Measurement of Fixation Disparity
There are several methods to quantify fixation disparity. The Mallett card, the Bernell lantern slide, the Wessen Card and the Disparometer may be used. A patient's associated phoria is the amount of prism needed to reduce their fixation disparity to zero minutes of arc.
The Mallett Fixation Disparity Unit
Instrument used to measure the associated heterophoria (or compensating prism). It consists of a small central fixation letter X surrounded by two letters O, one on each side of X, the three letters being seen binocularly, and two coloured polarized vertical bars in line with the centre of the X which are seen by each eye separately. The instrument can be swung through 90° to measure any vertical fixation disparity. The associated phoria is indicated by the misalignment of the two polarized bars when the subject fixates the X through cross-polarized filters in front of the eyes. The amount of associated phoria is given by the value of the base-in or base-out prism power necessary to produce alignment. The unit can also be used to detect suppression. See Disparometer; associated heterophoria; uncompensated heterophoria.
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