Optical Path Difference Scanning Research Articles

Comparison of objective visual quality between SMILE and FS-LASIK in moderate-to-high myopia.

This study aims to compare the changes in the corneal wavefront aberrations and the objective visual quality resulting from two types of eye surgery-small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK)-in patients with moderate-to-high myopia. A prospective analysis was performed on 98 eyes of 51 patients who underwent SMILE. Additionally, 88 eyes of 45 patients who underwent FS-LASIK were analyzed. All patients underwent ocular examination preoperatively and at 1 day, 1 week, 1 month, and 3 months postoperatively. Corneal aberrations and objective visual quality were measured using the Optical Quality Analysis System II (OQAS II) and Optical Path Difference Scan III (OPD-Scan III). At postoperative 1 day and 1 week, there was a statistically significant difference in uncorrected distance visual acuity (UDVA) between SMILE and FS-LASIK (P < 0.05). Postoperative spherical (S), cylinder (C) and spherical equivalent refraction (SE) were similar between the two groups (P > 0.05). In both groups, the absolute magnitude of total higher-aberrations (tHOA), piston, vertical tilt, vertical coma, and spherical aberration (SA) increased after surgery compared to preoperative values (P < 0.05). There was no significant difference in Δhorizontal tHOA, Δhorizontal tilt, Δhorizontal coma, and Δhorizontal trefoil between the two groups (P > 0.05), and the FS-LASIK had higher Δvertical trefoil and ΔSA (P < 0.05) but lower Δpiston, Δvertical tilt, and Δvertical coma than the SMILE group (P < 0.05). There was a rise in objective scattering index (OSI) and a decline in both modulation transfer function (MTF) cutoff and Strehl ratio (SR) after surgery compared to preoperative values in both groups (P < 0.05). There was a statistically significant difference in the OSI at 1 day and 3 months between the two groups (P < 0.05). Postoperative MTF cutoff and SR were similar between the two groups (P > 0.05). Postoperative OSI was positively correlated with corneal tHOA (0.261 ≤ R ≤ 0.483, P < 0.05) and was negatively correlated with vertical tilt and vertical coma (-0.315 ≤ R ≤ -0.209, P < 0.05) in both groups. While both SMILE and FS-LASIK can effectively correct moderate-to-high myopia, there is an increase in corneal aberrations and a postoperative delay in objective visual quality. The cornea may require a longer recovery period in the SMILE. OPD-Scan III combined with OQAS II is a useful supplementary inspection for assessing the optical quality following refractive surgery.

Whole-field principal angle and optical path retardation measurements using white light OPD scanning polariscope with first moment algorithm

White-light Optical Path Difference (OPD) scanning polariscope is currently an accepted technique for analyzing photoelastic specimens. While it has proven effective in overcoming limitations associated with phase-stepping and fringe scanning techniques, it is constrained when evaluating optical path retardation smaller than the light source coherence length. To address this limitation, a white-light OPD scanning polariscope, equipped with a first moment algorithm, has been introduced for the comprehensive examination of whole-field principal angle and optical path retardation. This article demonstrates the design and operational principles of the proposed polariscope, details an experimental setup for its deployment, and presents findings from practical applications of this setup. The results confirm the effectiveness and practicality of the proposed polariscope. Furthermore, these results highlight its precision, with standard deviations of 0.176° for principal angle and 6.4 nm for optical path retardation measurements.

White light OPD scanning polariscope for whole-field optical path retardation and principal angle examinations

