Biomedical subjects

Christopher Dainty

Publications and source records attributed to Christopher Dainty.

12 recordsLinked to original sources

PubMed2007

Lens axicons in oblique illumination.

Lens axicons, i.e., lenses or lens systems designed to work like axicons, can be a simple and inexpensive way of generating the characteristic axicon focal line. In the design of most lens axicons, only on-axis properties have been considered. We present the design of a lens axicon with improved off-axis characteristics. It is constructed from a singlet lens but with a double-pass feature that allows for a line of uniform width and a stop positioned to minimize aberrations. We perform off-axis analysis and experiments for this system and for another lens axicon, one designed for its on-axis characteristics. We conclude that the off-axis performance of the double-pass axicon is better than both that of an ordinary cone axicon and that of the other lens axicon.

Journal Article↗

PubMed2007

Maximum-likelihood methods in wavefront sensing: stochastic models and likelihood functions.

Maximum-likelihood (ML) estimation in wavefront sensing requires careful attention to all noise sources and all factors that influence the sensor data. We present detailed probability density functions for the output of the image detector in a wavefront sensor, conditional not only on wavefront parameters but also on various nuisance parameters. Practical ways of dealing with nuisance parameters are described, and final expressions for likelihoods and Fisher information matrices are derived. The theory is illustrated by discussing Shack-Hartmann sensors, and computational requirements are discussed. Simulation results show that ML estimation can significantly increase the dynamic range of a Shack-Hartmann sensor with four detectors and that it can reduce the residual wavefront error when compared with traditional methods.

Electrons↗

PubMed2006

Coherence function analysis of the higher-order aberrations of the human eye.

We measured the wavefront aberrations of the eyes of five subjects with a Shack-Hartmann sensor sampling at 21.2 Hz and decomposed the measurements into Zernike aberration terms up to and including the fifth radial order. Coherence function analysis was used to determine the common frequency components between the aberrations within subjects. We found the results to be highly subject dependent. The coherence values were typically <0.4. Possible reasons for this are discussed. Coherence function analysis is a useful tool that can be used in future investigations to determine correlations between the aberration dynamics of the eye and other physiological mechanisms.

Adult↗

PubMed2006

Adaptive optics system for investigation of the effect of the aberration dynamics of the human eye on steady-state accommodation control.

It is now known that defocus is not the only aberration in the eye that exhibits dynamic behavior during fixation. It is currently unknown what effects, if any, the dynamics of these other aberrations have on steady-state accommodation control. We constructed an adaptive optics system to serve as a tool for future investigations in this area. The system has several design features of interest, including automated precompensation of defocus and astigmatism and a method to bypass a scanner used to reduce speckle. It also has the facility to measure the eye's aberrations independent of the aberration manipulation device-a 37-actuator membrane deformable mirror. Coherence function analysis was used to assess the deformable mirror performance in terms of coupling between Zernike modes. Modes beyond third radial order showed severe coupling. Pilot data were collected on one subject to demonstrate the utility of this system in steady-state accommodation studies. The value of the system for future work in this area is discussed.

Accommodation, Ocular↗

PubMed2006

Singular-value decomposition for through-focus imaging systems.

Singular-value decomposition (SVD) of a linear imaging system gives information on the null and measurement components of object and image and provides a method for object reconstruction from image data. We apply SVD to through-focus imaging systems that produce several two-dimensional images of a three-dimensional object. Analytical expressions for the singular functions are derived in the geometrical approximation for a telecentric, laterally shift-invariant system linear in intensity. The modes are evaluated numerically, and their accuracy confirmed. Similarly, the modes are derived and evaluated for a continuous image representing the limit of a large number of image planes.

Journal Article↗

PubMed2006

Objective assessment of image quality. IV. Application to adaptive optics.

The methodology of objective assessment, which defines image quality in terms of the performance of specific observers on specific tasks of interest, is extended to temporal sequences of images with random point spread functions and applied to adaptive imaging in astronomy. The tasks considered include both detection and estimation, and the observers are the optimal linear discriminant (Hotelling observer) and the optimal linear estimator (Wiener). A general theory of first- and second-order spatiotemporal statistics in adaptive optics is developed. It is shown that the covariance matrix can be rigorously decomposed into three terms representing the effect of measurement noise, random point spread function, and random nature of the astronomical scene. Figures of merit are developed, and computational methods are discussed.

