Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Methods of use
Patent
1997-05-28
1999-05-04
Manuel, George
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Methods of use
A61B 300
Patent
active
059009241
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for determining absolute spatial coordinates of at least one location on a reflecting surface.
Methods for taking measurements of reflecting surfaces are known. These methods are based on the principle that a point with fixed spatial coordinates is reflected by the surface and that the virtual image is measured with a video keratometer. Several such methods and devices are known, for example, from U.S. Pat. No. 5,106,183, from U.S. Pat. No. 5,110,200 and from the German Offenlegungsschrift 40 30 002.
These methods are suitable for determining relative data points, they are, however, not suited for determining the exact spatial coordinates of individual locations on a reflecting plane.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method which can be used to determine individual spatial coordinates of a reflecting plane. This object is solved by a method in which on the surface there is reflected a location P.sub.1 having a distance h to an optical axis, with the line from location P.sub.1 perpendicular to the optical axis defining the location O, an arbitrary location D at a distance d from location O on the optical axis is defined as vertex of the fixation angle .phi. of P.sub.1, the distance I.sub.1 of the virtual image of location P.sub.1 to the optical axis is calculated with D as vertex of the fixation angle, the distance I.sub.2 of the virtual image of location P.sub.1 to the optical axis is measured, d is varied until the measured distance I.sub.1 * coincides with the calculated distance I.sub.2 * at D*, and a spatial coordinate of the reflecting surface is calculated from the respective fixation angle .phi.* and the distance I.sub.1 *=I.sub.2 *.
When individual spatial coordinates are measured with conventional methods, the problem arises that for measuring the exact dimensions of a virtual image, certain assumptions have to be made about the spatial location of a virtual image, which do not always correspond to the actual situation. For mathematical reasons, however, it is not possible to measure the dimensions of the virtual image and the distance of the virtual image from a fixed spatial coordinate at the same time.
The invention is based on the understanding that an arbitrary location of the virtual image in space can be assumed as long as the measured values are iteratively compared with the calculated values until they coincide. Consequently, the iterative method makes it possible to determine both the exact dimensions of the virtual image and the distance from the virtual image to a fixed location.
It is advantageous if the location P.sub.1 is a location of a Placido disc. By subdividing a Placido disc into segments, an arbitrarily large number of reflecting locations can be measured which can be easily located again based on their location and preferably their color in the measurement.
Preferably, the invention is applied for measuring the cornea of the eye, for example for determining the shape of rigid contact lenses.
In order to limit the possible location of the location D on the optical axis, it is advantageous if the distance a between the location O and the intersection S between the reflecting surface and the optical axis is determined by superimposing at least one centering object P.sub.2.
In one embodiment of the invention, the distance I.sub.1 * is calculated according to the formula ##EQU1##
The distance I.sub.2 is preferably measured with a video keratometer.
In another embodiment of the invention, the entire shape of a reflecting surface is determined by determining the spatial coordinates of a plurality of locations and subsequently interpolating therebetween.
BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS
An embodiment of the method of the invention is depicted in the drawings and is described in further detail hereinafter.
It is shown in:
FIG. 1 a schematic representation of the geometrical arrangement, and
FIG. 2 an example of a calibration function.
DETAILED DESCRIPTION O
REFERENCES:
patent: 5162811 (1992-11-01), Lammers et al.
Investigative Ophtalmology & Visual Science vol. 25, No. 12, Dec. 1984, pp. 1426-1435; S. D. Klyce: `Computer-Assisted Corneal Topography`.
Bende Klaus Thomas
Kielmann Michael Matallana
Oltrup Theo
Wallfeld Axel Von
Feiereisen Henry M.
Manuel George
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