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PubMed2005

Zener tunneling of light waves in an optical superlattice.

We report on the observation of Zener tunneling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice made of porous silicon. The structure was designed to have two photonic minibands, spaced by a narrow frequency gap. A gradient in the refractive index was introduced to create two optical Wannier-Stark ladders and, at a critical value of the optical gradient, tunneling between energy bands was observed in the form of an enhanced transmission peak and a characteristic time dependence of the transmission.

Journal Article↗

PubMed2004

New formula for calculating intraocular lens power after laser in situ keratomileusis.

PURPOSE: To assess the validity and accuracy of a proposed formula for keratometry (K) readings after laser in situ keratomileusis (LASIK). SETTING: The Eye Center and the Eye Foundation for Research, Riyadh, Saudi Arabia. METHOD: This studied comprised 34 eyes that had LASIK surgery. Refraction and an automated K-reading (auto-K) were performed preoperatively. Refraction, auto-K, and K-reading assessment by the clinical history method and the proposed formula were performed 4 to 12 weeks postoperatively. The proposed formula is K(postop) = K(preop) - [(N(c) - 1) x (R(a-postop) - R(a-preop))/(R(a-postop) x R(a-preop))], where K(postop) is the K-reading after LASIK, K(preop) is the K-reading before LASIK, N(c) is the index of refraction of the cornea (1.376), R(a-postop) is the radius of curvature of the anterior corneal surface after LASIK, and R(a-preop) is the radius of curvature of the anterior corneal surface before LASIK. RESULTS: Twenty patients (10 men, 10 women) were included in the study. The mean age of the patients was 30.58 years +/- 17.68 (SD) (range 18 to 44 years). Preoperatively, the mean spherical equivalent (SE) was -4.99 +/- 2.82 diopters (D) (range -1.12 to -15.00 D), the mean R(a) was 7.76 +/- 0.32 mm (range 7.33 to 8.50 mm), and the mean auto-K reading was 43.45 +/- 1.73 D (range 39.62 to 46.00 D). Postoperatively, the mean SE was +0.02 +/- 0.63 D (range -2.75 to +1.00 D), the mean R(a) was 8.63 +/- 0.53 mm (range 7.80 to 9.92 mm), and the mean K-reading assessed by auto-K, clinical history method, and the proposed formula was 39.17 +/- 2.35 D (range 34.00 to 43.25 D), 38.79 +/- 2.52 D (range 33.1 to 42.78 D), and 38.69 +/- 2.51 D (range 33.1 to 43.0 D), respectively. The results obtained by the proposed formula were similar to those obtained by the clinical history method (P =.098). Auto-K readings significantly overestimated the K-values (P<.0001) when compared to the proposed formula and clinical history method. CONCLUSION: The proposed formula was simple, objective, not dependent on refraction, and as accurate as the clinical history method in determining K-readings after LASIK.

Adolescent↗

PubMed1997

Incision sizes for foldable intraocular lenses.

OBJECTIVE: The purpose of the study is to determine the minimal incision sizes required for implantation of a variety of different foldable intraocular lenses (IOLs) and to evaluate the effect of incision size on tissue damage. DESIGN: Randomized experimental study. PARTICIPANTS: Sixty-nine fresh human cadaveric eyes: 15 (pilot study), 48 (main study), and 6 (scanning electron microscopy study). INTERVENTION: Implantation of foldable IOLs into cadaveric eyes. MAIN OUTCOME MEASURES: In 48 fresh human cadaveric eyes, limbal corneal tunnel incisions were made, and in a randomized fashion, 8 different foldable IOLs of 20.5 diopters (D) were inserted: 4 silicone (SI-30NB, Allergan Medical Optics, Irvine, CA; C10UB, Chiron Ophthalmics, Inc, Irvine, CA; LI41U, IOLAB, Chiron Ophthalmics, Inc, Irvine, CA; AA-4203, Staar Surgical Company, Monrovia, CA); two soft acrylic (MA60BM and MA30BA, both Alcon Laboratories, Inc, Ft. Worth, TX); and two hydrogel models (SH30BC, Alcon; H60M, Storz Ophthalmics, Inc, St. Louis, MO). For each IOL model, six insertions were performed with a recommended implantation device. Using calipers, the authors measured internal and external incision sizes before and after insertion. Scanning electron microscopy was performed on selected incisions in six additional human cadaveric eyes. RESULTS: Incision sizes after insertion ranged from 3.2 to 3.8 mm. The smallest incisions permitting IOL insertion were associated with the injectors. However, these incisions enlarged after insertion by approximately 11% and then were similar to the incision sizes after forceps insertion of the high-refractive-index silicone, the 5.5-mm optic acrylic, and the one-piece hydrogel IOL. The largest incisions were associated with the 6-mm acrylic IOL and the three-piece silicone IOL with a lower refractive index. The scanning electron microscopy showed tearing of corneal tissue after implantation through the smallest incision; this was more pronounced with injectors than with forceps. CONCLUSIONS: Corneal tunnel incisions enlarged up to 11% after insertion of foldable IOLs through the smallest possible incision. With current technology, the smallest postinsertion incision size of a 20.5-D foldable IOL is 3.2 mm.