Fringe-scanning (or TFP/RGB) and phase-stepping are now the most acceptable techniques for examining stressed photoelastic samples; however the former requires a calibration table, which is sensitive to the geometrical and optical parameters of the corresponding polariscope, and concentrates only on phase retardation measurements; the latter suffer from three potential issues: 1) the principal angle is indeterminate when the corresponding phase retardation is close or equal to an integer multiple of π, 2) the obtained phase retardation and principal angle are wrapped, and 3) the phase retardation and principal angle may couple together in certain situations. Free from the disadvantages of the fringe-scanning and phase-stepping polariscopes, a white light OPD (optical path difference) scanning polariscope composed of a white light laser, an OPD modulator, and a dark-field circular polariscope was proposed for whole-field optical path retardation (proportional to the phase retardation) and principal angle determinations. This paper introduces the theory of the polariscope, demonstrates the experimental setup for implementing the polariscope, and presents the results from the uses of the setup. The results manifest a standard deviation of 8 nm when repeatedly measuring an optical path retardation and 1.21ο when repeatedly measuring a principal angle; moreover, the results confirm the polariscope's validity and feasibility, but showing a limitation while adopting the polariscope to evaluate fringe orders < 3.

Optical path squeezing interferometry: boosting the resolution for Fourier transform imaging spectrometers.

We present an optical path squeezing interferometer dedicated to high-spectral-resolution Fourier transform imaging spectrometry. By incorporating a pair of gratings into a Sagnac interferometer, the short-wavelength light has a larger optical path difference (OPD) than the long-wavelength light. Interference fringes with different OPDs are squeezed into the same sampling window in data acquisition. As a result, the spectral resolution is greatly enhanced without large OPD scanning. The experimental results demonstrate that the proposed method is a promising technology for high-resolution spectral imaging.

Acute effect of cigarette smoking on pupil size and ocular aberrations: a pre- and postsmoking study.

Aim. To evaluate the acute effects of cigarette smoking on photopic and mesopic pupil sizes and wavefront aberrations. Methods. Cigarette smoker volunteers were recruited in the study. Photopic and mesopic pupil sizes and total ocular aberrations were measured before smoking and immediately after smoking. All volunteers were asked to smoke a single cigarette containing 1.0 mg nicotine. Pupil sizes and total ocular aberrations were assessed by optical path difference scanning system (OPD-Scan II ARK-10000, NIDEK). Only the right eyes were considered for statistical analysis. The changes of pupil size and total ocular aberrations after smoking were tested for significance by Wilcoxon signed ranks test. Results. Mean photopic pupil size decreased from 3.52 ± 0.73 mm to 3.29 ± 0.58 mm (P = 0.001) after smoking. Mean mesopic pupil size was also decreased from 6.42 ± 0.75 mm to 6.14 ± 0.75 mm after smoking (P = 0.001). There was a decrease in all the measured components of aberrations (total wavefront aberration, higher-order aberration, total coma, total trefoil, total tetrafoil, total spherical aberration and total higher-order aberration) after smoking; however the differences were insignificant for all (P > 0.05). Conclusion. Our results indicate that pupil constricts after smoking. On the other hand, smoking does not alter ocular aberrations.

The Change in Ocular Higher-Order Aberrations After Pterygium Excision With Conjunctival Autograft

To investigate the effects of pterygium excision with conjunctival autograft plus corneal polishing on ocular higher-order aberrations (HOAs) and to monitor the changes throughout the first postoperative year. A total of 18 eyes of 16 patients with primary pterygium who had an indication for surgical excision were included in the study. All patients underwent a detailed ophthalmologic examination preoperatively and at the third and twelfth postoperative months. Pterygium excision with conjunctival autograft plus corneal polishing was performed by the same surgeon (K.G.). Corneal topography and ocular HOAs were measured by NIDEK optical path difference scanning system ARK-10000 refractive power/corneal analyzer. Root mean square values of total aberrations and total HOAs were found to be significantly decreased both at 3 and 12 months after an uneventful pterygium surgery. Although solely root mean square values of coma and trefoil revealed a statistically significant reduction in the third postoperative month (P=0.043 and P=0.018, respectively), after 1 year, all parameters (total coma: P<0.001; total trefoil: P=0.004; total quatrefoil: P=0.010; and total high astigmatism: P=0.016) except for total spherical aberration (P=0.236) were found to be lower when compared with the preoperative values. Uneventful pterygium excision with conjunctival autograft plus corneal polishing can significantly reduce most of the pterygium-induced HOAs. Moreover, the reduced postoperative aberrations continue to change beyond the early postoperative period even into the 1-year postoperative period. Therefore, any refractive procedures should be postponed at least 1 year after the pterygium surgery until corneal stability occurs.

