Unified approach to tear film surface analysis with high-speed videokeratoscopy.

Placido disk videokeratoscopy, when used in a dynamic acquisition mode, can be utilized for noninvasive assessment of tear film quality. The different commercially available videokeratoscopes that incorporate an automatic option to evaluate tear film do not provide consistent measurements due to differences in their designs as well as differences in the algorithms used to analyze the images. Here, a standardized algorithm that can be applied to different instruments is developed. Results show that, by applying the proposed methodology in two different videokeratoscopes, the agreement among the estimated tear film parameters considerably improves. This represents a step forward in the normalization of tear film assessment.

Defocus vibrations in optical systems-considerations in reference to the human eye.

Experimental visual acuity (VA) of eight subjects was measured using the Freiburg vision test in a custom-made adaptive optics system. Measurements were conducted under one control and five defocus-induced conditions. In the defocus-induced conditions, 1 diopter of myopic defocus was added to the system using the Badal stage, and defocus vibrations with five different levels of amplitude were generated by a deformable mirror at 50 Hz. Computational simulations of the visual Strehl ratio (VSOTF) were performed using average aberrations of each subject recorded in the control condition. For the first time, to the best of our knowledge, it has been shown experimentally that both the simulated VSOTF and experimentally measured VA improve when defocus vibrations are added to a defocused eye.

Evaluating displacement of lamina cribrosa following glaucoma surgery.

PURPOSE: The purpose of the study is to assess the displacement of lamina cribrosa (LC) and prelaminar tissue area (PTA) changes following trabeculectomy and non-penetrating deep sclerectomy (NPDS) using spectral-domain optical coherence tomography (SD-OCT) with enhanced depth imaging technology. METHODS: A total of 30 patients underwent glaucoma surgery. Sixteen patients underwent trabeculectomy, and 14 patients undertook NPDS. Serial horizontal B-scan images of optic nerve head (ONH) were obtained using SD-OCT preoperatively, and at 2-week, 1-, 3-, and 6-month postoperative visit (6 pv). LC displacement magnitude and PTA changes were determined from selected B-scan images. Correspondingly, OCT retinal nerve fiber layer (RNFL) parameters were measured. RESULTS: Intraocular pressure (IOP) decreased from 27.4 ± 10.3 mmHg (mean ± standard deviation) to 10.2 ± 4.0 mmHg (P = 0.011) and from 19.9 ± 4.0 mmHg to 11.9 ± 3.6 mmHg (P = 0.012) at 6 pv, for trabeculectomy and NPDS, respectively. There was a significant decrease in the LC depth from a baseline glaucomatous LC displacement of 468.0 ± 142.4 to 397.6 ± 125.2 µm in the trabeculectomy group (P = 0.001) and from 465.2 ± 129.6 to 412.0 ± 122.4 µm in the NPDS group (P = 0.029) at 6 pv. The PTA differed between the procedures at baseline (P = 0.002), but was not statistically significant postoperatively. Multivariate analysis for all patients including age, magnitude of IOP reduction, baseline glaucomatous LC displacement, magnitude of LC displacement, and the type of surgery revealed that only the magnitude of LC displacement was associated with significant RNFL thinning on average (r2 = 0.162, P = 0.027) and in the following sectors: temporal superior (r2 = 0.197, P = 0.014), temporal (r2 = 0.150, P = 0.034), and nasal superior (r2 = 0.162, P = 0.027). CONCLUSIONS: Decrease in the LC depth after NPDS surgery can be observed at 6 pv. Regardless of the performed procedure, magnitude of LC displacement is associated with significant, focal RNFL thinning.

Blur detection thresholds in childhood myopia: single and dual target presentation.

There is some suggestion that the ability to detect blur may be altered in adults with myopia. Here, we address the question of whether children with myopia have worse blur detection than other children, and whether blur detection in myopic children is related to the rate of myopia progression. We recruited 20 myopes and 20 non-myopes aged between 8 and 12 years. Refractive errors, visual acuity, and contrast sensitivity were measured and the change in refractive error over the past year calculated from clinic records. Blur detection thresholds for two different types of black and white targets (text and scenes), two illumination conditions and two testing protocols were determined using a computer-based forced-choice testing procedure. The two testing protocols used were: (i) dual image presentation where subjects were asked to choose the clearer of the two images, one image always having zero blur, and (ii) single image presentation in which the subject reported whether the image was clear or blurred. Blur discrimination ability under all tested conditions was similar for both refractive error groups. Blur detection thresholds were 0.27+/-0.15 D (myopes) and 0.24+/-0.07 D (non-myopes) for text images. Thresholds were similar when measured with a one log unit reduction in lighting: 0.27+/-0.31 D compared to 0.23+/-0.14 D. Blur detection thresholds were greater for photographic scenes (myopes 0.41+/-0.36 D, non-myopes 0.44+/-0.36 D) and when only a single text image (myopes 0.51+/-0.21 D, non-myopes 0.59+/-0.01 D) was presented, but this increase was measured in both refractive error groups. There was no correlation between blur thresholds and refractive error magnitude, refractive error progression over the past year, or contrast sensitivity. We found that the blur detection ability showed greater individual variation in myopic children. Further work is required to determine whether blur detection ability is of relevance to myopia development.

