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Now showing 1 - 5 of 239
  • 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.
      13Scopus© Citations 1
  • Publication
    Multi-faceted digital pyramid wavefront sensor
    The modulated pyramid wavefront sensor is known for its high sensitivity and adjustable dynamic range. The need for mechanically moving parts in a modulated pyramid wavefront sensor can be overcome by using the recently proposed digital pyramid wavefront sensor. In this paper, a digital multi-faceted pyramid wavefront sensor is demonstrated with the use of a reflecting phase-only spatial light modulator. The four-pupil digital pyramid wavefront sensor with 4-facets is extended to 6 and 8-facets. It is noted from the experiments performed under identical low-noise conditions that the performance of the wavefront sensor in terms of the root mean square wavefront error remains nearly the same in cases of four, six and eight pupil configurations. Under the circumstances elucidated here, the results of simulations indicate that in the presence of scatter noise, the pyramid wavefront sensor with greater number of pupils could lead to an improvement over the standard four-pupil pyramid wavefront sensor. Noise from scattering makes the choice of optimal modulation radius critical while sensing in open-loop adaptive optics systems.
      16Scopus© Citations 11
  • Publication
    The retina and the Stiles–Crawford effects
    (CRC Press, 2017-01-01)
    Refraction by the anterior eye is essential for proper focusing of light in the formation of images of the exterior world onto the retina. However, it is the absorption by pigments located within the photoreceptor cells that triggers the visual system. Understanding light-photoreceptor interactions is therefore necessary to unravel the complexities in the last optical step in the eye prior to subsequent neural responses. In the human retina, there are two kinds of photoreceptors that are responsible for vision, namely, the rods and the cones. The rods are responsible for dim light (scotopic) vision, whereas the cones are responsible for vision in normal and bright light (photopic) conditions. The transition from pure rod to cone-mediated vision is a combination of the two (mesoscopic) whereby the visual system has an astonishingly large dynamical range that spans about 12 log units, most of which is accomplished by the retina as changes in pupil size account for little more than 1 log unit.
      21
  • 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.
      15Scopus© Citations 4
  • 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.
      16Scopus© Citations 8