Estimation of the ocular point spread function by retina modeling

Point spread function estimation from projected speckle illumination

The resolution of an imaging apparatus is ideally limited by the diffraction properties of the light passing through the system aperture, but in many practical cases, inhomogeneities in the light propagating medium or imperfections in the optics degrade the image resolution. Here we introduce a powerful and practical new approach for estimating the point spread function (PSF) of an imaging system on the basis of PSF Estimation from Projected Speckle Illumination (PEPSI). PEPSI uses the fact that the speckles' phase randomness cancels the effects of the aberrations in the illumination path, thereby providing an objective pattern for measuring the deformation of the imaging path. Using this approach, both wide-field-of-view and local-PSF estimation can be obtained by calibration-free, single-speckle-pattern projection. Finally, we demonstrate the feasibility of using PEPSI estimates for resolution improvement in iterative maximum likelihood deconvolution.

Image deconvolution by means of frequency blur invariant concept

Different blur invariant descriptors have been proposed so far, which are either in the spatial domain or based on the properties available in the moment domain. In this paper, a frequency framework is proposed to develop blur invariant features that are used to deconvolve a degraded image caused by a Gaussian blur. These descriptors are obtained by establishing an equivalent relationship between the normalized Fourier transforms of the blurred and original images, both normalized by their respective fixed frequencies set to one. Advantage of using the proposed invariant descriptors is that it is possible to estimate both the point spread function (PSF) and the original image. The performance of frequency invariants will be demonstrated through experiments. An image deconvolution is done as an additional application to verify the proposed blur invariant features.

Improving retinal imaging by corneal refractive index matching

Imaging the retina at high resolution requires a dilated pupil, which in turn exposes more corneal irregularities. We diminish the optical errors of the cornea by refractive index matching. Lens-fitted goggles were used for corneal immersion, to reduce its aberrations, while keeping the ocular power. An additional aspheric plate reduced the residual ocular spherical aberration. A comparison of the index-matching-based retinal images with those acquired directly shows resolution improvement for subjects with normal extent of ocular aberrations. A simulation of the point spread function, obtained from an averaged ocular and corneal wavefront error, also reveals substantial improvement when using corneal index matching. The demonstrated improvement using index matching may enable further improvement of current retinal imaging techniques or relaxing requirements for active ocular aberration correction.