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Retarders are made of bi-refringent material e.g. polyvinylalcohol PVAL oder modified polycarbonate. As dying does not occur in the process, the sheets remain transparent.

ITOS GmbH - Retarders

By stretching a polymer film it becomes birefringent meaning that it has two different refractive indices. Those two orientations produce wave shifts commonly quarter wave, half wave or full wave. The resulting two axes are considered “fast” or “slow” because the fast axis has not been retarded like the slow one has by a portion of the wavelength.

By shifting the sheets ¼ lamba, circular polarized light can be created.

Using retarders in this way allows for any kind of polarization to be created with linear polarized light. The sheets do not have any useful effect on unpolarized light. While it is possible to produce almost any degree of retardation, only the three common wave values are of technical interest.

Typ Phase Shift Retardance Function
WP140HE 90° 140nm / Lambda 1/4 circular polarizing light
WP280 180° 280nm / Lambda 1/2 Rotate 90°
WP560 360° 560nm / Lambda 1/1 Full wave film for 1st order red (used in microscopes and stress analysis)