Objective refraction from monochromatic wavefront aberrations via Zernike power polynomials.

With the increased popularity of wavefront sensor devices there has been significant interest in relating the subjectively measured sphero-cylindrical refractive error to the objectively measured wavefront aberrations. We investigate the applicability of four different measures for objective refraction assessment that are derived from a set of estimated Zernike wavefront coefficients. Two of the measures are based on the second- and higher-order Zernike polynomials while the other two measures are based on the estimated dioptric power representations. For the latter, closed-form expressions for dioptric powers based on the estimated focal length and the wavefront curvature are derived. We call these two new representations the 'refractive Zernike power polynomials' and 'curvature Zernike power polynomials'. Data from 120 eyes from 60 normal subjects were used to assess the correlation between the subjective and objective refractions. The results indicate that the objective sphero-cylindrical refraction calculated from the estimated refractive power map via the Zernike power polynomials is superior to the other considered representations and achieves best correlation with subjective sphero-cylindrical refraction.

Dynamics in longitudinal eye movements and corneal shape.

The magnitude and character of longitudinal movements of the eye were studied in the context of high speed videokeratoscopy. It was of interest to determine whether these dynamic changes in the eye movements can affect the corneal shape and its estimation. A high speed videokeratoscope with a sampling frequency of 50 Hz was used for measuring the corneal apex movements as well as for measuring variations in the best-fit sphere radius and central radius of curvature. The magnitude of the measured longitudinal apex movements could reach over 200 microm showing a slow trend. The estimated local changes of the apex movements that could be associated with the cardiopulmonary system were about 40-50 microm. The temporal variations in the equivalent estimated central radius of curvature ranged between 10 and 15 microm. Spectral analysis of the longitudinal eye movements revealed strong signatures of the pulse and respiration signals as well as the assumed blink control signal. No clear association between the longitudinal apex movements and the corneal curvature was found. However, very slow significant changes in the corneal shape were observed. The central radius of curvature of the cornea revealed slow changes of up to 120 microm. Understanding the nature of such changes will be of benefit in ophthalmic applications requiring highly accurate measurements of corneal shape, such as contact lens design and refractive surgery.

Approximating ocular surfaces by generalised conic curves.

Most of the optical models of the human eye use simple conic functions to represent its individual components such as corneal surfaces and the surfaces of the crystalline lens. Although a conic function provides an acceptable approximation for most anatomical eye surfaces, it also leads to a simple optical analysis of the whole eye system. To fill the gap between the classical use of conic surfaces and the use of more sophisticated functions that often invoke numerically expensive procedures in the optical analysis, a functional generalisation of the conic curve is proposed. A detailed derivation of the generalised conic function is presented for a two-dimensional (meridional) case. This is followed by a three-dimensional surface approximation. Examples are given in which the superiority of the proposed approximation over a classical conic function as well as the hyperbolic cosine approximation is evident. In particular, it is shown that for an average total corneal profile, the proposed generalisation results in a residual height error that is of an order smaller than those achieved with the conic and hyperbolic cosine approximations. In conclusion, the proposed generalised conic function can be a useful tool in eye modelling, where the simplicity of expression is often desirable.

Microfluctuations of wavefront aberrations of the eye.

To investigate fluctuations in the wavefront aberrations of the eye and their relation to pulse and respiration frequencies we used a wavefront sensor to measure the dynamics of the Zernike aberrations up to the polynomial fourth radial order. Simultaneously, the subject's pulse rate was measured, from which the instantaneous heart rate was derived. We used an auto-regressive process to derive the power spectra of the Zernike aberration signals, as well as pulse and instantaneous heart rate signals. Linear regression analysis was performed between the frequency components of Zernike aberrations and the pulse and instantaneous heart rate frequencies. Cross-spectrum density and coherence analyses were also applied to investigate the relation between fluctuations of wavefront aberrations, and pulse and instantaneous heart rate. The correlations between fluctuations of individual Zernike aberrations were also determined. A frequency component of all Zernike aberrations up to the fourth radial order was found to be significantly correlated with the pulse frequency (all R(2) >/= 0.51, p < 0.02), and a frequency component of nine out of 12 Zernike aberrations was also significantly correlated with instantaneous heart rate frequency (all R(2) >/= 0.46, p < 0.05). The major correlations among Zernike aberrations occurred between second-order and fourth-order aberrations with the same angular frequencies. Higher order aberrations appear to be related to the cardiopulmonary system in a similar way to that reported for the accommodation signal and pupil fluctuations.