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At least 235 records · Page 13Linked to original sources
PubMed2005

Characterization of AlF3 thin films at 193 nm by thermal evaporation.

Aluminum fluoride (AlF3) was deposited by a resistive heating boat. To obtain a low optical loss and high laser-induced damage threshold (LIDT) at 193 nm, the films were investigated under different substrate temperatures, deposition rates, and annealing after coating. The optical property (the transmittance, refractive index, extinction coefficient, and optical loss) at 193 nm, microstructure (the cross-sectional morphology, surface roughness, and crystalline structure), mechanical property (stress), and LIDT of AlF3 thin films have been studied. AlF3 thin films deposited at a high substrate temperature and low deposition rate showed a lower optical loss. The highest LIDT occurred at the substrate temperature of 150 degrees C. The LIDT of the films prepared at a deposition rate of 2 A/s was higher than that at other deposition rates. The annealing process did not influence the optical properties too much, but it did increase the LIDT and stress.

Journal Article↗
PubMed1996

Corneal aberrations increase with the magnitude of radial keratotomy refractive correction.

BACKGROUND: Refractive surgery induces optically abrupt changes in shape in the midperiphery of the cornea. The abruptness of this change is in part dependent on the magnitude of the surgically induced refractive change. Therefore, the optical aberrations of the cornea, as quantified by wavefront variance (WFV), may be expected to increase as the surgically induced change in the refraction increases. PURPOSE: It is the purpose of this study to test the hypothesis that as the surgery-induced change in refraction increases, so does the WFV of the cornea. METHODS: Fourteen radial keratotomy (RK) patients and seven normal patients served as subjects. Measurements were made before and 2 years after RK surgery. To quantify the WFV of the cornea, we used corneal topography measurements to calculate the surgically induced change in corneal WFV with respect to two different reference surfaces, a sphere and the presurgical cornea. To quantify the surgically induced change in the equivalent spherical correction (ESC), cycloplegic refractions were performed. The measurements were summarized by regressing the surgically induced change in the WFV against the surgically induced change in the ESC. RESULTS: For large pupils (7 mm diameter), the correlation between the change in the WFV referenced to a sphere and the change in the ESC was significant (p < 0.0001, r2 = 0.745) and dominated by fourth order aberrations. Similar results were found for the surgical lens. For small pupils (3 mm diameter), the effects were markedly reduced. CONCLUSIONS: (1) As the magnitude of the surgically induced refractive change increases so does the WFV of the cornea, particularly for large pupils. (2) The increase in corneal WFV for large pupils is dominated by fourth order aberrations. (3) The increase in corneal WFV is consistent with reported decreases in visual function (contrast sensitivity and low contrast visual acuity), particularly for large pupil diameters in combination with large surgically induced changes in refractive error.

Cornea↗
PubMed2006

Spatial refractive index measurement of porcine artery using differential phase optical coherence microscopy.

BACKGROUND AND OBJECTIVES: We describe a methodology to record spatial variation of refractive index of porcine renal artery using differential phase optical coherence microscopy (DP-OCM). STUDY DESIGN/MATERIALS AND METHODS: The DP-OCM provides quantitative measurement of thin specimen phase retardation and refractive index by measuring optical path-length changes on the order of a few nanometers and with a lateral resolution of 3 microm. The DP-OCM instrumentation is an all-fiber, dual-channel Michelson interferometer constructed using a polarization maintaining (PM) fiber. RESULTS: Two-dimensional en face dual-channel phase images are taken over a 150 x 200 microm region on a microscopic slide, and the images are reconstructed by plotting a two-dimensional refractive index map as the OCM beam is moved across the sample. CONCLUSIONS: Because the DP-OCM can record transient changes in the optical path-length, the system may be used to record quantitative optical path-length alterations of tissue in response to various stimuli. A fiber-based DP-OCM may have the potential to substantially improve in vivo imaging of individual cells for a variety of clinical diagnostics, and monitoring applications.

