Astigmatism Correction Method Research Articles

Predicting Residual Astigmatism in Cataract Surgery.

The purpose of this review is to evaluate the prediction of postoperative residual astigmatism and to determine the best prediction method for astigmatism correction. In recent findings for residual astigmatism in non-toric monofocal intraocular lens (IOL) implanted eyes, vector analysis can be used to correctly evaluate residual astigmatism by decomposing it. In predicting residual astigmatism, the with-the-rule (WTR) and against-the-rule (ATR) astigmatism components can now be almost predicted. This may be due to advances in inspection equipment and surgical technique. However, there are still issues with the oblique astigmatism component. In addition, corneal astigmatism is the most important predictor of postoperative residual astigmatism, and other predictors, such as refractive astigmatism, age, and lens thickness, have also been mentioned. However, all but corneal astigmatism are questionable because of the possibility of confounding variables. Total corneal astigmatism is more accurate in predicting residual astigmatism than anterior corneal astigmatism. Several predictions of residual astigmatism have been reported, but complete prediction has not been possible. Further research is needed, especially in predicting oblique astigmatism. However, I emphasize that the accuracy of predicting WTR and ATR astigmatism has improved considerably and can be predicted using regression equations with total corneal astigmatism.

Astigmatism correction of convex aspheres using the subaperture stitching hindle test

We propose an astigmatism correction method for the subaperture stitching Hindle test to measure hyperbolic convex aspheres. Astigmatic wavefront errors arise from misaligned Hindle setups, mechanical runout errors of the rotational motion for stitching, and the surface error of the target itself. Because these errors are combined, they cannot be separated in the conventional subaperture stitching Hindle tests. We exploited the rotational periodicity of each error to distinguish the surface figure error from other astigmatic error sources and rectified the Hindle test results with a third-order astigmatism. Using the subaperture stitching Hindle test, we averaged two sets of measurement data with a 180° rotational phase difference between them to calculate the astigmatic surface error. The proposed method was verified experimentally by comparing it with the results from a commercial stitching interferometer from QED Technologies; only subnanometer differences in the root-mean-square values were obtained. Therefore, the proposed method calibrated the system errors from the test surface wedge and the rotational decenter easily, thereby reducing the mechanical costs and alignment efforts and making it more accessible than a sophisticated mechanism.

Refractive surprise after phacoemulsification in a post-laser in situ keratomileusis case: December consultation #1

To evaluate the outcomes of femtosecond laser–assisted arcuate keratotomy combined with cataract surgery in eyes with low-to-moderate corneal astigmatism.Eyes of York Private Practice Ophthalmology Clinic, York, Pennsylvania, USA.Retrospective case series.This retrospective analysis included case records of patients with preexisting corneal astigmatism ranging from 0.5 to 2.0 diopter (D). Study parameters included corneal astigmatism, refractive astigmatism, and uncorrected (UDVA) and corrected (CDVA) distance visual acuities. The results, which were analyzed at 3 months postoperatively, included frequency distribution histograms, vector analysis, and single-angle polar plots.The study comprised case records of 189 eyes of 143 patients (56 men and 87 women). The postoperative refractive astigmatism was reduced significantly compared with preoperative corneal astigmatism to 0.14 D ± 0.23 (SD) from 0.92 ± 0.34 D (P < .001). One hundred eighty-one eyes (95.8%) demonstrated postoperative refractive astigmatism of 0.5 D or less. The mean surgically induced change along the preoperative steep axis was −0.59 ± 0.56 D, and the change along the orthogonal axis was 0.01 ± 0.35 D. Postoperatively, 171 eyes (90.5%) had astigmatism angle of error of 15 degrees or less. The postoperative mean UDVA and CDVA were 0.09 ± 0.16 logarithm of the minimum angle of resolution (logMAR) and 0.02 ± 0.05 logMAR, respectively. One hundred seventy eyes (90%) had a postoperative UDVA of 20/30 or better. The results demonstrated stability at 12 months postoperatively. No intraoperative or postoperative arcuate keratotomy-related events were observed.The results suggest that femtosecond laser–assisted arcuate keratotomy represents a safe and effective method for astigmatism correction at the time of cataract surgery with demonstrated stability of correction for at least 1 year postoperatively.

