"Refracting" a single retinal ganglion cell.
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Animals↗
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Results for “Optics and Refraction”
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Optical and mathematical corrections of micropipette measurements of red blood cell geometry during anisotonic perifusion.
Accurate knowledge of red blood cell (RBC) membrane surface area and cellular volume is critical to understanding the geometric factors that affect RBC deformability. A useful method to determine RBC geometry is via micropipette aspiration in which the cell partially enters the tip of a glass pipette with a diameter of about 2 microns. Measurement of the cell while in the pipette allows calculation of cell geometry, but such measurements are subject to several artifacts. Herein, we analyze the effects of three artifacts on calculated RBC geometry during experimental osmotic manipulation. 1) The pipette internal diameter, as measured optically, requires correction for refractive index and focusing errors; 2) the pipette barrel is tapered rather than cylindrical; 3) the observed RBC outline is affected by a diffraction phenomenon. These optical and mathematical errors were all found to influence the calculated RBC geometry; both absolute values and relative changes during osmotic manipulation were affected. For RBC in isotonic medium, use of the above corrections significantly (p < 0.001) reduced the calculated area by 3.7%, the volume by 7.0%, and the minimum cylindrical diameter by 3.9%. The calculated membrane area dilation for cells exposed to hypotonic media was also significantly reduced; at 200 mOsm, the dilation was 2.8% +/- 0.2% without correction vs. 1.5% +/- 0.2% when all corrections were employed (p < 0.01). We therefore suggest that micropipette aspiration of RBC requires use of these correction techniques to obtain accurate values for RBC geometry and that such correction methods are of particular importance when RBC of different sizes are compared.
Adult↗
[Aberrometer systems in laser refractive surgery].
The wavefront sensing device provides a new and objective way of mapping the profile of refraction and of higher order defocus. Higher-order optical errors of the human eye are often responsible for reduced visual acuity, in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can results from operations on the eye, involving optical structures. Nowadays, there are four different methods of wavefront technology available: Concept of "Outgoing" Reflective Aberrometry, Concept of "Retinal Imaging" Aberrometry, Concept of "Ingoing" Adjustable Aberrometry (Spatially Resolved Refractometer), Concept of "Double Pass" Aberrometry. Wavefront analysis is a more sophisticated method of defining aberrations that the surgeon is trying to correct through refractive surgery.
Corneal Surgery, Laser↗
Scanning laser tomography to evaluate optic discs of normal eyes.
PURPOSE: To investigate the effects of age, eye refraction, and disc area on topographic parameters of the optic nerve head in normal volunteers, using the Heidelberg Retina Tomograph. METHODS: Seventy-seven eyes of 77 normal volunteers were examined by scanning laser tomography. The topographic parameters analyzed were disc area, cup area, cup/disc area ratio, rim area, cup volume, rim volume, mean cup depth, maximum cup depth, cup shape measure, height variation contour, mean retinal nerve fiber layer thickness (MnRNFLT), and retinal nerve fiber layer (RNFL) cross-section area. The effect of age, refraction, and disc area on each parameter was analyzed by the multiple linear regression model. RESULTS: Significant declines in MnRNFLT and RNFL cross-section area were found with increasing age (P < .05). The mean cup depth and maximum cup depth were significantly deeper in myopic subjects (P < .05). Large discs had large cup area, cup/disc area ratio, rim area, cup volume, mean cup depth, cup shape measure (P < .01), and maximum cup depth (P < .05). The MnRNFLT was smaller in large discs (P < .01). Rim volume was unaffected by age, refraction, or disc area. CONCLUSIONS: The age, refraction, and disc area were related to several optic disc parameters obtained by the Heidelberg Retina Tomograph. Because of these relationships, care should be taken to analyze the appearance of the optic disc on the basis of these parameters in patients with glaucoma or other diseases. Rim volume appears to be a good parameter for evaluating the optic disc without considering age, refraction, or disc area.
Adult↗
Effect of optic edge design and haptic angulation on postoperative intraocular lens position change.
PURPOSE: To assess the effect of optic edge design and optic-haptic angulation of open-loop intraocular lenses (IOLs) on postoperative axial movement and the final position of the optic by measuring the anterior chamber depth (ACD) during the first postoperative year using partial coherence interferometry (PCI). SETTING: Department of Ophthalmology, Vienna General Hospital, Institute of Medical Physics, University of Vienna, Vienna, Austria. METHODS: In study 1, a 3-piece silicone IOL with nonangulated modified C-loop haptics (MicroSil, Dr. Schmidt) was implanted in 78 eyes of 39 patients; patients were randomized to receive a round-edged optic IOL in 1 eye and a sharp-edged optic IOL in the other eye. The ACD was measured by PCI 1 day, 1 week, 3 months, and 1 year after surgery. In study 2, a foldable, 3-piece acrylic IOL with modified 10-degree angulated J-loop haptics (AcrySof MA60BM, Alcon) was implanted in 32 eyes of 32 patients. The ACD was measured by PCI 1 day, 1 week, and 3 months after surgery. RESULTS: In eyes with a nonangulated silicone IOL, there was a significant postoperative change in ACD with both sharp-edged and round-edged designs (P<.01). There was forward movement of both IOL designs in the first week, with no significant difference between the 2 models. From 1 week to 3 months, there was backward movement of IOLs of both designs, with the sharp-edged IOL moving a significantly greater amount (P<.001). From 3 months to 1 year, IOLs with both optic edge designs moved slightly backward. Sixty-six percent of angulated IOLs showed continuous but variable forward movement and 34%, backward movement. CONCLUSIONS: Optic edge design influenced postoperative axial optic movement and thus had an impact on the development of postoperative refraction (refractive shift, deviation from target refraction). The influence of optic-haptic angulation proved to be significantly greater and more variable than edge design.
