Eyeglasses are available in two types of lens designs; Single vision and multifocal. Single vision is designed for distance vision while multifocal corrects both near and distance vision. Trifocals belong to the multifocal category. These lenses come with three different sets of correction hence their name. They are used in the treatment of presbyopia.
Presbyopia is a refractive condition where the eye gradually loses its ability for good near vision. Patients with presbyopia begin to have problems with near vision. The disease results from the aging lens of the eye. It becomes more rigid and has difficulty changing shape easily which leads to difficulty in reading, threading a needle and doing other close-up tasks.
Even though doctors prescribe a trifocal lens for adults over 40 years old, they can also prescribe them for children and young adults. This is because the children and young adults may have problems with focus that lead to eye strain when they are reading.
Besides trifocals for visual enhancement, there are also occupational trifocal eyeglasses that can assist with specialized vision. Occupational trifocal lenses are designed for performing specific tasks as opposed to general purpose wear. Such jobs may include mechanics.
Types of occupational trifocals include:
- The Double D - It is a popular trifocal lens with an upside-down segment for intermediate and near vision and a second section for near vision. The middle part of the Double D assists in distance vision. The mechanic would find the Double D trifocal lens ideal because he/she will see nearby objects as well as the area of work.
- The E-D trifocal is a special-purpose lens built for both intermediate and near vision. It is suitable for someone like a TV producer who needs near, intermediate and distance vision.
Types of Material
Eyeglasses are made from various materials:
- Glass - It is not as popular as it was before because of several disadvantages such as less flexibility, less safety and heaviness. Glass can be dangerous to the eye because it can break inside the eye and cause loss of an eye.
- Plastic - Modern trends have shifted towards using plastic as opposed to glass. Plastic is lighter, more flexible and safer. It cannot break and helps to block the harmful ultraviolet (UV) rays of the sun.
- Polycarbonate lens - These high impact-resistant lens are ideal for individuals who engage in activities that are likely to cause injuries such as sports and risky work engagements such as mechanical work.
- Trivex lens - They share similarities with polycarbonate lenses. These lenses cause fewer distortions than polycarbonate lens.
- High-index lens - The high-index lens is a thin, light lens made of plastic, ideal for individuals in need of high visual correction. It has fewer distortions with more aesthetic qualities compared to individuals who wear thick-lens glasses.
Indication & Uses
Trifocals correct distance, intermediate and near vision, all in one set of eyeglasses. The midsections look at objects from an arms' length such as computers. Driving is an example of distance vision while reading a map is an instance of near vision. Even if presbyopia patients have been using eyeglasses, they might need to start wearing trifocal glasses. This enables them to see clearly at all distances.
Presbyopia patients can choose from bifocal, trifocal and progressive lenses. While progressives are a favorite with many, trifocals have some advantages over them. For instance, trifocals provide a more comprehensive lens area for reading and working on the computer. Another advantage is that there are various types of specially made trifocal lenses. For example, glasses made specifically for relieving computer eye strain. There are also trifocal lenses designed specifically for those who wish to do tasks that require excellent near and intermediate vision.
The eyes need protection to keep them healthy. That is why a protective coating is included on the lens. There are several types of coatings which include:
- Anti-reflective coatings which have fewer reflections off the surface of the eyeglass. The result is that the wearer has an improved appearance and the presence of more effective eye contact with others. Other advantages include reduced eye strain, reduced glare and reflections, improved contrast, sensitivity and visual acuity.
- Some eyeglasses are coated with material which protects against UV light. Some plastic lenses come with inbuilt UV protection.
- Protection from prescription sunglasses with UV protection.
- Photochromic lenses suitable for both the indoors and outdoors. These lenses are made of a tint that changes according to the amount of light present. When there is sunlight, the darker tint is used while the lighter tint works when one is indoors. However, photochromic lenses do not work well in all environments. For instance, in a car or airplane, the windows may block the UV light that triggers a change in the tint. It takes time for the tint of the lens to adjust from dark to light.