Aberro-polariscope for the human eye

Combined effect of wavelength and polarization in double-pass retinal images in the human eye

A polychromatic double-pass setup was developed to study the effects of wavelength and polarization on retinal image quality. The results show that the central part of the images was similar for all wavelengths (543, 633 and 780 nm) and polarization states. However, the image tails increased significantly when using infrared light for all the polarization states used. For the set of subjects involved in the study, ocular diattenuation presented individual differences, however significant changes were not found across the different wavelengths. Moreover the Stokes vectors providing the maximum intensity transmittance varied across subjects and corresponded to elliptically polarized light. These non-negligible diattenuation effects might affect the performance of clinical devices which only take into account ocular birefringence.

Nonlinear registration for scanned retinal images: application to ocular polarimetry

Retinal images of approximately 1 degrees of visual field were recorded with a homemade scanning laser ophthalmoscope. The benefit of using a nonlinear registration technique to improve the summation process when averaging frames, rather than a standard approach based on correlation, was assessed. Results suggest that nonlinear methods can surpass linear transformations, allowing improved contrast and more uniform image quality. The importance of this is also demonstrated with specific polarization measurements to determine the degree of polarization across an imaged retinal area. In such a context, where this parameter of polarization is extracted from a combination of registered images, the benefit of the nonlinear method is further increased.

Improved scanning laser fundus imaging using polarimetry

We present a polarimetric technique to improve fundus images that notably simplifies and extends a previous procedure [Opt. Lett.27, 830 (2002)]. A generator of varying polarization states was incorporated into the illumination path of a confocal scanning laser ophthalmoscope. A series of four images, corresponding to independent incoming polarization states, were recorded. From these images, the spatially resolved elements of the top row of the Mueller matrix were computed. From these elements, images with the highest and lowest quality (according to different image quality metrics) were constructed, some of which provided improved visualization of fundus structures of clinical importance (vessels and optic nerve head). The metric values were better for these constructed images than for the initially recorded images and better than averaged images. Entropy is the metric that is most sensitive to differences in the image quality. Improved visualization of features could aid in the detection, localization, and tracking of ocular disease and may be applicable in other biomedical imaging.

Corneal polarimetry after LASIK refractive surgery

Imaging polarimetry provides spatially resolved information on the polarization properties of a system. In the case of the living human eye, polarization could be related to the corneal biomechanical properties, which vary from the normal state as a result of surgery or pathologies. We have used an aberro-polariscope, which we recently developed, to determine and to compare the spatially resolved maps of polarization parameters across the pupil between normal healthy and post-LASIK eyes. The depolarization distribution is not uniform across the pupil, with post-surgery eyes presenting larger levels of depolarization. While retardation increases along the radius in normal eyes, this pattern becomes irregular after LASIK refractive surgery. The maps of slow axis also differ in normal and post-surgery eyes, with a larger disorder in post-LASIK eyes. Since these changes in polarization indicate subtle structural modifications of the cornea, this approach can be useful in a clinical environment to follow the biomechanical and optical changes of the cornea after refractive surgery or for the early diagnosis of different corneal pathologies.

Polarimetric high-resolution confocal scanning laser ophthalmoscope

A polarimetric high-resolution confocal scanning laser ophthalmoscope has been developed. The system incorporates a fixed linear polarizer in the illumination path and a rotatory quarter-wave plate and another fixed linear polarizer in the registration path. Retinal areas that are smaller than those provided by commercial instruments can be imaged. Series of four fundus images for independent polarization states in the second pass were recorded for different eyes and retinal locations and the spatially resolved Stokes vectors calculated. From those images, the contrast across retinal blood vessels was maximized and the corresponding image was reconstructed. In terms of polarization, the analysis of small retinal areas might prove to be useful in the improvement of retinal imaging and the enhancement of structural details in the early diagnosis of ocular pathologies.