In the ophthalmic Shack-Hartmann wavefront sensor beacon light originates from various depths within the retina. We model the retina of the human and mouse eyes as formed by two backscattering planes, showing that the use of lenslets with low Fresnel number and/or too small search boxes in the centroiding algorithm will produce artifactual aberrations. We evaluate the impact of these errors for four common beacon illumination strategies: full circular, annular, small circular on-axis and small circular off-axis. We find that artifactual aberrations are larger for annular and off-axis beacon illumination, dominated by defocus plus spherical aberration and defocus plus coma, respectively. These artifactual aberrations can be almost completely eliminated by selecting the minimum centroid search box size based on a simple Gaussian optics model and ocular biometry, provided the lenslet Fresnel number is sufficiently large to introduce minimal cross-talk between images of adjacent lenslets.
Bruch’s membrane (BM) and retinal pigment epithelium (RPE) changes, thought to be the earliest signs of impending atrophy in dry age-related macular degeneration (AMD), are subtle and challenging to detect non-invasively. Here, we demonstrate imaging and quantification of BM and RPE thickness in the in vivo human eye using achromatized visible light optical coherence tomography (OCT) in normal human subjects. Our consistent visualization of BM and agreement of thickness values with ranges from prior histological studies is attributed to the increased axial resolution, and potentially also to the enhanced contrast of BM in the visible light range.
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.
Centroiding inaccuracies contribute to most of the wavefront reconstruction error in a Shack Hartmann sensor
based adaptive optics system. These errors primarily occur due to the presence of photon noise, readout noise,
finite background and strong scintillations. Elongation of the spots in the case of large apertures while using Laser
guide star as reference makes the situation further worse. A denoising procedure based on thresholded Zernike
reconstructor and pattern matching is suggested in this paper to largely overcome these problems. Individual
spot pattern images are reconstructed using Zernike polynomials and matched with ideal spot pattern without
distortion to arrive at accurate local centroid positions.
Simulation of the dynamic effects of atmospheric turbulence assists in understanding, testing and effective implementation
of adaptive optics systems. Statistical interpolation technique helps in retaining the spatial turbulence
statistics when atmospheric phase screens are required to be moved by non-integer multiples of the grid spacing.
We applied statistical interpolation in the simulation of temporally evolving phase screens using the multilayer
model of atmospheric turbulence. A comparison of the statistical method with bilinear interpolator and random
midpoint displacement method is presented. It is shown that underestimating Fried's parameter (r0) in the
interpolation leads to large errors and hence it is appropriate to choose a little larger value of r0 than estimated
from the phase screens.
Simulations of optical wavefronts propagating through the atmosphere are widely used in the design and testing of
adaptive optics systems. Phase screens are defined by their spatial and temporal statistics. In many applications,
a controlled production of phase is necessary. A linear combination of normalized Zernike polynomials can be
used for the generation of phase screens through the computation of Zernike moments following Kolmogorov
turbulence spectrum. In this paper, a technique for controlled production of normalized phase screens using a
known Fried's parameter, r0 is proposed by taking the advantage of the fact that with increasing radial index (n)
of Zernike polynomials, the spatial frequency increases. We arrived at an empirical relation between the index
interval of Zernike polynomials and r0. At large value of 'n', there is saturation in the minimum achievable r0
value.
The wavefront reconstruction accuracy in Shack Hartmann sensor based adaptive optics system depends on
accurate centroiding and phase estimation from measured slope values. Monte Carlo simulations of vector
matrix multiply and Fourier phase estimation methods show fluctuations in the value of wavefront reconstruction
accuracy leading to inconsistency. In this paper, it is shown that these fluctuations can be minimized and high
wavefront reconstruction accuracy can be maintained consistently by applying a dither signal on the Shack
Hartmann lenslet array. The information of the dither signal to be applied can be obtained from the wavefronts
of the past.
Real-time measurement of effective wind speed helps in continuous monitoring of the temporal bandwidth in
adaptive optics systems. In this paper, we propose a simple and efficient method for estimating wind speed
from wavefront sensor data. A small portion from the center of a wavefront arriving at time 't' is mapped at all
possible positions on phase screens coming later than 't' by a time n × Δtint (Δtint-integration time, n-integer)
to form correlation maps. Wind speed and direction is estimated statistically via the calculation of the vectors
formed by joining the central portion and the position of maximum correlation on the correlation maps. Number
of such realizations to be considered to arrive at an accurate estimate of wind speed was optimized to reduce
computing time and increase accuracy.
A hybrid centroiding technique involving Iteratively Weighted Center of Gravity (IWCoG) algorithm and correlation
technique for a Laser Guide Star (LGS) based Shack Hartmann wavefront sensor is proposed. A simple
method for simulating LGS elongated spots with photon noise and read out noise is demonstrated. The problems
associated with IWCoG are addressed (a) Error saturation is minimized by adding random numbers iteratively
to centroid positions, (b) non uniform convergence of Centroid Estimation Error (CEE) is reduced by using
the hypothesis that the iteration number with maximum correlation between the weighting function and the
actual spot image function is the iteration with minimum error, (c) convergence rate is improved by shifting the
weighting function to the point of maximum intensity in first iteration. The novelty of the algorithm is tested
by comparing with other centroiding algorithms.
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