By Eyecare NGTypes | Lens Material | Lens Coating
Spectacle lenses, also known as eyeglasses are lenses mounted on spectacle frames and worn some millimeters in front of the eyes. They are made from different lens materials and come in various designs depending on the error(s) they are designed to correct.
There are 4 major types of spectacle lenses namely single vision lenses, bifocals, trifocals, and multifocals.
Single vision lenses
These are also known as monofocal lenses. They are designed to improve vision at only one distance, that is, either at far or at near (for reading).
These are lenses that are designed to improve vision both at near and at far. It has 2 segments. The upper segment is for distance (far) vision and the lower segment is for near vision. Bifocal lenses are mostly used to correct presbyopia in a situation where the individual has an existing refractive error or needs to have clear distance and near vision while wearing eyeglasses.
There are different types of bifocals based on the style of the bottom or reading segment of the lens. These include:
- D top: This is also known as flat-top. The reading segment of the lens has the shape of a D turned to its side with the flat part facing up.
- Roundtop: The reading segment of the lens has the shape of a semicircle.
- Invisible: This is also known as blended bifocal. It is similar to the round top but the demarcation between the distance and near segments of the lens is not visible.
These lenses are no longer readily available. They are prescribed when there is a need for good intermediate vision (vision in between distance and near). The lenses are made of three segments demarcated by two horizontal lines. It is not cosmetically appealing.
These are also known as no-line multifocal or varifocal lenses. They are the most modern lenses available for the correction of presbyopia. The power of the lens increases gradually from top to bottom enabling the wearer to have clear vision at almost all distances. Compared to bifocals, it takes a little more time to adapt to progressive lenses. They are however more cosmetically appealing because of the absence of visible segments on them.
There are several materials that are used to make spectacle lenses. In the past, most of the lenses were made of glass. Glass lenses are heavy and they break easily. This led to the use of more technologically advanced materials for the manufacture of lenses. Modern day lenses are made of various kinds of durable plastics. Examples include:
- CR-39 plastic: A low-cost plastic much lighter than glass but is thicker than all other plastic lenses.
- High index plastics: Light-weight material with the thinnest lenses available.
- Polycarbonate: Impact resistant material suitable for sports eyewear. It is lighter than high-index lenses
- Trivex: Another impact resistant material with more superior optical quality than polycarbonate. It is lightest of all the lens materials
Lens coating is a layer of material applied on the surface of a lens to enhance its appearance, performance or durability. Examples of lens coatings include anti-reflective coating, ultraviolet coating, and anti-scratch coating.
- Anti-reflective coating is applied to lenses to cut off back surface reflections. This makes the eye more visible while the lens is on the face. The coating helps to reduce glare from headlamps of oncoming vehicles, computer screen and street lamps.
- Ultraviolet coating is applied on lenses to block the sun’s ultraviolet radiation from the sunlight. which can be harmful to the eyes. Trivex and polycarbonate lenses have inherent ultraviolet protection. Transition and photochromic lenses darken when exposed to ultraviolet rays acting like sunglasses and protecting the eyes from UV rays. This feature can be incorporated in all lens material.
- Anti scratch coating makes lenses more resistant to scratches.