Prism bars and variable Herschel prisms are most commonly used today to measure the angle of squint. Both have drawbacks, however. Prism bars are too large to carry around all day and are usually limited to 40 prism diopters (PD), while variable Herschel prisms have even smaller ranges. Hans Meester and I have therefore developed, at the Netherlands Ophthalmic Research Institute, Amsterdam, a hand-held prism that is continuously variable over a large range, horizontally as well as vertically. We propose to call it the "Variprism." It consists of a small housing containing two glass elements, ie, planoconvex and planoconcave, with equal radii of the convex and concave surfaces (Fig 1). The planoconvex front element is a half globe, freely rotatable in gimbals. The planoconcave lens is situated immediately behind it.

ophthalmology, variprism,
Archives of Ophthalmology
Department of Ophthalmology

Simonsz, H.J. (1988). The Variprism: A Large-Range Variable Prism for Measurement of the Angle of Squint. Archives of Ophthalmology, 106(9), 1166–1166. doi:10.1001/archopht.1988.01060140326014