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PubMed2005

Analysis of the refractive indices of TiO2, TiOF2, and TiF4: concept of optical channel as a guide to understand and design optical materials.

In an effort to gain insight into how optical properties of insulating materials are affected by a change in chemical composition, we investigated the dielectric functions of titanium dioxide, TiO(2), and its fluorine-substituted phases, TiOF(2) and TiF(4), by electron energy loss spectroscopy measurements and density functional theory electronic band structure calculations. The refractive indices of these compounds are found to be inversely proportional to their cell volumes per formula unit. This observation was explained by employing the concept of optical channels. Our study indicates that the light-scattering properties of insulating compounds can be controlled by modifying their cell volumes.

Journal Article↗
PubMed2001

Understanding refraction and accommodation through "retinal imaging" aberrometry: a case report.

PURPOSE: A new form of aberrometry based on Tscherning optics has been proposed that captures the refraction and high-order aberrations of the eye with and without accommodation. DESIGN: Experimental clinical optics study. METHOD: A green neodymium:yttrium-aluminum-garnet laser grid pattern is projected into the eye and viewed on the retina through a narrow, collimated aperture of 1 mm. The resulting aberrated pattern is photographically recorded in a normal eye in the unaccommodated and accommodated state through a pharmacologically dilated pupil without cycloplegia. MAIN OUTCOME MEASURES: Detection of pupil-dependent refraction and high-order optical aberration with and without accommodation. RESULTS: Subtle pupil-dependent errors in refraction and high-order aberrations (spherical aberrations and coma) are demonstrated in the unaccommodated normal eye. Accommodation reveals slightly more spherical power through the central 3-mm zone than through a 6.5-mm pupil without significant increase in aberration. CONCLUSIONS: Tscherning aberrometry based on 'retinal imaging' is useful in defining the refraction and optical aberrations in a normal eye. Accommodation increases spherical refractive power with only small aberration changes, including negative asphericity.

Accommodation, Ocular↗
PubMedDate not supplied

[Determination of the optical constants of intact microogranisms by the method of attenuated internal reflection spectroscopy].

The problems are considered of the determination of the optical constants (dispersion of refraction, n2, and absorption, K, indexes) of strongly dispersing biological objects on undestroyed cells of microorganisms (E. coli, strain C-85) by the method of repeated attenuated total refraction. The volume of unpacked intercellular space and the effect of repeated propagation of the light current through measuring elements with high refraction index were taken into account. Relationships R, s (n1, n2, O) K were obtained for determining the optical constants on the elements with the angle of light current incidence 0=45 degrees, made of infrared glass, IRG-24, IRF-24 and of Si and Ge as well. The optical constants of bacteria in the region of absorption bands 1660, 1550 cm-1 were also determined.

Actinomyces↗
PubMed1983

Refractive corneal surgery: the use of implantable alloplastic lens material.

A basic problem encountered in all forms of lamellar refractive surgery has been the predictability of the optical corrections. Corneal tissue used in refractive surgery behaves in a variable fashion because its shape is determined in vivo by the process of healing and graft repopulation. Therefore no preoperative measure of the ultimate power of the lathed corneal lens can be made. Another drawback is the limited supply of donor corneal tissue. In an effort to overcome these problems, we have used alloplastic materials in conjunction with lamellar refractive surgery to improve the predictability of the result. The use of this material would also allow accurate preoperative determination of lens power. Intralamellar implants appeared to heal quite rapidly after surgery. Their optical clarity was excellent and the corneal surface appeared stable. Further research into long-term stability and predictability are currently underway.

Animals↗
PubMed1999

Determinants of optic disc characteristics in a general population: The Rotterdam Study.

