It is known that certain measurements need to be taken in the manufacture of ophthalmic lenses (e. g., contact lenses and intraocular lenses) such as the center thickness of the lens, the radius of curvature of the posterior and anterior surfaces of the lens, the diameter of the lens, the dioptric power of the lens, etc. A variety of fixture devices have been proposed in the prior art to position an ophthalmic lens in preparation of taking such measurements. It is very important that the fixture device consistently position each lens to be measured on the fixture device so that the measuring device (e. g., a probe) is aligned with a consistent reference point on the lens. If the fixture device fails to consistently position a lens for measurement, inaccurate measurements will inevitably occur due to differences in the relative positioning of one lens compared to the next with regard to the measuring device.

Examples of prior art fixture devices include those that fixture the lens while the lens is submerged in solution, including those seen in U. S. Patents Nos. 4,684,246 issued Aug. 4, 1987; 5,719,669 issued Feb. 17,1998, and fixtures which capture the lens about the perimeter thereof which leave both the anterior (front) and posterior (back) surfaces of the lens unobstructed by the fixture so as to permit measurement to be made either from beneath or above the lens. In U. S. Patent No. 4,665,624 issued May 19, 1987, a probe rises from beneath the lens to deform the lens. The amount of lens deflection under a known applied force is used to derive the needed measurement. In the related U. S. Patent Nos. 5, 280,336 and 5,416,564, an optical system passes light through the lens while held in a three-point mount (ref. numeral 40 therein) to take the needed measurements of a lens held thereby.

In many of the fixture devices of the prior art, there is a certain amount of human involvement required to place the lens in the fixture. Thus, the more the fixture device and method relies on the human operator to correctly place the lens in the fixture device, the more the chance that inaccuracies in measurement of the lenses will occur. It is therefore a main object of the present lens fixture device and method to consistently position sequentially placed lenses on a fixture for measurement. It is a further object of the present lens fixture device and method to consistently center a lens on a fixture despite the involvement of a human operator in initially placing the lens on the fixture. It is another object of the present invention to provide a lens fixture device which is very simple to manufacture and use. It is yet a further object of the present invention to provide a lens fixture device and method which substantially eliminates the chance of lens damage due to interaction of the lens with the fixture. These and other objects will in part be apparent and in part appear in the description that follows.

Summary of the Invention The present invention solves the problem of inconsistent lens placement on a fixture by providing a fixture and method which consistently centers a wet lens on a fixture for measurement of various parameters of the lens including, but not limited to, the center thickness of the lens. The present invention furthermore achieves the objects and advantages stated above by providing a lens fixture device and method which is simple in construction and use, yet achieves a consistent lens centering on the fixture regardless of human error in initially placing the lens in a non-centered position on the fixture. The prior art simply fails to teach or suggest such a lens fixture device and method. The present inventive lens fixture device and method includes a fixture having a convex shaped lens-seating surface which has a radius of curvature smaller than the radius of curvature of the concave posterior surface (i. e., the surface which is placed against the cornea) of the lens to be measured. In this way, upon placing a wet, hydrophilic lens on the smaller radius lens-seating surface of the fixture, the lens will automatically slide on the lens-seating surface into its true center on the fixture due to the forces created by the differing radii of curvature of the fixture and lens. This movement of the lens on and with respect to the lens-seating surface of the fixture may thus be considered"automatic"in that no other external forces are required to move the lens into a centered position on the lens-seating surface of the fixture. Thus, if the operator were to purposely push a lens so positioned on the fixture away from its centered position, the forces between the posterior surface of the lens and the lens-seating surface of the fixture will cause the lens to slide back to the centered position on the fixture. This manner of wet lens centering on a fixture is neither taught nor suggested in the prior art.

Brief Description of the Drawings Figure 1 is a perspective view of the inventive fixture placed adjacent a common lens thickness measurement device shown in broken lines; Figure 2 is a perspective view of a prior art lens fixture which is used with the lens thickness measurement device shown in broken lines in Fig. 1; Figure 3 is an enlarged, side-elevational view in longitudinal cross-section of the inventive lens fixture; Figure 4 is an enlarged, side-elevational view of the upper portion of the fixture showing relative dimensions thereof with regard to a contact lens to be placed thereon for measurement; and Figure 5 is the view of Fig. 4 showing a lens positioned and centered on the lens-seating surface of the fixture in the intended manner.

Detailed Description Referring now to the drawing, there is seen in Figure 1 a lens measuring device 10 having a probe 12 used to measure the center thickness of a spherical contact lens 14 placed on a fixture in the form of a pedestal 16. Lens measuring device 10 may be of any known type, and it is understood that the lens measuring device 10 is shown and described herein for illustration purposes only. The inventive fixture device and method may of course be used in conjunction with a variety of measuring devices which require consistent lens centering on a fixture.

Briefly, measuring device 10 works by lowering probe 12 onto lens 14 until a sensor stops movement of the probe upon touching the outer (anterior or convex) lens surface. The pedestal 16 is mounted to table 18 so that the apex A2 (Fig. 4) of the pedestal 16 is set at a known location and, since the inner (posterior or concave) lens surface is in contacting relation to the apex A2 of the pedestal, the thickness of the lens measured at the lens apex Ai is calculated as the difference between the probe stop location and the pedestal apex A2 location. The center thickness of the lens 14 is then displayed on a digital read-out 20.