Femtosecond laser–assisted arcuate keratotomy at the time of cataract surgery for the management of preexisting astigmatism

To evaluate the outcomes of femtosecond laser-assisted arcuate keratotomy combined with cataract surgery in eyes with low-to-moderate corneal astigmatism. Eyes of York Private Practice Ophthalmology Clinic, York, Pennsylvania, USA. Retrospective case series. This retrospective analysis included case records of patients with preexisting corneal astigmatism ranging from 0.5 to 2.0 diopter (D). Study parameters included corneal astigmatism, refractive astigmatism, and uncorrected (UDVA) and corrected (CDVA) distance visual acuities. The results, which were analyzed at 3months postoperatively, included frequency distribution histograms, vector analysis, and single-angle polar plots. The study comprised case records of 189 eyes of 143 patients (56 men and 87 women). The postoperative refractive astigmatism was reduced significantly compared with preoperative corneal astigmatism to 0.14 D±0.23 (SD) from 0.92±0.34 D (P<.001). One hundred eighty-one eyes (95.8%) demonstrated postoperative refractive astigmatism of 0.5 D or less. The mean surgically induced change along the preoperative steep axis was -0.59±0.56 D, and the change along the orthogonal axis was 0.01±0.35 D. Postoperatively, 171 eyes (90.5%) had astigmatism angle of error of 15 degrees or less. The postoperative mean UDVA and CDVA were 0.09±0.16 logarithm of the minimum angle of resolution (logMAR) and 0.02±0.05 logMAR, respectively. One hundred seventy eyes (90%) had a postoperative UDVA of 20/30 or better. The results demonstrated stability at 12months postoperatively. No intraoperative or postoperative arcuate keratotomy-related events were observed. The results suggest that femtosecond laser-assisted arcuate keratotomy represents a safe and effective method for astigmatism correction at the time of cataract surgery with demonstrated stability of correction for at least 1year postoperatively.

Several controversial issues in astigmatism correction in cataract surgery

Cataract surgery has entered the era of refractive surgery, and patients have higher requirements for visual quality. Astigmatism correction in cataract surgery is increasingly valued, but the astigmatism correction results are not ideal, and the principles of corneal astigmatism correction are not well established. In this article, the issues including how to dialectically decide to treat the corneal astigmatism or not, how to choose the corneal astigmatism estimating method and the calculation formula for astigmatism correction, the range of corneal astigmatism suitable for applying multifocal intraocular lens, selection of intraoperative corneal astigmatism correction method, etc., were discussed based on relevant authoritative guidelines, expert consensus and literatures, and corresponding suggestions were given. (Chin J Ophthalmol, 2019, 55: 485-490).

A simplified focusing and astigmatism correction method for a scanning electron microscope

Defocus and astigmatism can lead to blurred images and poor resolution. This paper presents a simplified method for focusing and astigmatism correction of a scanning electron microscope (SEM). The method consists of two steps. In the first step, the fast Fourier transform (FFT) of the SEM image is performed and the FFT is subsequently processed with a threshold to achieve a suitable result. In the second step, the threshold FFT is used for ellipse fitting to determine the presence of defocus and astigmatism. The proposed method clearly provides the relationships between the defocus, the astigmatism and the direction of stretching of the FFT, and it can determine the astigmatism in a single image. Experimental studies are conducted to demonstrate the validity of the proposed method.

Astigmatism-free Czerny-Turner compact spectrometer with cylindrical mirrors.

A modified optical design for a broadband, high resolution, astigmatism-free Czerny-Turner spectrometer is proposed. Astigmatism is corrected by using cylindrical mirrors over a broad spectral range. The theory and method for astigmatism correction are thoroughly analyzed. The comparison between the modified Czerny-Turner spectrometer and the traditional Czerny-Turner spectrometer is also described in detail. The ray-tracing results show that the RMS spot radius has decreased to 4.2μm at the central wavelength and 17μm at the wedge wavelength.

Comparison Between Liquid Trial Lens and the Conventional Method for Astigmatism Correction on the Humphrey Field Analyzer III860

Comparison Between Liquid Trial Lens and the Conventional Method for Astigmatism Correction on the Humphrey Field Analyzer III860

The location of incision in cataract surgery and its impact on induced astigmatism.