Animals↗
PubMed1994

Optical method for estimating the equivalent refractive index of the crystalline lens in vivo.

In numerous studies it has been necessary to represent refractive indices of the crystalline lens by an equivalent refractive index (here denoted nL). Its traditional value of 1.416, based on in vitro measurements, may be in error by more than 0.025 in some cases. In vivo estimates are subject to recognized deficiencies of ultrasonography. This study describes and evaluates a new and exclusively optical technique. Using exact ray tracing, nL is found by combining routine optical section data with optical depths that may be measured by a ranging system termed OCDR. Gullstand's no. 1 eye is used to calculate the effects of errors in eye data on the accuracy of the technique. From pooled errors estimated for 22 parameters, a standard deviation of 0.0045 is anticipated for the estimate of nL, although allowance for additional uncertainty associated with the refractive index profile of the lens must await further research.

Humans↗
PubMed2006

Measurement of group refractive index wavelength dependence using a low-coherence tandem interferometer.

A technique for the measurement of the group refractive index wavelength dependence of optical materials using a low-coherence tandem interferometer and a spectrometer is proposed. Four channeled spectra resulting from interferences of light beams from different pairs of optical paths are used for the calculation of optical path differences. The group refractive index wavelength dependence is calculated from these optical path differences generated from the sample under measurement. No a priori information of the geometric thickness of a sample is required. The wavelength dependence of the group refractive index of the samples BK7 parallel plate of 5.200 and 10.025 mm from 675 to 850 nm is experimentally measured with an accuracy of the order of 10(-5) and a repeatability of the order of 10(-9).

Journal Article↗
PubMed2005

Negative refraction and focusing of circularly polarized waves in optically active media.

Analysis indicates that certain types of optically active media are capable of producing negative refraction and focusing of circularly polarized waves. It is established that a slab of such material acts just as Veselago's hypothetical left-handed media lens, providing subwavelength resolution as Sir Pendry's ideal lens, but for circularly polarized waves.

Journal Article↗
PubMed2005

X-Ray microanalytical techniques based on synchrotron radiation.

The development of 3rd generation synchrotron radiation sources like European Synchrotron Radiation Facility (ESRF) in parallel with recent advances in the technology of X-ray microfocusing elements like Kirkpatrick-Baez (KB) mirrors, diffractive (Fresnel zone plates, FZP) and refractive (compound refractive lenses, CRL) optics, makes it possible to use X-ray microscopy techniques with high energy X-rays (energy superior to 4 keV). Spectroscopy, imaging, tomography and diffraction studies of samples with hard X-rays at micrometre and sub-micrometre spatial resolutions are now possible. The concept of combining these techniques as a high-energy microscopy has been proposed and successfully realized at the ESRF beamlines. Therefore a short summary of X-ray microscopy techniques is presented first. The main emphasis will be put on those methods which aim to produce sub-micron and nanometre resolution. These methods fall into three broad categories: reflective, refractive and diffractive optics. The basic principles and recent achievements will be discussed for all optical devices. Recent applications of synchrotron based microanalytical techniques to characterise radioactive fuel particles (UO(2)) released from the Chernobyl reactor are reported.

Environmental Monitoring↗
PubMed2003

The effects of internal refractive index variation in near-infrared optical tomography: a finite element modelling approach.

Near-infrared (NIR) tomography is a technique used to measure light propagation through tissue and generate images of internal optical property distributions from boundary measurements. Most popular applications have concentrated on female breast imaging, neonatal and adult head imaging, as well as muscle and small animal studies. In most instances a highly scattering medium with a homogeneous refractive index is assumed throughout the imaging domain. Using these assumptions, it is possible to simplify the model to the diffusion approximation. However, biological tissue contains regions of varying optical absorption and scatter, as well as varying refractive index. In this work, we introduce an internal boundary constraint in the finite element method approach to modelling light propagation through tissue that accounts for regions of different refractive indices. We have compared the results to data from a Monte Carlo simulation and show that for a simple two-layered slab model of varying refractive index, the phase of the measured reflectance data is significantly altered by the variation in internal refractive index, whereas the amplitude data are affected only slightly.

