Exploring ocular aberrations with a schematic human eye model.

PURPOSE: Advances in ophthalmic technologies now offer both the measurement and reduction of ocular aberrations by surgically or otherwise honing refraction in the anterior eye. Ocular aberrations, however, are known to change with a multitude of factors, including field position, accommodation level, and age. Thus, although static correction of aberrations provides some vision improvement, this may be less than expected. METHODS: In this article, we use an aspheric, variable-focus, age-dependent, gradient index schematic eye to investigate where ocular aberrations arise in the eye and how these change with field position, accommodation, and age. Optical ray tracing was carried out using optical design software ZEMAX, and Seidel aberration analysis was performed with custom written software in MATLAB. RESULTS AND CONCLUSIONS: Our modeling is consistent with clinical findings that certain corneal aberrations almost balance those arising from the lens. Our calculations also support the general notion that, by optical sculpting, corneal aberrations can be adjusted to completely balance out those of the lens. This can effectively eliminate the eye's total monochromatic aberrations, but for only one retinal image point at a time. Centered on this point of minimal aberration is a region (the isoplanatic patch) within which the aberrations produce a point spread smaller than some tolerable limit. Also, using available evidence in the literature concerning changes in critical ocular parameters with age and accommodation, our modeling results parallel established clinical findings, and additionally indicate that the major source of aberration change can be attributed to the gradient index distribution in the lens.

Characteristics of the human isoplanatic patch and implications for adaptive optics retinal imaging.

Conventional adaptive optics enables correction of high-order aberrations of the eye, but only for a single retinal point. When imaging extended regions of the retina, aberrations increase away from this point and degrade image quality. The zone over which aberrations do not change significantly is called the "isoplanatic patch." Literature concerning the human isoplanatic patch is incomplete. We determine foveal isoplanatic patch characteristics by performing Hartmann-Shack aberrometry in 1 deg increments in 8 directions on 7 human eyes. Using these measurements, we establish the correction quality required to yield at least 80% of the potential patch size for a given eye. Single-point correction systems (conventional adaptive optics) and multiple-point correction systems (multiconjugate adaptive optics) are simulated. Results are compared to a model eye. Using the Marechal criterion for 555-nm light, average isoplanatic patch diameter for our subjects is 0.80+/-0.10 deg. The required order of aberration correction depends on desired image quality over the patch. For the more realistically achievable criterion of 0.1 mum root mean square (rms) wavefront error over a 6.0-mm pupil, correction to at least sixth order is recommended for all adaptive optics systems. The most important aberrations to target for a multiconjugate correction are defocus, astigmatism, and coma.

Refraction and aberration across the horizontal central 10 degrees of the visual field.

PURPOSE: The purpose of this study was to measure refraction and aberrations across the horizontal central visual field. METHODS: Cycloplegic refraction was measured on eight subjects at 13 points across the horizontal central 10 degrees of the retina using a Hartmann-Shack wavefront sensor. Refractions were converted into mean sphere (M), 90 degrees to 180 degrees astigmatism (J180), and 45 degrees to 135 degrees astigmatism (J45) components. For five subjects, higher-order aberrations were determined at the center and edges of the field. RESULTS: Subtle changes in refraction were found to exist across the central 10 degrees of the retina, with changes in mean best sphere varying by up to half a diopter across this region and with smaller changes in astigmatism. Horizontal coma, but no other higher-order aberrations, varied systemically across the visual field; it varied linearly with angle but at different rates for the different subjects. CONCLUSION: Subtle changes in cycloplegic refraction exist across the horizontal central 10 degrees of the retina. The results indicate the need for correct alignment when measuring objective refraction.

Spectra and lifetimes of fluorescence resonance energy transfer fluorophores under two-photon excitation.

We show two-photon spectra and lifetimes acquired using conventional confocal microscopes equipped with an ultra-short pulsed laser and a time-gated intensified charge coupled device. We report on the two-photon spectra and lifetimes of Alexa350, enhanced green fluorescent protein (EGFP), EGFP-CD46, and Cy3 labelled antibodies. Cellular and extracellular EGFP two-photon spectra and lifetimes are compared.

Effect of ultrashort pulsed illumination on foci caused by a Fresnel zone plate.

The focal distribution produced by a zone plate under ultrashort pulsed laser illumination is investigated under the Fresnel approximation. A comparison of the diffraction patterns in the focal region between pulsed and continuous-wave illumination shows that the focal shape produced by a zone plate can be significantly altered when an ultrashort pulse is shorter than 100 fs. In particular, the focal width in the axial and the transverse directions is increased by approximately 5 and 85%, respectively,from continuous-wave illumination to 10-fs pulsed illumination.