Evaluation of Total and Corneal Wavefront High Order Aberrations for the Detection of Forme Fruste Keratoconus

To investigate the application of anterior corneal and ocular aberrations in detecting mildly ectatic corneas. This study retrospectively reviewed the data of 220 eyes separated into three groups by the NIDEK Corneal Navigator System automated corneal classification software: normal (N) (n = 123); forme fruste keratoconus (N topography with contralateral KC) (n = 34); and KC (n = 63). Anterior corneal and ocular aberrations were obtained with the optical path difference scan and compared using a Kruskal-Wallis test. Evaluation of these data to discriminate between the three groups was assessed using a Receiver-Operating Characteristic curve analysis. Corneal and ocular tilt, vertical coma, and trefoil were significantly different in the FFKC as compared with the N group. The discriminant functions between the FFKC and the N group, and between the KC and the N group reached an area under the receiver operating characteristic curve of 0.98 and 0.96, respectively. Indices generated from corneal and ocular wavefront can identify very mild forms of ectasia that may be undetected by Placido-based neural network programs.

Accurate measurement of interferometer group delay using field-compensated scanning white light interferometer

Interferometers are key elements in radial velocity (RV) experiments in astronomy observations, and accurate calibration of the group delay of an interferometer is required for high precision measurements. A novel field-compensated white light scanning Michelson interferometer is introduced as an interferometer calibration tool. The optical path difference (OPD) scanning was achieved by translating a compensation prism, such that even if the light source were in low spatial coherence, the interference stays spatially phase coherent over a large interferometer scanning range. In the wavelength region of 500-560 nm, a multimode fiber-coupled LED was used as the light source, and high optical efficiency was essential in elevating the signal-to-noise ratio of the interferogram signal. The achromatic OPD scanning required a one-time calibration, and two methods using dual-laser wavelength references and an iodine absorption spectrum reference were employed and cross-verified. In an experiment measuring the group delay of a fixed Michelson interferometer, Fourier analysis was employed to process the interferogram data. The group delay was determined at an accuracy of 1×10(-5), and the phase angle precision was typically 2.5×10(-6) over the wide wavelength region.

Lokalisation und altersabhängige Verteilung von Aberrationen des Auges

The aim of this study was to analyze the distribution of total, corneal, and mainly internal human third- to sixth-order aberrations (HOA) and investigate possible correlations between wavefront errors and patient age. For this study 140 eyes of 140 subjects (age range 17-77 years) were measured three times each using Nidek Optical Path Difference scan. Mean value and standard deviation for each Zernike polynomial was computed for a 5-mm pupil diameter, and six different age groups were formed. There was wide individual variation in the internal wavefront aberrations. The mean root-mean-square value for HOA was 0.4563 microm+/-0.296 microm. No positive correlation was found between Zernike polynomials and increasing age. In total spherical aberration however a significant difference was found between the 26-35-year and > or =56-year age groups (p=0.0002), as well as to internal and total coma (Z8) (p=0.0487 and p=0.0117, respectively). The internal, corneal, and total wavefront aberrations varied widely among patients. Some significant differences were found between the age groups. No correlation between age and the internal, corneal, or total aberrations was found.

