Guillermo M. Pérez

An objective scatter index based on double-pass retinal images of a point source to classify cataracts.

Purpose To propose a new objective scatter index (OSI) based in the analysis of double-pass images of a point source to rank and classify cataract patients. This classification scheme is compared with a current subjective system. Methods We selected a population including a group of normal young eyes as control and patients diagnosed with cataract (grades NO2, NO3 and NO4) according to the Lens Opacities Classification System (LOCS III). For each eye, we recorded double-pass retinal images of a point source. In each patient, we determined an objective scatter index (OSI) as the ratio of the intensity at an eccentric location in the image and the central part. This index provides information on the relevant forward scatter affecting vision. Since the double-pass retinal images are affected by both ocular aberrations and intraocular scattering, an analysis was performed to show the ranges of contributions of aberrations to the OSI. Results We used the OSI values to classify each eye according to the degree of scatter. The young normal eyes of the control group had OSI values below 1, while the OSI for subjects in LOCS grade II were around 1 to 2. The use of the objective index showed some of the weakness of subjective classification schemes. In particular, several subjects initially classified independently as grade NO2 or NO3 had similar OSI values, and in some cases even higher than subjects classified as grade NO4. A new classification scheme based in OSI is proposed. Conclusions We introduced an objective index based in the analysis of double-pass retinal images to classify cataract patients. The method is robust and fully based in objective measurements; i.e., not depending on subjective decisions. This procedure could be used in combination with standard current methods to improve cataract patient surgery scheduling.

Optical Characterization of Bangerter Foils

PURPOSE Optical penalization is emerging as an alternative to patching for the treatment of amblyopia. Bangerter foils offer a form of optical penalization that is distinctly different from standard techniques making use of atropine or spectacle lens manipulation, or both, to produce defocus. The authors examined the optical properties of Bangerter foils and compared them with the effect of defocus. METHODS Bangerter foils were evaluated on an optical bench to calculate point spread and modulation transfer functions. Retinal images through the foils were also simulated and qualitatively compared with those with defocus and Gaussian blur. Subjective visual acuity and contrast sensitivity were compared in two subjects wearing spectacles with foils and with simple defocus. RESULTS The optical characteristics of the Bangerter foils do not correspond well with their labeled density designation. Bangerter foils and defocus affect the modulation transfer function similarly, with more attenuation of mid-range spatial frequencies than low spatial frequencies. However, Bangerter foils do not exhibit spurious resolution and phase shifts, as does defocus. CONCLUSIONS The blur resulting from Bangerter filters is qualitatively different from defocus. Whether this difference is of any consequence when these two methods of optical penalization are used for amblyopia treatment remains to be investigated.

Impact of scattering and spherical aberration in contrast sensitivity

We investigated the impact in spatial visual performance of the combined presence of different amounts of spherical aberration and intraocular scattering in the eye. In a group of subjects, contrast sensitivity at 6 cycles per degree was measured when viewing through holographic diffusers to produce different levels of scattering and with their spherical aberration simultaneously controlled using an adaptive-optics visual simulator. For elevated levels of scattering, the addition of small amounts of spherical aberration either does not decrease, or even may slightly increase, contrast sensitivity under some conditions. This seems to be due to an optical effect also demonstrated in an artificial eye. Although the visual effect is quite small, this finding could suggest a balancing mechanism where larger spherical aberration could keep relatively stable the retinal image quality under the presence of elevated scattering. This is actually the situation in older eyes with both spherical aberration and intraocular scatter being higher than in young eyes.

