The present invention relates generally to a lens for sunglasses, and more particularly to a uniquely configured quadroradial lens having a dual radius horizontal curvature and a constant radius vertical curvature for minimizing the prismatic shift of the lens. As discussed in greater detail below, embodiments of the present invention maximize the interception of peripheral light, while, at the same time, mitigating prismatic shift to enhance optical resolution for the wearer. An optional frusto-conical as worn configuration of the lens also permits construction of sunglasses which conform closely to the front and sides of the wearer's head. The resulting low profile glasses utilizing the lens of the present invention are particularly suited for demanding situations which require precise optical resolution and interception of peripheral light, such as competition skiing or bicycle racing.
Sunglasses have long been designed with the general objective of blocking the sun or other sources of bright light from one's eyes. Initially, numerous designs of dual lens glasses were developed, differing essentially only in aesthetic features. However, the unitary lens was later developed, and together with existing dual lens designs, has been geometrically modified in response to various optical considerations such as optical clarity and resolution, field of vision, light wave refraction, and others.
Although prior dual lens designs are useful for some purposes, the conventional dual lens system is inherently incapable of meeting the demands of certain activities. For example, the frame on dual lens glasses presents a substantial obstruction to one's peripheral vision, which can be extremely disadvantageous in any fast-paced activity. Even in dual lens glasses without a frame along the lower edges of the lenses, the edge of each lens disrupts peripheral vision. Simply providing a larger lens that is typically stamped or molded from a flat plane or a spherical blank, causes the glasses to extend too far tangentially away from the side of the head, leaving the glasses with an undesirably bulky profile.
At the same time, conventional dual lens glasses only intercept sunlight directly in front of the eye, leaving a large, unprotected periphery about each lens. Momentary flashes of light around the lens during activity cause constriction of the pupils, with a fleeting blindness as one attempts to readjust through the darkened lens.
Prior art attempts to block this peripheral light (aside from opaque blinders) included bending (e.g., thermoforming) a flexible lens in a posterior direction near the lateral edge. Although this improved the interception of peripheral light, the resulting optical resolution was unacceptable for high speed competition situations. This is due to the phenomena that even minor irregularities in the radius of curvature, which inherently result when bending a lens, cause an irregular diffraction of light waves passing through that region of the lens and distort the field of vision.
A unitary, molded, frusto-conical lens blank was then developed, such as that disclosed in U.S. Pat. No. 4,515,448 issued to Tackles. The frusto-conical lens was designed to conform closely to the wearer's head, having lateral edges that curved and extend interiorly to block peripheral light that would otherwise pass around the lenses of a dual lens design. In addition, diffraction gradients common in the bent, flexible lens, were minimized by molding the frusto-conical lens with a predetermined curvature. Nevertheless, the potential for improvement remained for several reasons.
The next improvement in specialty unitary eyewear utilized a unitary cylindrical lens which was curved about an axis having a substantially constant radius throughout, such that the lens defined a portion of the wall of a cylinder. This lens is the subject of U.S. Pat. No. 4,859,048, issued to Jannard. The cylindrical lens demonstrated improved optical properties and interception of peripheral light. Further, it provided a sleek, low profile design that also improved ventilation. Nevertheless, the cylindrical lens produced a measurable prismatic shift in each of the vertical and horizontal planes, particularly at off axis viewing angles.
Another improvement in specialty unitary eyewear utilized a toroidal lens that was curved along each of two substantially perpendicular axes to produce a lens of generally toroidal configuration. This lens is the subject of U.S. Pat. No. 4,867,550, also issued to Jannard. The toroidal lens configuration was characterized by having arcuately configured horizontal and vertical cross-sections. The arc of each respective cross-section corresponded to a given radius. Thus, the toroidal lens can be defined by two different radii, and is configured to substantially conform to the wearer's head. In addition, the lens thickness was tapered to further reduce prismatic shift.
Yet another improvement in unitary lens eyewear utilized an elliptical lens. This lens is the subject of U.S. Pat. No. 5,774,201, also issued to Tackles. The elliptical lens is characterized as having a horizontal cross-section that substantially conforms to an elliptical shape. The vertical cross-section of the elliptical lens can be defined by any of various geometric shapes, such as a cylinder, a cone, an ellipsoid, or an ellipsoid of revolution. The elliptical lens was introduced to improve the optical characteristics of specialty eye wear, including refraction between medial light entering at the front of the lens and peripheral light entering at the lateral ends of the lens.
Notwithstanding the many advantages presented by this progression of unitary lens designs, there is a continuing need for a specialty lens having excellent optical qualities and providing reduced light wave defraction of medial, lateral and peripheral light, while at the same time providing a low profile, aerodynamic configuration that allowed for adequate ventilation and maximum comfort. Preferably, the lens will exhibit good impact resistance, as well as minimal prismatic shift in both the horizontal and vertical axes.