Algorithms↗

PubMed2005

Double lateral shearing interferometer for the quantitative measurement of tear film topography.

A lateral shearing interferometer designed and built for the study of the precorneal tear film topography dynamics and its effect on visual performance is presented. Simple data processing algorithms are discussed and tested on data illustrating different tear topography features: postblink tear undulation, tear breakup, eyelid-produced bumps and ridges, bubbles, and rough precontact lens tear surfaces.

Cornea↗

PubMed2005

Weak correlation between the aberration dynamics of the human eye and the cardiopulmonary system.

It is fairly well established that the higher-order aberrations of the eye fluctuate over relatively short time periods, but as yet there is no conclusive evidence regarding the origin of these fluctuations. We measured the aberrations and the pulse pressure wave simultaneously for five subjects. The aberrations were measured by using a Shack-Hartmann sensor sampling at 21.2 Hz. We decomposed the aberration data into Zernike coefficients up to and including fifth order and also calculated the rms wave-front error. From the pulse data the heart rate variability signal was also derived. Coherence function analysis showed that for all subjects there was a weak correlation between many of the aberrations and the pulse and the derived heart rate variability. The pulse and the heart rate variability can account for only 11% +/- 2% and 20% +/- 2%, respectively, of the aberration dynamics.

Blood Pressure↗

PubMed2004

Wave-front reconstruction from shear phase maps by use of the discrete Fourier transform.

Fast wave-front reconstruction methods are becoming increasingly important, for example, in large astronomical adaptive optics systems and high spatial resolution shear interferometry, where pseudoinverse matrix methods scale poorly with problem size. Wave-front reconstruction from difference measurements can be achieved by use of fast implementations of the discrete Fourier transform (DFT), obtaining performance comparable with that of the pseudoinverse in terms of the noise propagation coefficient. Existing methods that are based on the use of the DFT give exact results (in the absence of noise) only for the particular case in which the shear is a divisor of the number of samples to be reconstructed. We present two alternate solutions for the more general case when the shear is any integer. In the first solution the dimensions of the problem are enlarged, and in the second the problem is subdivided into a set of smaller problems with shear amplitude equal to one. We also show that the retrieved solutions have minimum norm and calculate the noise propagation coefficient for both methods. The proposed algorithms are implemented and timed against pseudoinverse multiplication. The results show a speed increase by a factor of 50 over the pseudoinverse multiplication for a grid with N = 3 x 10(3) samples.

Journal Article↗

PubMed2003

Horizontal scintillation detection and ranging Cn2(z) estimation.

The strength of optical turbulence, Cn2(z), 2-3 m above ground level, was measured as a function of distance along a 1.23-km path by the simultaneous capture of the scintillation from two infrared laser sources. The data collected differ in a number of important aspects from the normal vertical scintillation detection and ranging (SCIDAR) data in astronomy. The SCIDAR inversion method for the horizontal path problem is outlined and demonstrated on experimental data collected from three field trials.

Journal Article↗

PubMed2002

Generalized scintillation detection and ranging results obtained by use of a modified inversion technique.

The measurement of the strength of atmospheric optical turbulence by use of a modified generalized SCIDAR (scintillation detection and ranging) inversion technique is outlined and demonstrated. This new method for normalizing and inverting scintillation covariances incorporates the geometry specific to generalized SCIDAR. Examples of profiles from two astronomical observation sites are presented.

Journal Article↗

PubMed2002

High-resolution imaging of the human retina with a Fourier deconvolution technique.

A high-resolution retinal imaging camera is described that uses a Shack-Hartmann wave-front sensor and a Fourier deconvolution imaging technique. The operation of the camera is discussed in detail and high-resolution retinal images of the human cone mosaic are shown for a retinal patch approximately 10 arc min in diameter from two different retinal locations. The center-to-center cone spacing is shown to be approximately 2.5 microm for the retinal images recorded at 2 degrees temporal from the central fovea and approximately 4 microm for the retinal images recorded at 3 degrees temporal from the central fovea.

Diagnostic Imaging↗