Visual Acuity and Optical Parameters in Progressive-Power Lenses

Does visual acuity predict visual preference in progressive addition lenses?

PURPOSE

We aimed to determine if visual acuity (VA) could differentiate the quality of vision with two ophthalmic lenses with unwanted astigmatism.

METHODS

Twenty presbyopic subjects (48 to 62 years old; VA better than 0.0 logMAR) graded the magnitude of their preference between two progressive addition lenses (plano addition 2.00D) and their visual acuities were measured with both lenses at various eccentricities from -12 to +12 mm from the near vision point every 3 mm in controlled conditions.

RESULTS

The Lens with the least peripheral astigmatism was preferred by 75% of the subjects. VA measured at the near vision point was statistically worse (p<0.01) with this lens whereas the contrary was observed in the periphery (± 12 and -9 mm of eccentricity). The Friedman test shows that the eccentricity (p<0.001) has a significant effect on visual acuity. However, the lens did not show any significant effect (p=0.76). The choice of the favorite lens was predicted for only 35% when considering central VA (up to 6mm) and 80% of the subjects when considering peripheral VA (9 to 12mm). However, the magnitude of the difference could be predicted by peripheral VA in only 60% of the subjects.

CONCLUSION

High contrast Visual acuity was clearly able to differentiate the 2 lens designs tested in our experiment. However, even under the controlled conditions of this study, it was not possible to predict the quality of vision, as measured by a subjective appreciation, through progressive addition lenses at various eccentricities from the near vision with an addition of 2.0D.

Optical performance of progressive addition lenses (PALs) with astigmatic prescription

The progressive addition lens (PAL) is a spectacle lens design with progressive refractive power changes across the lens surface to provide sharp vision at different viewing distances for patients with reduced accommodative strength. It has gained in popularity not just for presbyopic patients, but also patients with occupational (office, driving, or digital device) and therapeutic (e.g., myopia control) needs. However, despite the increasing prevalence of astigmatism in adults > 40 years old who rely on PAL correction, no metric is available to reflect the optical variation in PALs with astigmatic prescriptions. Based on recent studies, four novel optical metrics sensitive to variation of refractive power across the lens surface of PALs have been developed. These metrics were used to compare the optical performance of PALs of various prescriptions, designs, and manufacturers. For each lens, the refractive power profile was first measured with a Moire-deflectometry-based instrument.The data was then exported and analyzed using a two-dimensional error map for each of the four metrics. The results revealed significant impacts of astigmatic prescription, providing evidence for the usefulness of these metrics in quantifying the optical performance of PALs for patients with astigmatic prescriptions.

Subjective Evaluation of Defocus and Astigmatism Combinations Using Image Simulation in Presbyopes

SIGNIFICANCE

Image simulation is a useful and efficient tool to explore the impact of defocus and astigmatism combinations on visual acuity and image quality score when accommodation is taken into account.

PURPOSE

The goal of this experiment was to determine if a simulation is able to predict visual acuity and image quality score (IQS) with defocus and astigmatism combinations in presbyopes.

METHODS

We measured visual acuity and IQS in five defocus and astigmatism combinations in either real or simulated conditions. In real conditions, the subjects viewed a stimulus through an ophthalmic lens or a deformable mirror. In simulated conditions, subjects viewed images of the same stimulus with simulated blur. The amounts of defocus and astigmatism combinations of a progressive addition lens in near vision were generated through a static correction of the subject's aberrations. We simulated three levels of accommodation: subject could not accommodate (FOC0), subject could accommodate to the less hyperopic focal point (FOC1), or subject could accommodate to the circle of least confusion (FOC2).

RESULTS

Visual acuity or IQS did not differ between mirror and progressive addition lens conditions. Visual acuity measured in real blur conditions differed significantly from that in FOC0 simulated blur condition but were similar to that in FOC1 and FOC2 simulated blur conditions. Image quality score obtained in real conditions were between scores measured with the FOC0 and FOC1 simulated conditions, suggesting that the subjects were able to produce a low level of accommodation.

CONCLUSIONS

Accommodation may play a role when comparing optical and simulated defocus and astigmatism combinations. Presbyopic subjects are able to produce a low level of accommodation that may counterbalance a part of the deleterious effect of the astigmatism on image quality. Simulation remains a useful tool if the correct accommodation state is taken into account.

