Fidel Vega

Energy Distribution between Distance and Near Images in Apodized Diffractive Multifocal Intraocular Lenses

PURPOSE To determine the energy distribution between the distance and near images formed in a model eye by spherical and aspheric apodized diffractive multifocal intraocular lenses (IOLs). METHODS The IOL was inserted in a model eye with an artificial cornea with positive spherical aberration (SA) similar to that of the human cornea. The energy of the distance and near images, as a function of the pupil size, was experimentally obtained by image analysis. The level of SA on the IOL, which is pupil-size-dependent, was determined from simulations. The influence of the SA was deduced from results obtained in monofocal IOLs and by comparison of the experimentally obtained energy efficiency to theoretical results based solely on the diffractive profile of the IOL. RESULTS In contrast with theoretical predictions, the energy efficiency of the distance image strongly decreased for large pupils, because of the high level of SA in the IOL. The decrease was smaller in the apodized diffractive multifocal lens with aspheric design. As for the near image, since the diffractive zone responsible for the formation of this image was the same in the spherical and aspheric lenses and the apertures involved were small (and so the level of SA), the results turned out to be similar for both designs. CONCLUSIONS For large pupils, the energy efficiency of the distance image is strongly affected by the level of SA, although aspheric IOLs perform slightly better than their counterparts with a spherical design. For small pupils, there are no differences between the spherical and aspheric IOLs.

Laser application for optical glass polishing

Rapid generation of large area polished optical surfaces by high-power CO2 laser irradiation is shown. Results focus on glasses with high expansion coefficients (a>10 25 /°C) conventionally used in the op- tical industry. The technique involves active beam integration to obtain an intensity irradiation profile with a good uniformity over large spot sizes and is applied to preheated glass samples with initial rms roughness up to 500 nm. To find out the conditions for successful and reliable use of the proposed laser polishing method, the laser-driven heating process was monitored by means of the surface and depth temperature distribu- tions. Whereas the former was determined in situ from the IR radiation emitted by the glass surface, the latter was obtained by comparison of the IR radiation emitted by the bulk sample with a theoretical model. Laser polishing of 5000 mm 2 glass surfaces is reported and processes involved in the modification of the surface texture of the irradiated samples are described and discussed.

Spherical lens versus aspheric artificial cornea for intraocular lens testing

Recent versions of the International Organization for Standardization (ISO)-based model eye for intraocular lens testing include an artificial cornea whose spherical aberration is chosen to match the value of a human cornea. Aspheric lenses have been used as artificial corneas, but they are expensive and difficult to manufacture. This Letter demonstrates that the aspheric lens can be advantageously replaced with a simple biconvex spherical lens of the same focal length and similar wavefront error. A linear dependence is shown between the Coddington shape factor of the spherical lens and the value of the induced spherical aberration. This relationship is used to match the wavefront error of the spherical lens to the one produced by the aspheric lens. A modified ISO model eye, containing this alternative spherical lens as the artificial cornea, is proposed for the on-axis optical assessment of intraocular lenses. Its suitability for off-axis testing is discussed.

Comparison of far and near contrast sensitivity in patients symmetrically implanted with multifocal and monofocal IOLs.

Purpose A prospective, randomized, double-masked, clinical trial was designed to evaluate distance and near contrast sensitivity (CS) in patients symmetrically, and randomly, implanted with 4 different multifocal intraocular lens (MIOL) designs (ReSTOR SN6AD1, ReSTOR SN60D3, ReZoom NXG, and Tecnis ZMA00) and a monofocal control group (Tecnis ZA9003), 6 months after cataract intervention. Methods Photopic, mesopic, and mesopic with glare distance CS, as well as photopic near CS, was evaluated with the CSV-1000 CS test and the Vistech VCTS 6000 system, respectively, in a group of 180 patients attending the ophthalmology department of Sant Pau Hospital, Barcelona, for cataract intervention and lens implantation. Results Statistically and clinically significant differences were found between the monofocal and multifocal lens groups at all spatial frequencies and illumination conditions, both during distance and near CS evaluation (all p<0.05), with the monofocal lens offering the best performance in all cases. Contrast sensitivity was similarly compromised in all MIOL models at distance, although MIOLs with diffractive optics and aspheric profiles showed a non–statistically significant trend to perform better in mesopic conditions. Near CS was lower for refractive, distance dominant lens designs, particularly at medium to high spatial frequencies. Conclusions The present results, which reflect intraocular lens (IOL) characteristics in optics, profile, and add power, may contribute to help surgeons decide on the type of IOL most suitable for each patient by taking into consideration the individual needs for critical distance and near vision, both in terms of visual acuity and contrast sensitivity.

Halo and Through-Focus Performance of Four Diffractive Multifocal Intraocular Lenses

PURPOSE. To compare, as a function of pupil size, the through-focus performance and halo features of four diffractive intraocular lenses (IOLs). METHODS. Three diffractive bifocal IOLs (ReSTOR þ2.5 D SV25T0, Tecnis þ2.75 D ZKB00, and AT LISA þ3.75 D 809M) and a diffractive trifocal IOL (AT LISA tri þ3.33 D, þ1.66 D 839MP) were tested in vitro in a modified International Organization for Standardization eye model. The modulation transfer function (MTF) at the IOLs’ foci was obtained with pupils ranging from 2.0 to 5.0 mm. Through-focus MTF curves (at 50 cycles/mm) were compared among all the IOLs. The halo formation and characteristics were obtained from image analysis. RESULTS. The multifocal IOLs studied in this work showed, at their foci, secondary out-of-focus images, which originate halos and whose characteristics depend on the lens design and pupil size. The smallest halo occurred for the distance focus of the SV25T0. The distance and near foci of the SV25T0 yielded, respectively, the best and lowest optical quality among the studied IOLs. The distance focus of the ZKB00, AT LISA, and AT LISA tri were of similar quality, but the near focus of the ZKB00 outperformed the near foci of the rest of the IOLs. The IOLs’ optical performance gradually deteriorates as pupil increases. CONCLUSIONS. Differences in the design of the diffractive IOLs translate into differences in optical quality at their foci, through-focus performance, and halo features, which can offer further information to surgeons when selecting which IOL to implant.

