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However, the modern form that we are all familiar with, is the entire glasses lenses / frame combination, which was introduced to the general public in the year 1727 by Edward Scarlett, a British man who worked as an optician. Benjamin Franklin invented bifocals in 1784 on account of his myopia and presbyopia, while the first glasses lenses that were designed to correct astigmatism were developed in 1825 by a British inventor by the name of George Airy. The first textbook written on the process of examining the human eye and afterwards, diagnosing any visual problems was written by Dr F.C. Donders of Holland, and his textbook also covered the prescription of eyeglasses to a doctor's patients as well. Eventually this led to an increased interest in the human eye, and as a result many doctors set up shops and offices solely for eye examination and the prescription of eyeglasses. As it turns out, this was a rather profitable business; before we had  a set of standards to follow when trying to diagnose vision problems. Previously, patients would merely try on several different types of glasses until they found the pair that achieved the best optical performance for them. The process of eye examination and eyewear prescription eliminated this method, which, understandably, is now seen as being rather silly. This eventually evolved into the form of modern optometry as we know it today.

Types of glasses lenses

And now there are many different types of specific glasses lenses available as an option for those who require corrective lenses. Single vision lenses are the most popular type of glasses lenses for individuals who wear glasses before they are 40 years old. These are meant to correct nearsightedness, farsightedness, and astigmatism, and they are also recommended for patients with a combination of these conditions. Bifocals actually contain two different types of glasses lenses in one; the bottom lens is used for near vision such as reading or writing, while the upper part is used for viewing objects from a greater distance. A condition known as presbyopia is developed by many individuals over the age of 45, which leads to the inability of your eye's lens to properly focus on objects, and bifocals can be of tremendous help with patients of presbyopia. Varifocal lenses, also sometimes referred to as progressive lenses,  have no dividing lines in the glasses lenses and allow the wearer to transition smoothly between viewing things up close and seeing things from afar. Varifocal lenses have seen a tremendous boost in popularity of recent years because they resemble single vision glasses lenses, but still provide all of the assistance that your eyes require in order to properly focus.

Glasses Lenses shapes

Just like there are many different types of corrective Glasses Lenses, there is also a wide variety of Glasses Lenses shapes that are available on the market today, as well - however the most common lens shape is known as ophthalmic (convex-concave). What this means is that each side (both the front and the back) of your Glasses Lenses have a radius this is positive / convergent on the front side, and negative / divergent on the back side. The corrective power of the lens itself is what results of this difference in the curvature of each side the lens. While these are good for single vision Glasses Lenses, some of the more advanced prescriptions, such as bifocals and trifocals, as well as progressive (varifocal) lenses will require a much more complex lens shape - these are not comprised of two simple spherical surfaces like ophthalmic Glasses Lenses are. For patients who suffer from presbyopia and hyperopia, a convergent lens is required. In this case, the convergent (front!) side of the lens overrules the divergent (back) surface of the lens. It is important to keep in mind that this is exactly the opposite in the case of patients who are suffering from the visual impairment myopia, where the divergent (back) side is more powerful than the convergent (front) side. To achieve certain optic or even cosmetic looks in Glasses Lenses, the base curve of a particular lens can be changed to enhance the characteristics of the entire surface of the lens. There has been very detailed research that has been put in to determining some common base curve types that will be ideal for the vast majority of patients encountered.

High refractive index of glasses lenses

The refractive index is a system that is in place throughout the United Kingdom and the United States to classify the various types of materials that are used to create Glasses Lenses. The refractive index is used mainly for marketing and promotional purposes on behalf of the lens manufacturers - there is the normal index, the mid-index, the high-index, and the very high index. This is however, only a general classification - but there are some big advantages of having Glasses Lenses that feature a higher refractive index. First of all, lenses that are meant to treat myopia are thinner in the center, while still maintaining the same amount of refractive power. In severe cases of myopia the edges will be thinner as well, and all of this of course results in a lighter frame when it is all said and done, besides making them more fashionable. The disadvantages to having increased indices is that you will have less light transmission which increases the importance of having a proper anti-reflective coating on your Glasses Lenses, as well as a lower Abbe number. Apart from these disadvantages, glasses with a lower Abbe number are more likely to see manufacturing defects which could affect optical quality, and some may also see decreased light transmission out of Glasses Lenses that have an increased index.

