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At least 613 records · Page 34Linked to original sources
PubMed2004

Archer fish learn to compensate for complex optical distortions to determine the absolute size of their aerial prey.

Many animals, including humans, can visually judge the absolute size of objects regardless of changes in viewing distance and thus despite the resulting dramatic differences in the size of the actual retinal images. For animals that have to judge the size of aerial objects from underwater views, this can be a formidable problem; our calculations show that considerable and strongly viewpoint-dependent corrections are needed to compensate for the effects of light refraction. Archer fish face these optical difficulties because they have to shoot down aerial insects over a wide range of horizontal and vertical distances. We show here that these fish can learn to acquire size constancy with remarkable precision and are thus fully capable of taking complex viewpoint dependency into account. Moreover, we demonstrate that archer fish solve the problem not by interpolating within a set of stored views and distances but by learning the laws that connect apparent size with the fish's relative position to the target. This enables the fish to readily judge the absolute sizes of objects from completely novel views.

Animals↗
PubMed2006

Solution-deposited microstructures based on aluminum-tris-hydroxyquinoline.

Iridescent organic films consisting of quasi-parallel wire-like microstructures are grown by castings from ethanol solutions containing mixtures of aluminium-tris-hydroxyquinoline and 1,10-phenanthroline. Spectrophotometric measurements carried out in the ultraviolet-visible range indicate that the microstructured films have angular-dependent optical behaviour, which is motivated by a refractive-index modulation over dimensions that are comparable to visible-light wavelengths. According to the results of investigations carried out by means of optical microscopy, atomic force microscopy (AFM), and scanning near-field optical microscopy (SNOM), the refractive-index modulation originates from a thickness modulation and a phase separation that occurs as the aluminium-tris-hydroxyquinoline and phenanthroline co-crystallize, with the former material being arranged to form a green luminescent pattern on the top of the latter one.

Journal Article↗
PubMed2003

Colloidal gold-modified optical fiber for chemical and biochemical sensing.

A novel class of fiber-optic evanescent-wave sensor was constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled gold colloids. The optical properties and, hence, the attenuated total reflection spectrum of self-assembled gold colloids on the optical fiber changes with different refractive index of the environment near the colloidal gold surface. With sucrose solutions of increasing refractive index, the sensor response decreases linearly. The colloidal gold surface was also functionalized with glycine, succinic acid, or biotin to enhance the selectivity of the sensor. Results show that the sensor response decreases linearly with increasing concentration of each analyte. When the colloidal gold surface was functionalized with biotin, the detection limit of the sensor for streptavidin was 9.8 x 10(-11) M. Using this approach, we demonstrate proof-of-concept of a class of refractive index sensor that is sensitive to the refractive index of the environment near the colloidal gold surface and, hence, is suitable for label-free detection of molecular or biomolecular binding at the surface of gold colloids.

Biosensing Techniques↗
PubMed2006

How cold light was introduced to arthroscopy.

In 1954, in a landmark publication, H. H. Hopkins reported that images could be transmitted through glass fibers. This stimulated many investigations and, in 1957, B. Hirschowitz at the University of Michigan, succeeded in constructing a much-needed flexible gastroscope using optic glass fibers. Coating of the fibers by glass of lower refractive index was the next major step forward. Optical fibers were thus insulated and the accuracy of the transmitted image and the light transmission capacity of the fibers were enhanced. Working with American Cystoscope Makers Inc, L. Curtiss and H. Hett were the first to use glass fibers for illumination in an endoscope, producing a flexible ureteroscope in 1960. The light transmitted by the fibers was named "cold light" because it produced little heat, but the term had been used before for other types of illumination. The first arthroscope equipped with cold light was Watanabe's arthroscope No. 22 built by Tsunekichi Fukuyo in 1967. However, Watanabe was not satisfied with the new instrument and he still preferred the conventionally illuminated No. 21 arthroscope with the offset tungsten bulb at his tip. After R. W. Jackson reintroduced the technique to North America, most American pioneers used this conventionally illuminated arthroscope as well. But the early European arthroscopists had cold light instruments made by the German manufacturers R. Wolf and K. Storz since 1969. These rod-lens cold light instruments expanded in all markets during the following decade.

Arthroscopes↗
PubMed1999

How good is the match between the plane of the text and the plane of focus during reading?

