Shortcomings of plastic frame bifocal reading glasses
Bifocal reading glasses are a special category of prescription glasses. The most outstanding feature of them is that one bifocal lens has two separate powers. The upper power is used to correct nearsightedness and the lower power is for presbyopia. Compared with single vision reading glasses, bifocal glasses can still offer vision correction for nearsighted individuals. This is the most widely recognized advantage kept by bifocal glasses. In fact, bifocal reading glasses differ from reading glasses only in lens power. Other factors such as eyeglass frame materials and lens materials are almost the same. For instance, there are available plastic frame bifocal reading glasses as well as metal frame ones. The difference is similar to other forms of glasses that plastic frame bifocal reading glasses are often slighter. This articles focuses on introducing shortcomings of bifocal glasses, including plastic frame ones.
A superficial problem associated with bifocal glasses is that there is a visible line usually sitting in the center of the lens horizontally. Even the newly developed designs including the D-segment have still visible line or lines for power separation. What is the concern? In an obvious way, these visible lines on the lenses are always telling other individuals that the wearer need reading glasses, which is resulted from presbyopia. This condition is always associated with aging, so that reading glasses are sometimes considered as the symbol of an old age. For most individuals who are still in their middle ages, such an obvious expression caused by visible lines is really unwanted. In addition, these visible lines will surely compromise the wearer’s charming look. From this viewing point, bifocal reading glasses are quite similar to trifocal ones.
A functional shortcoming of bifocal glasses is visual distortion. In some instances, new users of bifocal reading glasses need a few days to make an adaption. In serious cases, even headaches and dizziness will be caused. For certain individuals, these reactions even persist for a long time. In fact, these symptoms are caused by the two separated powers in a single lens. A power for nearsightedness and the other one for presbyopia are strictly and suddenly separated using the center line described above. Such an obvious separation or jump will cause some people to experience visual discomfort. In fact, another functional shortcoming is that bifocal reading glasses can not deal with computer tasks professionally. This factor can be resolved by trifocal glasses. All these mentioned shortcomings are equally true in plastic frame reading glasses.
The progression of progressive reading glasses
Formally called progressive addition lenses (PAL), progressive lenses are the essential part of a pair of progressive reading glasses. These special reading glasses have won its fame because of incomparable features of these progressive lenses. In some cases, progressive lenses are also called progressive power lenses, graduated lenses, no-line bifocals and varifocal lenses. All of these variations indicate some features of progressive reading glasses. For instance, the names including progressive power lenses, varifocal lenses and graduated lenses suggest that these lenses have a gradient of increasing lens powers, helping the users correct other refractive errors. Such a gradient starts at a minimum at the top of the lens and reaches a maximum addition power at the bottom of the lens. Typically, the length of the power gradient on the lens surface usually stretches 15 to 22 mm. And the additional power is usually between 1.00 and 2.50 diopters. In addition, the name of no-line bifocals suggests that progressive lenses are superior to bifocal lenses. They will never exert visible lines on the wearer’s face. In this case, progressive reading glasses can also be named as no line bifocal reading glasses.
Evidence shows that the first commercially available progressive lenses were developed by Duke Elder in 1922. Sold by Gowlland of Montreal, these lenses were based on an arrangement of aspheric surfaces. Progressive lenses of a modern design could be seen only after the invention of Varilux lens, which was developed by Bernard Maitenaz and patented in 1953. It is understandable that those progressive reading glasses had relatively crude design and workmanship. Nowadays, progressive reading glasses are more mature and they have gained greater patient acceptance. In general, there are three vision zones in a progressive lens for near vision, distance vision and intermediate vision respectively. The near zone usually sits at the bottom; the distance section is usually situated at the top part; and the intermediate section is between the first two. Modern technologically advancements have developed several variations of such a basic design. For instance, progressive lenses can be customized for computer use. Moreover, they can be made of enlarged near or distance section. These customized progressive reading glasses are a great help for individuals with particular occupations.