Cadaver↗

PubMed2003

Optical analogue of electronic Bloch oscillations.

We report on the observation of Bloch oscillations in light transport through periodic dielectric systems. By introducing a linear refractive index gradient along the propagation direction the optical equivalent of a Wannier-Stark ladder was obtained. Bloch oscillations were observed as time-resolved oscillations in transmission, in direct analogy to electronic Bloch oscillations in conducting crystals where the Wannier-Stark ladder is obtained via an external electric field. The observed oscillatory behavior is in excellent agreement with transfer matrix calculations.

Journal Article↗

PubMed1989

Corneal topography and fluctuating visual acuity after radial keratotomy.

A high-resolution photokeratoscope using computer graphics to model corneal topography was used on patients who had undergone radial keratotomy. After radial keratotomy, central optical zones are created that can be characterized as round, oval or band-like, or dumbbell-shaped or split. The dumbbell form of optical zone was associated with larger amounts of refractive and keratometric astigmatism than the round or band-like zones. The authors correlated the shape of the optical zone with the presence or absence of diurnal variation (fluctuation) in visual acuity. Of the 26 eyes studied, 11 experienced fluctuation and 15 did not. Of those 11 eyes with fluctuating visual acuity, 10 (91%) had dumbbell-shaped or split optical zones and 1 (9%) had a round optical zone. Of the 15 eyes without fluctuation, 12 (80%) had round optical zones and 3 (20%) had band-like zones. The presence of a split or dumbbell-shaped optical zone after radial keratotomy indicates that the patient is likely to experience diurnal fluctuation of visual acuity.

Astigmatism↗

PubMed2000

Thickness of the peripapillary retina in healthy subjects with different degrees of ametropia.

PURPOSE: To evaluate the thickness of the peripapillary retina in healthy subjects with different degrees of ametropia. DESIGN: Cross-sectional study. PARTICIPANTS: Forty-three healthy normal subjects, ages 15 to 64 years (38+/-15 years) were recruited. METHODS: The method of retinal thickness mapping was applied to generate serial optical section images in the temporal peripapillary retinal area. The digitized images were analyzed to detect the separation between the vitreoretinal and chorioretinal interfaces. The separation was converted to absolute thickness by taking into account the magnification of the optical system and the optical dimensions of the eyes. MAIN OUTCOME MEASURES: The refractive error of the eyes ranged from +8.25 to -12.75 diopters (spherical equivalent). The axial length of the eyes ranged from 19.3 to 27.35 mm. The retinal thickness in a 2- x 2-mm temporal peripapillary retinal area was determined. RESULTS: The separation between the retinal interfaces decreased with increasing axial length (P<0.001). The thickness in the temporal peripapillary retinal area was not significantly correlated with the axial length of the eye (P = 0.3). A lack of statistically significant difference between the average thickness in the hyperopic, emmetropic, and myopic eyes (P> or =0.06) was found. CONCLUSIONS: The average thickness in the temporal peripapillary retinal area was not influenced by the axial length of the eye, when the magnification of the optical system and the optical dimension of the eyes were taken into consideration.

Adolescent↗

PubMed1993

Fluorescence decay of DPH in lipid membranes: influence of the external refractive index.