Because results from animal models demonstrate that retinal image defocus is a crucial factor in the visual control of eye growth, we have measured the precision of accommodation during reading at 1 m and at 30 cm distance. A newly developed photorefractor was used to sample both the refraction in the vertical meridian and direction of gaze at 25 Hz. Using these two parameters, a three-dimensional "refraction map" of the visual field was plotted. It showed the optic disc as an area with more myopic refractions and the course of refractions across a visual field of about +/- 25 deg. A special calibration scheme was employed to ensure that the precision of the refractions was 0.2 dpt or better (as estimated from the standard deviations of repeated measurements and the noise in the calibration curve). Twelve young adults (students from the lab) served as subjects. We found considerable inter-individual variability in the off-axis refractions but little variability among repeated measurements in the same subjects. Inter-individual variability reached a minimum in the foveal region. Both myopes wearing their spectacle corrections (n = 6) and emmetropes (n = 6) under-accommodated by about 0.3 D during reading at 30 cm distance but, at 1 m distance, only the emmetropes under-accommodated. Since both refraction groups under-accommodated similarly during reading at close distance, it remains unclear whether the small amount of defocus is critical for their future myopia development. Either accommodation errors differ at earlier times when myopia first appears (as suggested by the literature), or the subjects' eye growth was differently sensitive to defocus, or our simple protocol did not pick up existent differences in accommodation among the two groups.

Accommodation, Ocular↗
PubMed1995

A Monte Carlo estimation of tissue optical properties for use in laser dosimetry.

In certain clinical situations, such as photodynamic therapy, light dosimetry should be considered. The propagation of light in tissues is influenced by fundamental or microscopic optical properties, namely absorption mu a and scattering mu s coefficients, refractive index n and anistropy factor g. These optical parameters can be determined experimentally by direct and/or indirect methods when tissue macroscopic properties, such as reflectance, transmittance or collimated transmittance from a tissue slab, are measured. The method described in this work provides graphical, and in simple cases analytical, 'inverse' solutions to determine tissue microscopic properties from measured macroscopic parameters. The graphs necessary for this inversion have been calculated and are provided. The method can be applied in either direct or indirect techniques and it does not depend on limitations introduced by assumptions and approximations when using theoretical models. It can also be applied for any tissue type, detector geometry and experimental apparatus. The accuracy of the method is very good over a wide range, unlimited in practice, of values of optical properties. Finally, the results of this work are in good agreement with theoretical and experimental results of other investigators.

Animals↗
PubMed2002

Partial coherence laser interferometry vs conventional ultrasound biometry in intraocular lens power calculations.

AIMS: The purpose of the study was to compare optical biometry based on partial coherence laser interferometry (PCLI) principle to conventional ultrasound biometry in the accuracy of intraocular lens (IOL) power calculations. The role of partial coherence laser interferometry in pseudophakic axial length measurement was analysed in the study. METHODS: In a prospective randomised clinical trial, 100 patients undergoing phacoemulsification cataract surgery were randomised to undergo biometry by either partial coherence laser interferometry (IOL Master) or the applanation ultrasound technique. The IOL material, design and the IOL formula were standardized. The mean error and mean absolute error were calculated and compared using paired t-tests. RESULTS: One hundred patients were included in this prospective randomised trial, of whom 50 patients underwent optical biometry and 50 patients had biometry by applanation ultrasound. The mean age of patients in the PCLI group was 67 +/- 6 yrs as compared to 71 +/- 8 yrs in the ultrasound group (P > 0.05). The preoperative mean axial length was 23.47 +/- 1.1 mm in the PCLI group (range 20-27.6 mm) and 23.43 +/- 1.2 mm in the ultrasound group with a range of 20.1-27 mm (P > 0.05). The mean absolute error (MAE) in the PCLI group was 0.52 +/- 0.32 D (upper and lower 95% CI 0.62 and 0.42 respectively). The MAE in the ultrasound group was 0.62 +/- 0.4 D (upper and lower CI 0.73 and 0.50 D respectively). Eighty-seven per cent of patients were within +/- 1 D in the PCLI group as compared to 80% in the ultrasound group (P = 0.24). The MAE of axial length difference with optical biometry was 0.13 mm +/- 0.13 SD (range -0.42 to 0.78 mm) in the PCLI group and 0.19 +/- 0.13 mm in the ultrasound group. There was a mean shortening of the eye length in the PCLI group postoperatively. Optical biometry improved the post op refraction by 16% on retrospective IOL power calculations. Eight per cent failed biometry with IOL Master (dense cataracts (4%) and fixation instability due to macular degeneration (4%)). CONCLUSION: The non contact optical biometry using the partial coherence laser interferometry principle improves the predictive value for postoperative refraction and is a reliable tool in the measurement of intraocular distances in pseudophakic eyes.