Measurement of spectacle lenses using wavefront aberration in real view condition

A wavefront aberration analysis method for measuring spectacle lenses in real-view condition is proposed and verified using experimental apparatus based on the eye-rotation model. Two strategies-feedback positioning and posture adjustment of incident beams and Hartmann-Shack wavefront-aberration sensor calibration at each measurement subarea-are applied to improve measurement accuracy. By simulating the real-view condition, wavefront aberration and user power on the vertex sphere can be obtained. Comparison experiments demonstrate the validity and accuracy of the proposed method and experimental apparatus. Freeform progressive addition lenses are also measured and the results analyzed. The findings provide a possible approach for optimizing the design of spectacle lenses and evaluating their manufacturing and imaging quality.

Phenomenological model of visual acuity

We propose in this work a model for describing visual acuity (V) as a function of defocus and pupil diameter. Although the model is mainly based on geometrical optics, it also incorporates nongeometrical effects phenomenologically. Compared to similar visual acuity models, the proposed one considers the effect of astigmatism and the variability of best corrected V among individuals; it also takes into account the accommodation and the “tolerance to defocus,” the latter through a phenomenological parameter. We have fitted the model to the V data provided in the works of Holladay et al. and Peters, showing the ability of this model to accurately describe the variation of V against blur and pupil diameter. We have also performed a comparison between the proposed model and others previously published in the literature. The model is mainly intended for use in the design of ophthalmic compensations, but it can also be useful in other fields such as visual ergonomics, design of visual tests, and optical instrumentation.

Visual performance with lenses correcting peripheral refractive errors

PURPOSE

To design and manufacture lenses to correct peripheral refraction along the horizontal meridian and to determine whether these resulted in noticeable improvements in visual performance.

METHODS

Subjective refraction of a low myope was determined on the basis of best peripheral detection acuity along the horizontal visual field out to ±30° for both horizontal and vertical gratings. Subjective refraction was compared to objective refractions using a COAS-HD aberrometer. Special lenses were made to correct peripheral refraction, based on designs optimized with and without smoothing across a 3-mm diameter square aperture. Grating detection was retested with these lenses. Contrast thresholds of 1.25-min arc spots were determined across the field for the conditions of best correction, on-axis correction, and the special lenses.

RESULTS

The participant had high relative peripheral hyperopia, particularly in the temporal visual field (maximum, 2.9 D). There were differences >0.5 D between subjective and objective refractions at a few field angles. On-axis correction reduced peripheral detection acuity and increased peripheral contrast threshold in the peripheral visual field, relative to the best correction, by up to 0.4 and 0.5 log units, respectively. The special lenses restored most of the peripheral vision, although not all at angles to ±10°, and with the lens optimized with aperture smoothing possibly giving better vision than the lens optimized without aperture smoothing at some angles.

CONCLUSIONS

It is possible to design and manufacture lenses to give near-optimum peripheral visual performance to at least ±30° along one visual field meridian. The benefit of such lenses is likely to be manifest only if a subject has a considerable relative peripheral refraction, for example, of the order of 2 D.

Comparison of progressive addition lenses by direct measurement of surface shape

PURPOSE

To compare the optical properties of five state-of-the-art progressive addition lenses (PALs) by direct physical measurement of surface shape.

METHODS

Five contemporary freeform PALs (Varilux Comfort Enhanced, Varilux Physio Enhanced, Hoya Lifestyle, Shamir Autograph, and Zeiss Individual) with plano distance power and a +2.00-diopter add were measured with a coordinate measuring machine. The front and back surface heights were physically measured, and the optical properties of each surface, and their combination, were calculated with custom MATLAB routines. Surface shape was described as the sum of Zernike polynomials. Progressive addition lenses were represented as contour plots of spherical equivalent power, cylindrical power, and higher order aberrations (HOAs). Maximum power rate, minimum 1.00-DC corridor width, percentage of lens area with less than 1.00 DC, and root mean square of HOAs were also compared.

RESULTS

Comfort Enhanced and Physio Enhanced have freeform front surfaces, Shamir Autograph and Zeiss Individual have freeform back surfaces, and Hoya Lifestyle has freeform properties on both surfaces. However, the overall optical properties are similar, regardless of the lens design. The maximum power rate is between 0.08 and 0.12 diopters per millimeter and the minimum corridor width is between 8 and 11 mm. For a 40-mm lens diameter, the percentage of lens area with less than 1.00 DC is between 64 and 76%. The third-order Zernike terms are the dominant high-order terms in HOAs (78 to 93% of overall shape variance). Higher order aberrations are higher along the corridor area and around the near zone. The maximum root mean square of HOAs based on a 4.5-mm pupil size around the corridor area is between 0.05 and 0.06 µm.

CONCLUSIONS

This nonoptical method using a coordinate measuring machine can be used to evaluate a PAL by surface height measurements, with the optical properties directly related to its front and back surface designs.