The radiative decay rate of a fluorescent probe in an optically thin layer is known to depend on the orientation of the probe and on the refractive indices inside and outside the layer (W. Lukosz, Phys. Rev. B 22 (1980) 3030). Fluorescent probes in phospholipid bilayer membranes approximate such a system. The natural lifetime is expected to vary with the refractive index of the medium surrounding the bilayer. The lifetime variation with the refractive index depends on the orientation of the fluorescent probe. This can be used to retrieve the second-rank orientational order parameter, . The fluorescence decay of all-trans 1,6-diphenyl-1,3,5-hexatriene in L-alpha-dipalmitoyl-phosphatidylcholine large unilamellar vesicles (LUVs) was measured at a temperature well below that of the phase transition. The refractive index of the medium was varied by addition of glycerol or sucrose. The observed change of decay time with the refractive index followed the theoretical prediction. The value of the order parameter, , recovered is significantly lower than that obtained from fluorescence polarization data. Possible reasons for this disagreement are discussed.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1995

Oblique central refraction in spherocylindrical corrections with both faceform and pantoscopic tilt.

Thin lens equations, accurate to third order, are presented for the effective spherocylindrical parameters for oblique central refraction (OCR) through spherocylindrical lenses with both faceform and pantoscopic tilt. The equations can also be used to find the parameters of a lens that compensates for the tilts. Accuracy of the equations was checked by exact ray trace results.

Eyeglasses↗

PubMed2003

Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis.

PURPOSE: To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method. DESIGN: Interventional case series. METHODS: Retrospective analysis of consecutive cases from clinical practice. Two hundred randomly selected eyes from 200 patients were evaluated before and after LASIK surgery. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (Kpre) and post-LASIK K readings (Kpost). We then calculated for each case the pre- and post-LASIK refraction at the corneal plane and the amount of correction obtained by the refractive surgery (CRc). The cases were divided into two groups. Group I was used to derive the two formulas. The K values were calculated using the history-derived method (Kc.hd) in which Kc.hd = Kpre - CRc. Kc.hd was compared with Kpost. The average difference was 0.23 diopters for every diopter of myopia corrected. This value was used to calculate the corneal power using the refraction-derived method (Kc.rd) where Kc.rd = Kpost -0.23CRc. A regression equation was used to develop a clinically derived method (Kc.cd) where Kc.cd = 1.14Kpost -6.8. The values obtained with the two methods were compared with the Kc.hd values in group II to validate the results. RESULTS: Both Kc.rd and Kc.cd values correlated highly with Kc.hd when plotted on a scattergram (P <.001), and there was no statistically significant difference between the mean keratometric values (P >.5). CONCLUSIONS: The corneal power measurements for intraocular lens power calculations after LASIK need to be corrected to avoid hypermetropia after cataract surgery by either the history-derived method, the refraction-derived method, or the clinically derived method.

Adult↗

PubMed1991

Single beam optical trapping integrated in a confocal microscope for biological applications.

Confocal microscopy is very useful in biology because of its three dimensional imaging capacities and has proven to be an excellent tool to study the 3D organization of, for instance, cell structures. This property of confocal microscopy makes it also very suitable for observation during guidance of the three dimensional manipulation of single cells or cell elements. Therefore we decided to integrate a confocal microscope and a single beam optical manipulator into a single instrument. The advantage of optical manipulation over mechanical techniques is that it is non-invasive and therefore may be applied on living (micro-) organisms and cells. The creation of an effective single beam optical trap requires the use of a high numerical aperture (N.A.) objective to focus the laser beam. In this paper we briefly discuss the vertical or axial force exerted on a sphere in a single beam trap. The axial force on a sphere placed on the optical axis, caused by reflection and refraction, is calculated applying a electromagnetic vector diffraction theory to determine the field distribution in the focal region. One of the results is that the particle also experiences a vertical trapping force towards the focusing lens when it is in the strongly convergent part of the field in addition to the known negative signed trapping force in the divergent part of the field. Further we describe an instrumental approach to realize optical trapping in which the optical trap position is controlled by moving the focusing objective only.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗

PubMed1997

Clinical refraction in three-dimensional dioptric space revisited.

The traditional clinical designation of spherocylindrical power unambiguously specifies the refractive properties of a thin lens or refractive surface. This representation of dioptric power is not, however, optimum in mathematical terms, as is apparent when, for example, two spherocylindrical lens powers are added. Alternative systems have been described which are not subject to this same type of difficulty, and the essential feature of these other systems is that spherocylindrical power is defined in terms of a three-dimensional dioptric space in which the axes are usually orthogonal. The advantages of this orthogonality can be exploited in the practice of clinical refraction, provided lens powers in these three dimensions can be physically implemented. Systems using these characteristics have been introduced in the past, but the clinical community has not adopted them on a widespread basis. However, systems which take advantages of these features do have unique advantages relative to traditional clinical refraction procedures. These characteristics, and refractive procedures which exploit their advantages, are described.

