Tomasz J. Licznerski

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.

Ultrasonic system for accurate distance measurement in the air

This paper presents a system that accurately measures the distance travelled by ultrasound waves through the air. The simple design of the system and its obtained accuracy provide a tool for non-contact distance measurements required in the lasers optical system that investigates the surface of the eyeball.

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.

Novel double path shearing interferometer in corneal topography measurements

The paper presents an approach for measurements of corneal topography by use of a patent pending double path shearing interferometer (DPSI). Laser light reflected from the surface of the cornea is divided and directed to the inputs of two interferometers. The interferometers use lateral shearing of wavefronts in two orthogonal directions. A tilt of one of the mirrors in each interferometric setup perpendicularly to the lateral shear introduces parallel carrier frequency fringes at the output of each interferometer. There is orthogonal linear polarization of the laser light used in two DPSI. Two images of fringe patters are recorded by a high resolution digital camera. The obtained fringe patterns are used for phase difference reconstruction. The phase of the wavefront was reconstructed by use of algorithms for a large grid based on discrete integration. The in vivo method can also be used for tear film stability measurement, artificial tears and contact lens tests.

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.

Influence of the characteristics of tear film break-up on the point spread function of an eye model

The paper presents the complex structure of the human tear film. From the optics point of view the homogeneously distributed tear film on the corneal surface makes this diffused surface optically smooth. Every eye blink distributes a new portion of tears on the surface of the cornea or the contact lens. If the production of tears is not sufficient or their composition is inappropriate, the tear film is deteriorated and so called tear film break-up (TFBU) appears on the surface of the cornea. These local break-ups of the smooth tear film introduce disturbances in the phase of a quasiplane wavefront falling on the cornea from a far-located point source of light. Two types of the TFBUs are considered in this paper. Topography of a point one and a linear one form of the break ups are approximated by use of four different continuous functions. The modeled topography of the TFBU is used for calculation of the local variation of the wavefront phase. The Point Spread Function of the eye model is calculated by use of the Fast Fourier Transform without and with the different approximations of TFBUs. It is shown that different type of TFBU approximation influences significantly the form of the Point Spread Function of the eye.

Reconstruction of the corneal topography from lateral-shearing interferograms

The paper presents an approach for measurements of the corneal topography by use of lateral-shearing interferometry. In shearing interferometry the wavefront reflected from the object is split in two wavefronts. The introduction of lateral shift and superposition of the wavefronts produces interference pattern. We used phase shifting algorithm to obtain the phase differences of the wavefront reflected from the cornea. The measurements of the phase differences are carried out in two orthogonal directions. The phase of the wavefront is reconstructed by use of algorithm for the large grid based on the discrete integration. The computer-generated interferograms are used to test the procedure and to estimate its accuracy.

Interferometry for in-vivo testing of artificial tears on the surface of the cornea

The shearing interferometry is presented as in vivo method for testing artificial tears covering the cornea. The stability of two types of artificial tears (gel and liquid) on the surface of the cornea was estimated by the analysis of interferograms. The 3 mW He-Ne laser was used as the light source in the shearing interferometer and CCD camera for recording the interference images. The coherent light is reflected from the surface of the cornea covered with the artificial tears. The reflected wavefront is directed to the lateral shearing interferometer and forms the interference pattern. The sequence of the obtained interferograms were recorded on VHS video tape. Then selected frames were digitized and analyzed in computer by use of image processing software. The proposed method is noncontact and noninvasive.

Analysis of the lateral shearing interferograms of human tear film using modified FFT techniques

The paper presents a modified method of the Shearing interferogram analysis by the use of the Fast Fourier Transform. The interference fringes are obtained in Shearing interferometer and are related to a wavefront reflected from the tear film over the cornea. The method is based on the measurement of interference fringes disturbances by calculation of the 2nd momentum of Fourier spectra of the interferogram. The less smooth tear film covering the cornea the more disturbed interference fringes and higher value of the 2nd momentum calculated from its Fourier spectra. The interferograms obtained from the proposed setup can differ also in the shape of fringes and in its density. This is caused by the differences in distance between the interferometer objective and the eye. Since the living eye is a continuously moving object, the fixation of this distance is difficult to achieve in practice. Obviously the fringes variations affect the Fourier spectra distribution and results of the numerical analysis based on FFT. The proposed method compensates the variations by applying the correcting factor determined from spectra distance from the origin of Fourier domain.