Acrylic Resins↗
[Initial results of myopia correction with excimer laser].
The aim of the study was to demonstrate the efficacy of removal of the corneal tissue from the optical area by means of an excimer laser (Aesculap Meditec MEL-60) by the method of photo-ablation and by obtaining in that manner a change of refraction power of the optic system of a myopic eye. Fifty three corrections of myopia in 46 patients were performed from January 1991 to January 1992. Evaluated was the efficacy of this method i.e. the degree of the obtained correction in the relation to the expected one.
Follow-Up Studies↗
Photographic investigation of the human lens and cataract.
Slit-image photography of the anterior segment of the eye can provide important information about the dimensions and optical densities of the refractive structures. It is crucial that the slit-image photograph be in perfect focus and that the slit-beam cut in exactly the same section (along the optic axis) on each occasion. The equipment, principles and techniques which make such precision slit-imagephotography possible are discussed. Clinical and research applications, especially regarding the human lens and cataract, are summarized.
Adolescent↗
Analysis of signal propagation in optically coupled detectors for digital mammography: I. Phosphor screens.
The angular emission of light from turbid phosphor screens has been measured and modelled. As a first approximation, turbid phosphor screens have traditionally been modelled as Lambertian sources; however, Giakoumakis et al have previously shown that light emission from turbid phosphor screens is in fact more forward peaked. In this article, we extend the theory of Giakoumakis to include turbid phosphor screens that incorporate a transparent overcoat. The refractive index of the optical coupling medium in contact with the overcoat is shown to have a direct effect on both light output and angular emission from the screen. It was found that simple laws of refraction adequately describe this phenomenon. To model the angular emission of light from such phosphor screens, a term was included to describe the refraction from the overcoat into the adjacent coupling medium. The data obtained are required to calculate the propagation of the signal in a digital mammography detector in which a phosphor screen is optically coupled to a charge-coupled device (CCD) image array.
Biophysical Phenomena↗
Effective corneal refractive diameter as a function of the object tangent angle in visual space.
PURPOSE: To determine whether the currently accepted method of selecting a minimum optical zone diameter for laser refractive surgery that is equal to or slightly greater than the dark-adapted pupil diameter provides a sufficient diameter of corneal surface to focus light arising from objects in the paracentral and peripheral visual field. SETTING: Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA. METHODS: An optical model of the anterior segment was developed to calculate the effective corneal refractive diameter (ECRD), which is the diameter of the area of cornea that refracts all incident light rays arising from an object through the physical pupil (PP). This model incorporates the patient variables of central anterior chamber depth (ACD), central corneal curvature (K(c)), and the diameter of the apparent entrance pupil (EP). The model was expanded to incorporate distant objects off the line of sight (LOS), described by their angular displacement from the fixation object in visual space (the object tangent angle delta(ob)). Results were calculated for the 360 meridian degree visual field (ie, for all objects in visual space perceptually displaced from the fixation object by angle delta(ob)). The effect of the prolate nature of the cornea was also investigated. RESULTS: The ECRD expanded rapidly as a function of PP and delta(ob) but was minimally influenced by K(c). Beyond a critical object tangent angle delta(c), light rays striking the corneal vertex were not refracted through the PP, and the ECRD became an annular surface centered on the corneal vertex. The delta(c) was not a function of K, but increased as the PP increased and decreased as the ACD increased. The prolate nature of the cornea had little influence on the ECRD, even for very peripheral light rays. CONCLUSIONS: The ECRD expands rapidly when considering distant objects only slightly displaced from the LOS. A patient treated with an optical zone equal to or slightly greater than the dark-adapted pupil diameter may experience vision quality loss for paracentral and midperipheral objects even under conditions of ambient indoor lighting.
Cornea↗
Intraocular lens power formula based on vergence calculation and lens design.
