Now showing 1 - 3 of 3
  • Publication
    Retinal directionality and wavefront sensing using a digital micromirror device
    (University College Dublin. School of Physics, 2020)
    In the recent years, light modulation has been achieved by an array of binary-positioning micromirrors, namely Digital Micromirror Devices (DMD), which have gained popularity in vision science, such as in retinal imaging, psychophysical vision testing or coded wavefront sensing. In this thesis, a DMD is used to measure two aspects of vision characterization: a) retinal directionality through the Stiles-Crawford effect of the first kind (SCE-I) and b) crosstalk-free wavefront sensing for large ocular aberrations. For the first part of this work, a DMD is used as a key component in a uniaxial flicker system to characterize the SCE-I in Maxwellian view, capable of controlling pupil aperture size and location, as well as field intensity by adjusting the duty cycle of the micromirrors. The SCE-I is tested under different conditions including foveal and parafoveal retinal locations, different luminance levels ranging from scotopic to photopic conditions, wavelength dependence through the visible spectrum and into the near infrared, as well as differences for myopic subjects. The SCE-I is also characterized in normal viewing conditions (Newtonian view) by scanning of the pupil with small apertures, where the influence of the SCE-I on visual acuity is analysed. Furthermore, a method to directly measure the integrated SCE-I is described and analysed using a geometrical optics absorption model based on volumetric overlap of incident light with photoreceptor outer segments, without accounting for photoreceptor waveguiding. In the second part, a wavefront sensing system using a digital micromirror device is implemented to achieve a wavefront sensor with high dynamic range and speed, without the use of a lenslet array which, in turn, avoids crosstalk. By sequential scanning of the wavefront, very high sampling densities can be achieved when compromising speed. The system is tested by measuring aberrations in an artificial eye and extended to the human eye.
  • 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.
      781Scopus© Citations 24
  • 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.
      585Scopus© Citations 25