Berek Compensators

Berek Compensators

Berek compensators are variable retarders which allow to adjust the path difference and the axis orientation via changing the angle of incidence. They can be used as tunable wave plate. Additionally, they can be employed for the characterization of retardation plates. With our compact aXiscope test device one can visually determine the direction of the slow axis and the order number of arbitrary wave plates without the need of further equipment.

Berek compensators consist of a single plane parallel plate made from a uniaxial crystal. The crystal axis is oriented perpendicular to the polished surfaces. At normal incidence there in no birefringence. Tilting the surface normal with respect to the beam direction leads to a retardation which increases with tilt angle. The axis direction is determined by the tilt direction.

We manufacture Berek compensators from calcite with borosilicate cover plates without antireflection coating. The version with ring mount is suited for general purpose. The aXiscope is a visual test device for retardation plates which contains a Berek compensator mounted between crossed sheet polarizers. The aXiscope is used by looking into the aperture from short distance under ambient illumination. The tuning is induced by the field angle inside field of view without any moving parts. One observes a characteristic color pattern with a symmetric dark cross. The central minimum indicates the location of zero birefringence. Introducing a retardation plate between the polarizers shifts this minimum outwards to the locations in the field of view at which the path difference of the device under test is equal but with opposite sign to the path difference of the compensator. This results in two opposing dark sites. As calcite is known to have negative birefringence the line connecting these spots indicates the slow axis of the tested retarder independent of its type. The distance between the two minima allows to estimate the magnitude of the path difference of the retarder.

Version ring mount aXiscope
order. no. RKB 15.12 RKB 15.12FL
material calcite calcite
spectral range 400-2200 nm 400-2200 nm
plate thickness 1,5 mm 1,5 mm
clear aperture 12 mm 12 mm
mount diameter 25 mm 35 mm
mount length 10 mm 61 mm
order. no. RKB 15.12 RKB 15.12FL
price 1 piece [EUR] 342,- 500,-
price 2 piece [EUR] 314,- 480,-
price 3-5 pieces [EUR] 300,- 470,-
price 6-10 pieces [EUR] 277,- 454,-