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| Encyclopedia - Prism |  |
A prism is a triangular piece of glass or plastic that has flat sides and bends light rays. When used as a diagnostic instrument during an eye exam, a prism is helpful in measuring misalignment of an eye such as occurs in strabismus (cross-eye).
A prism bar is a plastic holder with a series of prisms in increasing strengths. A horizontal prism bar is used for measuring phoria and tropia. Phoria is a latent tendency of an eye to deviate left or right from normal straight-ahead vision when covered. Tropia is a defect caused by an imbalance of extraocular muscles that causes one eye to look more left or right than the other, even when both eyes are uncovered.
The vertical prism bar is used to measure hyperphoria and hypertropia. Hyperphoria is a tendency for an eye to look upward when it is covered. Hypertropia is the tendency of one eye to look upward, even when uncovered, when the other eye looks straight ahead.
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