Humans↗

PubMed2004

Visual manifestations of visible and buried optic disc drusen.

BACKGROUND: It has been reported that visible optic disc drusen are associated with decreased visual acuity and loss of peripheral visual field. Patients with buried optic disc drusen have not been as well characterized. METHODS: An observational, retrospective review was made of 92 eyes with funduscopic or ultrasonographic evidence of optic disc drusen. Demographics, presenting symptoms, visual acuity, refractive error, intraocular pressure, presence of an afferent pupillary defect, cup-to-disc ratio, appearance of the optic nerve, and visual field were recorded. RESULTS: Fifty-one (55%) of the eyes were symptomatic; among them, 63% had symptoms of visual acuity loss, and 49% had symptoms of visual field loss. Seventy-nine (86%) of the optic discs appeared abnormal on ophthalmoscopy, but only 42% of these had visible drusen. Forty-five (49%) of the eyes had a visual field defect, and 73% of these were nerve fiber bundle in type. Overall, 73% of optic discs with visible drusen had abnormal visual fields compared with only 36% of optic discs with buried drusen (P < 0.05). CONCLUSIONS: Among 92 eyes with optic disc drusen, only slightly more than 50% produced visual symptoms. Fewer than 50% of drusen were visible ophthalmoscopically. Buried optic disc drusen identified by ultrasound had a significantly lower frequency of associated visual field defects than did visible optic disc drusen.

Adolescent↗

PubMed2005

Optic disc topographic parameters measured in the normal cynomolgus monkey by confocal scanning laser tomography.

AIM: To study optic disc topographic parameters in normal cynomolgus monkeys by Heidelberg retina tomograph (HRT). METHODS: 12 optic disc topographic parameters were investigated in 36 normal eyes in 18 male monkeys. Mean (SD) and interocular differences were obtained for each parameter from three independent measurements made during a 1 week period. Correlations among the topographic parameters were analysed, too. RESULTS: No significant differences between right and left eyes were detected for any topographic parameters. Disc area, rim area, and height variation contour showed smaller right-left differences than other parameters. The coefficients of variation for rim area, height variation contour, rim volume, mean cup depth, maximum cup depth, mean retinal nerve fibre layer (RNFL) thickness, and RNFL cross section area were less than 10% (for rim area, less than 5%). Rim area and height variation contour showed relatively weak interrelations and neither showed a correlation with disc area. CONCLUSION: For evaluating time related changes in the optic disc by HRT in monkeys, rim area and height variation contour might be useful parameters because coefficients of variation and right-left differences were lower than for other parameters and because these parameters showed weak interrelations and no correlation with disc area.

Animals↗

PubMedDate not supplied

Use of the aphakic refraction in intraocular lens (IOL) power calculations for secondary IOLs in pediatric patients.

PURPOSE: Calculating the power of a secondary intraocular lens (IOL) requires axial lengths and keratometry measurements. In children, these measurements often need to be done under either sedation at the time of surgery or additional sedation. We devised a method of calculating IOL power using only the aphakic refraction and a standard keratometry value. PATIENTS AND METHODS: This retrospective study included children undergoing a secondary IOL procedure who had IOL calculations based on either a measured axial length and a measured keratometry value (with sedation if needed) or a calculated axial length and a standard keratometry value. Comparison of predicted postoperative refraction with actual postoperative refraction was used to evaluate each method. RESULTS: The mean difference between predicted and actual postoperative refractions was 2.34 D for the group with actual measurements and 2.50 D for the group with the calculated value method. CONCLUSION: The use of the aphakic refraction to calculate axial length and a standard keratometry value provides an alternative for secondary IOL calculations in pediatric patients when ultrasonic or nonsedated ultrasonic axial length measurements are not an option.

Adolescent↗

PubMedDate not supplied

Model for deriving the optical performance of the myopic eye corrected with an intracorneal ring.

BACKGROUND: The intracorneal ring has been advanced as a mechanical device for the correction of myopia. The device may reduce refractive error, but the effect on the overall optical performance of the eye has been neglected. This paper addresses this issue by presenting mathematical models that will predict the effect of the intracorneal ring on refractive error, corneal asphericity, and the spherical aberration of the eye. MATERIALS AND METHODS: Algorithms are derived for predicting the corneal apical radius, asphericity, and the change in myopia after inserting intracorneal rings of any thickness and diameter. RESULTS: The model predicts that the magnitude of myopia reduction is a function of both ring thickness and diameter. The large diameter (9 to 10 mm), thin (0.1 to 0.2 mm) intracorneal ring is less likely to adversely affect corneal asphericity and therefore not significantly enhance the spherical aberration of the eye. CONCLUSIONS: An intracorneal ring cannot correct more than 4.00 diopters (D) of myopia without significantly increasing ocular spherical aberration, which, in turn, will compromise the final visual outcome.