Computer Simulation↗
PubMed2003

[Phakic IOL].

Refractive errors can be corrected by three different means, as follows: optical devices, corneal refractive surgery and intraocular procedures. Each intraocular procedure has its advantages and disadvantages. In this paper the authors present the nowdays state of refractive phakic IOL's.

Humans↗
PubMed1983

Visual disability and blindness secondary to refractive errors in Africa.

Optical defects of the light-focusing apparatus of the eye are called errors of refraction. They are responsible for 13% of all significant vision loss in Kenya, ranking third of all causes, after cataract and trachoma. As the overall prevalence of such visual impairment in the Country is 3.7%, roughly 0.5% of the population can be said to need spectacles to obtain normal vision. The comparable figure for secondary school children in Nigeria has been found to be 2.4%. In addition, patients require special spectacles after cataract surgery to obtain adequate vision; without such spectacles these patients are still 'blind' by World Health Organization criteria. These glasses can be obtained through mission societies and various charitable organizations for a little as $5 per pair, and can be manufactured locally with available ophthalmic manpower and technology. Alternatives to spectacles such as contact lenses, intra-ocular lenses and Kerato-refractive surgery are not suitable for use in developing Countries.

Africa↗
PubMed2003

Optical development in the foetal bovine lens.

The refractive index variations along the optic axes of nine foetal bovine lenses were measured using an optic fibre reflectometer. The device measures refractive index directly, using the principle of Fresnel reflectance. These results show that lenses with a wet weight >0.7 g (corresponding to 4.5 months gestational age), have a refractive index profile which approximates a parabolic shape, akin to that found in post-natal and adult bovine lenses. In lenses of wet weight below 0.7 g, the refractive index distributions show irregularities. These findings suggest that the refractive index profile in the bovine lens starts to form into a parabolic distribution, as seen in adult lenses, about half-way through gestation. The trigger for this formation is not known.

Animals↗
PubMedDate not supplied

Wavefront-guided laser in situ keratomileusis: early results in three eyes.

PURPOSE: Wavefront optical aberrations induced by refractive corneal surgery correction of myopia are probably the reason for deterioration of visual performance in some eyes after surgery. Customized photoablation of the cornea to correct both the sphero-cylindrical refractive error as well as individual optical aberrations may improve postoperative visual acuity and visual performance. METHODS: In 3 eyes of 3 patients the wavefront deviations were measured by means of an aberrometer of the Tscherning-type. Based on these measurements an ablation pattern was determined and applied during a LASIK procedure using a Wavelight Allegretto scanning spot excimer laser with a spot size of 1 mm and a laser repetition rate of 200 Hz. The 3 eyes are part of a prospective study on wavefront-guided LASIK started in July 1999. RESULTS: At 1 month after LASIK, all 3 eyes had gained up to 2 lines of best spectacle-corrected visual acuity. Best spectacle-corrected visual acuity improved to 20/10 in all 3 eyes, uncorrected visual acuity was 20/10 in two eyes, and 20/12.5 in 1 eye 1 month postoperatively. The wavefront deviations were reduced by 27% on average. At 3 months, best spectacle-corrected visual acuity was 20/10 in 2 eyes and 20/12.5 in 1 eye. CONCLUSION: Wavefront-guided LASIK is a feasible approach in refractive corneal surgery. Optimized ablation patterns may further improve the visual results.

Adult↗
PubMed1991

Correlations between electroretinography, morphology and function in retinitis pigmentosa.

