How a Little Piece of Plastic Has Helped 125 Million People to See
More than half a millennium ago, Leonardo da Vinci first published Codex of the Eye, which explored the mechanisms of the eye and its ability to readjust in reaction to its environment — a milestone in the development of modern optometry. Five hundred years later, the concept of contact lenses — inspired by da Vinci’s speculation that submerging a person’s head in a bowl of water could alter their vision — has gone through quite a development. Scientists, such as René Descartes and Thomas Young, played around with the idea of corrective vision from the mid 1600s to the early 1800s, often venturing beyond the model of simple eyeglasses. Physicists and inventors have experimented with many transparent materials. At one point, spherical capsules of glass filled with animal jelly and water-filled lenses that would be affixed to the eyeball with wax or sugar solutions sounded like great ideas. Thankfully, our mothers’ words of wisdom to be careful with glass lest we poke our eyes out still stands. The developers of contact lenses have confronted and solved many problems, such as conforming lenses to the eye, allowing oxygen to pass through the lens and dealing with extended wear through durable and flexible silicone hydrogel technology. So how exactly do contacts work? How do they help 125 million people worldwide see every day?
Imagine a football player about to make a pass to his teammate. The path of his throw represents a light ray that is trying to reach the retina (the inner back lining of the eye) or, in our example, the receiver. A person with perfect vision is equivalent to all of the passes being “completes”, in which the ball reaches the receiver each time with accuracy and precision. Therefore, both nearsightedness and farsightedness signify incompletes. Nearsightedness is when the ball doesn’t fly far enough to even reach the receiver. This means that light rays focus too early within the eye, in front of the retina rather than directly on it. Contact lenses help to diverge the light rays far back enough so that their focus point hits the retina, thereby reducing the eye’s original focusing power. Farsightedness, on the other hand, means that the ball overshoots the receiver (light rays fail to form a focus point by the time they reach the retina). In this case, contact lenses correct vision by converging light rays to move the focus point forward, resulting in an increase in the eye’s original focusing power. In both nearsightedness and farsightedness, the focus power is corrected in the same way for both contact lenses and glasses.
Contact lenses offer a bigger unobstructed view with the added bonus of being thinner, lighter and more flexible in terms of movement.
So if we already have glasses then what’s different or better about contact lenses? After all, soft contact lenses need to be replaced frequently, cost more over time and introduce a slight risk of infection if improperly disinfected. Beyond cosmetic benefits, contact lenses do not break on impact or collect moisture from rain, snow, condensation and sweat. Contact lenses can also afford to be much thinner and smaller in comparison to their clunky eyeglass counterparts. Since they rest directly on the eye instead of almost half of an inch away from the eye’s surface, contact lenses can be much thinner. In terms of their size in comparison with glasses, contact lenses have smaller optic zones (the part of the lens that has the corrective power). Basically only the center area of a contact lens — the portion covering the pupil — is doing the work while the peripheral curves are only there so that the lens can stay relatively affixed to the eye. On the other hand, the optic zone for a pair of glasses spans the entirety of both lenses, which is why your view is limited beyond the rims of your glasses. Contact lenses offer a bigger unobstructed view with the added bonus of being thinner, lighter and more flexible in terms of movement.
Now, surely you’ve seen the commercials marketing contact lenses that correct for astigmatism, but what exactly does this mean? Using the football analogy, imagine that the first football player has just thrown the ball. Suddenly in midair, right before the ball is about to reach the hands of the receiver, it splits into two separate balls, one reaching the receiver and the other falling short. Similarly, astigmatism causes light rays, which represent a single image, to be focused in two different places in the eye, resulting in blurriness. To correct for this, astigmatism contact lenses have two powers in each lens and therefore need to be oriented in a specific and consistent manner on the surface of the eye. Most of the time, astigmatism lenses are heavier closer to the bottom, taking advantage of gravity, so that they don’t move too much and can keep their orientation relative to the pupil. However, if the wearer is moving a lot, such as in sports, or even just needs to blink, there needs to be something else keeping the contact in place. To solve this problem, many lenses have multiple zones that interact with the varying degrees of pressure across the eye’s surface. In this way, blinking actually resets this relationship by re-aligning the lenses.
Today, contact lenses for near- and far-sightedness and astigmatism grace the aisles of our local Walgreens. Customers can get them in nearly every prescription and eye color, as one-a-day contacts or twice-a-month contacts. They can even be shipped directly to your front door. Though contacts have turned into something of a simple mass-produced household item, it’s pretty fabulous to think that they are the products of the same minds that fathered analytical geometry, the wave theory of light and the parachute — and completely made from a little piece of silicone. How’s that for perspective?