Now showing 1 - 10 of 14
  • 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.
  • Publication
    Understanding the role of retinal cone photoreceptors in color perception, blur, and emmetropization
    (SPIE, 2020-09-04)
    The photoreceptors are responsible for the conversion of optical images into neural signals that are conveyed to the visual cortex where vision is triggered. Rod photoreceptors provide night vision whereas cone photoreceptors provide daylight vision and color perception. The photoreceptors have commonly been represented as discrete but dense array of pixels despite of their elongated cellular structure. Earlier studies have suggested that they act as biological waveguides transmitting images from the inner to the outer segments. However, this understanding may not fully encompass their role in vision which is more related to that of optical antennas organized in such a way that optical image contrast and resolution is optimized. Here, we discuss the role of the photoreceptors analyzed as three-dimensional adaptable detectors of light (voxels) using electromagnetic principles. We show that this understanding is compatible with how light is perceived when being incident onto the retina at different angles in the effect commonly known as the StilesCrawford effect. We discuss how this can explain the reduced sensitivity to aberrations and chromatic blur of the threedimensional retina when compared to the common two-dimensional understanding of image formation in the eye. We show how the same principles may impact on emmetropization and ultimately how it may play a key role to prevent the onset or progression of myopia.
  • 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.
      630Scopus© Citations 21
  • 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.
  • 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
  • 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
    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
    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.
      171Scopus© Citations 8
  • 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
    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.
      443Scopus© Citations 23