IL142861A - Method and apparatus for centring an ophthalmic lens - Google Patents

Abstract

Method of determining the optical center of an ophthalmic lens, of the type in which: in the absence of a lens, a first image of a radiation emitter (1) on a receiver (2) is analyzed, the emitter and the receiver being aligned on a reference axis (OO'); the ophthalmic lens (E) is placed on a support (3) which is transparent to the radiation and is located between the emitter (1) and the receiver (2), and the size and the position of the modified image obtained is analyzed; and the position of an approximate optical center (OC) of the lens is calculated from this analysis, characterized in that, in addition, the following operations are performed: the lens is displaced along a predetermined vector (OC1-OC2); the resulting displacement (A'1-A'2) of a predetermined point from the image is measured; and the position of the optical center of the lens is calculated from the latter measurement. 2752 י" ב בתשרי התשס" ה - September 27, 2004

Description

Method and apparatus for centring an ophthalmic lens Briot International C. 133040 Method and apparatus for centering an ophthalmic lens The present invention relates to a method of determining the optical center of an ophthalmic lens, of the type described in the preamble of claim 1.

It applies in particular to the automatic or semiautomatic placement of a grinding adapter on circular blanks of optical glass intended to be ground.

The conventional technique will firstly be explained with regard to figures 1 and 2, which are perspective diagrams illustrating the main parts of the apparatus used, and to figure 3, which is a schematic view in vertical axial cross section of an ophthalmic lens whose optical center it is desired to determine.

Existing apparatuses comprise a light emitter 1, especially a light beam or test pattern, and a receiver 2, especially a CCD photosensitive element or the like. In addition, the apparatus includes an ophthalmic lens support 3 placed between the emitter and the receiver. This support, which is transparent to the radiation emitted by the emitter, consists in practice of an XY table, possibly provided with rotational driving means .

In the absence of a lens, the receiver analyzes the position of a predetermined point in the image, typically its barycenter 0' , which is the image of the barycenter 0 of the emitter, and the size of the image of the emitter. This size is determined by measuring the distance separating two specific points in the image from the emitter on the receiver. The straight line 00' constitutes the vertical reference axis.

When a lens or optical glass or blank E is placed in the apparatus (fig. 2) on the support 3, the emitter is seen through the lens. The image produced on the receiver is therefore deflected/deformed according to the optical characteristics of the lens. Thus, considering the object focus F of the lens and the intersection M of the..00' axis with the mid-plane P of the lens, which is the plane passing through the middle of the thickness of the lens, the new position A' of the barycenter of the image lies on the straight line FM.

The image thus deformed is analyzed, the system seeks the new position A' of the barycenter of the image and stores the size of the image of the emitter. On the basis of this information, the system calculates the following parameters : (1) the optical center OC lies on the straight line defined by the intersection of two planes: - the mid-plane P of the lens, defined above and - the plane defined by the points 0, 0' and A' ; (2) the magnification factor Mag of the lens, which is the ratio of the size of the image taken without any glass to the size of the image taken through the glass; (3) the decentration distance d of the lens, that is to say the distance between the optical center OC of the lens and the 00' axis in the mid-plane of the lens. ■This distance is calculated by measuring the distance O'A' on the image and by applying this value to a function of the type d = f (0'A' ,Mag, L, 1) , where: - O'A' is measured on the image of the emitter on the receiver, - Mag is the magnification factor of the lens, - L is the distance between the emitter and the mid-plane P of the lens and - 1 is the distance between the mid-plane of the lens and the receiver.

The function f is a relatively simple function, which results from conventional geometrical optics calculations .

When the optical center 0C is determined in this way, it is aligned with respect to the 00' axis. A grinding, for example adhesive, adapter 5 is supported by an arm 6 which pivots about a vertical axis 7 parallel to the 00' axis. The axis of the adapter is aligned with respect to the 00' axis and then the adapter is lowered until it comes into contact with the lens .

The position of the optical center, determined in the manner explained above, proves in many cases to 5 be imprecise, especially for the following reasons: a) for weak correcting lenses, a decentration d of several centimeters generates a very small distance O'A' . For a decentration of a few millimeters, the distance O'A' becomes minute. Within this context, a 0 minute error in the O'A' measurement is multiplied, by a lever effect, in the calculated position of the " optical center of the lens. In addition, when O'A' becomes very small, the distance O'A' is smaller than the measurement resolution of the receivers normally 5 used; this generates large errors in the O'A' measurement and therefore in the position of the optical center; b) the formula allowing the distance d to be defined takes into account the quantities L and 1 which 0 are the respective distances of the emitter and of the receiver from the mid-plane P of the lens at the measurement point .

In fact, considering two measurements made on the same lens at two different points on two axes OiO'i 5 and 020'2 (fig- 3), the mid-plane Pi, P2 does not lie at the same level since, depending on the curvatures of the lens, its thickness and its camber vary. The quantities Li and li are consequently different from the quantities L2 and 12. 0 This problem is accentuated if all lenses to be treated, from the thinnest to the thickest, are to be considered, since the thickness and the camber of lenses vary.

