MXPA05000890A - Method and apparatus for measuring the refractive correction power in ophthalmic lenses. - Google Patents

Abstract

The present invention refers to a novel method for measuring the refractive corrective power of an ophthalmic lens using a lensometer. The refractive correction power of an ophthalmic lens refers to the dioptric power observed by the user through an ophthalmic lens commonly used for any observation angle. The method consists in positioning an ophthalmic lens to be tested at a predetermined distance and tilting angle with regard to the lensometer, so that said lensometer may carry out the measurement of the lens under the same conditions to those detected by the user's eye. Equations are included for the calculation of the aforementioned distance and tilting angle, as well as a mechanical device for simultaneously performing both calculations.

Description

METHOD AND DEVICE FOR THE MEASUREMENT OF REFRACTIVE CORRECTION POWER IN OPHTHALMIC LENSES DESCRIPTION OBJECT OF THE INVENTION The objective of this invention is to provide a new method for measuring the refractive correction power of an ophthalmic lens using a lensometer. Understanding by refractive correction power the dioptric power that the user will observe when viewing through the lens in its normal use for any angle of observation.

BACKGROUND In terms of ophthalmic optics, it is customary to specify a lens by its vertex power, the power of its frontal surface, its thickness and the refractive index of the material it is made of. Of these parameters, the most important is the vertex power, since it represents the power necessary to correct the refractive error of the eye, which finally is the raison d'être of the lens itself.

The lensometer, also called the phocimeter or vertometer, is an instrument used to measure the apex power of ophthalmic lenses, or its inverse: the posterior focal length. Currently there are several types of lensometers, but in general they can be classified into two types: optical lensometers and digital lensometers. Traditional optical lensometers are manual instruments that base their operation on the observation of an operator through a simple optical system. On the other hand, digital lensometers incorporate optical components with modern and sophisticated electronic and computational systems, in order to obtain efficient automated measuring instruments, with the deployment of graphic information and storage options for information and statistical analysis. It should be mentioned that despite the enormous technological advantages of digital lensometers over optical lensometers, the latter have remained firmly in the market due to their relatively low cost, compared to the former.

Optical lensometers are generally based, except for small variants, on a principle called a focimeter1, hence they are also known by this name and can measure both types of lenses. As shown in Figure 1, in this method a light source (1) illuminates a grid (2) placed at a distance x from a converging lens (3) with a back focal distance fp. The ophthalmic lens (4) to be measured is placed at a distance d from the converging lens (3). Given the distance d, a distance x is chosen in such a way that a collimated beam emerges from the ophthalmic lens, this collimation is checked through a telescope (5) focused to infinity, where the observer (6) must see the image perfectly focused. In the optical lensometers, the operator places the ophthalmic lens with its vertex firmly attached to the head of the instrument, then, through a mechanism, varies the distance x to see the focus in focus, the measurement of the vertex power obtained for the lens, it is read from a graduated scale in the same adjustment mechanism.

As a result of the vertiginous advances in science and technology in optics, electronics and computing, a new generation of lensmeters has emerged whose main characteristic is automated measurement, which omits the dependence of an observer. Currently there are several commercial teams of this type (Humphrey2, Humphrey et al.3, Ikezawa and Kobayashi4, Abitol et al.5, Oana and Yanagi6, Kajino et al.7, Ikezawa et al.), Which are based primarily on variations of the Hartmann test9. Essentially, variations of the so-called Hartmann test with four holes proposed by Malacara and Malacara10 are used. Figure 2 shows the bases of this method where a collimated lighting source (1) is used to illuminate a portion of the lens under test (2), the beam altered by the lens passes through a four-hole Hartmann grid (3) , the resulting beam is registered by an electronic opto device (4). Figure 3 shows the detail of the Hartmann grid used which is a circular plate with four circular holes at, b, c, and d orthogonally distributed at a distance p from the center of the plate. In spite of small differences, all conclude in the analysis of the deviation of the points of the diagram from their reference position, which is invariably done by means of electronic and computational devices, so these instruments deliver the result of the measurement automatically through digital indicators or graphic screens, usually having the option to store these readings and keep statistical records.

For both types of cases, as shown in Figure 4, the lens under test (1) is held by recharging it on the head (2) of the lensometer (3) to make the measurement.

Generally when it is desired to measure the power in an off-axis ophthalmic lens with a lensometer, ie with an observation angle different from the optical axis of the lens, as shown in FIG. 5, the lens under test (1) is moves off axis on the head (2) of the lensmeter (3), always recharging the rear surface of the lens against the base of the head. This may seem like an adequate solution to determine the power of the lens at different points outside its optical axis. However, as demonstrated by Trujillo-Schiaffino11, the measurement made in this way presents an error with respect to the power that the eye would "see" through the lens for that same angle of observation.

