Melanie C. W. Campbell

Adaptive optics scanning laser ophthalmoscopy.

We present the first scanning laser ophthalmoscope that uses adaptive optics to measure and correct the high order aberrations of the human eye. Adaptive optics increases both lateral and axial resolution, permitting axial sectioning of retinal tissue in vivo. The instrument is used to visualize photoreceptors, nerve fibers and flow of white blood cells in retinal capillaries.

Presbyopia and the optical changes in the human crystalline lens with age.

Lenses from 27 human eyes ranging in age from 10 to 87 years were used to determine how accommodation and age affect the optical properties of the lens. A scanning laser technique was used to measure focal length and spherical aberration of the lenses, while the lenses were subjected to stretching forces applied through the ciliary body/zonular complex. The focal length of all unstretched lenses increased linearly with increasing age. Younger lenses were able to undergo significant changes in focal length with stretching, whereas lenses older than 60 years of age showed no changes in focal length with stretching. These data provide additional evidence for predominantly lens-based theories of presbyopia. Further, these results show that there are substantial optical changes in the human lens with increasing age and during accommodation, since both the magnitude and the sign of the spherical aberration change with age and stretching. These results show that the optical properties of the older presbyopic lens are quite different from the younger, accommodated lens.

Biometric, optical and physical changes in the isolated human crystalline lens with age in relation to presbyopia

The biometric, optical and physical properties of 19 pairs of isolated human eye-bank lenses ranging in age from 5 to 96 years were compared. Lens focal length and spherical aberration were measured using a scanning laser apparatus, lens thickness and the lens surface curvatures were measured by digitizing the lens profiles and equivalent refractive indices were calculated for each lens using this data. The second lens from each donor was used to measure resistance to physical deformation by providing a compressive force to the lens. The lens capsule was then removed from each lens and each measurement was repeated to ascertain what role the capsule plays in determining these optical and physical characteristics. Age dependent changes in lens focal length, lens surface curvatures and lens resistance to physical deformation are described. Isolated lens focal length was found to be significantly linearly correlated with both the anterior and posterior surface curvatures. No age dependent change in equivalent refractive index of the isolated lens was found. Although decapsulating human lenses causes similar changes in focal length to that which we have shown to occur when human lenses are mechanically stretched into an unaccommodated state, the effects are due to nonsystematic changes in lens curvatures. These studies reinforce the conclusion that lens hardening must be considered as an important factor in the development of presbyopia, that age changes in the human lens are not limited to the loss of accommodation that characterizes presbyopia but that the lens optical and physical properties change substantially with age in a complex manner.

Measurement of refractive index in an intact crystalline lens

A knowledge of the refractive index distribution within the crystalline lens is necessary to define the optical properties of the eye. This report describes the first nondestructive method for measuring the refractive index of intact crystalline lenses based on the work of Chu (1977) [Electron Lett. 13, 736-738] and Barrell and Pask (1978) [Opt. Commun. 27, 230-234] for optical fibre preforms. The method is simpler and more accurate than previous methods and shows that the rat crystalline lens has a smooth and almost parabolic distribution of refractive index.

Multifocal lenses compensate for chromatic defocus in vertebrate eyes.

Abstract The focal length of the vertebrate eye is a function of wavelength, i.e. the eye suffers from longitudinal chromatic aberration. Chromatic defocus is a particularly severe problem in eyes with high light-gathering ability, since depth of field is small due to a pupillary opening that is large in relation to the focal length of the eye. Calculations show that in such eyes only a narrow spectral band of light can be in focus on the retina. For the major part of the visual spectrum, spatial resolution should be limited by the optics of the eye and far lower than the resolving power achievable by the retinal cone photoreceptor mosaic. To solve this problem, fishes with irises unresponsive to light have developed lenses with multiple focal lengths. Well-focused images are created at the wavelengths of maximum absorbance of all spectral cone types. Multifocal lenses also appear to be present in some terrestrial species. In eyes with mobile irises, multifocal lenses are correlated with pupil shapes that allow all zones of the lens, with different refractive powers, to participate in the imaging process, irrespective of the state of pupil constriction.

