Analysis of the Cross Cylinder and Chevron Techniques for Finding the Axis of Astigmatism

Abstract

ABSTRACT: This paper presents a theoretical analysis of cross cylinder (CC) and chevron tests for the axis of astigmatism. Both tests depend on the detection of a difference in blur between two out of focus targets. Blur differences are calculated for a wide range of simulated test conditions which may be encountered in clinical practice. With the CC optimum performance is achieved by initially locating cylinder axis with a small refractor head cylinder and a large CC, and by refining axis with a small CC after cylinder power has been determined. With the chevron, optimum performance over a wide range of conditions is achieved with a 60 chevron angle, and a fogging sphere that places both focal lines in front of the retina.

When conditions for the two tests are optimised they are found to produce very similar blur differences, suggesting that their performances should be of similar accuracy.

Key words:

• astigmatism
• subjective refraction
• cross cylinder
• chevron