Corrective lens for different needs will not have the same shape, protection index or properties. Lens indices, mineral glass or organic glass, spherical or aspheric, univocal, multifocal or progressive: choose your glasses well.
Lenses for eyeglasses can be made thinner according to the needs of the wearer. The degree they can be thinned is linked to the material of the lens.
An index is associated with each material; the higher the index, the thinner the lens.
- Organic materials: from 1.5 to 1.74
- Mineral materials: from 1.5 to 1.9
The first lenses were made from a mineral-based material: glass. Glass is solid and resistant to chemicals, but it’s also fragile at room temperature.
The colors and tints of these lenses are also rather limited. The principal advantage of mineral glass is in its refraction index (from 1.5 to 1.5). The higher this number, the thinner the glass.
As this type of lens is breakable, it’s forbidden for use by those younger than 16 and not recommended for athletes.
Organic, that is to say in plastic lenses, are today the most widely used lenses in eyewear optics. Several ranges and types of lenses are available from different manufacturers.
Very light and shock-resistant, these lenses can have a wide variety of tints and even untinted can offer 100% protection against UV rays.
Organic materials are indexed to a number (from 1.5 to 1.74) that corresponds to the degree to which they can be thinned. The higher the number, the thinner the glass.
Organic lenses scratch easily, so applying a scratch-resistant coating is recommended.
Spherical and aspheric lenses
Spherical lenses are unifocal, which means they have a single optical focal point. They correct simple visual defects such as myopia (nearsightedness), hyperopia (farsightedness) or presbyopia.
Aspheric lenses are more sophisticated unifocal lenses, modified so that they have a better esthetic appearance. Flatter and thinner, they correct myopia, hyperopia or presbyopia but can also be used for strong corrections. Compared with spherical lenses, they offer better optical quality with reduced distortion in the periphery of the lenses, so that vision is clear from the edge to the center.
Unifocal lenses have a single optical focal point. They correct hyperopia, myopia and astigmatism.
People with presbyopia can also improve their close-range vision with unifocal lenses, often called half-moon or reading glasses.
There are also mid-range lenses for presbyopia, usually for use with a computer. Unifocal lenses can be spherical or aspheric.
Bifocal lenses have two focal points. There are also trifocal lenses. These lenses are separated in more than one region for each type of correction: close and long-range vision for bifocals and close, mid and long-range vision for trifocals. Multifocal lenses are used less and less today in favor of progressive lenses.
More comfortable, they soften the harsh transition between the different focal points of a lens.
Progressive lenses correct presbyopia when combined with other visual problems such as farsightedness, astigmatism and myopia. The bottom of the glass allows for close-range vision and the top for long-range. The transition between the different parts is gentle. Interior progressive lenses allow for clear close and mid-distance vision. Standard lenses are suitable for a large number of people, whereas personalized lenses take into account the specific needs of the wearer, as well as the frames they will go into. Individualized lenses are totally personalized and have a very large field of vision.