Influences of reference plane and direction of measurement on eye aberration measurement

Comparison of wavefront aberrations in the object and image spaces using wide-field individual eye models

Wavefront aberrations in the image space are critical for visual perception, though the clinical available instruments usually give the wavefront aberrations in the object space. This study aims to compare the aberrations in the object and image spaces. With the measured wavefront aberrations over the horizontal and vertical ±15° visual fields, the in-going and out-going wide-field individual myopic eye models were constructed to obtain the wavefront aberrations in the object and image spaces of the same eye over ±45° horizontal and vertical visual fields. The average differences in the mean sphere and astigmatism were below 0.25 D between the object and image spaces over the horizontal and vertical ±45° visual fields under 3 mm and 6 mm pupil diameter. The wavefront aberrations in the object space are a proper representation of the aberrations in the image space at least for horizontal visual fields ranging from -35°to +35° and vertical visual fields ranging from -15°to +15°.

A new coordinate system convention in schematic eye modeling

PURPOSE

To discuss standardizing methods for presenting results from schematic eye calculations where light is entering the eye and forming an image on the retina.

METHODS

Published methods, measurement equipment, and software were evaluated.

RESULTS

The methods already used by ray tracing software are proposed, where everything is viewed from behind the eye and a conventional (x,y) coordinate system is used for images and plots. For meaningful evaluation in comparison to the image, any representations of the cornea, pupil, imaging wavefront, point spread function (PSF), and spot diagram are rotated by 180° about the optical axis for display to match the image orientation of a standard text chart. If a Zernike sag surface is used for a corneal surface of a schematic eye, this is also defined from the rear; however, care must be taken to ensure that the ordering of the Zernike terms is correct.

CONCLUSIONS

Simulated images can be viewed from behind the eye and oriented so that text has a normal appearance. This requires a rotation of the imaging wavefront and PSF by 180° about the optical axis.

Single dispersive gradient-index profile for the aging human lens

We provide a single gradient-index (GRIN) profile for the crystalline lens in an updated age-dependent emmetropic-eye model. The parameters defining the GRIN profile include their variation with age and the dispersion of the refractive index in order to account for the increase in the positive-wave spherical aberration, for the constant chromatic difference in the refraction of the human eye, as well as for the decrease in the retinal-image quality with aging. In accounting for these ocular properties, the results show that first, the value of the dispersion parameters are invariant with age. Second, those parameters defining the distribution of the lens index cause the lens-center-index value to decrease slightly, and its position along the lens axis changes with age. Furthermore, these findings are in agreement with the lens paradox.

Effect of defocus on on-axis wave aberration of a centered optical system

We derive equations for defocus and primary spherical wave aberration coefficients caused by a shift in image plane of a perfect optical system. The spherical aberration equation is accurate at describing changes in the spherical aberration of an aberrated schematic eye.

Neural and optical limits to visual performance in myopia

We investigated the relative importance of neural and optical limitations to visual performance in myopia. A number of visual performance measures were made on all or subsets of 121 eyes of emmetropic and myopic volunteers aged 17-35 years. These tests included visual measures that are mainly neurally limited (spatial summation out to +/-30 degrees in the horizontal visual field and resolution acuity out to +/-10 degrees in the horizontal visual field) and central ocular aberrations. We found that myopia affected the neurally limited tests, but had little effect on central higher order aberration. The critical area for spatial summation increased in the temporal visual field at 0.03 log units/dioptre of myopia. Resolution acuity decreased at approximately 0.012 log units/dioptre of myopia. Losses of visual function were slightly greater in the temporal than in the nasal visual field. The observed visual deficit in myopia can be explained by either global retinal expansion with some post-receptor loss (e.g. ganglion cell death) or a posterior polar expansion in which the point about which expansion occurs is near the centre of the previously emmetropic globe.

Optical models for human myopic eyes

Data from the author's investigations and other studies are used to construct refractive dependent models. These models include a gradient index lens and aspheric corneal, lens and retinal surfaces. Elements that alter with refraction are anterior corneal radius, vitreous length and retinal shape (vertex radius of curvature and asphericity) and decentration. Two versions of the models are produced, one with centred and symmetrical optical elements, and one with tilts of the lens and decentrations and tilts of the retina. The centred model predicts increase in spherical aberration in myopia. It predicts the relative change in mean sphere in the periphery between the horizontal and vertical meridians that has been observed in a recent experimental study. It overestimates peripheral astigmatism by about 50%. The decentred version has limited success in predicting changes in peripheral refraction of average eyes.