Adult↗
PubMed2000

[Intra-individual comparison of intraocular lenses of highly refractive silicone (Allergan SI40NB) and hydrophobic acrylate (Alcon Acrysof MA60BM). 1-year follow-up].

BACKGROUND: A prospective, randomized study was performed to evaluate intra-individually the biocompatibility of foldable, highly refractive silicone and hydrophobic acrylic intraocular lenses (IOL). MATERIALS AND METHODS: We studied 35 patients who underwent phacoemulsification using a self-sealing tunnel incision. In a randomized fashion one eye received a 6-mm optic IOL made of high-refractive index silicone (Allergan SI40NB) and the other eye a hydrophobic acrylic 6-mm optic IOL (Alcon AcrySof MA60BM). All patients were examined 7 days, 1-3 and 6 months, and 1 year postoperatively. RESULTS: The mean best-corrected visual acuity (BCVA) was 0.9 +/- 0.12 vs. 0.89 +/- 0.13 (SI40NB vs. MA60BM) after 1-3 months. One-year postoperatively BCVA was still 0.9 +/- 0.12 vs. 0.87 +/- 0.14. The flare values (photon counts/ms) increased slightly 7 days after surgery (14.2 +/- 8.68 vs. 15.49 +/- 7.2, n.s.). Three months after surgery these values were again in the normal range. The mean IOL decentration was 0.29 +/- 0.14 vs. 0.3 +/- 0.15 mm 1 year postoperatively. Scheimpflug slit photography showed 40% of MA60BM IOLs to have "glistenings." No significant difference regarding posterior capsular opacification was found. CONCLUSION: One year after implantation of foldable, highly refractive silicone and hydrophobic acrylic IOLs using a self-sealing tunnel incision and phacoemulsification, no significant functional or morphological differences between the two IOL types were observed.

Acrylates↗
PubMed1995

Comparison of elevation, curvature, and power descriptors for corneal topographic mapping.

Corneal topography systems sample thousands of surface points and from these data different descriptors are derived to create maps of the cornea. Without visualizing and comparing the maps, it is difficult to appreciate the implications of each descriptor for mapping. We created and compared several maps of an ellipsotoric cornea based on the following surface descriptors: relative elevation, dioptric curvature, and refractive power. Each map presented a different picture of the same cornea. Only elevation maps show true topography but must be calculated relative to an appropriate reference surface to reveal important features. Axial and to a greater degree instantaneous curvature maps bring out optically significant shape asymmetries but misrepresent refractive power away from the apex. Ray tracing maps display optical properties that are not apparent from the elevation or curvature maps, including spherical aberration. Oblique astigmatism can be described using a pair of maps for the sagittal and tangential powers at each surface point. A knowledge of these principles is necessary to interpret color maps of the corneal surface correctly.

Cornea↗
PubMed1998

Live-epikeratophakia for keratoconus.

PURPOSE: To assess the clinical feasibility of refractive live-epikeratophakia (L-EPI) for early keratoconus to cap, rather than reverse, the ectatic cornea. SETTING: Eye Department, Martin-Luther-Hospital, Bochum, Germany. METHODS: Twenty-seven consecutive cases with keratoconus stage I-II had L-EPI. Lenticules were obtained from eye-bank eyes. The lenticules were cut on the artificial anterior chamber bench of the Barraquer-Krumeich-Swinger set. Optical power was generated over refractive dies to achieve postoperative emmetropia. The lenticule was sutured into a peripherally undermined 7.0 mm trephination with a double-running torque suture. RESULTS: Re-epithelialization was complete within 4 to 6 days. In all but three cases that had corneal ulcers and experienced partial melting of tissue following severe neurodermatitis, all lenticules remained stable during the 10 year follow-up. The 1 month spherical equivalent remained almost stable over the entire follow-up. There was no sign of progression of keratoconus as induced myopia or astigmatism indicated by keratometry readings. Mean best spectacle-corrected visual acuity (BSCVA) was 0.45 (n = 27) preoperatively and 0.19 (n = 25) at 1 week, 0.39 (n = 26) at 1 month, 0.53 (n = 20) at 6 months, 0.58 (n = 13) at 1 year, and 0.64 (n = 8) at 2 years. Preoperative BCVA was achieved or improved in 13 eyes at 1 month. Postoperative astigmatism was < or = 3.25 diopters. Neither rejection nor lenticule opacification was observed. CONCLUSIONS: Live-epikeratophakia is a safe and minimally invasive extraocular procedure suitable for keratoconus stage I-II. Progression of keratoconus may be arrested. If unsuccessful, the procedure is complementable and there is no interference with a later penetrating keratoplasty.

