Waldemar Kowalik

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.

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.

Corneal topography measurement of the eye by means of radial shearing interferometer

The method of the measurement of the corneal topography was worked out. This measurement system uses an interferometer based on radial shearing. This paper presents the preliminary results of the experiments. The results are compared with other methods.

Corneal topography measurement by means of radial shearing interference: Part I – theoretical consideration

Summary The paper presents the principle of radial shearing interference; the build of a simple and stable device for interference measurement; the comparison of this structure with other types of interferometers; presents the results of the examination of corneal topography and discusses the prospects for its future uses.

Interferometric measurements of fine corneal topography

The cornea is the most refractive element in the eye. Its refractive power is about 70% of the power of the whole eye. The shape of the cornea is aspheric, and almost always has no rotational symmetry. Even small surface irregularities can cause a perceptible reduction in visual acuity. Standard methods for evaluation of the corneal topography used in clinical practice include keratometry, photokeratoscopy, and computer assisted videokeratography. All of these methods used the principles of geometrical optics, and their accuracy is about 0.25 D. An application of interference phenomenons to examine the corneal contour map significantly increase the accuracy. Using the interferometric inspection of the corneal shape one can easily observe the fine corneal topography, the fast, dynamic changes of the corneal surface, and the topology of the tear film and its irregularities. The paper presents the Twyman Green interferometer, used in experiments, an example of sequence of interferograms and their 3D presentations.

Corneal topography measurement by means of radial shearing interference: Part II – experiment results

Summary The method of the measurement of the corneal topography was worked out. This measurement system uses an interferometer based on radial shearing. This paper presents the preliminary results of the experiments. The results are compared with other methods.

Two interference techniques for in-vivo assessment of the tear film stability on a cornea and contact lens

The paper presents preliminary results of applying two different interference techniques for evaluation of the breakup characteristics of the tear film. The Twyman-Green interferometer (TGI) and Lateral Shearing technique (LST) were applied in two separates set ups. To find irregularities in the tear film distribution, the sequence of interferograms of the cornea or contact lens were stored and processed in a computer by use of modular frame grabber. The interferograms of in vivo precorneal tear film breakup formation are presented for both techniques. The proposed methods have the advantage of being noncontact and applying the low energy laser beam.

Corneal topography measurement by means of radial shearing interference: Part III – measurement errors

Summary This work contains results of computer simulation researches, which define requirements for measurement conditions, which should be fulfilled so that measurement results ensure allowable errors. They define: allowable measurement errors (interferograms scanning) and conditions, which should fulfill computer programs, so that errors introduced by mathematical operations and computer are the smallest.

Optical Coherence Tomography as a Tool for Ocular Dynamics Estimation

Purpose. The aim of the study is to demonstrate that the ocular dynamics of the anterior chamber of the eye can be estimated quantitatively by means of optical coherence tomography (OCT). Methods. A commercial high speed, high resolution optical coherence tomographer was used. The sequences of tomographic images of the iridocorneal angle of three subjects were captured and each image from the sequence was processed in MATLAB environment in order to detect and identify the contours of the cornea and iris. The data on pulsatile displacements of the cornea and iris and the changes of the depth of the gap between them were retrieved from the sequences. Finally, the spectral analysis of the changes of these parameters was performed. Results. The results of the temporal and spectral analysis manifest the ocular microfluctuation that might be associated with breathing (manifested by 0.25 Hz peak in the power spectra), heart rate (1–1.5 Hz peak), and ocular hemodynamics (3.75–4.5 Hz peak). Conclusions. This paper shows that the optical coherence tomography can be used as a tool for noninvasive estimation of the ocular dynamics of the anterior segment of the eye, but its usability in diagnostics of the ocular hemodynamics needs further investigations.

Interference Measurement Of Waveguide Preform Refractive Index Distribution - Destructive Method With Plane Wave Of Interference

Exploitation parameters of a waveguide mostly depend on the distribution of the refractive index in it. Therefore, control over the distribution of the refractive index in the waveguide preforms out of which the waveguides are drawn is necessary in waveguide production and in working out a proper technology of preform production. Interference methods prevail in refractive index distribution measurements because of their great measurement accuracy. Observation of the preform along its axis provides us with information about the actual refractive index distribution in this preform. In order to obtain this information: it is necessary to cut a slice out of the preform and to place it in the measurement setup so that the light beam goes along the symmetry axis of the preform, slice. Owing to the fact that the refractive index does not change along the run of the beam parallel to the preform axis, this method makes it possible to determine the actual refractive index distribution. The method of three interferograms is a destructive method and it makes it possible to measure the refractive index distribution with an accuracy of 1x10-6 for a slice of 1mm thickness as well as the sample thickness distribution. Although the method presented here is simpler, it constitutes a model method for nondestructive methods. This method makes it possible to make measurements of varying accuracy whether we take into account the distribution of the thickness of the examined slice based on other measurements, or whether the slice wedge alone is considered, or whether the slice thickness is assumed as constant.