Now showing 1 - 10 of 14
  • Publication
    Monocular foveal, parafoveal, and perifoveal accommodation response to random defocus step changes
    (SPIE, 2020-08-21) ;
    Accommodation of the human eye relies on multiple factors, including – object size, monochromatic and chromatic aberrations, and vergence, and corrects defocus even in monocular conditions. Previous studies have been done to understand whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. Yet, findings have not been unambiguous and questions remain. Thus, in this study we tried to understand how accommodation makes use of defocus blur to detect the sign of defocus by performing experiments using a fast wavefront sensor in a vision testing system while eliminating other visual cues that may otherwise confound the analysis. A new automated method has been introduced to study monocular accommodation by using a currentdriven tunable lens (TL) to induce a random sequence of defocus step changes within the accommodative range of each observer. The response was captured in real time using a Hartmann-Shack wavefront sensor (HS-WFS) operating at 20 Hz while detecting aberrations and Zernike coefficients until 4th radial order across a 3 mm limited pupil. Foveal, parafoveal and perifoveal accommodation has been studied for young emmetropes and myopes to determine until which eccentricity accommodation is triggered. Our findings show that the accommodative range diminishes with eccentricity and at 14° (diameter) and beyond it becomes largely absent.
  • Publication
    Analysing the impact of myopia on the Stiles-Crawford effect of the first kind using a digital micromirror device
    Purpose: Photoreceptor light acceptance is closely tied to the Stiles-Crawford effect of the first kind (SCE-I). Whether the SCE-I plays a role in myopic development remains unclear although a reduction in directionality has been predicted for high myopia. The purpose of this study is to analyse the relationship between foveal SCE-I directionality, axial length, and defocus for emmetropic subjects wearing ophthalmic trial lenses during psychophysical measurements and for myopic subjects with their natural correction. Method: A novel uniaxial flicker system has been implemented making use of a Digital Micromirror Device (DMD) to flicker between a 2.3 visual degrees circular reference and a set of circular test patterns in a monocular Maxwellian view at 1 Hz. The brightness of the test is adjusted by the duty cycle of the projected light to an upper limit of 22727 Hz. The wavelength and bandwidth are set by a tuneable liquid-crystal filter centred at 550 nm. A total of 4 measurement series for 11 pupil entrance points have been realized for the right eye of 5 emmetropic and 8 myopic subjects whose pupils were dilated with tropicamide. The emmetropic subjects wore ophthalmic trial lenses in the range of -3 to +9 dioptres to mimic hyperopic to highly myopic vision and resulting visibility plots have been fitted to a Gaussian SCE-I function. In turn, the myopic subjects wore their natural correction during the analysis of the SCE-I. All subjects had their axial length determined with an ultrasound device. Results: A SCE-I directionality parameter for well-corrected vision in the range of 0.03 to 0.06/mm2 was found for the emmetropic subjects with corrected vision in fair agreement to values in the literature. The results also revealed a marked reduction in directionality in the range from 16% to 30% with every 3 dioptre increase of simulated myopia, as well as a 10% increased directionality in simulated hyperopic eyes. For both emmetropic and myopic subjects a decrease in directionality with axial length was found in agreement with theoretical expectations. Conclusion: The study confirms a clear link between SCE-I directionality, uncorrected defocus, and axial length. This may play a role for emmetropization and thus myopic progression as cone photoreceptors capture light from a wider pupil area in elongated eyes due to a geometrical scaling.
      591Scopus© Citations 25
  • Publication
    Hartmann–Shack wavefront sensing without a lenslet array using a digital micromirror device
    The common Hartmann–Shack wavefront sensor makes use of a lenslet array to sample in-parallel optical wavefronts. Here, we introduce a Hartmann–Shack wavefront sensor that employs a digital micromirror device in combination with a single lens for serial sampling by scanning. Sensing is analyzed numerically and validated experimentally using a deformable mirror operated in closed-loop adaptive optics with a conventional Hartmann–Shack wavefront sensor, as well as with a set of ophthalmic trial lenses, to generate controllable amounts of monochromatic aberrations. The new sensor is free of crosstalk and can potentially operate at kilohertz speed. It offers a reconfigurable aperture that can exclude unwanted parts of the wavefront.