OBJECTIVE: To determine whether age, gender, height, and refractive error are associated with optic disc morphology in a general elderly population. DESIGN: Population-based, cross-sectional study. PARTICIPANTS: A total of 5114 subjects 55 years of age or older participated in this study, representing 76% of a population-based sample of 6777 ophthalmologically examined white patients from a geographically well-defined suburb in Rotterdam, The Netherlands. MAIN OUTCOME MEASURES: Disc area, neural rim area, cup area, vertical and horizontal cup-to-disc ratios, and parapapillary atrophy. METHODS: Disc characteristics were measured on stereoscopic simultaneous optic disc transparencies using an image analyzer. The presence and location of parapapillary atrophy, differentiated into zones alpha and beta, were assessed from disc transparencies of both eyes in a random sample of 894 persons. Subjects with open-angle glaucoma were excluded. RESULTS: The mean disc area was 2.42 mm2 (standard deviation [SD], 0.47), mean neural rim area was 1.85 mm2 (SD, 0.39), mean cup area was 0.57 mm2 (SD, 0.34), mean vertical cup-to-disc ratio was 0.49 (SD, 0.14), and mean horizontal cup-to-disc ratio was 0.40 (SD, 0.14). Age was not a determinant of any disc characteristic. Disc and rim areas were 3.2% (P < 0.0005; 95% confidence interval [CI], 2.7%-3.7%) and 4.3% (P < 0.0005; 95% CI, 3.5%-4.6%) larger in men than in women. For each diopter increase toward myopia, the disc area increased by 0.033 mm2 (P < 0.0005; 95% CI, 0.027-0.038) and neural rim area by 0.029 mm2 (P < 0.0005; 95% CI, 0.025-0.034). The disc area increased by 0.02 mm2 (P = 0.02; 95% CI, 0.005,0.05) for each 10-cm increase in height. The prevalence of zone alpha slightly decreased by 0.4% per 10 years of age (P = 0.035; 95% CI, 0.03%-0.8%), whereas the prevalence of zone beta increased by 1.3% (P = 0.0003; 95% CI, 0.57%-1.9%) for each diopter increase toward myopia. CONCLUSIONS: In a general population, statistically normal discs may vary twofold in disc area and threefold in rim area. Age is not associated with any disc characteristic, whereas disc area and neural rim area are slightly larger in men than in women. Refractive error is weakly related to disc area and neural rim area. Height is weakly related to disc area in persons of medium height. The prevalence of zone beta is higher in myopic eyes.

Age Factors↗
PubMed2006

Characterization of high refractive index semiconductor films by surface plasmon resonance.

Si-based surface plasmon resonance (SPR) in the Kretschmann-Raether geometry is considered as a platform for the optical measurement of high refractive index films. The implementation of the SPR effect becomes possible due to the relatively high index of refraction of Si compared to most materials. As examples we study the SPR responses for some important semiconductor-based films, including laser-ablated porous silicon and thin germanium films. Using SPR data, we determine the refractive indices of these films for different parameters (thickness and porosity) and ambiences. We also discuss novel SPR biosensor architectures with the use of these solid films.

Journal Article↗
PubMed2004

Mechanism of linear and nonlinear optical effects of chalcopyrite AgGaX2 (X=S, Se, and Te) crystals.

The electronic band structures for AgGaX(2) (X=S, Se, Te) chalcopyrites have been calculated using a pseudopotential total energy method. First-principles calculations of the linear and nonlinear optical properties are presented for these crystals, with the electronic band structures obtained from pseudopotential method as input. The theoretical refractive indices and nonlinear optical coefficients are in good agreement with available experimental values. The origin of the nonlinear optical effects is explained through real-space atom-cutting analysis. The contribution of the GaX(2) group (X=S, Se, Te) for second harmonic generation (SHG) effect is dominant while that of the cation Ag is negligible. In addition, the percentage contribution to the SHG coefficients from the different bonds increase with increase of the bond order.

Journal Article↗
PubMed2001

Optical effects of ocular surgery including anterior segment surgery.

