A calibration set for ocular aberrometers: Manufacture, testing and application

Abstract

Purpose. To manufacture and test a set of phase plates for the calibration of ocular aberrometers and eventually to apply it to the calibration of an ocular laser ray tracing aberrometer. Methods. The set of phase plates is made by a grey-scale single-mask photosculpture in photoresist method. Each plate induces a given amount of a particular aberration (Zernike) mode. The set contains two subsets: (1) different pure Zernike modes to test the accuracy among different orders (from 3rd to 7th, about 0.3 - 0.4 microns); and (2) plates having different amounts of the same mode, 3rd order coma ranging from 0.11 to 0.47 microns. Right after manufacturing, the plates were tested twice, as a cross-check, measuring the aberration pattern of each plate with a Mach-Zehnder interferometer and a single-pass Hartmann-Shack wavefront sensor. The set was then applied to the calibration of an ocular double-pass laser ray tracing aberrometer. Results. We found a close agreement between the three types of measurement. The maximum difference between H-S and LRT measurements was 0.032 μm (that is about λ/20, half of the typical measuring error in human eyes). This permitted us to detect a small bias in the ocular laser ray tracing aberrometer. Conclusions. The calibration set may be a powerful tool for the assessment of accuracy and reliability in ocular aberrometry. It allowed us to discover a small bias, that is almost impossible to detect working with human eyes or trial lenses. This type of calibration tool is especially important in clinical environments.