      787Scopus© Citations 25
  • Publication
    Rate of riboflavin diffusion from intrastromal channels before corneal crosslinking
    (Lippincott Williams & Wilkins, 2016-03) ; ;
    Purpose To determine the diffusion of riboflavin from intrastromal channels through the effective diffusion coefficients compared with traditional axial diffusion with epithelium on or off. Setting Advanced Optical Imaging Laboratory, University College Dublin, and Wellington Eye Clinic, Sandyford, Dublin, Ireland. Design Experimental study. Methods The rate of diffusion in whole-mounted porcine eyes was monitored for a 30 minutes using an optical setup with a charge-coupled device camera and a bandpass filter (central wavelength 550 nm and 40 nm bandpass) to image the fluorescence under ultraviolet illumination (365 nm wavelength). For comparison, an isotropic corneal stroma with an annular channel was modeled numerically for different diffusion constants and boundary conditions. Results Numerical and experimental results were compared, allowing determination of the effective diffusion coefficient for each case. Experimental results for 6 different riboflavin solutions were in all cases found to be higher than for the common crosslinking (CXL) riboflavin protocol, where the diffusion constant is D0 = 6.5 × 10-5 mm2/sec. For the intrastromal channel, 2 isotonic solutions containing riboflavin 0.1% correlated with a diffusion constant of 5D0 = 32.5 × 10-5 mm2/sec. Hypotonic solutions and transepithelium had a higher diffusion coefficient approaching 10D0 = 65.0 × 10-5 mm2/sec, which is an order-of-magnitude increase compared with the typical diffusion coefficient found in standard CXL. Conclusions In this study, riboflavin had a faster stromal diffusion when injected into a corneal channel than when applied as drops to the anterior corneal surface. Further numerical modeling might allow optimization of the channel structure for any specific choice of riboflavin.
      100Scopus© Citations 6
  • Publication
    Geometrical scaling of the developing eye and photoreceptors and a possible relation to emmetropization and myopia
    (Elsevier, 2021-12-01)
    In this study the role of vergence in relation to age-dependent scaling of eye and photoreceptor parameters is studied. The underlying hypothesis is that the size and packing of outer segments is matched to the pupil size outdoors in photopic conditions. Vergence is analysed in relation to the angular spectrum of waves being incident using age-dependent data from the literature for the actual geometry and density of photoreceptor cones and rods. This approach is used to derive simple relations for the angular confinement of light along outer segments. Only with a small photopic pupil can leakage and crosstalk for both central and peripheral photoreceptors be entirely ruled out due to the finite length of the outer segments. A limiting 3 mm pupil size is found for children in the school age. Larger pupils will increase the likelihood of leakage and crosstalk that may therefore impact on emmetropization. This study has introduced a new paradigm in myopia research by considering vergence across the 3-D retina as being matched to the angular spectrum of waves being incident from the eye pupil. Emmetropization suggests a delicate balance between photoreceptor outer segment length and density in relation to pupil size. Only when balanced will leakage and crosstalk between adjacent outer segments be effectively suppressed thereby ensuring the highest possible light capture efficiency by visual pigments in the outer segments whether an image is formed on the retina or not.
      101Scopus© Citations 8
  • Publication
    Uniaxial flicker analysis of the psychophysical Stiles–Crawford effects
    (Taylor & Francis, 2017-02-21) ;
    Purpose: We report on a semi-automated system for frequency analysis of the Stiles–Crawford effect of the first kind (SCE-I) using flicker methodology designed to gain insight into the temporal dynamics of the perceived visibility for alternating pupil entrance points. We describe the system and its calibration in detail and discuss psychophysical measurement data obtained for the two authors. Methods: A uniaxial system is used for SCE-I characterization of two emmetropic subjects as a function of flicker frequency for narrow wavelength bands chosen in the range of 450–700 nm using a fibre-guided tungsten–halogen lamp as light source. The flicker is realized using two orthogonally mounted galvanometric scanning mirrors that allow linear trajectories at any angle across the pupil. A fast tuneable liquid-crystal neutral density filter is used for brightness adjustment and another liquid-crystal filter is used for wavelength adjustment at each pupil point allowing simultaneous hue-shift determination for the Stiles–Crawford effect of the second kind (SCE-II). Results: Validation of the system is realized with a CCD camera, a spectrometer and a powermeter, and the data obtained are used in the software to calibrate all subsequent human subject measurements. The psychophysical data obtained show a strong frequency dependence of the Gaussian SCE-I with a characteristic directionality parameter, ρ, that is found to increase from 0.03 to 0.06/mm2 with flicker in the range of 1–10 Hz. The simultaneously determined hue shift could not be determined beyond 1 Hz due to the longer time required for a subjective determination. Conclusion: We have reported on a fast uniaxial system for temporal characterization of the SCE-I. The psychophysical results obtained show that accurate specification of frequency in flicker analysis is mandatory when comparing SCE-I visibility and directionality curves obtained with those obtained using quasi-static bipartite fields. A uniaxial design offers unique advantages over that of common two-channel systems by completely eliminating spectral errors or brightness differences in the two branches that otherwise will impose on those of the visual system and degrade the psychophysical data. Future work with more subjects will be used to narrow the uncertainty and the causes of the effects observed.
