Precision composite article

Details Precision composite article Precision composite article

2001-12-05

2003-05-27

Acquah, Samuel A. (Department: 1711)

Optical: systems and elements

Lens

Eyepiece

C264S001100, C264S001320, C264S001360, C264S001380, C264S001700, C525S089000, C525S090000, C525S091000, C525S09200D, C525S094000, C525S098000, C525S107000, C525S123000, C525S191000, C525S330300, C522S006000, C428S441000, C428S500000, C428S515000, C428S522000

Reexamination Certificate

active

06570714

ABSTRACT:

BACKGROUND OF THE INVENTION
Ophthalmic lenses are used to correct vision by changing the focal length of the light rays entering the pupil of an eyeglass-wearer. When the patient is near-sighted or far-sighted, the correction is rather simply made using a single vision lens in which the outer and inner surfaces of the lens are both spherical, but have different radii of curvature. An added level of complication occurs when a patient exhibits astigmatism in one or both eyes. In this case the back surface of the lens is made toroidal by imposing two different radii of curvature on the same surface. In order to properly correct for astigmatism, the rotational position of the toroidal surface must be fixed with respect to the pupil of the eyeglass-wearer (typically accomplished with the eyeglass frames). Patients who require multi-vision lenses, such as bifocals and progressives, introduce yet another level of complication. In this case, a bifocal or progressive pocket (an “add” pocket) is molded into the front surface of the lens, providing a lens that corrects to various focal lengths across the lens depending on the spatial distribution of the add pocket. The most common example of this is someone who is both near-sighted (needs eyeglasses to see objects at a distance) and far-sighted (needs a bifocal pocket to read text).
When a patient needs both multi-vision lenses and astigmatic correction, the toroidal back surface must be fixed rotationally with respect to the location and orientation of the bifocal pocket. This presents an obstacle to high-throughput manufacturing of plastic ophthalmic lenses for reasons that will be discussed below.
Polycarbonate is widely used as an optical material for the production of ophthalmic lenses. It has a refractive index of 1.586, reasonably good light transmission, and extremely good impact resistance. Imparting scratch resistance to polycarbonate lenses must typically be accomplished with a secondary coating.
Polycarbonate ophthalmic lenses are formed by injection molding. Injection molding is a process that requires high injection and clamping pressures. As a result, molds are quite expensive for industrial-scale equipment. In addition, changing molds from one to another is time-consuming and involves a significant amount of down-time for the injection molding system, as well as significant start-up time before obtaining quasi-steady-state operation.
Typical ophthalmic lenses have a prescription range of +2 to −6 diopters in ¼ diopter increments, a bifocal pocket of 0 to +3 diopters in ½ increments, and an astigmatic correction from 0 to 2 in ¼ increments and a specified rotational angle of 0 to 90 degrees in 1-degree increments. Thus, taking into account all of the possible variations, there are roughly 10
5
different prescriptions possible. In terms of injection molding, there would have to be approximately 150 different front molds and 720 different back molds in order to accommodate the prescription ranges covering multi-vision lenses with astigmatic correction. These numbers increase even more when other design features such as aspherical lenses or progressives are considered. The high-volume production of polycarbonate lenses with only a few variations can be quite economical. However, since molds are expensive and change-out time is excessive, injection molding of multi-vision lenses incorporating astigmatic corrections is not practical due to the large number of variations. Even if such a manufacturing process could be economically carried out, long tooling change-out times would require stocking the entire range of prescriptions, adding substantially to the cost of the lens. When the number of substrate variations is small, they may be produced economically by injection molding or other techniques. Thus, what is needed in order to produce the relatively large number of prescription variations is a method by which a lens or lens blank (i.e., substrates) can be imparted with either the desired back toroidal surface or the desired front multi-focal surface after the substrate fabrication process. While some work has been done in this area (e.g., U.S. Pat. Nos. 4,873,029 and 5,531,940), the resins used have been liquids, which creates a new set of problems and complexities in keeping the liquid resins in place in a mold prior to cure.
A further difficulty in the ophthalmics industry relates to the production of photochromic lenses, said lenses incorporating photochromic dyes that undergo a change in color upon exposure to sunlight. Unfortunately, photochromic dyes are well known to be sensitive to the lens manufacturing processes. Either the dyes are attacked or degraded by the peroxide initiators used to polymerize the lens casting resins, or the dyes lose their activity upon incorporation into the lens material due to steric hindrances or other factors. In an attempt to circumvent these problems, the dyes are often added after lens fabrication by means of an “imbibition” process in which the dyes are imbibed or absorbed partially into the lens in a hot water bath. In this case, long soaking times at high temperatures and softer lens materials must often be used in order to achieve acceptable dye uptake. The resultant thin layer of photochromic dye concentrated in the near-surface region of the lens shows problematic behavior in terms of both degree of tint obtained in the darkened state, as well as fatigue of the photochromic dye over time.
To overcome these performance limitations, polymer matrices have been developed that successfully incorporate photochromic dyes throughout the lens material during the fabrication process (see for example, Henry and Vial, U.S. Pat. No. 6,034,193). However, the resultant material is relatively expensive since the photochromic dye is dispersed throughout the material. Because the product is typically a semi-finished lens blank, of which 20-90% may be ground away during the subsequent surfacing process, much of the valuable photochromic dye is discarded and photochromic lenses produced by this technique are expensive. Thus, it would be desirable if the photochromic-containing material could be applied to the lens surface in such a way as to provide a layer of material to the front surface of the lens such that very little or none of the photochromic containing material was lost during surfacing. Further, it would be desirable if such a layer could be approximately 0.3 mm to 2.0 mm thick, such that photobleaching and/or fatigue problems over the lifetime of the lens were minimized.
Yet another problem in the ophthalmics industry concerns the production of polarized lenses. Such lenses are currently produced by fixing a polarizing film within a gasketed mold assembly, filling the mold on both sides of the polarizing film with a curable liquid resin, then curing the resin to produce a semi-finished lens blank with an embedded polarizing film. This approach is problematic because in order to achieve a thin final lens product, the spacing between the polarizing film and the lens molds must be kept small (approximately 1 mm , but preferably less than 1 mm ) in order to produce a finished lens of acceptable thickness. Small spacings between the film and molds present difficulties in keeping the film in place due to capillary forces. Fill-time delay and incorporation of bubbles are other problems associated with this manufacturing scheme. Additionally, since the liquid casting resins typically used in this process shrink anywhere from about 7% to about 15% or more, there can be a large stress gradient at the interface between the polarizing film and the cured resin. Since stress gradients at interfaces typically hinder the adhesion between two surfaces, lenses manufactured by this processing scheme often suffer from delamination failures.
Alternatively, lens substrates formed by casting, injection molding, or other techniques can be bonded to both sides of the polarizing film using optical adhesives. Such a processing scheme for multi-focal lenses in outlined in U.S.

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