Modern technologies used by manufacturers are still trying to minimize unwanted aberrations on progressive reading glasses. They improve mathematical modeling of surfaces and conduct extensive wearer trials. Furthermore, computer-controlled machines are used to cut the complex surfaces of a progressive lens.
Formally called progressive addition lenses (PAL), progressive lenses are the essential part of a pair of progressive reading glasses. These special reading glasses have won its fame because of incomparable features of these progressive lenses. In some cases, progressive lenses are also called progressive power lenses, graduated lenses, no-line bifocals and varifocal lenses. All of these variations indicate some features of progressive reading glasses. For instance, the names including progressive power lenses, varifocal lenses and graduated lenses suggest that these lenses have a gradient of increasing lens powers, helping the users correct other refractive errors. Such a gradient starts at a minimum at the top of the lens and reaches a maximum addition power at the bottom of the lens. Typically, the length of the power gradient on the lens surface usually stretches 15 to 22 mm. And the additional power is usually between 1.00 and 2.50 diopters. In addition, the name of no-line bifocals suggests that progressive lenses are superior to bifocal lenses. They will never exert visible lines on the wearer’s face. In this case, progressive reading glasses can also be named as no line bifocal reading glasses.
Evidence shows that the first commercially available progressive lenses were developed by Duke Elder in 1922. Sold by Gowlland of Montreal, these lenses were based on an arrangement of aspheric surfaces. Progressive lenses of a modern design could be seen only after the invention of Varilux lens, which was developed by Bernard Maitenaz and patented in 1953. It is understandable that those progressive reading glasses had relatively crude design and workmanship. Nowadays, progressive reading glasses are more mature and they have gained greater patient acceptance. In general, there are three vision zones in a progressive lens for near vision, distance vision and intermediate vision respectively. The near zone usually sits at the bottom; the distance section is usually situated at the top part; and the intermediate section is between the first two. Modern technologically advancements have developed several variations of such a basic design. For instance, progressive lenses can be customized for computer use. Moreover, they can be made of enlarged near or distance section. These customized progressive reading glasses are a great help for individuals with particular occupations.
Modern technologies used by manufacturers are still trying to minimize unwanted aberrations on progressive reading glasses. They improve mathematical modeling of surfaces and conduct extensive wearer trials. Furthermore, computer-controlled machines are used to cut the complex surfaces of a progressive lens.
Differences between Trifocals and Bifocals
Trifocals are a new group of special reading glasses which have three divisions on the lenses for correcting vision. It is different from traditional bifocals which have only two such divisions. The lower sections of the lens of trifocals are demarcated by a thin line which is a semi circular arc. One area is for near and the other for intermediate vision correction. And the part for intermediate rectification lies above the nearsighted correction area which is near the bottom of the lens. For those who used to wear bifocals may think it is strange to reading with glasses having three lines etched on it. As trifocals are new to most of people, it actually did need time for patient who uses this type of glasses to adjust to them. However, once you used to wear them, you will find it pretty easy to use and it is more convenient than bifocals.
After eye exam by the eye doctor, those people with extreme presbyopia, which means their net optical power with a special unit of diopter is very high, will be recommended to use trifocals. Because, unlike bifocals, trifocals can correct all ranges of vision, from nearby, intermediary to far distance, when bifocals can correct only two of them, either near vision or distance vision. Since trifocals can offer three levels of correction, it can give you a better vision. So, if you are one of the extreme presbyopia, you will get the better and more clarified vision when you use trifocals.
Trifocals are not only for those who have extreme presbyopia, it can also correct intermediate distance which can’t be provided by bifocals. For example, if your have a vision of -4 diopters, usually you will be recommended to use a glasses with a correction of 3 units, which means you vision is -1 diopter when you wear your reading glasses. The intermediate correction for such situation may be end up with a power of -2.5, which may take patients a little time to get used to it.
So with a single pair of trifocals, you can solve all problems that are related to presbyopia.