-M4 has a shape like an "N" letter and so that light of incidence L1 has the same optical axis as exit light L2. An optical center O is a point that a line segment that connects a point of incidence A with an exit point D intersects an optical path that connects the reflecting surface M2 with the reflecting surface M3. The reflecting surface M2 is an ellipsoid of revolution having foci of the optical center O and the point of incidence A. The reflecting surface M3 is an ellipsoid of revolution having foci of the optical center and the exit point D. A reflecting surface rotating device rotates M4 at the exit point D in conjunction with the reflecting surface M1 that is rotated at the point of incidence A. The reflecting surface M5 is a parabolic mirror that makes light reflected from the reflecting surface M1 parallel. M6 is a parabolic mirror that converges light reflected from the reflecting surface M5 onto the reflecting mirror M4.

A photopolarimeter for the simultaneous measurement of all four Stokes parameters of light. The light beam, the state of polarization of which is to be determined, strikes, at oblique angles of incidence, three photodetector surfaces in succession, each of which is partially specularly reflecting and each of which generates an electrical signal proportional to the fraction of the radiation it absorbs. A fourth photodetector is substantially totally light absorbtive and detects the remainder of the light. The four outputs thus developed form a 4x1 signal vector I which is linearly related, I=A S, to the input Stokes vector S. Consequently, S is obtained by S=A.sup.-1 I. The 4x4 instrument matrix A must be nonsingular, which requires that the planes of incidence for the first three detector surfaces are all different. For a given arangement of four detectors, A can be either computed or determined by calibration.