Measurement of eye aberrations in a speckle field

Measuring aberrations in the rat brain by coherence-gated wavefront sensing using a Linnik interferometer

“…With scattering samples, the amplitude distribution of the electric field on a sublens is speckled. As a consequence, the discrete Fourier transform also shows a speckle structure in the focal plane [ 11 , 22 ] ( Fig. 3(c) ).…”

“…In some experiments (see figure captions), a larger M value is used and the shifted positions are placed on a grid with 3 µm spacing. At each position, (i) a center of mass algorithm [ 23 ] is used to locate the centroid of the intensity distribution, which is preferable to other techniques [ 22 ], (ii) the slopes of the local wavefront are calculated, (iii) the wavefront (described with Zernike coefficients) is reconstructed by least-square fit to the array of local slopes, (iv) the obtained individual Zernike coefficients are finally averaged over the M neighboring positions. This spatial averaging reduces the maximal theoretical rate at which the phase can be extracted (typically to 2 to 3 Hz if the sample or the beam is moved fast and for M = 5).…”

Clinical comparison of 6 aberrometers. Part 1: Technical specifications

“…WASCA (Carl Zeiss Meditec): a high-resolution Hartmann-Shack system. MultiSpot 250-AD Hartmann-Shack sensor 9 : a custom-made Hartmann-Shack system, engineered by the Laboratory of Adaptive Optics at Moscow State University, that includes an adaptive mirror to compensate for accommodation. Allegretto Wave Analyzer (WaveLight): an objective Tscherning device…”

Speckle reduction in wave-front sensing

“…The resulting nonlinear response and saturation in the sensor, and therefore the accuracy of the reconstructed wave-front, are strongly affected [1]. A number of different methods have been employed in an effort to reduce the speckle effect, such as scanning and descanning mirrors [1,2], diffusers such as rotating scatter-plates, multi-mode or dispersive fibres [3]. Decreasing the coherence of the light also achieves this, such as by the use super-luminescent diodes (SLDs), which have a coherence length of 20 to 30 µ m [3] or even less coherent femtosecond lasers as used in optical coherence tomography, and even white light.…”

“…In the case of ocular wave-front measurements, the retina creates most of the unwanted noise in the form of a strong speckle field in the reflected laser light, due to the random interference of the coherent light in the highly anisotropic tissues comprising the retina. The resulting nonlinear response and saturation in the sensor, and therefore the accuracy of the reconstructed wave-front, are strongly affected [1]. A number of different methods have been employed in an effort to reduce the speckle effect, such as scanning and descanning mirrors [1,2], diffusers such as rotating scatter-plates, multi-mode or dispersive fibres [3].…”