Carlos Illueca

Optical analysis of PresbyLASIK treatment by a light propagation algorithm

PURPOSE To evaluate the changes in the optical quality of corneas of hyperopic patients operated using central multifocal presbyLASIK treatment through the study of light patterns around the retinal plane. METHODS This study included eight hyperopic eyes that underwent central presbyLASIK surgery with Presby-one software using an H. Eye Tech Technovision excimer laser platform. Mean patient age was 57 years. Mean preoperative spherical equivalent refraction was 1.28 +/- 0.87 diopters (D). Mean distance decimal visual acuity with correction was 1.02 +/- 0.13 and without correction was 0.37 +/- 0.15. The optical quality was characterized by the Strehl ratio, the spot size on the retina, and objective decimal visual acuity calculated based on measured corneal topography using Fresnel propagation algorithm based on a realistic eye model. RESULTS The postoperative spherical equivalent refraction was -0.55 +/- 0.48 D. The distance decimal visual acuity with correction was 0.94 and without correction was 0.70. The pseudoaccommodation range was 1.60 +/- 0.40 D. A good Pearson correlation coefficient was obtained (r2 = 0.86) for the linear fit between the real and calculated decimal visual acuity for the eight eyes. The Strehl ratio value on the retinal plane increased after the surgery by a mean factor of 4.7. CONCLUSIONS With a complete characterization of the eye and a complete propagation algorithm (that takes into account all refractive surfaces in the eye at the same time), it is possible to evaluate the optical quality in eyes of patients who have undergone central presbyLASlK treatment.

Measurement of wide frequency range structural microvibrations with a pocket digital camera and sub-pixel techniques

Analysis of vibrations and displacements is a hot topic in structural engineering. Although there is a wide variety of methods for vibration analysis, direct measurement of displacements in the mid and high frequency range is not well solved and accurate devices tend to be very expensive. Low-cost systems can be achieved by applying adequate image processing algorithms. In this paper, we propose the use of a commercial pocket digital camera, which is able to register more than 420 frames per second (fps) at low resolution, for accurate measuring of small vibrations and displacements. The method is based on tracking elliptical targets with sub-pixel accuracy. Our proposal is demonstrated at a 10 m distance with a spatial resolution of 0.15 mm. A practical application over a simple structure is given, and the main parameters of an attenuated movement of a steel column after an impulsive impact are determined with a spatial accuracy of 4 µm.

Optical surface reconstruction technique through combination of zonal and modal fitting

Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From height data it is possible to adjust analytical surfaces that will be later used for aberration calculation. Zernike polynomials are often used as adjusting polynomials, but they have shown to be not precise when describing highly irregular surfaces. We propose a combined zonal and modal method that allows an accurate reconstruction of corneal surfaces from height data, diminishing the influence of smooth areas over irregular zones and vice versa. The surface fitting error is decreased in the considered cases, mainly in the central region, which is more important optically. Therefore, the method can be established as an accurate resampling technique.

Noninvasive measurement of eye retraction during blinking.

We present a noninvasive technique for high-speed measuring of eye retraction and eyelid position during blinking. The anterior chamber of the eye is illuminated by the slit lamp of a biomicroscope and eye dynamics during a blinking sequence are captured with a high-speed camera working at 500 frames per second. Digital image processing allows quantitative analysis of cornea and eyelid positions during the closing and opening phases of the blinking process. Our method allows simultaneous measuring of corneal retraction, duration of down and up phases, total blinking duration, and average and peak speeds of the eyelids in both phases, thus providing a complete analysis of the blinks transversal motions.

Correlation between the dioptric power, astigmatism and surface shape of the anterior and posterior corneal surfaces

A knowledge of the shape of the cornea is of major importance for the planning and monitoring of surgery, and for the correct diagnosis of corneal diseases. Many authors have studied the geometry of the second corneal surface in the central region and it has been stated that there is a high correlation between the central radii of curvature and asphericities of the two corneal surfaces. In this work we extend this study to a larger, central, 6 mm diameter of the cornea. Surface height data, obtained with an Oculus Pentacam from 42 eyes of 21 subjects, were analysed to yield surface power vectors. Corneal heights of both surfaces were also decomposed into low‐order Zernike polynomials and the correlations between each of the power vectors and low‐order Zernike coefficients for the two surfaces were studied. There was not only a strong correlation between spherical powers and Zernike defocus coefficients, but also between the astigmatic components. The correspondence between the astigmatism in both surfaces found here can be of the utmost importance in planning optical surgery, since perfect spherical ablation of the first surface does not assure total correction of corneal astigmatism.

Complete algorithm for the calculation light patterns inside the ocular media

Realistic mathematical models are of great importance for studying the optical performance of the human eye. Light propagation algorithms provide robust methods to calculate field distributions inside a homogeneous medium, and thus they can be applied to the study of light patterns inside the anterior and posterior chambers of the eye. Dealing with real eyes implies using very short propagation distances together with highly refractive power surfaces. Thus, the general solutions of the field equation are used instead of paraxial Fresnel solutions. Conditions of application and sampling conditions of the method are clearly stated here. Numerical evaluation of the different refractive surfaces is also analysed. The main result is that a complete algorithm to obtain light patterns at any axial distance inside the eye is proposed. The method uses real corneal measures and axial distances together with crystalline models. Statistical results and individual predictions show the validity of the model. Application of the method is illustrated with the study of a bifocal intraocular lens.

Refractive analysis of the human cornea through propagated fields

A new technique for analysing the optical quality of the human cornea is presented. Corneal maps are obtained through keratographies and then converted into phase maps. The propagated fields generated from this surface are plotted and studied. It is shown that any irregularity in the cornea affects the propagated field and the energy distribution at the focal plane. Simple applications are also indicated.

The use of Newton's rings for characterising ophthalmic lenses

The interference technique of Newtons Rings is widely used for the quality control of optical surfaces because the precision obtained with this method proves to be very satisfactory. The dimensions of the rings permits calculation of the radii of curvature of the analysed surfaces and deformation of the interference pattern can be utilised to calculate other parameters, such as astigmatism. We describe the study of progressive surfaces by means of this technique, whereby the analysis of the various points of the progressive corridor is made, and also include information on the power function for these lenses, as well as the addition and corridor length.

Near-field light distributions propagated from human corneas: determination of relevant patterns

A procedure to determine the best image range of distances that can be obtained from an aphakic eye is presented. Corneal surfaces are calculated from videokeratographic data. Light distributions at different distances inside the eye are calculated with a near-field Fresnel diffraction algorithm. These patterns are evaluated using three different quality functions which quantify the appearance of the energy distributions in each plane. Local extremes in those functions will serve to locate relevant distributions as Sturm focal planes and even the best image plane which is affordable by the cornea. The study is applied to strongly aberrated corneas.

Scale corrections for faster evaluation of convergent Fresnel patterns

Numerical calculation of Fresnel patterns through fast Fourier transforms usually requires an extremely large number of samples in order to fulfil the Nyquist sampling condition. In many applications, the cut-off frequency of the system is much below the limit fixed by our calculations. As a consequence of this, correct sampling may result in heavy processes that produce results of useless accuracy. Unfortunately, subsampling may introduce aliasing that may distort the final appearance of the diffracted pattern. In this paper, we present a simple method that permits subsampling the Fresnel pattern while maintaining the Nyquist condition, and thus preventing the appearance of aliasing effects in the calculation. Secondary effects of the subsamplings are rescaling the illuminating wavelength and the introduction of an effective low-pass filter. Some applications relative to the propagation of the light inside the human eye are also suggested in the text.