The purpose of the present study is a systematic review of previous studies on choosing the best incision site for the correction of astigmatism in cataract surgery and assessing the amount of surgically induced astigmatism (SIA) with each approach. Regardless of astigmatism axis, studies show that using an on-axis incision is associated with favorable results for 0.5-1.0 diopter (D) of astigmatism. In cases with more than 1.0 D astigmatism, paired on-axis incisions can be appreciably efficient in astigmatism correction and cause at least 1.5 D SIA. Considering the amount of SIA, a temporal incision is the best approach when the patient has minimal amounts of corneal astigmatism preoperatively. At higher levels of astigmatism, if no other astigmatism correction method is used simultaneously, the temporal incision is used less frequently; however, since it is associated with the least SIA, it is still the choice site when another correction method is used. The temporal incisions in cataract surgery are associated with little SIA and are appropriate choices for mild preoperative astigmatism. At higher levels of preoperative astigmatism, superior incisions are associated with better results when combined methods are not applied.

星载宽波段大气痕量气体临边探测仪光学设计

In order to meet the urgent requirements of detecting atmospheric trace gas in limb observation geometry,an optical system of spaceborne broadband limb sounder for detecting atmospheric trace gas is designed.The system is an imaging spectrometer with the working wavelength band from 0.3 μm to 0.7 μm,and its full field of view is 2.4°,focal length is 120 mm,and the relative aperture is 1∶6.To avoid the problems of the classical Czerny-Truner spectrometer,such as low spatial resolution caused by large astigmatism,a modified Czerny-Turner spectrometer is designed,in which astigmatism can be corrected simultaneously in a wide band.By matching the modified Czerny-Turner spectrometer with a off-axis parabolic telescope,an example of limb sounder optical system is designed.Ray tracing,optimization and analysing are performed by ZEMAX software.The analyzed results demonstrate that the astigmatism is substantially corrected,and the MTF for different spectral band is more than 0.69,which satisfies the pre-designed requirement and proves the feasibility of the astigmatism-correction method.

Defocus and twofold astigmatism correction in HAADF-STEM

A new simultaneous autofocus and twofold astigmatism correction method is proposed for High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). The method makes use of a modification of image variance, which has already been used before as an image quality measure for different types of microscopy, but its use is often justified on heuristic grounds. In this paper we show numerically that the variance reaches its maximum at Scherzer defocus and zero astigmatism. In order to find this maximum a simultaneous optimization of three parameters (focus, x- and y-stigmators) is necessary. This is implemented and tested on a FEI Tecnai F20. It successfully finds the optimal defocus and astigmatism with time and accuracy, compared to a human operator.

The efficacy of multi-zone cross-cylinder method for astigmatism correction.

The purpose of this study is to assess the efficacy of the multi-zone cross-cylinder method as compared with the single method for astigmatism correction using LASIK. This prospective study enrolled 40 patients (52 eyes) who underwent the cross-cylinder method using LASIK, and 52 patients (60 eyes) who underwent the single method using LASIK: all patients were given a diagnosis of complex myopic astigmatism from the department of ophthalmology of this hospital between January 2002 and July 2003. Preoperatively, the mean spherical equivalent refraction was -3.85 +/- 1.13 D in the cross-cylinder group and -4.05 +/- 1.20 D in the single method group (p = 0.23). The mean cylinder was -2.05 +/- 1.58 D in the cross-cylinder group and -1.95 +/- 1.12 D in the single method group (p = 0.31). 6 months after treatment the results were a mean spherical equivalent refraction of -0.26 +/- 0.30 D in the cross-cylinder group and -0.34 +/- 0.35 D in the single method group (p = 0.13). The mean cylinder was -0.38 +/- 0.29 D in the cross-cylinder group and -0.45 +/- 0.30 D in the single method group (p = 0.096). There were no statistically significant differences between the two groups. The mean BCVA was not different from mean preoperative BCVA in both groups (i.e., 0.98 +/- 0.10, 0.96 +/- 0.25, p = 0.86). Postoperatively, patient complications that included night halo, glare and corneal haze were not noted in either group. In conclusion, the results of cross-cylinder method are no different from the single method for the correction of a complex astigmatism. In the future, studies will have to be conducted to assess the efficacy of the cross-cylinder method in consideration of those factors that can affect the postoperative outcome.

