According to documented history, the first eyeglass was made 732 years ago in Italy. It was as crude as it could be, for most of the time, the wearers held them with the hand or with ribbons around the head. They only had capacity to solve one problem at a time, either to help people who couldn’t see clearly at far or people who couldn’t read properly. Through consistent creativity and desire to make life easier, innovative science has made modern day eyeglasses very fashionable; for the medical correction of diverse problems of vision, and even for beautification of the wearer. Understanding the various designs of eyeglass present today will help us explore all the opportunities that abound in the eyewear industry.
Currently, there are a vast variety of eyeglass designs, ranging from prescription glasses, occupational glasses, aesthetic glasses and Smartglasses.
In the next couple of editions we would look at these varieties in much detail.
Let’s begin
PRESCRIPTION GLASSES
These are a group of optical glasses with specific refractive power used for the medical correction of anomalies of vision. It is usually given to a patient after series of test by an Optometrist. Prescription glasses are of various types and they are prescribed depending on the type of visual problem. We hope to fully explore all the types available, with the intent of making you an expert after reading this piece.
Before we look at the various prescriptions glass designs, let’s familiarize ourselves with key terminologies in the industry
Myopia: This is a vision condition in which people can see close objects clearly, but objects farther away appear blurred.
Hyperopia: This is a vision condition in which distant objects can be seen clearly, but close ones do not come into proper focus.
Astigmatism: This is an imperfection in the curvature of the cornea (the clear covering on the black of the eye) resulting in the formation of blur images both at near and distance
Presbyopia: This is an age-related condition (usually starts at age 40) characterised by inability to read tiny prints at normal reading distances because of a loss of flexibility by the eye’s crystalline lens
Eyeglass Frames: These are special designs used to hold lenses in front of the eyes. They have two temples that rest on the ear and two eye wires, where lenses are mounted.
Optical Lenses: These are optical materials that focus or diverge light depending on their refractive power
Spectacle: when you mount a lens into a frame, the combination of both is called spectacle.
Common Prescription designs
Convex lenses
A convex lens is thicker at the middle than at the edges. When rays of light pass through the lenses, they are converged to a focal point.
Convex lenses are primarily used to correct hyperopia
Concave lenses
A concave lens is thinner at the middle than at the edges. When rays of light pass through the lenses, they spread out (diverged).
Concave lenses are primarily used to correct myopia
Cylindrical lenses
A cylindrical lens focuses light into a line instead of a point. they help to resolve the imperfection in the curvature of the cornea as found in astigmatism
Bifocal lenses
Bifocal lenses are prescription designs with two portions. Top portion serves for distance and the lower portion serves for reading. They are used for the correction of presbyopia. A myopic, hyperopic or astigmatic presbyope will benefit from bifocals.
Progressive lenses
Progressive lenses (Varifocals) are an improvement to the conventional bifocals. They provide an additional intermediate portion and the powers progressively increase downwards. For people that want to see far objects, read at near and also see clearly things that are not far and not near (like computer monitors), progressive lenses are best suited.
Additional effects
- Photochromic effect
Whether prescription or non-prescription glasses; photochromic lenses (commonly called transition lenses) are adaptive lenses that change colour with exposure to varying light intensity. Under extreme sunlight, the lenses will form a dark film on the lens surface to protect the eyes from excessive sunlight. The same lens becomes clear in low ambience (absence of sunlight).
- Antireflective coatings
These are properties incorporated into lenses that help in reducing the amount of light that enters into the eyes especially blue lights from digital screens. They eliminate reflections from the back and front surface of the lenses. This type of lenses helps reduce the glare from car head lamps especially at night and brings comfort to computer use.
Next: Occupational glasses
