Eyeglasses are used to correct vision in individuals with refractive errors such as astigmatism, myopia (nearsightedness), presbyopia and hyperopia (farsightedness).
The thickness of a lens follows the Food and Drugs Administration (FDA) guidelines which include:
- Convex lenses - They correct hyperopia and their edges should be the thinnest.
- Concave lenses - Concave lenses correct myopia. FDA recommends that the central area of the lens should be the thinnest.
The thickness of a lens is also affected by the size and shape of the frame. Smaller frames can reduce the weight and thickness of the lenses. Frames might also give one a good physical appearance. The material used to make the lenses is very important since it has an impact on comfort, appearance, safety and vision.
Types of Materials
Whether single vision, bifocal, trifocal or progressive lenses, the material used includes:
In the past, glass has been the material of choice due to its exceptional performance in enhancing vision. It is also low on cost. However, one downside to using glass is that it can break and injure the eye. Sometimes the injury may be so serious as to lead to the loss of an eye. It is the reason glass is hardly used nowadays to make eyeglass lenses. Besides, glass tends to be heavy and consequently, uncomfortable for the wearer.
Plastic has become more prevalent in modern times because it is light. It is also less costly with excellent vision solutions. A plastic polymer called CR-39 is popularly used to make eyeglass lenses.
Developed in the '70s, polycarbonate lenses remain popular for making safety glasses. Polycarbonate carries the advantages of being lighter than high-index plastic. It is also more impact-resistant than plastic. That is why it is the preferred material to make eyewear for children, safety glasses and sports eyeglasses.
This newer material has the same properties as polycarbonate such as being light. Just like polycarbonate, it has impact-resistant properties. The Abbe value for Trivex is higher than polycarbonate. An Abbe value refers to the objective measurement of how wide the lens disperses different wavelengths of light as light passes through it. A low Abbe value for the lens material means there will be high dispersion leading to an optical error-a colored halo around objects.
Tribrid enjoys the qualities of being light and thin. It has better impact-resistance than CR-39 plastic and high-index plastic. It also has a higher Abbe value than polycarbonate or Trivex. The problem with Tribrid is that it is not yet available in a variety of designs.
- High-index plastic
High-index lenses are lighter and slimmer than CR-39 plastic lenses due to their higher index of refraction. These lenses block 100% of ultraviolet (UV) light, thus protecting the eyes from the harmful rays of the sun. High-index plastic with a refractive measurement of 1.70 to 1.74 is costlier than a 1.6 to 1.67 high-index plastic.
There are two ways in which lens material can be measured. These are the Abbe value and refractive index.
- Abbe value
An Abbe value of lens materials for eyeglasses ranges from 59 (glass) to 30 (polycarbonate). If the Abbe value is low, it may likely cause chromatic aberration (failure to focus).
- Refractive index
The lens’ material refractive index measures how efficiently the material refracts light which is dependent on the speed at which light travels through the material. It is a number, a ratio derived at after dividing the speed of light in the lens material and the speed of light in a vacuum. For instance, 1.498 is the index of refraction measurement for CR-39. The pace at which light travels is approximately 50% slower through CR-39 compared to a vacuum.
If the refractive index of the material is high, the light will move slowly through it. The result is that more light rays are bent, meaning less material is needed to bend light. A lens made from material with a lower refractive index is thicker than one with a higher index. The refractive index currently ranges from 1.498 to 1.74. High-index plastic measuring 1.74 will be thinner than CR-39 because it requires less lens material.
Types of Coating
- The coating that protects against UV rays
- Photochromic lenses which protect the eye
- Prescription sunglasses fitted with UV coating
- Anti-reflective coatings lessen reflections off the surface of the eyeglass.
Types of Eyeglass Designs
- Conventional designs. They are no longer popular because they cause a more unwanted magnification of the eyes. These designs also may negatively affect an individual's appearance.
- Aspheric Design. Aspheric designs have their lens curvature change gradually from the centre to the edges. The result is a flatter curve that is equally excellent in optical performance. These lenses do not magnify the eye hence give the wearer a better appearance. Sometimes, aspheric designs improve an individual’s side vision. Some of the high-index plastic lenses come already made in aspheric designs. Polycarbonate and CR-39 plastic do not have aspheric designs but can be incorporated at an increased cost.