Evaluation of Internal Refraction with the Optical Path Difference Scan

To evaluate internal refraction and its relation to other optical properties of the eye across a large range of refractive errors, which can provide useful information for the assessment and design of intraocular lenses and corneal ablation patterns. Cohort study. Three hundred ninety-four eyes of 197 healthy subjects with a mean age of 27+/-7 years (range, 18-42). All eyes underwent optical path difference scans to evaluate corneal topography, whole and internal ocular refraction (determined by the subtraction of corneal refraction from whole ocular refraction), and total and higher-order aberrations (HOAs) were assessed. After the conversion of any spherocylindrical refractive errors to vectorial data (sphere equivalent, blurring strength; cylinder, J(0) [power of Jackson cross cylinder at 90 degrees and 180 degrees] and J45 [power of Jackson cross cylinder at 45 degrees and 135 degrees]), the distribution of internal refraction among refraction groups (high myopes, low to moderate myopes, hyperopes, mixed astigmats, and emmetropes) and relationships between internal refraction, corneal refraction, and wavefront aberrations were analyzed. The compensation relation and its rate between corneal and internal astigmatism was assessed by the compensation factor (CF). Whole ocular power and astigmatism, corneal power and astigmatism, internal power and astigmatism, CF for astigmatic data, location of the highest internal refraction zone, and wavefront aberrations. The highest refraction zone was mostly (90%) located in the center in myopes, whereas it was located at the nasal side (71%) in hyperopes. There was a significant correlation between whole ocular and internal powers (P<0.01), but no correlation was observed between corneal and internal powers (P>0.05). Internal astigmatism was mostly against the rule. The mean CF J(0) was 0.63+/-3.78 and CF J(45) was 0.57+/-2.47. The magnitude of the internal astigmatism under the 3-mm zone was correlated with the magnitude of corneal astigmatism (P<0.05). The distribution of astigmatic CF differed among refraction groups. There were significant correlations between internal power and spherical aberration (P<0.05) and internal cylinder under the 5-mm zone and HOAs (P<0.001). There is a remarkable tilt in internal refraction in hyperopes. Although there is a tendency of undercompensation of the corneal astigmatism by internal astigmatism in the entire group of eyes, the compensation differs among refraction groups.

Higher Order Aberrations in Normal Eyes Measured With Three Different Aberrometers

To assess the repeatability of measurements of higher order aberrations using three different aberrometers and to compare higher order aberration measurements between optical path difference (OPD) scanning and the Hartmann-Shack method. Wavefront aberration data obtained using the NIDEK OPD-Scan, Bausch & Lomb Zywave wavefront aberrometer, and VISX CustomVue wavefront analyzer were compared. A total of 19 subjects were included in the study. The repeatability in each machine was assessed by calculating the difference between measurements and the mean of three consecutive measurements in the same eye. Subsequent analysis of the distribution of these differences yields the mean difference, the standard deviation of the differences, and the 95% confidence interval for repeated measurements, also termed the "repeatability coefficient". Repeatability errors in all three machines were found to be low, suggesting that all three machines are reliable in their repeated measurements. Significant differences were demonstrated between OPD scanning and Hartmann-Shack aberrometers. All three machines showed statistically significant differences in several higher order aberration parameters when compared to each other. The three different aberrometers provided repeatable measurements but statistical differences were noted in the measurement of higher order aberrations when comparing the machines. No instrument was superior over the other and all three were reliable.

Precision of Higher Order Aberration Repeatability With NIDEK OPD-Scan Retinoscopic Aberrometry

To evaluate repeatability of the total high order aberrations with a retinoscopic wavefront sensor. This prospective case series analyzed 12 eyes from 6 patients who underwent wavefront measurement using retinoscopic aberrometry with the NIDEK Optical Path Difference Scan (OPD-Scan). Four consecutive wavefront measurements of each eye were taken by two trained examiners for 5.8+/-0.8-mm and 8.2+/-0.6-mm pupils (P=.002, Wilcoxon test). Total high order aberrations out to the eighth order were assessed including third order coma, third order trefoil, fourth order quadrafoil, fourth order secondary astigmatism, and fourth order spherical aberrations. Differences between measurements of all of the variables were analyzed. A P value <.05 was considered statistically significant. Repeatability analysis of the root-mean-square of total higher order aberrations, coma, trefoil, quadrafoil, secondary astigmatism, and spherical aberrations for both dilated and nondilated pupils did not show a statistically significant difference among all repeated measurements, except for trefoil and secondary astigmatism (analysis of variance and the Friedman test). The repeatability of total higher order aberrations was 0.15 microm for nondilated pupils and 0.18 microm for dilated pupils. Except for trefoil measurements, all high order aberrations showed reproducibility >0.15 microm when Zernike coefficients were analyzed individually. The NIDEK OPD-Scan aberrometer measures total higher order astigmatism and most individual aberrations with acceptable repeatability. However, measurement of trefoil with this instrument is less repeatable.