Ocular surgery may alter the refraction and the corneal power of an eye. However, refraction and corneal power are not fundamental optical properties of the eye as such. Refraction is a fundamental property of a lens in front of the eye and an indirect or derived property of the eye, while corneal power is a fundamental property of only part of the eye. Nor are they the only clinically relevant paraxial optical effects of surgery. There are also effects on size, shape, and orientation of retinal images, for example, which are not simply related to refraction and corneal power. Quantitative analyses of the effects of surgery on the eye that account only for refraction and corneal power are necessarily incomplete. This paper offers an approach to the analysis of the optical effects of surgery that is complete within the limitations of paraxial optics. The key is the concept of the ray transference, which contains the 4 fundamental optical properties of an optical system. The surgery is represented by a hypothetical optical system, the anterior surgery-equivalent system, in front of the preoperative eye. The transference of this system, the anterior surgical transference, is a complete paraxial quantification of the optics of the surgery and is what needs to be studied in analyses of surgery. In this approach, ocular surgery in general is optically equivalent to a thick system placed in front of the preoperative eye. A simple model is developed for the case of anterior segment surgery in particular. In terms of the model, anterior segment surgery is shown to be equivalent to placing a thick bitoric lens in front of the preoperative eye. In particular cases, the surgery may be equivalent to an anterior thin lens. Astigmatism is an aspect of optical effects in general. It is not directly amenable to quantitative analysis in isolation; quantitative analyses of astigmatism should be done in terms of the concept of antistigmatism. In thin systems, astigmatism manifests only in power. However, in thick systems such as the eye, astigmatism can manifest in 4 fundamentally different ways. Among them are some phenomena currently described as irregular astigmatism. The foundation is laid for the complete analysis of paraxial astigmatism in general and in ocular surgery in particular.

Astigmatism↗
PubMed2000

Three-dimensional optical measurement of instantaneous pressure.

Local perturbations in material density induced in a material by a compressional wave give rise to local perturbations in refractive index. Accurate, high-resolution, three-dimensional, optical measurements of an instantaneous refractive index perturbation in a homogeneous, optically transparent medium may be obtained from measurements of scattered optical intensity alone. The method of generalized projections allows incorporation of optical intensity measurements into an iterative algorithm for computing the phase of the interrogating optical pulse as the solution of a fixed point equation. The complex optical field amplitude, computed in this manner, is unique up to a constant unit magnitude complex coefficient. The three-dimensional refractive index distribution may be computed via the Fourier slice reconstruction algorithm from the optical phase data under the assumption of weak optical scattering. The refractive index perturbation is related to local instantaneous pressure under a linear, small-displacement model for the mechanical wave. A numerical simulation of the measurement experiment, phase recovery, and reconstruction process for a plane piston ultrasound transducer with a semicircular aperture and center frequency of 1.5 MHz is described and corresponds very well with experiment. Experimental data obtained using an 810-nm laser source are used to reconstruct the three-dimensional pressure field from two elements of a 2.5-MHz linear array. Comparison with a measurement obtained via a 500-microm needle hydrophone shows excellent agreement.

Acoustics↗
PubMed1993

Hyperopia as a risk factor for nonarteritic anterior ischemic optic neuropathy.

The most convincing morphologic component in the pathogenesis of nonarteritic anterior ischemic optic neuropathy is the anatomically small and crowded optic nerve head. Because the appearance of the optic disk is presumed to be related to underlying refractive error, we sought to determine if a particular refractive error places patients at additional risk for, or affords protection from, nonarteritic anterior ischemic optic neuropathy. We compared refractive errors of 50 patients with nonarteritic anterior ischemic optic neuropathy to an age-matched and eye-matched control population. The mean refractive error (in spherical equivalents) for the nonarteritic anterior ischemic optic neuropathy group was +0.26 diopter (SD +/- 2.08); the mean refractive error for controls was -0.86 diopter (SD +/- 2.91) (P = .027). Our results suggest that patients who have nonarteritic anterior ischemic optic neuropathy are less myopic than a control population. Whether it is myopia that protects from, or hyperopia that predisposes to nonarteritic anterior optic neuropathy, our results imply another anatomically based factor that characterizes eyes that develop nonarteritic anterior ischemic optic neuropathy. Those eyes tend to be minimally hyperopic.

Arteritis↗
PubMed2002

A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis.