Algorithms↗

PubMed2005

Inducing form-deprivation myopia in fish.

PURPOSE: To induce form deprivation myopia in fish and investigate the role of the lens in the development of refractive error. METHODS: Tilapia (Oreochromis niloticus), approximately 4 months old and from 26 to 63 g, were divided into three groups. Translucent goggles were directly sutured over the right eye for 4 weeks to induce form-deprivation myopia; the left eye served as an untreated contralateral control. The refractive state was measured by retinoscopy. Ocular dimensions were determined from frozen sections and with ultrasound biomicroscopy, and a scanning laser system was used to determine the optical quality of excised lenses. RESULTS: After 4 weeks of form-deprivation treatment, all the deprived fish eyes showed development of significant amounts of myopia ranging from -3.75 to -26.25 D, with the average amounting to -10.27 +/- 1.14 D. Eye dimension measurements show that the vitreous and anterior chambers of the treated eye are significantly longer axially than those of the contralateral eyes. No significant change in optical quality was found between lenses of the myopic and nonmyopic eyes. The fish recovered completely from the myopia 5 days after the goggle was removed. CONCLUSIONS: Although lower vertebrates are capable of lifelong growth, their eyes are susceptible to form-deprivation myopia. Thus, the visual environment is an important factor controlling ocular development in lower vertebrates, as well as in higher ones, and eye development is not strictly genetically determined. This study also indicates that lens growth and optical development are largely independent from the refractive development of the whole eye.

Animals↗

PubMed2003

Aberrations of the human eye in visible and near infrared illumination.

PURPOSE: In most current aberrometers, near infrared light is used to measure ocular aberrations, whereas in some applications, optical aberration data in the visible range are required. We compared optical aberration measurements using infrared (787 nm) and visible light (543 nm) in a heterogeneous group of subjects to assess whether aberrations are similar in both wavelengths and to estimate experimentally the ocular chromatic focus shift. METHODS: Ocular aberrations were measured in near infrared and visible light using two different laboratory-developed systems: laser ray tracing (LRT) and Shack-Hartmann. Measurements were conducted on 36 eyes (25 and 11 eyes, respectively), within a wide range of ages (20 to 71 years), refractive errors (-6.00 to +16.50), and optical quality (root mean square wavefront error, excluding defocus, from 0.40 to 9.89 microm). In both systems, wave aberrations were computed from the ray aberrations by modal fitting to a Zernike polynomial base (up to seventh order in laser ray tracing and sixth order in Shack-Hartmann). We compared the Zernike coefficients and the root mean square wavefront error corresponding to different terms between infrared and green illumination. RESULTS: A Student's t-test performed on the Zernike coefficients indicates that defocus was significantly different in all of the subjects but one. Average focus shift found between 787 nm and 543 nm was 0.72 D. A very small percentage of the remaining coefficients was found to be significantly different: 4.7% of the 825 coefficients (25 eyes with 33 terms) for laser ray tracing and 18.2% of the 275 coefficients (11 eyes with 25 terms) for Shack-Hartmann. Astigmatism was statistically different in 8.3% of the eyes, root mean square wavefront error for third-order aberrations in 16.6%, and spherical aberration (Z4(0)) in 11.1%. CONCLUSIONS: Aerial images captured using infrared and green light showed noticeable differences. Apart from defocus, this did not affect centroid computations because within the variability of the techniques, estimates of aberrations with infrared were equivalent to those measured with green. In normal eyes, the Longitudinal Chromatic Aberration of the Indiana Chromatic Eye Model can predict the defocus term changes measured experimentally, although the intersubject variability could not be neglected. The largest deviations from the prediction were found on an aphakic eye and on the oldest subject.

Adult↗

PubMed1991

[Eliminating reflection of intraocular lenses].

Reflections on the refractive surfaces of intraocular lenses can be a nuisance when performing retinoscopy or refractometry. Some patients with IOLs also see reflections which disturb their vision. This article examines the conditions for coating IOLs. It also explains why conventional coating methods cannot be used for IOLs.

Humans↗