Optical Factors in Increased Best Spectacle-corrected Visual Acuity After LASIK

To study the factors that correlate with improved best spectacle-corrected visual acuity (BSCVA) after LASIK. This was a nonrandomized, prospective clinical trial of 850 eyes from 480 patients undergoing LASIK for myopia, hyperopia, and mixed astigmatism. The mean preoperative spherical equivalent refraction was -3.75+/-4.82 diopters (D) (range: -13.88 to 6.00 D). From this population, 72 eyes (including 22 amblyopic eyes) from 43 patients were found to have improved BSCVA 6 months after LASIK. All patients underwent LASIK with the NAVEX platform. These eyes were analyzed to evaluate factors that correlated with improved BSCVA. Pre- and postoperative BSCVA, refraction, pupil diameter, corneal topography, asphericity (Q value), total aberrations, and higher order wavefront aberrations were analyzed. All wavefront aberrations were measured using the NIDEK Optical Path Difference Scan aberrometer (OPD-Scan) preoperatively and at 6 months postoperatively. Postoperatively, the mean sphere was -0.44 1.30 D (range: -4.50 to +2.50 D). The mean increase in BSCVA was 0.15+/-0.09 logMAR. A statistically significant negative correlation was observed between the increase in BSCVA and the preoperative BSCVA (P<.01). Mixed astigmatic and highly myopic eyes are more likely to gain BSCVA after LASIK than moderately myopic (P<.05) and hyperopic eyes (P<.001). In patients with myopia, the amount of BSCVA improvement correlated with the magnitude of the correction (P<.05). The induction of spherical aberration negatively correlated with the increase in BSCVA (P<.05). There were no significant differences between normal eyes and amblyopic eyes with respect to postoperative improvement in BSCVA (P>.05). Decreased preoperative BSCVA, lower total spherical aberration induction, and preoperative mixed astigmatism and high myopia correlate with an increase in BSCVA after LASIK.

Development of a parallel demodulation system used for extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented, which is based on a Michelson interferometer and combines the methods of low-coherence interference and a Fourier-transform spectrum. The parallel demodulation theory is modeled with Fourier-transform spectrum technology, and a signal separation method with an EFPI and FBG is proposed. The design of an optical path difference scanning and sampling method without a reference light is described. Experiments show that the parallel demodulation system has good spectrum demodulation and low-coherence interference demodulation performance. It can realize simultaneous strain and temperature measurements while keeping the whole system configuration less complex.

The effect of corneal flap on optical aberrations

Purpose To evaluate the changes in ocular aberrations induced by corneal flap creation. Design Prospective interventional nonrandomized clinical trial. Methods This study included 15 patients who were scheduled for laser in situ keratomileusis. A nasal hinge flap was created, using the Nidek MK-2000 microkeratome and then replaced without performing laser ablation. The ocular aberrations were measured before and after flap creation using the Nidek Optical Path Difference Scanning System ARK-10000. Results The root mean square wavefront errors of the higher-order optical aberrations (third-, fourth-, fifth-, and sixth-order aberrations) were not significantly altered at 1 week postsurgery compared with the preoperative values ( P > .35). Conclusions Creating a corneal flap with the Nidek MK-2000 microkeratome did not induce changes in higher-order optical aberrations as measured with the Nidek Optical Path Difference Scanning System ARK-10000 during the early postoperative period.

Infrared single-mode fiber-optic Fourier-transform spectrometry and double Fourier interferometry

An infrared single-mode fiber-optic Fourier-transform spectrometer constructed for the demonstration of the technique is described, in which beam splitting and beam combining are performed with fiber-optic directional couplers and the optical path difference scan is generated by stretching fibers. Experiments done with this fiber-optic Fourier-transform spectrometer are presented. A spectral resolution of 1328 at 2.2 µm can be achieved. The experimental results show the feasibility of fiber-optic Fourier-transform spectrometry and double Fourier interferometry.