Repeatability of in vitro power profile measurements for multifocal contact lenses

Highlights

• Power profiles were measured with high repeatability values.

• The repeatability was homogeneous along the optical zone.

• Three measurements of the power profile are needed to get a tolerance of 0.05 D.

Abstract

Purpose

To evaluate the repeatability of an optical device (NIMO TR1504, Lambda-X, Belgium) for measuring multifocal contact lens power profiles.

Methods

The NIMO TR1504 was used to measure power profiles 30 times for each of 10 different contact lenses from 4 major companies. All contact lenses were labelled as −3 D for distance vision; half were for high addition and half for low addition. The optical zone in all measurements was set to 3-mm radius. For each lens, the median power profile and the residuals of the 30 measurements were calculated. The 95% confidence bands and two indices that summarize measurement errors were calculated: the repeatability limit and an index of repeatability heterogeneity, quantifying heterogeneity of measurement errors over the optical zone.

Results

The repeatability limit was good (from 0.04 D to 0.12 D), for all multifocal contact lenses. Variability of measurement errors of power profiles was quite homogeneous along the optical zone for all lenses, although for some lenses variability was slightly higher in the centre than peripherally.

Conclusions

The repeatability of measured power profiles obtained by the NIMO TR1504 is lower than 0.12 D for the multifocal contact lenses.

Introduction

Accurate repeatable measurement of the power profile and of its variability is necessary to assess the quality of a multifocal contact lens and how well the final product meets the design specifications. Objective measurements of the power profiles provide helpful information for prescribing lenses on presbyopic patients [1], give practitioners a better understanding of the behaviour of these contact lenses, and enhance fitting nomograms. For example, it has been reported [2] that modifying the distance nominal power in a multifocal contact lens has double effect: change both the near addition, and the pupil diameter for which the lens provides the distance correction.

There are different techniques that have been used to measure contact lens power profiles objectively: Meier et al. [3] used a series of double aperture stops on a lensometer to measure the power distribution across the Bausch & Lomb bifocal lenses; Bullimore et al. [4] assessed power variation across the surface of a multifocal lens using a video-keratographic method; and Collins et al. [5] measured power variations across the optic zone using a computer-interfaced vectometer. More sophisticated methods are used nowadays to measure in vitro power profiles, such as in Ehrmann et al. [6], where an optical system was developed for power mapping intraocular and contact lenses using a paraxial laser scan. Papas et al. [7] evaluated the refractive power within the optical zone with a contact lens power analyser. Plainis et al. [8], measured the power profile of several multifocal contact lenses with a lens profiler based on ptychographic imaging. And Wagner et al. [1] investigated the power profile of single vision and multifocal contact lenses with a wavefront sensing instrument.

The NIMO TR1504 (Lambda-X, Belgium) is an optical device that measures refractive power and optical aberrations of spherical, toric, and multifocal lenses. It is based on a deflectometry technique and combines the Schlieren principle with a phase-shifting method [9], [10]. With a single measurement, this device obtains information about Zernike coefficients, power profiles, sphere, cylinder and axis. Joannes et al. [11] found a reproducibility standard deviation for monofocal contact lenses with the NIMO device of 0.05 D (Table 5 in their paper [11]); being then more precise than any current ISO referenced method. Although they did not report repeatability data directly, they stated that the repeatability standard deviation for monofocal contact lenses was of the same order of magnitude as the reproducibility one.

Repeatability of NIMO for multifocal contact lenses has not been assessed up to now. Thus, the aim of this study is to evaluate the repeatability of the NIMO for multifocal contact lenses when it comes to measure their power profiles.