      97Scopus© Citations 4
  • Publication
    The impact of aberrations in a 3D retinal model eye
    (SPIE, 2021-08-05)
    Aberrations of the eye degrade the ocular point-spread function thereby reducing the attainable visual acuity. It is common practice to distinguish between lower and higher-order monochromatic aberrations of the eye when differentiating between what can be corrected with sphere and cylinder, and what cannot. Nevertheless, at the retina it matters more whether light is incident along or obliquely onto the elongated photoreceptors. In this contribution, I discuss the impact of different Zernike aberration terms not at the pupil, but at the retina. Even-ordered monochromatic Zernike aberrations have an associated wavefront slope at the retina whereas odd-ordered Zernike aberration modes have no wavefront tilt across the point-spread function. In other words, even and odd-ordered Zernike modes are affected differently by the Stiles-Crawford effect of the first kind that relates to obliqueness of light at the retina. Understanding this is essential to decode how vision is triggered in normal viewing conditions as well as when probing vision and photoreceptors with psychophysical methods in the analysis of vision or for ophthalmic design. Finally, a uniaxial pupil flicker system is used to directly measure the integrated Stiles-Crawford effect in the author’s eye in order to assess apodization of oblique light in normal vision.
      102Scopus© Citations 1
  • Publication
    Virtual pyramid wavefront sensor for phase unwrapping
    Noise affects wavefront reconstruction from wrapped phase data. A novel method of phase unwrapping is proposed with the help of a virtual pyramid wavefront sensor. The method was tested on noisy wrapped phase images obtained experimentally with a digital phase-shifting point diffraction interferometer. The virtuality of the pyramid wavefront sensor allows easy tuning of the pyramid apex angle and modulation amplitude. It is shown that an optimal modulation amplitude obtained by monitoring the Strehl ratio helps in achieving better accuracy. Through simulation studies and iterative estimation, it is shown that the virtual pyramid wavefront sensor is robust to random noise.
      67Scopus© Citations 2
  • Publication
    Defocus-corrected analysis of the foveal Stiles-Crawford effect of the first kind across the visible spectrum
    (IOP Publishing, 2013-10-18) ;
    The Stiles-Crawford effect of the first kind describes a gradually diminished visibility of light that enters the eye towards the pupil rim. Although of retinal origin, it is commonly described by a Gaussian pupil apodization whose width is determined by a directionality parameter that depends on retinal eccentricity, wavelength and spatial coherence of the light. As the measurements are done psychophysically they are prone to subjective variations and difficult to obtain across the visible spectrum. In this work, requirements for accurate refractive correction when determining the directionality parameter at any given wavelength are discussed and we show that a current-controlled tunable liquid-polymer lens provides a convenient means to accomplish this without requiring mechanical readjustments. This may be the most convenient way to combat defocus across the visible spectrum in the analysis of the Stiles-Crawford effect as demonstrated through experiments and with a detailed Zemax eye-and-system analysis. The results obtained are discussed in relation to myopia and a reduced directionality for highly myopic eyes. © 2013 IOP Publishing Ltd.
      77Scopus© Citations 8
  • Publication
    Monocular accommodation response to random defocus changes induced by a tuneable lens
    (Elsevier, 2019-12) ;
    Accommodation of the human eye relies on multiple factors and visual cues that include object size, monochromatic and chromatic aberrations, and vergence. Yet, even in monocular conditions, accommodation corrects for defocus. Studies of eye growth in chicks have addressed whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. However, findings have not been unambiguous and questions remain. Here, we report on monocular accommodation studies of emmetropic and myopic human subjects to clarify whether foveal vision drives accommodation in the correct direction by removing out-of-focus blur potentially before relying on other cues. Subjects viewed monocularly a green target at 1-meter distance while being presented with a random sequence of negative defocus step changes induced by a pupil-conjugated currentdriven tuneable lens. The natural pupil was constricted by a pupil-conjugated motorized iris using three different diameters and target brightness was set with a liquid crystal variable attenuator. A Hartmann-Shack wavefront sensor with an infrared beacon captured real-time changes of defocus and Zernike polynomial coefficients up to 4th radial order. We find that the young adult eye accommodates reliably in the correct direction but with a latency of 300 – 700 ms. The findings are discussed in relation to an absorption model of light in outer segments 2 that breaks the defocus symmetry and thus may serve as a plausible guide for accommodation and emmetropization.
      301Scopus© Citations 12