On the origin of recalescence in amorphous Ge films melted with nanosecond laser pulses

Rapid solidification phenomena have been studied in amorphous germanium films on silicon substrates by means of real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30–180 nm. Two different solidification scenarios are observed depending on the ratio between film thickness (d) and the thermal diffusion length (l) of amorphous germanium (l≈80 nm for 12 ns laser pulses). In the thinner films (d<l), reamorphization occurs upon solidification. Recalescence is observed in the thicker ones (d⩾l) when the melt depth induced is above of ≈80 nm. Above this threshold, crystalline phases are nucleated upon solidification. The origin of this melt depth threshold is discussed in terms of the heat flow into the substrate, the supercooling prior to solidification, and the need of a minimum amount of initially solidified mate...

Evidence for surface initiated solidification in Ge films upon picosecond laser pulse irradiation

Amorphous Ge films on Si films have been melted by single 30 ps laser pulses at λ=583 nm and a solidification process has been followed by means of real time reflectivity (RTR) measurements with nanosecond time resolution. Evidence is provided for the occurrence of surface initiated solidification for films with thicknesses in the 80–130 nm range. This process occurs at high fluences following complete melting of the film and of a thin layer of the Si substrate which undergoes mixing with the liquid Ge. The release of the solidification enthalpy of the latter layer together with its lower solidification temperature favored by constitutional undercooling are proposed as the origin of the initial gradient inversion required for such a process. This scenario leads also to the formation of a secondary solidification front counterpropagating towards the film surface, as suggested by optical simulations of the experimental RTR transients. A transition from a surface initiated solidification process towards a bu...

Design of a Test Bench for Intraocular Lens Optical Characterization

The crystalline lens is the responsible for focusing at different distances (accommodation) in the human eye. This organ grows throughout life increasing in size and rigidity. Moreover, due this growth it loses transparency through life, and becomes gradually opacified causing what is known as cataracts. Cataract is the most common cause of visual loss in the world. At present, this visual loss is recoverable by surgery in which the opacified lens is destroyed (phacoemulsification) and replaced by the implantation of an intraocular lens (IOL). If the IOL implanted is mono-focal the patient loses its natural capacity of accommodation, and as a consequence they would depend on an external optic correction to focus at different distances. In order to avoid this dependency, multifocal IOLs designs have been developed. The multi-focality can be achieved by using either, a refractive surface with different radii of curvature (refractive IOLs) or incorporating a diffractive surface (diffractive IOLs). To analyze the optical quality of IOLs it is necessary to test them in an optical bench that agrees with the ISO119679-2 1999 standard (Ophthalmic implants. Intraocular lenses. Part 2. Optical Properties and Test Methods). In addition to analyze the IOLs according to the ISO standard, we have designed an optical bench that allows us to simulate the conditions of a real human eye. To do that, we will use artificial corneas with different amounts of optical aberrations and several illumination sources with different spectral distributions. Moreover, the design of the test bench includes the possibility of testing the IOLs under off-axis conditions as well as in the presence of decentration and/or tilt. Finally, the optical imaging quality of the IOLs is assessed by using common metrics like the Modulation Transfer Function (MTF), the Point Spread Function (PSF) and/or the Strehl ratio (SR), or via registration of the IOLs wavefront with a Hartmann-Shack sensor and its analysis through expansion in Zernike polynomials.

Delayed melting at the substrate interface of amorphous Ge films partially melted with nanosecond laser pulses

The dynamics of melting-rapid solidification of amorphous Ge films on transparent substrates upon irradiation with nanosecond laser pulses has been analyzed by means of real time reflectivity measurements performed both at the air-film and film-substrate interfaces. The effect of the heat flow conditions on the rapid solidification process has been studied by comparing the behavior of films with thicknesses ranging from 50 to 180 nm on substrates with different thermal conductivities like glass, quartz, and sapphire. The films deposited onto substrates of low thermal conductivity ~glass and quartz! undergo a local delayed melting process in the vicinity of the film-substrate interface, the process being dependent on the film thickness and/or the laser fluence. This delayed melting process is never observed in films deposited on sapphire. The comparison of the results suggests that the solidification heat released from the primary melt is responsible for the delayed melting process at the film-substrate interface whenever the heat-transfer ratio to the substrate is low enough.

Solidification phenomena in Ge films upon nano- and pico-second laser pulse melting

Abstract Melting has been induced in amorphous Ge films upon irradiation with both nano- and pico-second laser pulses. The role of undercooling and heat flow in the subsequent rapid solidification process has been investigated by analyzing the behavior of films with different thicknesses (30–180 nm) grown on Si(100) substrates by means of real time reflectivity measurements in the ns timescale. Recalescence is observed in films with a thickness above a threshold value which depends on the pulse duration. An additional solidification scenario, i.e. surface initiated solidification, is observed upon ps pulse irradiation in films with intermediate thicknesses.