The optical transverse chromatic aberration on the fovea of the human eye

The horizontal component of optical transverse chromatic aberration (TCA) at the fovea between 486 and 656 nm is measured in a sample of 8 eyes by two novel methods, both using vernier adjustment tasks with a retinal illumination of approx. 780 td and for a pupil size of approximately 5.5 mm dia. Initially, in an indirect method, TCA is derived along the line of sight from chromatic parallax. Secondly, TCA is measured directly using a semi-Maxwellian view and compensating for longitudinal chromatic aberration (LCA). Both techniques are unaffected by coma or by the Stiles-Crawford effects, thus optical TCA rather than the TCA perceived in normal view is measured. On average, optical TCA is in the same direction but less than previously predicted by eye models and predictions of the optical quality of the eye in white light are modified. Factors underlying the lower average value of optical TCA and variability among subjects, especially pupil centration and foveal position, are discussed. The relationship of optical TCA to TCA perceived in normal view and to chromostereopsis is analysed. The results suggest that the optical design of the human eye is optimized to reduce the wavelength dependent phase shift in the optical transfer function, which could be produced by optical TCA.

An analytic, gradient index schematic lens and eye for the rat which predicts aberrations for finite pupils

Abstract Schematic eyes with a homogeneous equivalent lens are inadequate for non-paraxial optics and available versions have been invalidly fitted to non-paraxial properties. A new model eye is here analytically derived from the refractive index profile of the crystalline lens and anatomical measurements of the rat eye. It predicts spherical abberration, coma, paraxial properties and the variation of refractive state with pupil size in accord with experimental measurements. The cornea counteracts the spherical aberration and linear coma of the lens so that overall aberration is reduced and the eye is of good optical quality.The nodal point is invariant with object eccentricity in a manner advantageous to a species whose visual axis is displaced from the optic axis. The potential of the model lies in its extension to a study of such off-axis optics.

The Julius F. Neumueller Award in Optics, 1989: change of pupil centration with change of illumination and pupil size.

Pupil centration is important to the optical blur on the retina. Using a dual Maxwellian view system we measured the centration of the pupil with respect to the achromatic axis of the eye as a function of pupil size. Significant shifts of the pupil center (up to 0.6 mm) with pupil dilation were measured in both nasal and temporal directions. The effect was usually symmetrical between the two eyes and the shift was linear with pupil size in one-half of the subjects. From their initial positions the linear pupil center shifts with dilation were in the direction of the achromatic axis.

Refractive index distribution and spherical aberration in the crystalline lens of the African cichlid fish haplochromis burtoni

Refractive index distribution in the teleost crystalline lens was measured with a nondestructive method in freshly excised lenses of the African teleost fish Haplochromis burtoni. Independently, spherical aberration was measured in a parallel set of lenses. The measured refractive index profiles show a continual decrease of refractive index from the center to the surface of the lens. The H. burtoni lens is of high optical quality and slightly overcorrected for spherical aberration. Details of the small residual spherical aberration were accurately predicted by ray-tracing model calculations based on the measured refractive index profile. The refractive index profile and the spherical aberration both show more complex characteristics than suggested by earlier measurements and lens models.

Psychophysical measurement of the blur on the retina due to optical aberrations of the eye

The blur on the retina in the horizontal meridian due to monochromatic and chromatic aberrations has been measured using a novel psychophysical technique. Longitudinal chromatic aberration gives the dominant blur for pupil sizes of 4-5 mm, followed by monochromatic aberrations, and blur due to optical transverse chromatic aberration. In some eyes, coma was present as a result of a displacement of the axis of symmetry from the centre of the pupil, but in three eyes, coma was present without spherical aberration. The technique also allows a measurement of the effective pupil centre relative to the geometric centre and a partial analysis of the relative positions of the reference axes of the eye.