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.
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.
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.
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.
The shearing interferometry is presented as an in vivo method for testing the stability of the tear film covering the contact lens. The material and quality of a contact lens and its correct fitting to the surface of cornea influence the stability of the tear film. By observation of the tear film distribution, the precise assessment of the quality of the contact lens surface can be performed. Moreover, the overused or damaged contact lens can be detected by the analysis of interferograms. In this study the following types of soft contact lenses were used: Dura Soft D3 (r equals 8,7 mm, r equals 8,4 mm), Medalist (r equals 8,7 mm) and Johnson and Johnson (r equals 8,8 mm). The contact lenses were placed on the patients' cornea. The 3 mW HeNe laser was used as the light source in shearing interferometer and the CCD camera for recording the interference images. The coherent light reflected from the surface of the contact lens covered with the tear film formed the interference pattern. The sequence of the obtained interferograms were recorded and then analyzed. The accuracy of the contact lens fitting can be estimated by the interferogram analysis. The examples of correct and incorrect fitting of the overused or damaged contact lenses are presented. The proposed method is noncontact, nondestructive and of the high accuracy.
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.
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.
The tear film fulfills several important functions in the eye. Apart of its physiologic functions like maintaining a moist environment for the epithelial cells of the cornea and conjunctiva, bacterial properties, transporting metabolic products etc., this film causes that the corneal surface has the optical quality. This smooth surface allows to apply interferometry for measurements. The paper presents tear's layer distribution on the soft contact lens and the cornea in comparison. Tv frame speed registration in the Twyman- Green interferometer was used to observe an unstable biomedical objects like the eye. The proposed method has the advantage of being noncontact and applies the low energy laser beam in interferometric set-up. This provides non- invasive testing of human cornea in vivo and enables observation the kinetics of its tear layer deterioration. The evaluation of non-invasive tear breakup time is possible by use of proposed setup. Further analysis of recorded interferograms helps to examine the matter of the breakup process and can be used for detection of the 'dry eye' symptoms.
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