Finally, the quantities L and 1 vary from one 5 lens to another and they also vary over the same lens if two different measurement points are considered." For all these reasons, the optical- center determined as above is in fact an approximate optical center of the lens .

To obviate these difficulties, most systems take into account a mid-plane which is a plane lying at the middle of the thicknesses of lenses to be treated. The quantities L and 1 thus fixed have, of course, a negative effect on the precision with which the optical center is detected.

It is an object of the invention to provide a method and an apparatus which are capable of achieving a precision in the positioning of the optical center of ophthalmic lenses of the order of 0.1 mm, while still using a standard commercial receiver of reasonable cost and by inexpensively modifying the existing apparatuses .

For this purpose, the subject of the invention is a method of the aforementioned type, characterized by the characterizing part of claim 1.

The method according to the invention may comprise one or more of the characteristics of claims 2 to 7, taken in isolation or in any technically possible combination thereof.

The subject of the invention is also a centering apparatus intended for implementing the method defined above.

This apparatus is as described in claim 8.

Further characteristics of this apparatus are described in claims 9 to 11.

Examples of ways of implementing the invention will now be described with regard to the appended figures 4 to 9, in which: - figure 4 is a diagram similar to figure 2, illustrating the method according to the invention; figures 5 and 6 are schematic views in vertical cross section of two different lenses to which this method is applied; - figure 7 is a graph in which the power of the lens in diopters is plotted on the x-axis 'and' the' corresponding magnification factor in the apparatus used is plotted on the y-axis; figure 8 is a graph in which the power of the lens in diopters is plotted on the x-axis and the minimum useable displacement of the lens is plotted on the y-axis; and - figure 9 shows schematically the centering apparatus used.

To explain the method of the invention, the lens E will firstly be likened to a thin lens having no thickness .

Figure 4 illustrates two positions of the lens in the same plane P: - a first position Ei in which the approximate optical center lies at a point OCi offset by a distance di with respect to the 00' axis. The points 0, 0', F1( A'i and OCi are coplanar, as described above with regard to figure 2 ; a second position E2 in which the approximate optical center lies at a point 0C2 offset by a distance d2 with respect to the 00' axis. The points 0, 0', F2, A' 2 and 0C2 are coplanar.

With the lens occupying the first position, it is moved to its second position by a displacement according to a known vector Ex,Ey. The resulting displacement A'x,A'y of the barycenter A', from ΑΊ to A' 2, is measured.

When the positions Εχ and E2 are close to the centered position of the lens, in a sense which will become apparent later, the displacements of the point A' are practically proportional to those of the optical center, along each principal axis of an orthonormal coordinate system: A'x = aEx, A'Y = bEY.

Having thus determined the coefficients a and b, the displacement of the lens which brings its optical center exactly onto the 00' axis, that is to say which brings the point A' to 0' , is calculated.

Figures 5 and 6 show that the more powerful the lens, that is to say the greater its correction; the closer the points OCi and 0C2 must be to the 00' axis in order to avoid any loss of precision. This is because, for a weakly correcting lens (figure 5) , the mid-plane P varies little over a relatively wide range of positions of the measurement point with respect to the optical center. In contrast, for a strongly correcting lens (figure 6) , the same precision in the position of the mid-plane P assumes that one remains much closer to the optical center.

To avoid any difficulty from this standpoint, and to disregard variations in the position of the mid-plane P, two positions Ei and E2 of the lens which are substantially symmetrical with respect to its position of alignment of the optical center on the 00' axis are chosen.

Moreover, it is clear that another condition for correctly operating the method is that the distances 0ΆΊ and 0'A'2 be sufficient to be detected, reliably by the receiver 2.

These considerations result in the following operating mode.

The lens E is placed on the support 3 in any position. A first image is taken and analyzed and an approximate position of the optical center is calculated in the conventional manner described above with regard to figure 2.

As indicated, the measurement of this first image provides the magnification factor Mag of the lens in the apparatus. The graph in figure 7 makes it possible to calculate therefrom the power or correction factor C of the lens. For example, as illustrated in figure 7, a magnification of 1.2 has been measured, which gives a factor C of +3 diopters.

Reference is then made to the graph in figure 8. This indicates, for a given factor C, the displacement D of the lens, in mm, which is needed in order to observe a displacement of the point A' by an elementary unit (1 pixel) on the CCD receiver. Thus, in the above example, since the factor C is +3 diopters, the receiver is capable of detecting the displacement of the point A' for a displacement of the lens of about 0.15 mm.

Consequently, two positions Ei and E2 are chosen in which OCi and OC2 are separated by the same distance di = d2, which is markedly greater than 0.15 mm, from the 00' axis. In practice, the positions Ei and E2 are chosen so that OCi and OC2 are symmetrical with respect to the 00' axis, as shown.