DETAILED DESCRIPTION OF THE INVENTION The present invention consists in a method for measuring the refractive correction power, that is, the power that the user will observe when viewing through the ophthalmic lens in normal use for any angle of observation.

To perform the measurement of the refractive correction power of an ophthalmic lens using a conventional lensmeter, as shown in Figure 6, the lens to be tested (1) must be provided with a distance d 'from the head (2) of the lensometer. (3) at the same time that an inclination f is provided with respect to the normal of the head.

The separation distance d 'is calculated with the expression: cos T - d Eq. (2) Where r is the radius of curvature of the posterior surface of the lens, d is the separation between the apex of the lens and the center of rotation of the eye which for practical purposes can be considered as 28 mm and T is the angle of observation with with respect to the optical axis of the lens.

The angle of inclination of the lens with respect to the normal head of the lensometer is given by the expression: Where r, d and T represent the same parameters described above.

By performing these two movements simultaneously, the measurement obtained in the lensometer will correspond to the power that the user of the lens will observe when viewing through it in normal use for any angle of observation.

These two movements can be applied simultaneously by means of a mechanical mount, adapted to the lensometer and on which the lens is held. Figure 7 illustrates this device.

The lens under test (1) is held by the clamping mechanism (2) against the head (3) of the lensmeter (9), The clamping mechanism (2) is held by a straight connector (4) that allows it to rotate on the upper pivot (5), the upper pivot (5) is mounted on a horseshoe connector (6) attached to the lensometer support (8) through the central pivot (7). This mechanism is placed in such a way that the central pivot (7) is positioned at 28 mm from the head on the optical axis of the system. By turning the fastener (2) with the lens under test (1) on the upper pivot (5) and the central pivot (7), the separation d 'and the angle of inclination necessary to carry out the measurement of the diopter power of the lens in the same conditions as the eye of the user will find when observing through the lens in that same direction, thereby obtaining the measurement of the refractive correction power of the lens being tested. This device can be adapted to almost any commercial lensmeter. And it can also be used to correct the measurement in interferometric assemblies.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1. Figure 1 shows a schematic representation of the method of measuring diopter power in ophthalmic lenses known as a phocimeter, which is used in most commercial optical lensometers.

FIG 2. Figure 2 shows a schematic representation of the dioptric power measurement method in ophthalmic lenses known as a four-hole Hartmann test, which is used in most commercial digital lensometers.

FIG. 3. Figure 3 shows the detail of the grid used in the four-hole Hartmann test.

FIG. 4. Figure 4 shows the conventional way of performing the measurement on the optical axis of diopter power of an ophthalmic lens using a commercial lensmeter.

FIG. 5. Figure 5 shows the conventional way of making the measurement outside the optical axis of diopter power of an ophthalmic lens using a commercial lensmeter.

FIG 6. Figure 6 shows the proposed method for the off-axis measurement of the refractive correction power of an ophthalmic lens using a commercial lensmeter, applying a d 'distance of the head of the lensometer simultaneously with an inclination f with respect to the normal of the head.

FIG 7. Figure 7 shows a possible mechanical device proposed to effect the separation d 'of the head of the lensometer simultaneously with an inclination f with respect to the normal of the head

Claims (3)

CLAIMS Having sufficiently described my invention, I consider as a novelty and therefore claim as my exclusive property, what is contained in the following clauses:

1. The method of measuring the refractive correction power of an ophthalmic lens with a lensometer consisting of positioning the lens under test with a spacing of the head of the lensometer and an angle of inclination f with respect to the normal of the head, given respectively by the Expressions Where r is the radius of curvature of the back surface of the lens, d is the separation between the apex of the lens and the center of rotation of the eye and T is the angle of observation with respect to the optical axis of the lens. Where r, d and T represent the same parameters described above

2. The application of the method described in the previous clause as an improvement for the measurement of the dioptric power of an ophthalmic lens.

3. The principle of the mechanical device is formed by a holding mechanism for ophthalmic lenses supported through a support on an upper pivot which is mounted on a horseshoe support connected through a central pivot to the support for lensometer. The use of the mechanical device described in the previous clause as a mounting frame for ophthalmic lenses on a lensmeter in order to measure the diopter power of the lens under test. The use of any other clamping mechanism for ophthalmic lenses on a lensmeter that provides the characteristics of the method described in clause 1.