An Abbe number is the statistic that really determines the optical quality of your Glasses Lenses. This measures the lens dispersion, and having a low Abbe number will result in chromatic aberrations (such as color fringes or blurriness) - this is especially true with larger types of Glasses Lenses. Typically, lower Abbe numbers mean unavoidable higher index lenses, and vice versa. It is important to remember when comparing the Abbe numbers of two or more particular Glasses Lenses, there is not much of a difference between, for example, a lens that has an Abbe number of 27 or whether it has an Abbe number of 30 - most will not even notice a difference at all. On the other hand, one will notice a great amount of difference between for example, a lens with an Abbe number of 30 when compared to one that has an Abbe number of 45; and this remains true regardless of whether you are dealing with a spherical, aspheric, or atoric design. The eye also has its own Abbe number - however, this is unimportant when determining what particular lens is right for you. Power error (-D corrections for myopia) represents an optical power change of the lens itself when an eye is looking through different parts of the lens. Lens induced oblique astigmatism (+D corrections for presbyopia) involves peripheral vision when looking through the corrective Glasses Lenses. If you are a wearer of glasses that frequently suffers from blurry vision as a result of power error or lens induced oblique astigmatism, you may be a better candidate for contact lenses instead. If this is not a viable option, you may opt to go with a lens that is smaller in vertical size, or a lens that has the highest available Abbe number while maintaining an acceptable lens thickness level.

Lighter glasses lenses

One big worry that many people have in common when they are choosing their Glasses Lenses and particularly the frames that they want is the style - everybody is concerned about how other people will view them with their brand new glasses. Luckily, frames are now being designed with this in mind, and are now more lightweight, and more sleek, stylish, and up-to-date with the latest fashion trends than they ever have been in the past. Choosing smaller Glasses Lenses and frames to hold these lenses is a good way to decrease the weight of your eyeglasses, and this changing this aspect will provide the most noticeable difference in the overall weight of your glasses. However, there is a minimum lens thickness that is needed by your Glasses Lenses to support the lens shape, and this is determined by an FDA ball-drop-test, and some of the newer materials that are being used in eyeglasses today only have a minimum thickness of 1.0 millimeters, while traditional glass lenses have a minimum thickness requirement of a whole 2.0 millimeter, as determined by the FDA. Additionally, going with an aspheric design may help eliminate magnification of the wearer's eye when observed by others at certain angles. Conversely, it can also eliminate the appearance of the wearer having smaller eyes, as well.

Materials used for glasses lenses

As Glasses Lenses that are made of actual glass have become less common, newer lens materials are becoming more and more common today. The reason for this is simple: glass lenses are much more prone to shatter and break, which could serious injury the wearer. Optical crown glass (B270) is one of these types, but it has more or less been replaced. It has an Abbe number of 59, and offers 79%/20% UVB/UVA protection. Plastic (CR-39) Glasses Lenses are now the most commonly material that is prescribed today; not only because they are much safer than actual glass, but it is much easier and cost-efficient to produce plastic (CR-39) Glasses Lenses, too. They have an Abbe number of 58 and 100%/90% UVB/UVA protection. Trivex is a newer form of lens material that is being used, with an Abbe number of 44 and boasting 100%/100% UVB/UVA protection. While it may require a slightly thicker lens, Trivex provides great UV protection, shatter resistance and optical quality. A more light weigh alternative than plastic Glasses Lenses are lenses made out of Polycarbonate. With an Abbe number of 30, polycarbonate also provides 100%/100% UVB/UVA protection to the wearer. This is the most common type of Glasses Lenses that are used in creating glasses that are designed for sports activities and glasses that are meant for young children or teenagers because of their extremely high durability. High-index plastics (polyurethane) allow wearers to go with thinner Glasses Lenses, but this doesn't necessarily mean that the glasses themselves will be more lightweight than some of the other options. The Abbe number of these high-index plastics ranges from 42-32, and the amount of UVB/UVA protection varies greatly, as well.

Lens coatings

No pair of Glasses Lenses is complete without some type of lens coating, as well - and once again, there are several different types and options to choose from. Anti-reflective Glasses Lenses are meant so that the wearer's eyes are more visible behind the lens to others who are looking at them. But anti-reflective lenses have one serious drawback - they are notoriously easy to scratch and damage. However, some of the newer anti-reflective coatings that are being offered address and attempt to fix this problem. Ultraviolet protection is provided by a type of UV coating applied to the Glasses Lenses themselves. This helps to keep out harmful UVA rays which can lead to permanent retina damage, as well as UVB rays which increase your chances of developing cataracts. Certain kinds of lens materials that are in use today resist both UVB and UVA rays naturally, and therefore do not even require this kind of coating to be applied. Scratch resistant lenses are becoming more popular these days, and are now highly recommended to those who are purchasing a new pair of glasses. Glasses Lenses that have some kind of scratch resistant coating even last longer than their uncoated counterparts; however many labs, depending on the exact type of lens material that is being used, apply a scratch resistant coating to the Glasses Lenses automatically.

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