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Risley Prism provides horizontal and vertical prismatic affect with touch of a dial. Aperture of 23mm (1 inch) provides adequate view for patient to see prism affect. Using this Risley away from the phoropter permits great flexibility in real space including close-up viewing by the patient. In one application of the Risley, motor binocularity can be shown if a patient is able to overcome the effect of variable prisms and maintain ocular alignment. The patient views an accommodative target and the prism power is slowly increased until a ‘breakpoint’ is achieved as judged by ocular deviation in the directions of the prism or by patient report. Two versions are offered with different prism ranges: +/- 20 PD and +/- 30 PD with handles. The prism power is controlled with a convenient knurled rotating knob. Accurately calibrated dial shows prism diopter selected and rotates thru 30PD right-exo to 0 PD to 30PD left-eso. Changing the handle from horizontal to vertical changes the position of the base to cover both horizontal and vertical measurements. Removable Red Maddox filter available to fit any of these Risley Prisms. Less expensive instruments called Maddox Phoria Measures are available for less extensive examinations. These instruments work in a similar manner. Additonal information on Risley and maddox Phoria measures available at the Technical Data tab. Richmond’s Risley Prisms feature smooth operation and will provide accurate results for many years.