In particular, the first position Ei may be the position that was used to determine the point OC, if the distance d from figure 2 is suitable.

From the two positions of the lens, the aforementioned coefficients a and b are calculated, from these coefficients the displacement that the lens must perform in order for the optical center to be aligned with the 00' axis is calculated, and this displacement is performed.

Finally, having thus accurately centered the lens, the adapter 5 is brought onto the 00' axis and lowered until it is attached by adhesion to the lens. The adapter · is therefore more or less perfectly centered with respect to the optical center of the lens .

Figure 9 shows schematically the apparatus for implementing the method described above. Shown again are the emitter 1, the receiver 2 and the lens support 3, consisting of an XY table with an X motor 8 and a Y motor 9. The arm 6 supporting an adapter 5 is mounted on a vertical shaft 7 so as to be able to pivot and move in vertical translation, as shown schematically by the arrows . The means for driving the arm 6 are shown schematically at 10.

Also shown in figure 9 is an information processing and control unit 11, interfaces 12 to 15 between this unit and the members 1-2, 8, 9 and 10, respectively, a man-machine interface 16 linked to the unit 11, and a display 17 linked to this same unit.

Of course, the unit 11 is fed with the data and the computing means necessary to execute the program described above, and this is the only modification as compared with the conventional centering apparatuses.

With such an apparatus, it is possible to place the adapter on a lens E automatically, or at least semiauto atically, in order to allow the operator to choose between two positions El and E2.

As a variant, it will be understood that the lens may be placed on a stationary support, while the aforementioned displacements are performed by the emitter-receiver assembly and by the shaft 7.

Claims (11)

1. Method of determining the optical center of an ophthalmic lens, of the type in which: - in the absence of a lens, a first image of a radiation emitter (1) on a receiver (2) is analyzed, the emitter and the receiver being aligned on a reference axis (00'); the ophthalmic lens (E) is placed on a support (3) which is transparent to the radiation and is located between the emitter (1) and the receiver (2), and the size and the position of the modified image obtained is analyzed; and the position of an approximate optical center (OC) of the lens is calculated from this analysis, characterized in that, in addition, the following operations are performed: the lens is displaced along a predetermined vector (0Ci-0C2) ; - the resulting displacement (ΑΊ-Α'2) °f a predetermined point from the image is measured; and the position of the optical center of the lens is calculated from the latter measurement.

2. Process according to claim 1, characterized in that, after having determined the' approximate optical center of the lens, the lens (E) is displaced in order to bring this optical center approximately to a first defined position (OCi) , then to a second defined position (OC2) offset substantially by the same distance (di, d2) with respect to the reference axis (00' ) as said first position, and the displacement of said predetermined point in the image of the emitter (1) when the approximate optical center is moved from the first predetermined position (OCi) to the second defined position (0C2) is measured.

3. Method according to claim 2, characterized "in that said first and second defined positions (OCi, 0C2) are substantially symmetrical with respect to the reference axis (0').

4. Method according to any one of claims 1 to 3 , characterized in that said predetermined point is the barycenter (Ο' , A') of the image.

5. Method according to any one of claims 1 to 4, 5 characterized in that an amplitude of the or each displacement of the lens (E) is chosen which is all the smaller the larger the power of the lens is.

6. Method according to any one of claims 1 to 5, characterized in that, after the optical center of the 0 lens (E) has been determined, this optical center is brought onto the reference axis (00')·

7. ' - 7. Method according to claim 6, characterized in that, after having brought the optical center onto the reference axis {00'), an adapter (5) intended to grind 5 the lens, is placed thereon.

8. Apparatus for centering an ophthalmic lens, of the type comprising a radiation emitter (1) , a radiation receiver (2), a lens support (3), transparent to the radiation and located between the emitter and 0 the receiver, and means (11) for analysis of the radiation ■ detected and for control, said means being used to make the lens (E) perform relative movements with respect to the reference axis (00' ) defined by the emitter and the receiver, characterized in that the 5 analysis and control means (11) are designed to control a . displacement of the lens between two positions (Εχ, E2) , to measure the corresponding displacement (ΑΊ, A' 2) of the image of the emitter (1) on the receiver (2), to calculate therefrom the displacement of the 0 lens needed to place its optical center on the reference axis, and to control this displacement.

9. Apparatus according to claim 8, characterized in that said two positions (Ei, E2) are substantially offset by the same distance with respect to the 5 reference axis (00' ) .

10. Apparatus according to claim 9, characterized in that said two positions (Ei, E2) are substantially symmetrical with respect to the reference axis {00').

11. Apparatus according to any one of claims 8 to 10, characterized in that the analysis and control means (11) contain data making it possible to determine a minimum displacement of the lens (E) for ensuring a detectable displacement of the image on the receiver (2) . For the Applicants REINHOLD COHN AND PARTNERS By a