While it is a simple matter to align probe 12 and pedestal 16 along a common axis x- x, ensuring that the geometric center of the lens 14 at apex A, thereof is also aligned along axis x-x is a much more challenging prospect as appreciated by those skilled in the art of ophthalmic lens metrology. This is because in the measurement of soft (hydrophilic) contact lenses, the lens must be measured while in the hydrated state where the lens is very soft and pliable, and thus difficult to handle. The wetness of the lens in combination with the smoothness of the fixture surface necessarily causes the lens to easily slide about on the lens- seating surface of the fixture. Thus, even if an operator were to centrally place the lens on the fixture (which is difficult to do consistently), the lens is still prone to slide off center due to the extremely low coefficient of friction between the lens and the fixture. This sliding of the lens on the fixture could occur from even low vibrations emanating from the surrounding manufacturing environment, for example.

A prior art fixture device is seen in Figure 2 to comprise a spherical ball 22 suspended in a frame 24. The radius of curvature of the ball 22 equals the radius of curvature of the posterior surface of the lens to be seated thereon, the thinking being that the curves need to match to ensure the lens surface is not distorted during measurement with the probe. It will be appreciated, however, that the problem of centering the lens on the ball 22 is pervasive and introduces much error in the measuring process. This is again due to the extremely low coefficient of friction between the hydrated lens and the ball surface, and the fact that there are no means for retaining the lens in the centered position on the ball. Thus, even if a very skilled operator were to successfully center a lens on the ball for measurement (which is difficult to do, nevertheless consistently over a multitude of lenses), the lens is still prone to sliding off-center, which even if only microns would introduce error into the measurement process.

Referring now to Figures 3-5, the inventive pedestal 16 is seen to comprise a cylinder portion 30 having a bottom surface 30'which includes means such as a threaded bore 32 for fixing the pedestal 16 to a surface such as table 14 in Figure 1. Other means for fixing the pedestal 16 to a surface are of course possible (e. g., gluing, soldering, integrally forming the pedestal with a surface, etc.). Opposite bottom surface 30'a cap 34 having a convex lens- seating surface 36 is provided. As seen best in Fig. 4, the dimensions of the lens-seating surface 36 relative to the lens 14 to be measured are given. In particular, lens-seating surface 36 includes a spherical central surface portion 36a having a first radius of curvature Ri which is slightly smaller than the radius of curvature R2 of posterior surface 14a of lens 14. In a preferred embodiment, Rl is between about 0. 005 to 0.100mm less than R2, is more preferably between about 0.010 to. 050mm less than R2, and is yet more preferably between about 0.020 to 0.040mm less than R2. In the most preferred embodiment, Rl is about 0.035mm less than R2. Thus, when measuring a lens having a posterior surface having a radius of curvature R2 of 8.6mm, the radius of curvature of lens-seating surface 36 is selected to be between about 8.5 and 8.595mm; is more preferably selected to be between about 8.55 and 8.59; is yet more preferably selected to be between about 8.56 and 8.58mm; and is most preferably selected to be about 8.565mm. By making Rl only slightly smaller than R2, the interacting forces between a hydrated lens placed upon surface 36a cause lens 14 to find its center upon surface 36a, with the apex Al of the lens 14 aligned with the apex A2 of lens seating surface 36a along the center axis x-x.

It is of course understood that the lens should be initially placed on the lens-seating surface as close to center as possible since if the lens is initially placed too far off center, the interacting forces generated by the differing radii of curvature between the lens and pedestal surfaces will not be strong enough to overcome the distance the lens is placed off-center and the lens will not slide into its centered position. This"breaking point"on the pedestal can be judged fairly quickly by an operator without undue experimentation and the operator will soon realize the approximate placement required of the lens to ensure the interacting forces will be sufficient to move the lens into its centered position on the pedestal. For a lens and pedestal of the preferred dimensions described herein, it has been found that the peripheral edge of the lens should be placed no less than 2mm from the apex Al of the lens in order for the lens to center itself on the pedestal.

In the preferred embodiment of the invention, the diameter D, of pedestal cap 36 at the perimeter of second surface 36b is slightly smaller than the diameter D2 of hydrated lens 14. In the case of a lens 14 having a diameter D2 of about 14.2mm, the preferred diameter D of cap 36 is preferably about between 13.5 to 14.2mm, is more preferably about between 13.75 to 14. 0mm, and is most preferably about 13.8mm. As seen in Figure 5, the outer surface 36b of cap 36 forms a bevel which extends slightly inwardly relative to the curvature of central cap surface 36a such that the corresponding perimeter 14b of lens 14 is spaced slightly from cap surface 36b. This spacing between the lens perimeter and bevel creates a small gap 40 at the perimeter of the lens to facilitate removal of the lens 14 from pedestal cap 36 once the measurement of lens 14 is complete. In the preferred embodiment, the width W of bevel 36b is about between 1.0 and 2.0mm, is more preferably about between 1.5 and 2.0mm, and is most preferably about 1.085mm.

The present invention thus provides a lens-centering pedestal for centering a hydrated ophthalmic lens in preparation of taking a measurement of the lens, for example, measuring lens thickness, although it is understood that the present invention is applicable to other processes where lens centering on a fixture is required. For example, measurement of the optics or cosmetic inspection of a lens could be carried out by making the pedestal from a transparent material such as glass for passage of light therethrough. The present invention achieves lens centering on the pedestal automatically by providing a radius of curvature on the pedestal lens-seating surface which is slightly smaller than the radius of curvature of the posterior surface of the lens to be seated thereon. This differing radii of curvature between these two contacting surfaces, while small, generates sufficient interacting forces of adhesion which urge the lens to slide into the centered position on the lens-seating surface of the pedestal. This is neither taught nor suggested by the prior art, and fulfills an urgent and long- felt need in the industry for a robust yet simple apparatus and method for centering hydrated lenses upon a fixture for measurement. While the invention has been shown and described herein with regard to a preferred embodiment and manufacturing environment, it is understood that variations may be made thereto without departing from the full spirit and scope of the invention as defined by the claims which follow.