PURPOSE: To present a new method of intraocular lens (IOL) power calculation that takes into account the power-dependent variation in lens design. SETTING: Department of Ophthalmology, Aalborg, Denmark. METHODS: Information on exact IOL design is derived from the manufacturers' cutting cards. These data were built into an IOL calculation formula based on exact ray tracing and vergence calculation. All algorithms are demonstrated. RESULTS: The method is optically correct as all refractive surfaces are characterized with respect to both position and refractive power. In simulation models, the Naeser and the Holladay formulas performed similarly, while the SRK/T formula predicted higher postoperative refractions for low-power IOLs. CONCLUSION: It is possible to incorporate the exact IOL design into an IOL power calculation formula. Theoretically, the Naeser formula should increase the accuracy of IOL power calculation; however, this has yet to be proved from empirical data. The formula provides an advantage in analysis of postoperative pseudophakia for experimental/scientific purposes because all intraocular distances and powers may be measured or calculated.
Algorithms↗
Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography.
PURPOSE: To measure total corneal power using optical coherence tomography (OCT). SETTING: Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. METHODS: Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. RESULTS: The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001). CONCLUSIONS: Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.
Anterior Eye Segment↗
Second mode transition in microstructured optical fibers: determination of the critical geometrical parameter and study of the matrix refractive index and effects of cladding size.
We carried out a numerical study of the second mode transition in finite-sized, microstructured optical fibers (MOFs) for several values of the matrix refractive index. We determined a unique critical geometrical parameter for the second mode cutoff that is valid for all the matrix refractive indices studied. Finite size effects and extrapolated results for infinite structures are described. Using scaling laws, we provide a generalized phase diagram for solid-core MOFs that is valid for all refractive indices, including those of the promising chalcogenide MOFs.
Journal Article↗
Optic disc size in a population based study in northern China: the Beijing Eye Study.
AIM: To determine the optic disc size in the adult Chinese population in an urban and a rural region of Beijing. METHODS: The population based, cross sectional cohort study included 4439 subjects out of 5324 subjects invited to participate (response rate 83.4%). It was divided into a rural part (1973 (44.4%) subjects) and an urban part (2466 (55.6%) subjects). Mean age was 56.2 (SD 10.6) years (range 40-101 years). Colour optic disc photographs were morphometrically examined. Main outcome measure was optic disc area. RESULTS: Optic disc photographs were available for 4027 (90.7%) subjects. Mean optic disc area measured 2.65 (0.57) mm2 (range 1.03 mm2-7.75 mm2). Optic disc area was significantly (p<0.001) correlated with myopic refractive error, with a steep decrease in optic disc area from high myopia to the mid-range of refractive error, a slightly horizontal course in the refractive error range between -8 dioptres and +4 dioptres, and a further decrease in optic disc area towards higher hyperopia. Optic disc area was not related to age (p = 0.14) or sex (p = 0.93) (optic disc area, males: 2.65 (0.56) mm2 versus females: 2.65 (0.57) mm2). "Microdiscs" may be defined as smaller than 1.51 mm2, and "macrodiscs" as larger than 3.79 mm2. CONCLUSIONS: Compared with data of preceding studies, mean optic disc size is larger in Chinese people than in white people. In Chinese people highly hyperopic eyes have significantly smaller optic discs, and highly myopic eyes have significantly larger optic discs than emmetropic eyes.
Adult↗
FEATURES OF A CLINICALLY NORMAL OPTIC DISC.
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Aging↗
Substrate temperature effect on the refractive index and a two-step film method to detect small inhomogeneities in optical films.
Nb2O5 films were deposited by a reactive magnetron sputtering technique. The average refractive index was found to increase with the rise of substrate temperature. Modulated interference transmittance spectra were observed in the two-step films, which were prepared by stopping the deposition process in the middle of the designed sputtering time, and then, after a full cooling down to room temperature, starting the same deposition process again to complete the whole preparation of the films. A linearly graded-index model was used to explain the interference behavior. It was proved that the two-step film method was sensitive to the small inhomogeneity in the films. We also suggest that the inhomogeneity of sputtered films can be minimized by controlling the substrate temperature at a constant value.
Journal Article↗
Application of the Jones calculus for a modulated double-refracted light beam propagating in a homogeneous and nondepolarizing electro-optic uniaxial crystal.
The Jones matrix calculus is applied to an electro-optic crystal with uniaxial symmetry when the light beam is incident nearly normally on the crystal face. The approach allows one to treat refracted waves and rays that diverge in the crystal and are modulated by an external low-frequency field. The effect of partial interference of overlapping refracted beams is allowed for and calculated for the case of uniform intensity of the beam over its cross section. The method is employed to analyze optical systems containing an imprecisely cut and aligned electro-optic crystal plate.
Journal Article↗
Complete ocular component analysis by vergence contribution.
Traditionally, the components of ocular refraction are analyzed in surface powers and their linear separations in the eye. More recently it has been suggested that the refractive effects of these components are better studied in terms of their vergence contributions. This study presents an improved method of vergence analysis which takes into account the refractive status of the eye. Results show a significant interaction of all components and indicate that their relative contributions are dependent on refractive status.
Cornea↗