Spectral characteristics of longitudinal corneal apex velocities and their relation to the cardiopulmonary system.

PURPOSE: To study the naturally occurring kinetic characteristics of corneal surface. METHODS: The right eyes of three subjects (young, early presbyope, and presbyope) were examined. Cardiac signal and longitudinal corneal apex movements were simultaneously measured with electrocardiography (ECG) and a high-speed videokeratoscope, respectively. Time, frequency, and combined time-frequency representations of the acquired signals were derived to establish their temporal and spectral contents. Coherence analysis was used to assess the correlation between the corneal apex velocities and the cardiopulmonary system. RESULTS: In all measurements, longitudinal corneal apex velocity signals showed close correlation with the corresponding ECG signals. The signatures of the pulse frequency, which was inferred from the ECG spectra and their variations in time, were clearly visible in the spectral content of corneal apex velocities. For the young subject, the correlation was the strongest and all of the spectral content of the pulse signal including the harmonics was propagating to the corneal apex velocities. For the other two subjects, there was a clear propagation of the pulse signal itself but not of all pulse harmonics. CONCLUSIONS: Longitudinal movements of the corneal apex are closely related to the cardiopulmonary system. The differences in propagation of pulse harmonics to the corneal apex velocities for different subjects suggest that the frequency characteristics of apex velocity could be related to pulsative variations in the intraocular pressure and biomechanical properties of the eye. These findings could potentially be used in noninvasive assessment of the hemodynamic status of the eye with high-speed videokeratoscopy.

[Evaluation of tear film stability by means of laser interferometry]. / Pomiar stabilnosci filmu lzowego za pomoca prototypowego interferometru laserowego.

PURPOSE: The paper presents a new method for evaluation of the tear film stability on the human eye. METHODS: The tear film distribution on the cornea is measured by the lateral shearing interference technique. The eye is kept open during approximately a two-minute recording, when the blinking has to be prevented. Continuous recording and viewing of interferograms enables registration of the changes in disturbances of interference fringes during elapsed time. The changes in fringes are caused by evaporation of tears from the ocular surface and appearance of the breakups. CONCLUSIONS: The noninvasive tear breakup time (NITBUT) can be evaluated by comparing the recorded consecutive interferograms.

New approximation for the whole profile of the human crystalline lens.

This paper presents a new approximation for the whole profile of the human crystalline lens by the use of only one analytical function for both unaccommodated lens and the lens on accommodation. Approximation of the anterior and posterior lens profile is composed of hyperbolic cosine functions and is given in polar coordinates. Each of the hyperbolic cosines is modulated by the function of hyperbolic tangent type. The curvature of the hyperbolic cosine in polar coordinates is discussed and some results concerning the stability of its central radius of curvature are shown. Fitting of the hyperbolic cosine type curve to various results of the lens curvature measurements is presented. Examples of profiles of the lens under accommodation are given. It is shown that this approximation can be used for the description of iso-indical profiles inside the lens.

A new schematic eye model incorporating accommodation.

PURPOSE: To create a schematic eye model with four refracting surfaces and with the new function describing the crystalline lens profile. METHODS: The new function describing the crystalline lens profile is a combination of hyperbolic cosine functions modulated by hyperbolic tangent functions. It ensures the continuity of the radius of curvature along the whole lens profile. Recent experimental results on cornea and lens shape measurements have been applied to the proposed model. The new function describing the lens profile has been used to model the eye during accommodation. RESULTS: Spherical aberration and modulation transfer function have been computed to test the eye model. Results of calculations are in agreement with experimental findings. CONCLUSIONS: The calculations show that spherical aberrations of the cornea and crystalline lens cancel each other. The new description of the crystalline lens offers a reduction of longitudinal spherical aberration. The image quality of the unaccommodated eye model is slightly better than that of the accommodated one.

Generalized algorithm for photoelastic measurements based on phase-stepping imaging polarimetry.

A general method for measuring the birefringence of nondichroic, linear retarder media has been presented. The method is based on phase-stepping imaging polarimetry and permits the azimuth angle, phase retardation, and transmission coefficient of a sample to be calculated. The method uses a simple setup, a sample at rest, and permits fast acquisition of data. With the mathematical description applied, various algorithms for different optical configurations can be used and any number of intensity patterns can be generated. Experimental results for photoelastic samples and the results of measurements of the birefringence of optical components and biological samples are also presented.

Application of twyman-green interferometer for evaluation of in vivo breakup characteristic of the human tear film.

The paper presents an interferometric method of assessing the in vivo stability of the precorneal tear film. To observe dynamic effects on a human cornea the Twyman-Green interferometer with television frame speed digital registration synchronized with a laser flash was used. The instrument was applied to the human cornea in vivo. The results of the experiment, both tear film distribution and its dynamics, are presented. The proposed interferometric setup can be used to evaluate the breakup characteristics of the tear film, its distribution, and to examine its dynamic changes. The breakup profiles and their cross sections calculated from the interferogram analysis are presented. The depth of recorded breakup, calculated on the basis of interferogram analysis, amounts to about 1.5 µm. The proposed method has the advantage of being noncontact and applies only a low-energy laser beam to the eye. This provides noninvasive viewing of human cornea in vivo and makes it possible to observe the kinetics of its tear film deterioration. © 1999 Society of Photo-Optical Instrumentation Engineers.

Analysis of shearing interferograms of tear film using fast fourier transforms.

A new method for evaluating tear film stability on the human eye is reported. The tear film distribution on the cornea is measured by the lateral shearing interference technique. The eye is kept open during approximately a 2-min recording, when blinking has to be prevented. Continuous recording and viewing of interferograms allows the changes in disturbances of the interference fringes to be registered during elapsed time. The changes in fringes are caused by the evaporation of tears from the ocular surface and appearance of the breakups. For precise and repetitive assessment of the tear film breakup time, a fast fourier transform (FFT) is applied to consecutive interferograms. Larger fringe disturbances result in wider Fourier spectra. The tear breakup time can be evaluated noninvasively by comparing the value of the second momentum of Fourier spectra calculated from the consecutive interferograms. © 1998 Society of Photo-Optical Instrumentation Engineers.

A model of inhomogeneous expansion of the cornea and stability of its focus.

A simple derivation of an analytical condition for expanding the cornea to get the stable position of the corneal focus is given. The corneal thickness increases from the apex till the limbus and the corneal shell expands inhomogeneously as a result of changes in intraocular pressure conditions. It expands more in the centre of the cornea and less at its apex. As a result the expansion changes the paraxial radius of the corneal curvature by the value of delta r as well as displaces the position of the centre of the central corneal curvature by the value of s. Presented paraxial calculations show that the position of the corneal focus is stable and insensitive on the expansion of the cornea if the ratio of both values amounts to about 0.34. Comparison of analytical results with published results of experiments on expanding intact cornea in vitro indicates their high accordance. This suggests a new, important role of the thickness distribution of the cornea.