PURPOSE:

We investigate whether a structured probe beam that creates the beacon for use in a retinal imaging adaptive optics system can provide useful side effects. In particular we investigate whether a Bessel beam that is seen by the subject as a set of concentric rings has a dampening effect on fixation variations of the subject under observation. This calming effect would allow longer periods of observation, particularly for patients with abnormal fixation.

METHOD:

An experimental adaptive optics system developed for retinal imaging is used to monitor the fluctuations in aberrations for artificial and human subjects. The probe beam is alternated between a traditional beacon and one provided by a Bessel beam created by SLM.

RESULTS:

Time-frequency analysis is used to indicate the differences in power and time variation during fixation depending on whether the Bessel beam or the traditional beacon is employed. Comparison is made with the response for an artificial eye to discount systemic variations.

CONCLUSION:

Significant evidence is accrued to indicate the reduced fluctuations in fixation when the Bessel beam is employed to create the beacon.

PURPOSE:

We tested the hypothesis that pupil apodization is the basis for central pupil bias of spherical refractions in eyes with spherical aberration.

METHODS:

We employed Fourier computational optics in which we vary spherical aberration levels, pupil size, and pupil apodization (Stiles Crawford Effect) within the pupil function, from which point spread functions and optical transfer functions were computed. Through-focus analysis determined the refractive correction that optimized retinal image quality.

RESULTS:

For a large pupil (7 mm), as spherical aberration levels increase, refractions that optimize the visual Strehl ratio mirror refractions that maximize high spatial frequency modulation in the image and both focus a near paraxial region of the pupil. These refractions are not affected by Stiles Crawford Effect apodization. Refractions that optimize low spatial frequency modulation come close to minimizing wavefront RMS, and vary with level of spherical aberration and Stiles Crawford Effect. In the presence of significant levels of spherical aberration (e.g. C4 0 = 0.4 μm, 7 mm pupil), low spatial frequency refractions can induce -0.7 D myopic shift compared to high SF refraction, and refractions that maximize image contrast of a three cycle per degree square-wave grating can cause -0.75 D myopic drift relative to refractions that maximize image sharpness.

DISCUSSION:

Because of small depth of focus associated with high spatial frequency stimuli, the large change in dioptric power across the pupil caused by spherical aberration limits the effective aperture contributing to the image of high spatial frequencies. Thus, when imaging high spatial frequencies, spherical aberration effectively induces an annular aperture defining that portion of the pupil contributing to a well-focused image. As spherical focus is manipulated during the refraction procedure, the dimensions of the annular aperture change. Image quality is maximized when the inner radius of the induced annulus falls to zero, thus defining a circular near paraxial region of the pupil that determines refraction outcome.

PURPOSE:

We propose the Bessel Circular Functions as alternatives of the Zernike Circle Polynomials to represent relevant circular ophthalmic surfaces.

METHODS:

We assess the fitting capabilities of the orthogonal Bessel Circular Functions by comparing them to Zernike Circle Polynomials for approximating a variety of computationally generated surfaces which can represent ophthalmic surfaces.

RESULTS:

The Bessel Circular Functions showed better modelling capabilities for surfaces with abrupt variations such as the anterior eye surface at the limbus region, and influence functions. From our studies we find that the Bessel Circular Functions can be more suitable for studying particular features of post surgical corneal surfaces.

CONCLUSIONS:

We show that given their boundary conditions and free oscillating properties, the Bessel Circular Functions are an alternative for representing specific wavefronts and can be better than the Zernike Circle Polynomials for some important cases of corneal surfaces, influence functions and the complete anterior corneal surface.

PURPOSE:

An impediment for the clinical utilisation of ophthalmic adaptive optics imaging systems is the automated assessment of photoreceptor mosaic integrity. Here we propose a fully automated algorithm for estimating photoreceptor density based on the radius of Yellott's ring.

METHODS:

The discrete Fourier transform (DFT) was used to obtain the power spectrum for a series of images of the human photoreceptor mosaic. Cell spacing is estimated by least-square fitting an annular pattern with a Gaussian cross section to the power spectrum; the radius of the resulting annulus provides an estimate of the modal spacing of the photoreceptors in the retinal image. The intrasession repeatability of the cone density estimates from the algorithm was evaluated, and the accuracy of the algorithm was validated against direct count estimates from a previous study. Accuracy in the presence of multiple cell types and disruptions in the mosaic was examined using images from four patients with retinal pathology and perifoveal images from two subjects with normal vision.