Cornea↗
PubMed1989

Assessment of intraocular lens power on the basis of precataract refractive measurements.

Assessment of intraocular lens (IOL) power may rely, when biometric measurements of the eye are not available, on refraction previous to cataract development. Such an assessment, however, is generally considered unreliable. To improve the predictability of this method, we correlated refraction measurements with predictions of IOL power. In contrast to previous studies which used retrospective assessment of precataract refraction, this study included only noncataractous eyes and correlated the direct refractive measurements obtained with hypothetical IOL power values. These values were calculated by placing axial length and corneal power measurements of the same eyes in the SRK formula. We conclude that the regression formula obtained may improve the clinical judgment required for predicting lens implant power in cataractous eyes, on the basis of precataract refractive measurements.

Adolescent↗
PubMed1987

Embryonic lens of the human eye as an optical structure.

The spherical aberration of the excised embryonic (18 to 22 weeks) human lens was determined by photographing the refractive effects of the lens on fine parallel laser beams. All lenses showed little evidence of focal variation with laser position although a slight tendency toward positive (undercorrected) spherical aberration was noted. One lens from the eye of a newborn showed slight negative (overcorrected) aberration. It is assumed that the continued growth of the lens, with central compression of old tissue, produces a refractive index distribution which contributes to lens optical quality, even in the uterine environment. Frozen sections of embryo eyes of the same period (18 to 22 weeks) indicate that a major part of the change from a spherical to an elliptical lens shape takes place between the fourth and fifth month of development.

Fetus↗
PubMed1964

AXES AND ANGLES.

Explore the source record for details and available documents.

Biophysical Phenomena↗
PubMed1990

Accommodative intraocular lens with spring action. Part 1. Design and placement in an excised animal eye.

We designed an intraocular lens (IOL) with spring action, which we call the Spring IOL, with the idea of providing a slight but adequate accommodative power. The Spring IOL consists of two 6.0-mm optics, held 4.38 mm apart, and four flexible loops. The optics are made of polymethylmethacrylate (PMMA) and the loops of polyvinylidene fluoride. The lens's refractive power is provided by the anterior optic; the posterior optic acts as a footplate to restrict the anteroposterior movement of the anterior optic. By torsionally compressing the obliquely arranged loops, a 10.0-mm 3-dimensional IOL can be reduced to 6.0 mm and inserted in the capsular bag through a 3.5-mm capsular opening. Because the Spring IOL is made of the same materials as conventional IOLs, most of the problems hindering the development of a refilling material (which could also provide accommodation) are avoided. In this first report we discuss the design of the lens, the refractive theory involved, and our experience inserting it in an excised animal eye.

Accommodation, Ocular↗
PubMed1985

Accommodation-dependent model of the human eye with aspherics.

We consider a schematic human eye with four centered aspheric surfaces. We show that by introducing recent experimental average measurements of cornea and lens into the Gullstrand-Le Grand model, the average spherical aberration of the actual eye is predicted without any shape fitting. The chromatic dispersions are adjusted to fit the experimentally observed chromatic aberration of the eye. The polychromatic point-spread function and modulation transfer function are calculated for several pupil diameters and show good agreement with previous experimental results. Finally, from this schematic eye an accommodation-dependent model is proposed that reproduces the increment of refractive power of the eye during accommodation. The variation of asphericity with accommodation is also introduced in the model and the resulting optical performance studied.

Accommodation, Ocular↗
PubMed1999

Numerical study of the influence of the shell structure of the crystalline lens on the refractive properties of the human eye.

In this paper, the numerical model of the refractive properties of the human eye is given. A special account is given of the laminated structure of the crystalline lens. The crystalline lens is presented in the form of hundreds of shells with rotational symmetry, and with the refractive index constant within each shell. The shells have the form of two semi-ellipsoids joined along the equator. The refractive index increases from the cortical shell to the inner one, according to the exponential dependency. The cornea, approximated by two ellipsoidal surfaces, is added in front of the crystalline lens. A ray-tracing procedure is applied to study the refractive properties of such a system: refractive power, spherical aberration and energy distribution. The optical properties of the given model are analysed by changing some parameters such as refractive index profile and number of shells. Calculations show that the gradient of the refractive index inside the crystalline lens results in the generation of many focal planes in such an optical system.

Computer Simulation↗