Original papers: A combination horizontal/vertical prism bar A precisely calibrated tool for use in front of one eye

Abstract

The authors used an optical bench to investigate calibration and errors from improper positioning of prism bars manufactured by R.O. Gulden and Luneau. They urged Gulden to redesign its prism bars to be used back-to-back in front of one eye. The horizontal bar is held with its flat face posteriorly within a channel on the flat face of the vertical bar; the interface is positioned perpendicular to the direction of the fixation object, which demands that the horizontal prism bar be calibrated in the frontal plane position and the vertical bar in the Prentice position. Analysis of calibration demonstrated that Gulden's new combination horizontal/vertical prism bar can be used without significant error (within + 0.5δ of labelled values). Gulden's old vertical prism bar is also calibrated in the Prentice position. Luneau's horizontal and vertical prism bars are calibrated close to the frontal plane position (within +1.0δ and +0.4δ respectively). Improper positioning demonstrated an increasing error with larger prisms. Luneau's 25δ segment measured 27.8δ in the Prentice position, the 40δ segment 67.5δ. The 25δ segment of Gulden's new and old vertical prism bars measured 23.25δ in the frontal plane position. Gulden's vertical prism bars should always be held with the flat surface toward the examiner. Luneau's horizontal and vertical prism bars should be held one in front of each eye when used simultaneously; thus, neither eye is looking directly at the fixation object and defining primary and secondary deviations is not possible. The combination horizontal/vertical prism bar is manufactured by Gulden Ophthalmics, 225 Cadwalader Avenue, Elkins Park, PA 19117-2097. The Luneau prism bars are manufactured by Luneau Ophtalmologie, B.P. 252, 28005 Chartres Cedex, France. The authors have no proprietary or financial interest in these products or in the companies involved.