Salvador Bará

Nonparaxial design of generalized axicons

The geometric law of energy conservation is utilized in evaluating the phase transmittance function for axicons with arbitrary distribution of the on-axis intensity. Several simple analytical solutions are presented, and a computer-generated holographic version of the uniform-intensity axicon is examined.

Phase plates for wave-aberration compensation in the human eye

We present a method for manufacturing phase plates to compensate for the wave aberration in the human eye. The wave aberration of the eye is measured in vivo by a new laser ray-tracing method and then compensated for by a phase plate placed in front of the eye. This plate is made from a gray-level single-mask photosculpture in photoresist. Two experiments were carried out, first with an artificial eye and then with a human eye: 80% compensation for the wave aberration was achieved in both cases.

Phase retardation of the uniform-intensity axilens

A method for determining the phase-retardation function of the uniform-intensity axilens is discussed and compared with that of an earlier publication [Opt. Lett. 16, 523 (1991)]. Within the presented formulation good agreement is achieved between the geometrical-optics prediction and the numerically evaluated diffraction integral.

Positioning tolerances for phase plates compensating aberrations of the human eye

The positioning tolerances for phase plates used to compensate human eye aberrations are analyzed. Lateral displacements, in-plane rotations, and axial translations are considered, describing analytic and numerical procedures to compute the maximum degree of compensation achievable in each case. The compensation loss is found to be dependent both on the kind and the amount of misalignment and on the particular composition of the aberration pattern of each subject in terms of Zernike polynomials. We applied these procedures to a set of human eye aberrations measured with the laser ray-tracing method. The general trend of results suggests that lateral positioning, followed by angular positioning, are the key factors affecting compensation performance in practical setups, whereas axial positioning has far less stringent requirements.

Direct transformation of Zernike eye aberration coefficients between scaled, rotated, and/or displaced pupils

In eye aberrometry it is often necessary to transform the aberration coefficients in order to express them in a scaled, rotated, and/or displaced pupil. This is usually done by applying to the original coefficients vector a set of matrices accounting for each elementary transformation. We describe an equivalent algebraic approach that allows us to perform this conversion in a single step and in a straightforward way. This approach can be applied to any particular definition, normalization, and ordering of the Zernike polynomials, and can handle a wide range of pupil transformations, including, but not restricted to, anisotropic scalings. It may also be used to transform the aberration coefficients between different polynomial basis sets.

The time course of the effects of central and peripheral cues on visual processing: an event-related potentials study

OBJECTIVE The varying results of visual event-related potential (ERP) studies of central and peripheral cueing suggest that these types of cue may modulate stimuli processing with different time courses. The aim of this study was to investigate differences in the time course of facilitatory effects on the visual processing induced by peripheral and central cues. METHODS ERPs were recorded for visual target stimuli that were preceded by informative-central, informative-peripheral or uninformative-peripheral cues with stimulus onset asynchronies (SOAs) of 100, 300, 500 or 700 ms. RESULTS Validly cued stimuli elicited an enhanced P1 component with peripheral cueing at 100 ms SOA. P1 amplitude in valid trials was reduced at 300, 500 and 700 ms SOAs with uninformative-peripheral cueing, but only at 500 ms SOA with informative-peripheral cueing. With informative-central cueing, there was no validity effect on P1. CONCLUSIONS These results suggest that the automatic attraction of attention by a peripheral cue results in improved sensory processing at the cued location. This facilitation is replaced by an inhibitory effect when SOA increases, although cue informativeness may modulate this effect. Central cueing does not affect sensory processing at the P1 level.

Contrast improvement of confocal retinal imaging by use of phase-correcting plates

We have developed a custom scanning laser ophthalmoscope that uses phase plates produced by photolithography to improve the contrast of human retinal images. We combined the scanning engine from a commercial real-time confocal microscope with custom optics to provide medium magnification imaging of the human retina (3 degrees field of view). Defocus and astigmatism were corrected with conventional trial lenses. Higher-order aberrations of the eye were corrected with a phase plate. A 633-nm laser was used for illuminating the retina. Inserting the phase plate into the optical system increased the contrast of a sample retinal vessel by 26%. Additionally, a number of small features of the retina, which were not visible with standard commercial imaging systems, became visible. There results illustrate that, with the rapid development of custom fabrication techniques for refractive corrections, improved diagnostic imaging with little added complexity to existing ophthalmic imaging systems may be realistic.

The Light Sword Optical Element—a New Diffraction Structure with Extended Depth of Focus

Abstract This letter introduces a novel diffraction structure, the light sword optical element, with extended depth of focus.

Position and displacement sensing with Shack–Hartmann wave-front sensors

The use of a Shack-Hartmann wave-front sensor as a position-sensing device is proposed and demonstrated. The coordinates of a pointlike object are determined from the modal Zernike coefficients of the wave fronts emitted by the object and detected by the sensor. The position of the luminous centroid of a moderately extended incoherent flat object can also be measured with this device. Experimental results with off-the-shelf CCD cameras and conventional relay optics as well as inexpensive diffractive microlens arrays show that axial positioning accuracies of 74 microm rms at 300 mm and angular accuracies of 4.3 microrad rms can easily be achieved.

Measurement and compensation of optical aberrations using a single spatial light modulator

We describe a compact adaptive optical system using a spatial light modulator (SLM) as a single element to both measure and compensate optical aberrations. We used a low-cost, off-the-shelf twisted nematic liquid-crystal display (TNLCD) optimally configured to achieve maximum phase modulation with near constant transmittance. The TNLCD acts both as the microlens array of a Hartmann-Shack wavefront sensor and as the aberration compensation element. This adaptive setup is easy to implement and offers great versatility.