OBJECTIVE: To evaluate the accuracy of computerized videokeratography, keratometry, and the Gaussian optics formula for measuring corneal refractive power in patients after myopic laser in situ keratomileusis (LASIK). DESIGN: Noncomparative case series. PARTICIPANTS: One hundred eyes of 63 patients (mean age, 45.0 +/- 10.9 [standard deviation] years) who underwent LASIK were included in the study. METHODS: Using the clinical history method as the standard, we evaluated the accuracy of values of corneal refractive power derived from computerized videokeratography (the EffRP value of the EyeSys Corneal Analysis System, which averages corneal refractive power over the central 3 mm), keratometry (K), the Gaussian optics formula (GauRP), and values of EffRP and keratometry as modified according to the amount of LASIK-induced refractive change. MAIN OUTCOME MEASURES: Correlation of measured corneal power values to those obtained using clinical history method (HisRP). RESULTS: Although the values for HisRP were significantly correlated with postoperative EffRP and K values and with GauRP, postoperative EffRP and K values were higher than HisRP (0.87 +/- 0.68 diopters [D] and 1.16 +/- 1.10 D, respectively), and GauRP were lower than HisRP (0.44 +/- 0.66 D) (P < 0.001 for all three comparisons). The differences between HisRP and both postoperative EffRP and K values increased significantly with the amount of myopic correction. The most accurate results were obtained by modifying the postoperative values of EffRP according to the amount of LASIK-induced refractive change; 70% of these values were within +/- 0.5 D and 94% within +/- 1 D of HisRP values. CONCLUSIONS: Using the clinical history method as the standard, we found that the most accurate method for determining corneal refractive power in post-LASIK eyes was to adjust the postoperative corneal measurement according to the amount of LASIK-induced refractive change.

Adult↗
PubMed2004

Matrix-based calculation scheme for toric intraocular lenses.

BACKGROUND AND PURPOSE: While a number of intraocular lens power prediction formulas are well established for determination of spherical lenses, no common strategy is published for the computation of toric intraocular lenses. The purpose of this study is to describe a paraxial computing scheme using 4 x 4 system matrices to describe the 'optical system eye' containing astigmatic refractive surfaces with their axes at random. METHODS: Based on the definition of a centred optical system in the paraxial Gaussian space containing astigmatic surfaces using 4 x 4 refraction and translation matrices, we derived a methodology for calculating the refractive power of thin and thick toric intraocular lenses by solving a linear equation system. In a second step, we derived a methodology for prediction of the residual spectacle refraction after implantation of any toric lens implant with any orientation. RESULTS: The capabilities of this computing scheme are demonstrated with three examples. In example 1 we calculate a 'thin toric lens' for compensation of a corneal astigmatism to achieve a spherical target refraction. In example 2 we compute a 'thick toric lens', which has to compensate for an oblique corneal astigmatism and rotate the spectacle cylinder to the 'against the rule' position to enhance near vision. In example 3 we predict the residual refraction at the corneal plane after implantation of a thick toric lens, when the cylinder of the lens implant is compensating the corneal cylinder in part and the axis of implantation is not fully aligned with the axis of the corneal astigmatism. CONCLUSION: We present an en bloc matrix-based strategy for the calculation of thick or thin toric intraocular lenses, with the flexibility of crossing an unlimited number of cylinders with restrictions to paraxial optics. The resulting system matrix S is written as a product of 4 x 4 refraction and translation matrices. Residual refraction at the corneal (contact lens) or spectacle plane can be derived by inverting the order of matrices for calculation of the system matrix.

Astigmatism↗
PubMed2003

Optic disc morphology in south India: the Vellore Eye Study.

AIM: To evaluate the morphology of the optic nerve head in an unselected population group in south India. METHODS: The study included 70 subjects forming a population based sample, selected in a random manner. Mean age was 47.5 (SD 8.7) years, mean refractive error measured -0.07 (1.11) dioptres (range -4.50 to +2.50 dioptres). Optic disc slides were morphometrically analysed. RESULTS: Mean optic disc area measured 2.58 (0.65) mm(2). It was statistically independent of age and refractive error. Optic disc shape was slightly vertically oval. Mean neuroretinal rim area was 1.60 (0.37) mm(2). It was significantly and positively correlated with optic disc size and optic cup size. It was independent of age, sex, refractive error, and axial length. In all subjects included in the study, the rim was smallest in the temporal horizontal optic disc sector. Mean horizontal cup/disc diameter ratio (0.66 (0.07)) was significantly (p<0.001) higher than the mean vertical cup/disc diameter ratio (0.56 (0.08)). Both ratios were highly significantly (p <0.001) and positively correlated with optic disc size. The alpha zone of parapapillary atrophy (0.84 (0.29) mm(2)), and beta zone (0.13 (0.38) mm(2)), respectively, occurred in 69 (98.6%) subjects and in eight (11.4%) subjects, respectively. They were significantly larger in the temporal horizontal sector. The alpha zone was significantly (p<0.001) larger and occurred significantly more often than beta zone. Retinal arterioles and venules were wider, and in spatial correlation, the visibility of the retinal nerve fibre layer was significantly better, in the temporal inferior disc arcade and the temporal superior arcade than in the nasal superior arcade and the nasal inferior vessel arcade. Except for the absolute size measurements these optic nerve head parameters did not differ markedly (p >0.05) from the values found in white people. CONCLUSIONS: South Indians and white people do not show marked differences in the morphology of the optic nerve head as measured by morphometric optic disc parameters, with the possible exception of the absolute optic disc dimensions.

