- A new method and algorithm for measuring optical linear birefringence is proposed. The method allows the measurement of both the principal axis orientation angle and the retardation simultaneously by a three-step measurement. The average absolute error of the retardation of an achromatic quarter-wave plate (QWP) is found to be better than 10 -4 parts of the wavelength over the whole spectrum, and its principal axis system orientation is determined with accuracy better than 0.18 deg. In comparison to other methods, the current technique holds several advantages: wavelength independence, low cost, compact setup, ease of alignment, use of a simple algorithm, no polarization reflectance dependence, and possesses high accuracy. The method was applied also to the measurement of an arbitrary retardation of a sapphire plate and to the assessment of the dynamic retardation of a liquid crystal device.