Many people need some sort of corrective lens or magnifier in order to read or see objects at a distance, and so wear eyeglasses. Many such people also wear sunglasses outside in the sun and a practical problem arises if they want to read a book, or otherwise need their eyeglass aided vision. For example, he or she must purchase a pair of prescription sunglasses or endure the sun without the benefit of sunglasses while reading. The purchase of a pair of prescription sunglasses is expensive, but a large part of the expense is due to the cost of the frame. Broken lenses and new prescriptions for the old frames are always much less expensive to replace than the original cost of the frame and lenses combined. Such people have the further practical problem of carrying around at least two pairs of prescription lens and frame sets (i.e., the prescription sunglasses and the reading or distance eyeglasses).
Even for those sunglass wearers whose vision does not require assistance, sunglass lenses break or get scratched, or different activities suggest or require different shapes and/or colors of lenses. For instance, in bright sunlight, a darker shade may be indicated, while in overcast light, a yellow or rose tint can brighten the view; bicycle athletes want a lens with substantial wrap depth to protect their eyes and to improve aerodynamics, but for running or apres-cycle they may want a different shape or color. Sometimes the dictates of fashion simply require a different color or shape lens set.
One approach to deal with this problem has been to use a pair of prescription eyeglasses that are tinted in such a way that there is a minor tint when the ambient light is relatively low and a more severe tint as the ambient light becomes brighter. However, such eyeglasses are relatively expensive and the tint of the lenses may change at times when it is not desirable for it to change.
Another attempt to solve the problem of the need for multiple pairs of corrective or sunglass eyewear is to use "clip-on" or "snap-in" tinted lenses in connection with a pair of clear prescription spectacles. Clip-on lenses are attached to a pair of eyeglasses by a clip, typically located at the center of the clip-on lenses. The clip attaches over the nosepiece of the eyeglasses. Sometimes the clip snaps over the sides of each of a pair of separate lenses formed to the shape of specific eyeglasses for which they are provided. In another variation, a tinted lens is snapped into a circumferential groove inboard of the clear lens. In another attempt to solve the problem, clear prescription lenses are hung, hinged or otherwise detachably mounted inboard of a pair of conventional sunglass lenses. Typically, some additional structure is added to the sunglass frames for holding a prescription lens or pair of lenses behind the sunglass lenses; however in one species, the frames themselves are modified with threads to receive a "screwed-in" prescription lens for each lens of the sunglasses. Clip-on lenses can be disadvantageous, however, because they result in less than desirable optics, typically through an inability to control the spacing and angular relationship between the sunglass lens and its clear lens with any precision or degree of stability, and because they are cosmetically unattractive.
Ultimately, the route everyone seems to take, if they can afford it, is to simply own more than one pair of eyewear, and to carry for a given day or activity the pairs they think they will need. Of course, each pair has its own frameset, and typically its own case, the combined bulk and weight of which will generally discourage all but fanatics from having ready to hand the right eyewear for the right activity or light condition, or simply for the right fashion statement at the moment. All of which is to say nothing of the disastrous expense of having broken a sunglass lens or lenses, or otherwise marred them to the point of needing replacement. Sunglass manufacturers, unlike prescription eyeglass purveyors, do not generally provide ready means for lens replacement; generally, damage to a pair of expensive sunglasses means another trip to the store to buy a new pair, frames and all.
There is therefore a need for a simple, relatively inexpensive lens replacement system for a given frame set that may be employed to support a pair of fashion or performance sunglass lenses, and various colors and shapes of them, and also to support a corrective pair of lenses or a pair of prescription sunglass lenses, that can be used comfortably and effectively for sustained periods of time, and that will provide for interchangeability or quick and cost effective replacement of the various lens types on a single frame set.