The wide-angle point spread function of the human eye reconstructed by a new optical method

The point spread function (PSF) of the human eye spans over a wide angular distribution where the central part is associated mostly to optical aberrations while the peripheral zones are associated to light scattering. There is a plethora of optical methods for the direct and indirect measurements of the central part of the PSF as a result of monochromatic and polychromatic aberrations. The impact of the spatial characteristics of this central part of the PSF on the retinal image quality and visual function has been extensively analyzed and documented both by optical and psychophysical methods. However, the more peripheral areas of the PSF in the living human eye, ranging from about 1 to 10 degrees of eccentricity, have been investigated only psychophysically. We report here a new optical method for the accurate reconstruction of the wide-angle PSF in the living human eye up to 8 degrees. The methodology consists of projecting disks of uniform radiance on the retina, recording the images after reflection and double pass through the eyes optics and performing a proper analysis of the images. Examples of application of the technique in real eyes with different amount of scatter artificially induced are presented. This procedure allows the direct, accurate, and in vivo measurement of the effect of intraocular scattering and may be a step toward the comprehensive optical evaluation of the optics of the living human eye.

Wavelength dependence of the ocular straylight.

PURPOSE Ocular straylight is the combined effect of light scattering in the optical media and the diffuse reflectance from the various fundus layers. The aim of this work was to employ an optical technique to measure straylight at different wavelengths and to identify the optimal conditions for visually relevant optical measurements of straylight. METHODS The instrument, based on the double-pass (DP) principle, used a series of uniform disks that were projected onto the retina, allowing the recording of the wide-angle point spread function (PSF) from its peak and up to 7.3° of visual angle. A liquid crystal wavelength tunable filter was used to select six different wavelengths ranging from 500 to 650 nm. The measurements were performed in nine healthy Caucasian subjects. The straylight parameter was analyzed for small (0.5°) and large (6°) angles. RESULTS For small angles, the wavelength dependence of straylight matches the transmittance spectrum of hemoglobin, which suggests that diffuse light from the fundus contributes significantly to the total straylight for wavelengths longer than 600 nm. Eyes with lighter pigmentation exhibited higher straylight at all wavelengths. For larger angles, straylight was less dependent on wavelength and eye pigmentation. CONCLUSIONS Small-angle straylight in the eye is affected by the wavelength-dependent properties of the fundus. At those small angles, measurements using wavelengths near the peak of the spectral sensitivity of the eye might be better correlated with the visual aspects of straylight. However, the impact of fundus reflectance on the values of the straylight parameter at larger angles did not depend on the measuring wavelength.

Impact of scatter on double-pass image quality and contrast sensitivity measured with a single instrument

We compared objective Double-Pass (DP) image quality data with subjective visual parameters measured within the same modified instrument for different amounts of scatter. The original DP imaging channel of a clinical instrument was maintained intact and two additional channels were included, one for visual testing and another for tear film (TF) imaging by using a retro-illumination technique. Contrast sensitivity (CS) was compared with measurements of the Objective Scattering Index (OSI) obtained from DP retinal images corresponding to different scatter levels induced by pre-defined filters. OSI values were correlated with the change in CS for different spatial frequencies measured with the same instrument. Since TF and DP images were recorded at the same rate, this provided additional information about the dynamic spatial stability of the tear film. This new DP instrument has been proven to provide accuracy and repeatability, and to be suitable for clinical diagnosis, with a complete evaluation of the eyes performance by a simultaneous objective and subjective assessment under the same experimental conditions.

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.

Cause of Monocular Diplopia Diagnosed by Combining Double-pass Retinal Image Assessment and Hartmann-Shack Aberrometry

PURPOSE To report an advanced optical procedure developed for the diagnosis of a particular case of diplopia. METHODS This approach combined the quantification of the level of intraocular scattering by using an Objective Scatter Index provided by a double-pass instrument (Optical Quality Analysis System) with the analysis of higher order aberrations using a Hartmann-Shack wavefront sensor. RESULTS The value of the Objective Scatter Index revealed increased intraocular scattering; the Hartmann-Shack images showed the existence of an optically differentiated area at the upper region of both crystalline lenses. Simulation of retinal images computed from the wavefront maps confirmed that, under low luminance conditions, this inhomogeneous region of the lens was included in the pupil, generating a secondary image and therefore the diplopia. CONCLUSIONS This report demonstrates the potential of combining two objective optical methods to show the presence of minor lens opacities that may severely degrade quality of vision.