(from the article `radiation`) ...is so selected that the path difference for the ordinary and the extraordinary rays is one-quarter the wavelength of the single-wavelength, or ...
used to change linearly polarized light to circularly polarized light
in a waveguide capable of propagating two waves of orthogonal polarizations having the same phase change coefficient, a device which introduces a phase-shift of p/2 radians of one wave with respect to the other
<microscopy> A compensator giving a retardation of about 130 nm, and a phase shift of 1/4 ~, thus constituting a device used with a polarizer and analyser designed to produce circularly polarized light. ... (05 Aug 1998) ... 
a crystal thin enough to cause a phase difference of 90° between the ordinary and extraordinary rays of polarized light, thereby converting circularly polarized light into plane polarized light. Cf.