Light in nature addresses the optical phenomena we see in our everyday lives, ranging from rainbows and auroras to insect wing colors. Explanations of these phenomena draw from fields that include optical science, physics, engineering, chemistry, biology, astronomy, photography, and art. What is light? How do the beautiful and often colorful features in nature appear? What are the best ways of observing and studying these things? How does human vision affect what we see with these phenomena? Join this virtual live event panel discussion for an exploration into light in nature.
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.
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.
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.
We report on the combined far-field measurement of the three involuntary eye movements, drift, microsaccades, and ocular microtremor (OMT), using a noncontact far-field optical method. We review the significance of the smallest and least measured, and thus least understood, of the three, OMT. Using modern digital imaging techniques, we perform detailed analysis, present experimental results, and examine the extracted parameters using a noncontact far-field sensor. For the first time, in vivo noncontact measurements of all three fixational in-plane movements of the human eye are reported, which simultaneously provide both the horizontal (left-right) and vertical (up-down) displacement results.
Sensing and compensating of optical aberrations in closed-loop mode using a single spatial light modulator (SLM) for ophthalmic applications is demonstrated. Notwithstanding the disadvantages of the SLM, in certain cases, this multitasking capability of the device makes it advantageous over existing deformable mirrors (DMs), which are expensive and in general used for aberration compensation alone. A closed-loop adaptive optics (AO) system based on a single SLM was built. Beam resizing optics were used to utilize the large active area of the device and hence make it feasible to generate 137 active subapertures for wavefront sensing. While correcting Zernike aberrations up to fourth order introduced with the help of a DM (for testing purposes), diffraction-limited resolution was achieved. It is shown that matched filter and intensity-weighted centroiding techniques stand out among others. Closed-loop wavefront correction of aberrations in backscattered light from the eyes of three healthy human subjects was demonstrated after satisfactory results were obtained using an artificial eye, which was simulated with a short focal length lens and a sheet of white paper as diffuser. It is shown that the closed-loop AO system based on a single SLM is capable of diffraction-limited correction for ophthalmic applications.
Here we report on a new variation of the Z-scan method to characterize the third-order optical nonlinearity of photonic
materials. By exploiting a Hartmann-Shack wavefront sensor on a Z-scan set up we demonstrate an improvement in
sensitivity of the method. We also show that the method is suitable for the evaluation of samples having strong nonlinear
absorption. The nonlinear indices of refraction values have been obtained by analyzing the variation of the fifth-order
Zernike coefficients C5 that describe defocus as function of the sample position on the Z-scan setup. Here the method is
demonstrated by evaluating the nonlinear optical properties of CS2 and Coumarin as standard materials, using a 1 KHz
repetition rate Ti-Sapphire laser delivering 100fs pulses.
We have developed a new prototype of a confocal scanning laser ophthalmoscope that incorporate relatively low-cost adaptive optics to correct for wavefront aberrations induced in the exit path of the eye and the optical setup components. The scanning part of the system consists of two galvanometric scanners, and the adaptive optics part contains a membrane deformable mirror in conjunction with a Hartmann-Shack wavefront sensor. The system allows to register images of the retina with infrared illumination at a 15 Hz frame rate and with a variable viewing angle in the range of 1° to 10°. We show first results obtained with the system with images of a test target in an artificial eye and with imaging of the living human retina. We compare images obtained without and with the adaptive optics part activated. In preliminary images, retinal features down to a size of ~25 μm have been resolved with the application of adaptive optics.
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