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H Burek

Publications and source records attributed to H Burek.

7 recordsLinked to original sources
PubMed1996

Extent and effect of surface tilt on the data display of the EyeSys videokeratoscope.

AIMS: This study examined the occurrence, magnitude, and the consequences of a possible tilt between the corneal surface and optical axis of the EyeSys videokeratoscope. METHODS: Initially, a theoretical model was developed to calculate the angle of tilt. The predictions of the model were verified empirically using a convex conicoid surface and were found to predict the tilt to within 0.5 degree of the actual tilt. The likely effects of the tilt on the corneal power were also examined. The angle of tilt was then measured on the human cornea and the effect of neutralising the tilt on the videokeratoscopic data display was observed. RESULTS: The angle of tilt was found to lie between 1 degree and 6 degrees in a temporal direction. CONCLUSION: When the corneal tilt on the human subjects was neutralised, then a reduction in the nasal/temporal asymmetry was observed.

Cornea↗
PubMed1995

The Bausch and Lomb keratometer does not measure the tangential radius of curvature.

The purpose of this research was to determine if the conventional keratometer measures tangential or sagittal radius. A Bausch and Lomb keratometer was modified to enable an assessment of vertex radius and p-value to be made on convex conicoidal buttons of known surface form. Calculation of these two parameters was achieved assuming that the keratometer was measuring tangential radius and then repeated assuming a sagittal measurement. The resulting values could then be compared with those obtained by Form Talysurf analysis and by the use of an autokeratometer designed to measure sagittal radius. There was good agreement between the three instruments when the modified Bausch and Lomb keratometer was assumed to measure sagittal radius. We conclude that the Bausch and Lomb keratometer makes a radius measurement closer to the sagittal than the tangential radius of a conicoid of revolution.

Anthropometry↗
PubMed1994

Mass and centroid of rigid lenses.

By subdividing a lens design into simpler units it is possible to evaluate its mass and locate its centre of mass (the centroid). The constituent subcomponents form plane sections of the sphere, or, in the case of aspheric lenses, sections of a conicoid. The theory of lens analysis in terms of the conicoidal (and spherical) frustrum is discussed and a BASIC computer program is presented to perform the calculations for spherical multicurve contact lenses.

Contact Lenses↗
PubMed1994

Tri-meridional analysis using arbitrary meridians: a new solution.

A new solution is presented for the evaluation of the spherocylindrical components of curvature or refraction from measurements taken along three distinct, arbitrary, meridians. Unlike previous approaches, the axis direction is found first, thereby making sphere and cylinder evaluation relatively simple. The derivation of the solution is reasonably uncomplicated using straightforward trigonometrical operations.

Mathematics↗
PubMed1993

Mathematical models of the general corneal surface.

Although the curvature of the general cornea's two principal meridians can be acceptably modelled (within limits) by ellipses, this is insufficient to delineate the overall corneal surface. Applying different assumptions in either case, two corneal models are advanced which interpolate x, y, z points over the whole corneal surface, dependent only on the specifications of the plane curves of the principal meridians.

Anthropometry↗
PubMed1990

Tri-meridional analysis using arbitrary meridians.

The evaluation of the sphero-cylindrical components of curvature or refraction from measurements taken along three distinct meridians has been solved for various sets of three fixed meridians. A constraint of those solutions has been that the meridians of measurement are necessarily predetermined, which introduces an unwelcome restriction in experimental or clinical applications of tri-meridional analysis. A new solution is presented (in three formats: analogue computer, graphical and algebraic), which enables any set of three meridians to be used for tri-meridional analysis. A computer program listing (in BASIC) is included.

Computers, Analog↗