In a retrospective study, data from 116 patients suffering from different forms of retinitis pigmentosa were analysed, and 15 categories comprising altogether 34 symptoms or clinical signs were tabulated from each patient's record. The 15 categories evaluated were: visual acuity, visual field diameter, ring or central scotoma, nyctalopia, susceptibility to glare, refraction, cataract, electroretinography, colour of the optic disc, bone-spicule pigmentation of the retina, retinal vessel diameters, tapetoretinal reflex, sex, heredity, and age. Correlations between the tabulated 34 subcategories or symptoms were calculated. The results of a factorial analysis of the data showed a high number of highly significant correlations between the different categories. It seemed possible to discriminate between two groups of categories, with the members of each group being closely correlated but correlations with members of the other group being much smaller, if not nonexistent. We tentatively associated the first group with the functional state of the central retina and the second group with the state of the peripheral retina.

Adolescent↗
PubMed2003

Quantitation and test of enantiomeric purity of the L-ketohexoses by liquid chromatography with dual refractive index and laser-based chiroptical detection.

Experimental data for the synthetic methods of preparation of the L-ketohexoses have been lacking an analytical method for monitoring the chemical or enzymatic reactions described. Dual refractive index and laser-based chiroptical detection provides an ideal method for following the reactions, since the refractive index detector quantifies the amount of analyte, while the ratio of optical rotation to refractive index response allows the enantiomer mole fraction to be determined. Sulfonated polystyrenedivinylbenzene resin in the Ca form as the stationary phase with H2O at 80 degrees C as the eluent gives base-line separation of sorbose, fructose, tagatose, and psicose. Dependent on the complexity of the reaction mixture, analysis times range from 20 to 60 min.

Calibration↗
PubMed1982

A comparison of clinical refractive data obtained by three examiners.

Clinical refractive data were obtained on 32 subjects by each of three examiners, each examiner using standard subjective refraction procedures with an endpoint criterion of "maximum plus for best visual acuity." The results of the three examiners were compared in terms of spherical equivalent refraction, spherical power, cylinder power, cylinder axis, and anisometropia. In addition, results were compared to those of previous studies involving both clinical refraction and the American Optical Corporation's SR III Subjective Refraction System.

Adolescent↗
PubMed2001

Aging of the optics of the human eye: lens refraction models and principal plane locations.

Biometric data describing the geometry and spacings of emmetropic human eyes were combined with lens shape and placement within the globe to generate paraxial models of image formation as a function of age. Three different representations of the shape of the internal refractive index gradient of the lens were evaluated--a Gullstrand-type model consisting of cortical and nuclear regions with different refractive indices, a power series model, and a linear-gradient model. All three refractive models satisfy the requirements for focus for all the data sets, indicating that lenticular refractive index gradient shape is essentially underdetermined in the paraxial limit. Lens refractive power decreases by almost 2 D during a 50-year period, and the concomitant decrease in system refractive power is due almost entirely to this effect. The reduction in spacing between the lens principal planes is a function of this, as is their anterior movement with age, and suggests that the compensatory processes maintaining far focus at the expense of near are not exactly balanced. Despite these changes in the lens contribution and their effect on the location of the system principal planes, which also move anteriorly, the spacing between the system principal planes remains constant. However, the trend toward reduced overall system power with age indicates the primary role of the lens in mediating image formation onto the retina over time.

Accommodation, Ocular↗
PubMed1995

[New knowledge of echographic imaging of the optic nerve. Experimental and clinical studies].

The optic nerve can be displayed in vivo by A- and B-scan techniques. To achieve correlations between the anatomical structure of the optic nerve and the echograms and to answer the question if by the usual examination techniques the diameter of the nerve can be measured, experimental studies on bovine optic nerves were carried out: examination of the bovine optic nerve in vitro with the A-scan technique; measurement of the sound velocity in bovine optic nerves. Additionally, we compared the two most common A-scan techniques for the display of the optic nerve in vivo in 47 normal persons and in 35 patients with changes of the optic nerve. The experimental studies showed that the orbital fat and outer sheaths of the optic nerve cause refraction of the sound beam. The sound velocity of ultrasound in bovine optic nerves was measured to 1567 +/- 25 m/s. The two most common examination techniques in vivo showed no differences in normal persons, but significant differences in patients with increased subarachnoidal fluid.

Adipose Tissue↗