Prescription eyeglass lenses > Types and materials of eyeglass lens

Nowadays we put more and more demands on our sight. A good visual acuity is a prerequisite for success not only at work but also in leisure time. We can't imagine life without a good sight. This is the reason why we protect our sight or improve its limitations. Today, there are many ways how to correct sight limitations (errors of refraction). There is a high probability that at present, as well as in the future, the most common solution for these limitations and for protection of eyes would be eyeglasses or contact lenses.

Prescription eyeglass lenses are the main part of eyeglasses. Eyeglass lenses themselves take part in better seeing. Prescription eyeglass lenses correct errors of refraction, such as shortsightedness (myopia), longsightedness (hypermetropia), astigmatism or presbyopia. Even though prescription lenses are used and manufactured for a long time already, their development is still going forward and the biggest world manufacturers come up with something new and better every year. The aim is to get the best and most comfortable sight.

We can divide eyeglass lenses according to the material used and optical effect.

Types of prescription eyeglass lenses according to the material used

Glass lenses

The characteristics of glass are well-known. Glass is a hard, poorly conductive material, which can crack under rapid temperature changes. In the past, glass was the only material used for lens manufacture. The advantage of glass is very good optical characteristics, high resistance against scratching and thin profile. With diopters of 10 and more and when using glasses with a higher refractive index, it is possible to achieve esthetically very nice marginal thickness.

Plastic material – CR39, Orma 1000

Plastic eyeglass lenses are the most commonly used type of lenses today. They have very good optical characteristics in comparison with mineral glass they are very light and resistant against braking. It is possible to use them in all types of frames and to use all kinds of colors and surfacing. They can be manufactured even with high refractive index, which is suitable for higher diopter. Their slight disadvantage is that they are softer than mineral glass which causes that they get scratched faster.

Polycarbonate

It was developed in 1957 in the USA. In the 60s of the 20th century it was used to manufacture visors on the helmets of astronauts and windows of spaceships in the Apollo programme. This material belongs to a group of thermoplastic materials, which are softer, but a lot more flexible, more impact-resistant than CR39 and other, they have higher refractive index (1.59), thin center thickness, small weight and high resistance against scratching. They also absorb 100% of UV rays. Thanks to these characteristics they have been used for lenses for higher risk jobs and sports.

Trivex

Trivex is the first material used in lenses that combines three very important characteristics – brilliant optical characteristics, excellent impact- and cracking-resistance, and very small weight, which is 10% less than with polycarbonate material. Its disadvantage is that it can be manufactured with refractive index up to 1.53, which means it can't be used with higher diopter, because the profile would be too thick.

Types of prescription eyeglass lenses according to optical effect

• Unifocal eyeglass lenses
• Bifocal eyeglass lenses
• Trifocal eyeglass lenses
• Multifocal eyeglass lense

Unifocal prescription lenses

Under unifocal eyeglass lenses we understand all spherical lenses, cylindrical lenses and lenses with aspherical surface and one focal point, or two focal points with the cylindrical lenses.

Spherical prescription lenses

They are the most commonly used type of eyeglass lenses for correction of rotary symmetrical (spherical) errors of refraction. The radius is constant on functional optical surface.

Cylindrical prescription lenses

They are used for correction of astigmatism. The way how they refract light beams is how they differ from spherical lenses. Parallel light beams are not focused in one point, but there are two focal points in different distances.

Aspherical prescription lenses

Eyeglasses should fulfill all demands on good vision but also satisfy requests and wishes of their wearers, e.g. look good and be the lightest possible. Aspherical eyeglass lenses fulfill al these demands with their aspherical surface. It is a surface with variable radius of curvature. So it is not a round surface, but rotary aspherical surface, which gets flatter towards the edge. This enables to manufacture a thin and light lens, which has very good displaying abilities on the periphery. Aspherical prescription eyeglass lenses play an important role with plus diopter and very high minus diopter.

Bifocal prescription lenses

Bifocal eyeglass lenses, or lenses with two focal points, have another “additional” lens connected with the “basic” one-focal point lens. The basic part is used for looking long-distance and the additional piece for looking short-distance. The additional part is either sealed or cut into the basic part. The shapes of the segments were changing with the development. Starting from flat boundary, through round piece for short distance, to a segment shaped like a lying letter D with slightly arched course of the upper part of the segment.

The biggest disadvantage of bifocal lenses is a big difference in the picture when changing the distance of looking. This type of lenses is in a recession.

Trifocal prescription lenses

Trifocal eyeglass lenses are used when the additional part for short distance is over two diopters and at the same time the drop of accommodative width is under 2 diopters. The difference between bifocal and trifocal eyeglass lenses is in inserting a middle piece for middle distance. This type of lenses never really got popular and is hardly used.

Multifocal prescription lenses

Multifocal (progressive) lenses, unlike bifocal lenses, enable a sharp vision in all distances and have a fluent transition from the short-distance piece to the long-distance piece. The dioptrical value changes continuously from the zone of long-distance vision, through a corridor (progressive canal) for middle-distance vision to the zone for reading. They are manufacture from one piece of optical material. It is the ideal type of lenses for active people with presbyopia.

The quality of the prescription eyeglass lenses comes not only from the material used, but also from the surfacing. Every good lens should have the antireflex layer, which eliminates unpleasant reflections from the lens itself. With the plastic eyeglass prescription lenses it is desirable to harden them to prevent scratching, because plastic material is too soft. The hydrophobic adjustment is used for easier cleaning, crystal surface adjustment is used with the highest categories of lenses and these adjustments drive away the dirt particles because of extremely smooth surface. They also decrease the risk of contamination and trapping of dust particles, which can lead to scratching the lens and make lenses easier to clean.

If you choose prescription sunglasses or sports prescription eyeglasses, or eyeglasses with greater curvature, it is not a problem nowadays. Also, every high quality lens, colored or transparent contains a UV filter.