Polymeric sheets as phase retardation elements

The spectral dependences of the retardance and of the birefringence for thr ee polymeric sheets of polyethylene, polyamide and polyethylene-polyamide b lend between 420 and 680 nm are studied. Retardance measurements are carrie d out using a recently developed model which allows for calibrating two pha se plates simultaneously from a single adjustment of the optical system. A new parameter that expresses the retardance introduced when light crosses 1 mum of the material is found to be useful in characterizing birefringent m aterials and in constructing phase plates for use at single wavelengths and achromatic phase plates.