Those days when a visually impaired person could only rely on glasses in a plastic frame with thick glass and heavy lenses are long gone. Modern ophthalmology can offer a lot of different options on the topic: "Lenses for glasses." Yes, and frames today produce a very different form and from a variety of materials.
A bit of history ...
The first mention of glasses can be found in documents dated to the 13th century AD. The material from which the first frame was made was the tortoise shell. It happened in China 2 thousand years ago. Around the year 1000 e. when copying manuscripts, the monks began to actively use a magnifying glass.
When books became an object available to the public (this is a period in the region of the 15th century), glasses became more popular. Frames, lenses - everything was far from perfect, and their design was somewhat different than now. Then the person either held the glasses in his hand, or hoisted them on the bridge of the nose.
By the beginning of the 17th century, glasses equipped with temples appeared in London. And the end of the 19th century was marked by the fact that high-quality spectacle lenses were invented in Germany. They were glass, and this situation continued until 1940, when a new type of plastic was developed in Pittsburgh , which became a worthy competitor to glass - brittle and quite heavy.
Over the next 75 years, optics developed by leaps and bounds. Today, modern lenses are classified according to the mass of parameters and can help a person with any eye disease.
Materials for the manufacture of lenses
As mentioned earlier, lenses for glasses can be glass (inorganic) or plastic (organic). Glass for the production of lenses has been used for a long time. This material has excellent optical properties, effectively protects the eyes from UV radiation. Its (glass) surface is quite scratch resistant. However, glass lenses are much heavier and thicker than plastic ones, and installing them in modern frames is not always possible.
Polycarbonate eyeglass lenses are thinner and lighter than glass. In addition, they are shockproof, which makes it possible to safely wear such glasses to children and people involved in sports. This material has the ability to protect the eye from ultraviolet radiation.
Since 2000, another material for the manufacture of lenses for glasses has appeared on the market - triyvex.
Optical Lens Options
The lenses for glasses for vision (or rather, its correction) are spherical, astigmatic and afocal in their optical effect.
For people suffering from hyperopia or nearsightedness, spherical lenses are most suitable. The name astigmatic speaks for itself. The main area of their application is astigmatism correction. Moreover, with simple astigmatism, cylindrical lenses are necessary, and with complex or mixed toric lenses.
Afocal lenses do not have any optical capabilities at all. Such glasses can be worn either by people without any problems with visual perception, or those who suffer from aniseikonia (the eye has a significant difference in the size of the perceived images). In this case, the manufacture of glasses will require eikonic lenses. If a person suffers strabismus - glasses are made with afocal prismatic lenses.
Optical zones and their number
There can be several optical zones in the lens, therefore, according to their number, classification into categories such as monofocal and multifocal is possible. Multifocal, in turn, can be divided into bifocal, trifocal and progressive lenses for glasses.
Monofocal ones have one focus and are used only when correction of any one distance is needed - near or far. The scope of these lenses is limited by the correction of astigmatism and poor accommodation (the ability of the eye to change focus due to weakening or increased contraction of the ciliary muscle) associated with age.
When it is necessary to correct vision at several distances at once, one speaks of multifocal lenses.
An example of their use is presbyopia, when a person cannot see the small print or small objects at close range. Glasses with multifocal lenses will save a person from having to take them off when moving his gaze from close range into the distance and vice versa.
Types of coating lenses
Lenses for glasses, no matter what material they are made from, will not have ideal characteristics. Meanwhile, the application of various coatings makes it possible to significantly improve their quality and capabilities, ranging from increased resistance to damage and pollution and ending with improved visual comfort. Consider the lens coatings most commonly used and in demand among consumers.
Photochromic - makes it possible to protect the eye from the aggressive effects of ultraviolet radiation due to its ability to change light transmission depending on the light. Polarizing lenses are coated with a special film (filter) and transmit only vertically polarized or completely non-polarized rays, so the eye will not suffer from glare from water, roadway or snow.
To reduce discomfort when reflecting light rays from the sclera, cornea or lens surface, an antireflection (anti-reflex, anti-reflective) coating is used.
The hardening coating increases the scratch resistance of the lens, and the hydrophobic coating gives them smoothness, prevents the accumulation of water, dirt and dust and facilitates the care of glasses. For those who spend a lot of time in the sun, glasses with a UV-blocking lens coating will be useful.
For the manufacture of sunglasses, a mirror coating is used, which is applied only to the outer surface of the lens and can be of different color shades. In addition to everything described above, color coatings are widely used , that is, the color of the lenses of glasses can be very different.
Difficulties of choice: glass or plastic?
At present, glass "eyes" have practically no advantages over their polymer counterparts. Plastic lenses for glasses, the price of which can vary significantly depending on the optical parameters and hardening coatings, are leading (and by a wide margin) in the optics market.
However, when it comes to correcting a high degree of myopia (a big minus from 10.0 diopters or more), from an aesthetic point of view, mineral lenses will look more advantageous due to the fact that their edge will be thinner than that of plastic.
In addition, glass lenses for glasses are traditionally used by manufacturers that produce eye protection from the sun.
Refractive index: which one to prefer?
The refractive index of lenses made of polymers varies in the range from 1.5 to 1.74. The thinner and stronger the lens, the higher its ratio. It weighs less and costs more. When choosing the right one will be guided by a prescription from a doctor and a rim, which is preferred.
Monofocal or progressive lenses for glasses with small diopters (from -2 to +2) can be with a refractive index in the range from 1.5 to 1.6. With average diopters (from -6 to -2 and from +2 to +6), the optimal coefficient will be from 1.6 to 1.7. If the diopter values are high enough, it is best to prefer lenses with a refractive index in excess of 1.7.
If the consumer opted for a frame made of plastic, then a thick lens with a small coefficient will not be as noticeable as in a frame on a fishing line or on screws. If the frame is helical, then a thin and strong lens, i.e. with a higher coefficient, is preferable.
Interchangeable lenses with glasses
There are people who do not have any vision problems, but nevertheless they wear glasses to maintain their image. Interchangeable lenses of different colors and shades in them enable its owner to meet the environmental conditions in which he is located, it is excellent to see under any weather conditions, in the light and dark. Gray lenses will protect your eyes on a sunny day, blue lenses will be in demand with variable cloud cover, transparent lenses are designed for cloudy weather, and yellow lenses perfectly polarize the light from the streetlights in the evening.
Athletes, as a rule, also prefer glasses with interchangeable lenses, which can easily be changed in one motion, but at the same time firmly fixed in place. The shape of these lenses is designed to fit snugly to the face, but at the same time provides excellent visibility. A water-repellent coating is also an important element of such lenses; water does not accumulate on them, but flows down freely, leaving no stains.
Price range
For eyeglass lenses, the price can fluctuate very much. Products are considered inexpensive, the cost of which ranges from 1290 to 1700 rubles, lenses of an average price category will cost from 2700 to 9000 rubles and even more. To expensive include products with a cost of from 12,000 to 26,000 rubles. Everything is purely individual. The price depends on the mass of indicators: material, color and design of the lens, shade of anti-glare, diameter and thinning of the lens, its coating. In general, the wider the financial possibilities of the buyer, the more elegant, advanced and at the same time comfortable lenses for his glasses he can afford.