RESULTS:

Intrasession repeatability of the power spectrum method was comparable to a fully automated direct counting algorithm, but worse than that for the manually adjusted direct count values. In images of the normal parafoveal cone mosaic, we find good agreement between the power-spectrum derived density and that from the direct counting algorithm. In diseased eyes, the power spectrum method is insensitive to photoreceptor loss, with cone density estimates overestimating the density determined with direct counting. The automated power spectrum method also produced unreliable estimates of rod and cone density in perifoveal images of the photoreceptor mosaic, though manual correction of the initial algorithm output results in density estimates in better agreement with direct count values.

CONCLUSIONS:

We developed and validated an automated algorithm based on the power spectrum for extracting estimates of cone spacing, from which estimates of density can be derived. This approach may be used to estimate cone density in images where not every single cone is visible, though caution is needed, as this robustness becomes a weakness when dealing with images from patients with some retinal diseases. This study represents an important first step in carefully assessing the relative utility of metrics for analysing the photoreceptor mosaic, and similar analyses of other metrics/algorithms are needed.

PURPOSE:

In children, time spent outdoors has a protective effect against myopia development. In animal models, bright light reduces the development of experimental myopia. This study investigates how an increase in daily light exposure, presented either continuously during the day or periodically at different times of day, influences the development of experimental myopia in the chick.

METHODS:

Myopia was induced in Cobb Chicks (Gallus domesticus) by monocular deprivation (MD) of form vision with a translucent diffuser for 3 days (from 4 days of age) under a 12:12 light: dark cycle. MD control chicks were exposed to constant 300 lux (n = 11) during the light period. MD treatment groups received either constant 2000 lux (n = 11) during the light period or 300 lux for 10 h with a 2 h period of bright light (10 000 lux), either in the morning (n = 10), midday (n = 10) or evening (n = 10), giving the same total daily light exposure as the 2000 lux group. After 3 days of MD, refractive status, corneal curvature and axial eye dimensions were measured for all eyes under anaesthesia.

RESULTS:

Myopia in the constant 2000 lux group (-4.94 ± 1.21 D) was significantly less than in the 300 lux control group (-9.73 ± 0.96 D; p = 0.022). However, compared to the 300 lux control group, 2 h periods of 10 000 lux did not produce significant effects on refraction when presented either in the morning (-9.98 ± 0.85; p = 1.00), midday (-8.00 ± 1.26; p = 0.80), or evening (-13.14 ± 1.16 D; p = 0.20), although significantly less myopia was induced in the midday group compared to the evening group (p = 0.018). Orthogonal regression showed that myopia development was matched by changes in vitreous chamber depth (R(2)  = 0.69; p < 0.0001).

CONCLUSIONS:

In chicks, an increase in daily light exposure continuously during the day is more effective at inhibiting myopia than adding an equivalent dose within a 2 h period of bright light. A weak time-of-day effect also appears to be present in the response to bright light exposure. Our results suggest that future light-based myopia therapies in humans may be more effective if light levels are increased over the whole day, rather than through short periods of bright light exposure.

In the following point-counterpoint article, internationally-acclaimed myopia researchers were challenged to defend the two opposing sides of the topic defined by the title; their contributions, which appear in the order Point followed by Counterpoint, were peer-reviewed by both the editorial team and an external reviewer. Independently of the invited authors, the named member of the editorial team provided an Introduction and Summary, both of which were reviewed by the other members of the editorial team. By their nature, views expressed in each section of the Point-Counterpoint article are those of the author concerned and may not reflect the views of all of the authors.

In the following point-counterpoint article, internationally-acclaimed myopia researchers were challenged to defend the two opposing sides of the topic defined by the title; their contributions, which appear in the order, Point followed by Counterpoint, were peer-reviewed by both the editorial team and an external reviewer. Independently of the invited authors, the named member of the editorial team provided an Introduction and Summary, both of which were reviewed by the other members of the editorial team. By their nature, views expressed in each section of the Point-Counterpoint article are those of the author concerned and may not reflect the views of all of the authors.

In the following point-counterpoint article, internationally-acclaimed myopia researchers were challenged to defend the two opposing sides of the topic defined by the title; their contributions, which appear in the order, Point followed by Counterpoint, were peer-reviewed by both the editorial team and an external reviewer. Independently of the invited authors, the named member of the editorial team provided an Introduction and Summary, both of which were reviewed by the other members of the editorial team. By their nature, views expressed in each section of the Point-Counterpoint article are those of the author concerned and may not reflect the views of all of the authors.

In the following point-counterpoint article, internationally-acclaimed myopia researchers were challenged to defend the two opposing sides of the topic defined by the title; their contributions, which appear in the order, Point followed by Counterpoint, were peer-reviewed by both the editorial team and an external reviewer. Independently of the invited authors, the named member of the editorial team provided an Introduction and Summary, both of which were reviewed by the other members of the editorial team. By their nature, views expressed in each section of the Point-Counterpoint article are those of the author concerned and may not reflect the views of all of the authors.