Risley Rotary Prism
Large aperture (1"/23mm) rotating prisms permit more flexibility compared to your Phoropter. Two versions are offered; +/- 20 PD or +/- 30 PD with handles. The prisms are controlled with a convenient knurled rotating knob. Accurately calibrated dial shows prism diopter selected and rotates thru 30 PD right-exo to 30 PD left-eso.