A Quarter-Wave Compensator with a Sensitive Half-Shadow Device

Abstract

Details are given of an instrument capable of measuring large and small optical phase differences between the components of an elliptically polarized radiation. The lowest limit is between 10⁻⁴ and 10⁻⁵ 2π. The compensator consists of an accurate quarter-wave plate (Sénarmont compensator) for the wavelength used (5461A) and a Nakamura half-shadow device. The quarter-wave plate, suitably orientated, reduces the elliptically polarized light to a linearly polarized vibration, the azimuth of which is simply related to the phase difference. The azimuth is determined with a Glan-Thompson polarizing prism. The full theory is given with special reference to possible sources of error. These are calculated for a series of values. An expression for the half-shadow sensitivity is found. The method of use and of construction are fully described. The instrument is particularly suitable for the measurement of streaming double refraction.