disparometer
The disparometer is a near-point instrument that measures fixation disparity, the small misalignment of the eyes that remains while single binocular vision is maintained for the point of fixation.1,2 The fixation disparity is read directly, and the associated phoria is the prism needed to neutralize it.1 The model in common use in North America is the Sheedy Disparometer.3
The instrument presents two 1.5-degree circular targets, each containing two oppositely polarized nonius lines, and each circle provides the fusion lock.2 The upper circle measures vertical fixation disparity and the lower circle measures horizontal deviations, and the disappearance of an entire line shows suppression.2 The Sheedy Disparometer has only a parafoveal fusion lock and lacks the central lock of the Mallett unit, so the fixation disparities it records are larger.3
More detail
Prisms change the fixation disparity, so the disparometer can plot fixation disparity in minutes of arc against prism power in prism diopters.3 The disparometer generates these forced vergence fixation disparity curves at 40 cm.2 Type I is the most frequent curve, with a flatter middle where fixation disparity changes little over low prism powers and a steep rise at higher powers. At the limit of the fusional reserves diplopia appears.3
Sources (7)
- AOA Clinical Practice Guideline, Care of the Patient with Accommodative and Vergence Dysfunction
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 15
- Pickwell's Binocular Vision Anomalies, ch. 4
- Khurana, A. K. (2019). Theory and practice of optics and refraction. Elsevier Health Sciences.
- Remington, L. A. (2012). Clinical anatomy and physiology of the visual system. Elsevier Health Sciences.
- Rutstein, R., Daum, K. (1997). Anomalies of Binocular Vision: Diagnosis & Management. Mosby.
- Von Noorden, G. K., & Campos, E. C. (2002). Binocular vision and ocular motility: Theory and management of strabismus. Mosby.
Reviewed October 2026