Adult↗
PubMed1990

Dioptrics of the facet lenses of male blowflies Calliphora and Chrysomyia.

1. The dioptrics of the facet lenses of two blowfly species, Calliphora erythrocephala and Chrysomyia megacephala, was investigated. Measurements were performed on facet lenses ranging in diameter from 20 to 80 microns. 2. The radius of curvature of the front surface of the facet lenses, measured by microreflectometry, increases approximately linearly with the facet lens diameter. 3. The optical path difference of the facet lens and water, measured by interference microscopy, depends on the distance from the optical axis according to a parabolic function. Average refractive index values, calculated from the optical path difference profile together with estimates of the thickness profile, are between 1.40 and 1.43, with the lowest values in the largest lenses. 4. The F-number calculated from the experimental data ranges from 1.5 to 2.2. It is argued that the range of effective F-numbers is 2.1-2.4.

Animals↗
PubMed2006

Four corneal presbyopia corrections: simulations of optical consequences on retinal image quality.

PURPOSE: To investigate the possibility of multifocal or aspherical treatment of the cornea with optical ray tracing. SETTING: Institute for Refractive and Ophthalmic Surgery, Zurich, Switzerland. METHODS: The optical consequences of 4 corneal shapes-global optimum (GO) for curvature and asphericity, central steep island (CSI), decentered steep island (DSI), and centered steep annulus (CSA)-for presbyopia correction were analyzed using a modified Liou-Brennan eye model and ray tracing with a commercial optic design software (Zemax, Zemax Development Corp.). The ocular optical configuration for far vision was a point light source at a distance of 5 m, 1 degree up, and a pupil diameter of 5.0 mm and for near vision, 0.4 m distance, 1 degree up, and a pupil diameter of 2.5 mm. The curvature radius (R) of the cornea and its asphericity (Q) were used as operands to optimize (simultaneously for near and far vision) the quality of the retinal image described by means of the minimum spot diameter or the root-mean-square (RMS) wavefront error. RESULTS: Starting from an emmetropic eye optimized for R and Q, the RMS wavefront error in the retina was 0.07 microm (far) and 1.42 microm (near). The GO resulted in a wavefront error of 1.42 microm (far) and 0.52 microm (near); improvement of near vision using reading glasses is possible. The CSI yielded 0.91 microm (far) and 0.13 microm (near); spectacles did not improve far or near vision. The DSI and CSA had significantly worse results for near and far vision. CONCLUSIONS: Of the options studied, GO and CSI seemed the most promising alternatives for corneal presbyopia correction. Although reading glasses can improve near vision in GO, reading glasses did not improve near vision in CSI-treated eyes. The CSI treatment is critically dependent on centration and a reverse treatment is difficult to achieve.

Computer Simulation↗
PubMedDate not supplied

Optical scattering, absorption, and polarization of healthy and neovascularized human retinal tissues.

The optical scattering, absorption, and polarization properties of human retinal tissues are investigated for a number of laser wavelengths in the visible range. The indices of refraction of these tissues are determined by applying Brewster's law. The inverse adding doubling method based on the diffusion approximation and radiative transport theory is applied to the measured values of total diffuse transmission, total diffuse reflection, and index of refraction to determine the optical absorption, scattering, and scattering anisotropy coefficients of the intact retinal tissues from healthy and diseased (neovascularized) human eyes. The polarization studies show that the retinal tissues possess significant intrinsic polarization characteristics, that are more pronounced in diseased tissues than in healthy tissues.

Absorption↗