Eyeglass lenses can contain prisms. These prisms are what help to correct eye vision. When the light passes through the lens, the prism causes the light to bend. This is known as the lens prism effect. Incorrect prisms can cause many vision problems and related eye conditions such as headaches, blurry vision and even nausea. Vertical prism is used to compensate for vertical fixation disparity or vertical phoria. Vertical prism can be measured using a focimeter, also known as a lensmeter.
Set the focimeter power to zero. Turn the eyepiece as far as you can in an counterclockwise direction and then slowly turn it clockwise until the graticule comes into focus. The eyepiece is know focused.
Place the glass lens into the focimeter. The ocular surface points away from you when you sit to use the machine. The middle of the lens is against the lens stop. Adjust the lens horizontally and vertically until the target sits at the centre of the graticule. Fix the lens in place with a lens retainer when this position is achieved.
Center the right lens in the focimeter so that the middle of the graticule is in the optical centre of the lens. Clamp the right glass lens in place so that it is secure against the lens rest. Adjust the lens table so that the bottom of the spectacle rests against the lens table. Dot the lens with the focimeter marking device at the centre of the graticule.
Unclamp the lens and move the entire frame over to measure the left lens. Do not change the height of the table; only move the machine horizontally.
Look through the eyepiece at the target. Dot the centre of the graticule on the left lens. Compare the dots on the two lenses. If the dot on the left lens is above or below the dot on the right lens, then a vertical prism is present. If the left lens dot is above the right lens dot, then a base-up prism is in the lens. It the dot is lower, there is a base-down prism in the lens.
Measure the amount of prism using the graticule circles. Each circle represents one prism dioptre starting at zero from the centre. The circle where the centre of the graticule sits on the left lens measures the prism.