Checking the calibration of Rotary Prisms is a part of every Preventive Maintenance.
We have a special factory calibrated chart which allows us to check the horizontal and vertical calibration of the Rotary Prism from 0 to 20 diopters.
A pinhole of light is projected through the Rotary Prism onto the calibration chart. When the Refractor is placed at the correct distance from the chart, the pinhole will be centered at 0 diopters. By moving the Prism Knob the light travels across and up and down the chart. Calibration is checked at 0, 5, 10 & 20 diopters, by verifying the light is centered over the corresponding mark on the chart.