Biomedical subjects

Luis Diaz-Santana

Publications and source records attributed to Luis Diaz-Santana.

4 recordsLinked to original sources

PubMed2006

Translational and rotational pupil tracking by use of wavefront aberration data and image registration techniques.

We present a methodology with which to evaluate translations and rotations of wavefront aberration measurements of systems in which the exit pupil suffers displacements and rotations with respect to the reference frame of the measuring device. We propose to use image registration techniques to account for rotations, translations, and scale changes of the pupil. We present a proof of principle, using an artificial eye in addition to computer simulations. The method is software based and requires no additional hardware.

Algorithms↗

PubMed2003

Aberrations of the human eye in visible and near infrared illumination.

PURPOSE: In most current aberrometers, near infrared light is used to measure ocular aberrations, whereas in some applications, optical aberration data in the visible range are required. We compared optical aberration measurements using infrared (787 nm) and visible light (543 nm) in a heterogeneous group of subjects to assess whether aberrations are similar in both wavelengths and to estimate experimentally the ocular chromatic focus shift. METHODS: Ocular aberrations were measured in near infrared and visible light using two different laboratory-developed systems: laser ray tracing (LRT) and Shack-Hartmann. Measurements were conducted on 36 eyes (25 and 11 eyes, respectively), within a wide range of ages (20 to 71 years), refractive errors (-6.00 to +16.50), and optical quality (root mean square wavefront error, excluding defocus, from 0.40 to 9.89 microm). In both systems, wave aberrations were computed from the ray aberrations by modal fitting to a Zernike polynomial base (up to seventh order in laser ray tracing and sixth order in Shack-Hartmann). We compared the Zernike coefficients and the root mean square wavefront error corresponding to different terms between infrared and green illumination. RESULTS: A Student's t-test performed on the Zernike coefficients indicates that defocus was significantly different in all of the subjects but one. Average focus shift found between 787 nm and 543 nm was 0.72 D. A very small percentage of the remaining coefficients was found to be significantly different: 4.7% of the 825 coefficients (25 eyes with 33 terms) for laser ray tracing and 18.2% of the 275 coefficients (11 eyes with 25 terms) for Shack-Hartmann. Astigmatism was statistically different in 8.3% of the eyes, root mean square wavefront error for third-order aberrations in 16.6%, and spherical aberration (Z4(0)) in 11.1%. CONCLUSIONS: Aerial images captured using infrared and green light showed noticeable differences. Apart from defocus, this did not affect centroid computations because within the variability of the techniques, estimates of aberrations with infrared were equivalent to those measured with green. In normal eyes, the Longitudinal Chromatic Aberration of the Indiana Chromatic Eye Model can predict the defocus term changes measured experimentally, although the intersubject variability could not be neglected. The largest deviations from the prediction were found on an aphakic eye and on the oldest subject.

Adult↗

PubMed2003

Repeatability of ocular wavefront measurement.

PURPOSE: To assess the repeatability of measurements of ocular aberrations using wavefront sensing in a small group of observers and to assess the potential effect of measurement error on custom corneal correction due to this variability. METHOD: A Shack-Hartmann wavefront sensor was used to measure the ocular wavefront in nine eyes. Head position was stabilized using a dental bite bar, and the pupil was centred using a cathode ray tube monitor and circular grating. Twenty Shack-Hartmann images were collected for each measurement. Each observer had three sets of measurements taken; the first and the second after careful alignment and the final after regrasping the bite bar in the same position as for the second measurement, but without pupil realignment. The modulation transfer functions for each set were calculated, and the effect of best-aligned custom treatments on the modulation transfer function was estimated. RESULTS: There were highly statistically significant differences in a large number of Zernike modes between the three sets of measurements. The modulation transfer functions calculated for the residual wavefronts after aligned custom treatment were below the diffraction limit. The root mean square wavefront errors were consistently better for the residual wavefronts obtained using the realigned data than using data taken without pupil realignment. CONCLUSIONS: Sequential measurement of ocular aberrations shows statistically significant differences in a large number of Zernike modes. If aberrations determined by a single measurement are to be used in a custom correction, the resulting modulation transfer function is likely to remain below the diffraction limit. Pupil realignment is critical in reduction of the residual root mean square wavefront values to a minimum.

Adult↗

PubMed2002

Ocular aberrations with ray tracing and Shack-Hartmann wave-front sensors: does polarization play a role?

Ocular aberrations were measured in 71 eyes by using two reflectometric aberrometers, employing laser ray tracing (LRT) (60 eyes) and a Shack-Hartmann wave-front sensor (S-H) (11 eyes). In both techniques a point source is imaged on the retina (through different pupil positions in the LRT or a single position in the S-H). The aberrations are estimated by measuring the deviations of the retinal spot from the reference as the pupil is sampled (in LRT) or the deviations of a wave front as it emerges from the eye by means of a lenslet array (in the S-H). In this paper we studied the effect of different polarization configurations in the aberration measurements, including linearly polarized light and circularly polarized light in the illuminating channel and sampling light in the crossed or parallel orientations. In addition, completely depolarized light in the imaging channel was obtained from retinal lipofuscin autofluorescence. The intensity distribution of the retinal spots as a function of entry (for LRT) or exit pupil (for S-H) depends on the polarization configuration. These intensity patterns show bright corners and a dark area at the pupil center for crossed polarization, an approximately Gaussian distribution for parallel polarization and a homogeneous distribution for the autofluorescence case. However, the measured aberrations are independent of the polarization states. These results indicate that the differences in retardation across the pupil imposed by corneal birefringence do not produce significant phase delays compared with those produced by aberrations, at least within the accuracy of these techniques. In addition, differences in the recorded aerial images due to changes in polarization do not affect the aberration measurements in these reflectometric aberrometers.

Adult↗