Thursday, December 22, 2011

What are Prisms?

We sometimes use prisms in both your eye exam as well as in your eyeglass prescription. A prism is a triangular, or wedge-shaped, piece of plastic or glass that has the property of displacing a bundle of light toward the base of the prism. A prism has flat sides, an apex, and a base. If the prism is placed before the eye, an object viewed in front of the prism will appear to be displaced toward its apex. Prisms are employed in measuring the presence of the amount of any tropias (an obvious misalignment of the eyes) or phorias (a tendency toward ocular misalignment that is held in check by the fusion effort of the extra ocular muscles). 

Types of prisms that are available are loose prisms, horizontal and vertical prism bars and Risley's rotary prism. The loose or individual prism is made of plastic or glass. These prisms are supplied in low powers in standard trial lens sets and in full range of powers in individual prism boxes.

Horizontal and vertical prism bars are fused prisms amalgamated into a single bar of gradually increasing strengths. These prisms may be set in a horizontal direction (base in or out) or in a vertical direction (base up or down). The prism bar is principally employed to measure the amplitude or power of fusion.

Risley's rotary prism consists of two counter-rotating prisms mounted in rings, one in front of the other. These rings are easily rotated in opposite directions by a small thumbscrew. Risley's rotating prism provides a rapid and simple increase in prism power strength so that a deviation may be rapidly adjusted and measured without the delay in introducing individual prisms before the eye.

If you have questions about eye exams and how we use prisms please feel free to contact us at Doctor & Associates-203-22-4113.