Biomedical subjects

Henryk T Kasprzak

Publications and source records attributed to Henryk T Kasprzak.

4 recordsLinked to original sources

PubMed2006

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.

Cornea↗

PubMed2006

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.

Accommodation, Ocular↗

PubMed2003

Linear birefringence measurements of the in vitro human cornea.

This paper presents results of an in vitro investigation of the optical anisotropy of the human cornea. Computer controlled imaging polarimetry and the Jones formalism were used to investigate the distribution of the birefringence and the azimuth angle across the surface of 12 corneas taken from the corneal bank. Each map of the birefringence and the azimuth angle has no rotational symmetry and is different for each cornea. The minimal value of the birefringence always appears in the corneal centre and varies from 9 to 43 degrees for different corneas. Birefringence of the cornea increases monotonically more than one order in the direction of the corneal periphery. Close to the centre of the cornea the azimuth angle is almost constant and varies significantly away from the optical part of the cornea.

Anisotropy↗

PubMedDate not supplied

Interferometric measurements of dynamic changes of tear film.

Tear film stability plays an important role in the quality of vision. We present an interferometric method for assessing the stability of precorneal tear film in real time. A lateral shearing technique is applied as a noninvasive and sensitive method for investigating tear film stability and the smoothness of the tear film surface by quantitative evaluation of the interference fringe pattern. The evaporation of tears and the appearance of tear film break-up between blinks cause changes in the fringe geometry. For quantitative assessment of the fringe smoothness and consequently of the tear film structure, the fast Fourier transform (FFT) is applied. Four parameters are used to quantitatively estimate dynamic changes in the tear film stability of the cornea 20 s after the eye blink. Examples of examined interferograms recorded in patients with healthy eyes, patients suffering from dry-eye syndrome, and patients wearing contact lenses are given. Significant differences between the stability of the tear film in the healthy eye and that in the dry eye and the eye with a contact lens are observed. The favorable influence of artificial tears applied in patients with dry-eye syndrome or contact lenses is also discussed.

Dry Eye Syndromes↗