Automatic Focusing and Astigmatism Correction Method based on Fourier Transform of Scanning Electron Microscope Images

A new method for automatic focusing and astigmatism correction of electron optics has been proposed. The method consists of the following 6 steps: (1) Obtain scanning electron microscope (SEM) images i1(x,y), i2(x,y) under two focusing conditions. (2) Obtain Fourier transformed images of i1(x,y) and i2(x,y), I1(kx,ky) and I2(kx,ky). (3) Obtain a characteristic function R(kx,ky)=ln(I1(kx,ky)/I2(kx,ky)). (4) Repeat the above steps under different focusing conditions. (5) From the change in R(kx,ky), calculate the change in beam sharpnesses in different directions. (6) Correct the astigmatism and adjust the focus. R(kx,ky) is independent of the sample shape, and is independent of the beam shape in the case of a character beam. Therefore, this method can be applied both to a SEM and to an electron beam writer with a character beam.

A robust focusing and astigmatism correction method for the scanning electron microscope—Part III: An improved technique

AbstractAs described in a previous work, a new technique has been developed to perform automatic focusing and astigmatism correction on any general scanning electron microscope (SEM) sample ranging from gold‐on‐carbon to integrated circuit (IC) tracks. In this work, various improvements made to this technique are reported. They include the implementation of direct control of the SEM and the development of three new algorithms, namely the adaptive fast Fourier transform (FFT) algorithm, the coarse focusing algorithm, and the fine focusing algorithm. Direct control reduces the communication time with the SEM while the three new algorithms are integrated with the existing technique to make it even more robust to noise and to extend it to correct images with any degree of defocus and astigmatism. The enhanced focusing and astigmatism correction algorithm is able to perform the correction in a shorter time while maintaining the accuracy of the original algorithm even under noisy conditions.

A robust focusing and astigmatism correction method for the scanning electron microscope—Part II: Autocorrelation‐based coarse focusing method

AbstractThis paper describes the development of a new coarse focusing algorithm for the scanning electron microscope (SEM). The algorithm is based on a new focus parameter that has been developed for quantifying the sharpness of an image. This parameter is found to be more robust to noise compared with other conventional focus parameters. A peak detection technique is also incorporated to shorten the number of iterations needed for the coarse focusing. Results show that the coarse focusing algorithm is able to bring a defocused and astigmatic image to either the best focus, the upper line focus, or the lower line focus in fewer iterations compared with the conventional approach in which the full focus range is searched.

A robust focusing and astigmatism correction method for the scanning electron microscope

AbstractThis paper discusses a new approach to focusing and astigmatism correction based on the fast fourier transforms (FFTs) of scanning electron microscopy (SEM) images. From the FFTs, it is possible to obtain information on the severity of the defocus and astigmatism. This information is then processed by an algorithm to perform real‐time focusing and astigmatism correction on the SEM. The algorithm has been tested on defocused and astigmatic images of different samples, including those with highly directional features. Experiments show that the images obtained after running the algorithm can be as good as those that an experienced SEM operator can achieve.

Advanced e-beam lithography

The acceleration voltage dependence of electron-beam (EB) lithography was investigated. A low acceleration voltage is suitable for a mask (reticle) of several magnifications, because the proximity effect correction is not required. A reticle writing system EX-8 with an acceleration voltage of 12.5 to 20 kV has been developed. A high acceleration voltage is preferable for the direct writing of fine patterns. A direct writing system EX-7 with a typical acceleration voltage of 40 kV has been developed. A new shaped beam calibration method and a new astigmatism correction method have enabled the EX-7 to write a pattern with 0.1 μm. High accuracy shaped beam calibration and alignment as required by small patterns were realized for a high acceleration voltage.

An automatic focusing and astigmatism correction method for high resolution electron microscopy

When based on the image formation theory of an object studied in high resolution electron microscopes, the amount of focusing associated with the astigmatism was derived from measurements of the phase transfer function of the computer generated diffractograms, together with a line spectrum integration using the one-dimensional Fourier transform of the specimen. It was successfully found that an automatic focusing and astigmatism correction system with higher accuracy can be achieved by an on-line computer system.