How Do Contact Lenses Work?
Before we dive into how contacts work, let’s first establish how your eyes work. Contact lenses aim to correct the way that light enters your eyes so you can focus and see clearly.
Ideally, this is how vision occurs. The light that meets your eyes:
- Enters the eye through the cornea (the clear dome that covers the iris – the part of the eye that gives your eye color)
- Passes through the pupil (the black spot in the center of the iris, that is actually just a hole)
- Passes through the crystalline lens (which is immediately behind the pupil and fine tunes focus for you)
- Passes through the vitreous (a clear “jelly” that fill the eye)
- Focuses on the most important point in the back of the eye, the fovea (which lies in the center of an area called the macula which is the most important part of the retina)
When light does not focus properly on the fovea, the distance that the point of focus is away from the fovea is considered a person’s “refractive error”.
How Contact Lenses Correct Vision
To correct this refractive error, light has to be moved (corrected) onto the fovea. The cool thing is, we know how to move focus points around with lenses. This is where a contact lens’s “power” comes in. To move a focal point closer to a lens, we use converging lenses. Converging lenses have plus (+) dioptric power. To move a point away from a lens, we use diverging lenses. Diverging lenses have minus (-) dioptric power.
The distance from the front of the cornea to the correct focus point in the back of an eye is called “axial length”. As our eyes grow through adulthood, our axial length grows as well. If our eyes are to maintain a perfect focus relationship, then the optics of the eye have to keep up with the growing eye. When it does not keep up or was never right in the first place, optical correction (with glasses, soft or rigid contact lenses, or ocular manipulation) can help move the optics into place.
The average adult eye has to bend light onto the fovea perfectly at a distance of about 23.5mm from front to back. If light is even a fraction of a millimeter off of foveal focus, then that light can create visual blur.
To help you understand how we can fix this error, imagine having the ability to measure the distance off target the focal point is with a ruler. If you measured a distance of 1 millimeter out of focus and you had to move the point backward, we might say that the power is -1.00 millimeter. This would make sense if we measured moving focal distances in millimeters, but we don’t, instead we light using the designation of Diopters. Therefore, we would move the focal point -1.00 Diopter to reach the foveal focus point. All forms of optics discussed earlier (glasses, contact lenses, and ocular manipulation) can apply -1.00 diopter of power to the eye.
Do Contacts and Glasses Correct Vision in the Same Way?
Before we get directly into soft contact lenses, think about a window versus optical spectacles (“glasses”). Since windows and optical glasses can be made of the same materials, have you ever wondered why windows don’t have “prescription power” like an optical glass? Here’s why… a window has the same thickness and the front curve is the same as the back curve. Think about it, a flat window is equally flat on both sides and has equal thickness throughout, so no optical power exists. A curved window is curved on both sides equally, and thickness does not change, so no optical power exists there either.
An optical lens however, has a different curve on the front compared to the back of the lens. When the curves are thinner in the center, then it is considered a divergent lens, or minus lens. When the curves are thicker in the center, it is considered to be a convergent lens, or plus lens. So it is the direction of the curve that gives an optical lens its power, and so it is for contact lenses as well.
We can make about 10 million specific combinations of optical spectacle powers when taking into account refractive error corrections, such as:
- Spherical power (one point of focus)
Cylinder power (when there is a second point of focus as with astigmatism)
- Axis (alignment when there are two focus points)
- Add power (for presbyopic patients – people losing their near focus too as they age).
With soft contact lenses, there are only about 5 thousand specific customizable combinations available. Realistically, the vast majority of all of these combinations are not used by anyone, but the simple point is that spectacles powers are much more customizable than contact lens powers and that spectacle power does not usually directly convert into a contact lens power. Even the distance that glasses sit away from the eye requires a conversion of power when moving to contact lenses.
Structure & Material of a Contact Lens
Today’s soft contact lenses are formed from transparent plastic hydrogels. A hydrogel is a matrix of material that has water absorbing properties, but will not swell or change shape as more water is added. It can also be formed into a specific shape. This is important for creating prescription power within the lens – the front curve has to hold a specific curve design as compared to the back curve, and the hydrogel matrix is perfect for doing this because it’s also flexible and soft on the eye and will not lose these curve characteristics.
A dramatic leap in contact lens evolution occurred around the turn of the 21st century with the advent of silicone hydrogel lenses. Silicone was known to be very porous, so it had excellent oxygen transmission possibilities, but it repelled water (like silicone sealants). Therefore, developing a silicone hydrogel matrix for contact lens use was an obstacle to get over. However, contact lens manufacturers did find a way by including components and polymers within the matrix that would absorb the water despite the silicone. This created a new era of maximal breathability, tear function, and comfort combined, that flourishes within current lens designs.
Interaction with the Eye: Fitting Contact Lenses
For light to travel to the fovea in the back of the eye, it has to travel through the pupil. Therefore, the optics of the contact lens only has to align the two separate curves (front and back) that form the prescription in this area over the pupil when placed onto the cornea. This zone of the contact lens over the pupil is called the “optical zone” of the contact lens, and this is where the prescription power corrects the eye’s refractive error.
The area that surrounds the optical zone is then left to create a good fit. This fitting zone area of a soft contact lens is often referred to as the “peripheral zone”. This area of the contact lens is designed to keep the optical zone consistently aligned over the pupil and to establish a comfortable fit on the eye. It tends to taper the closer it gets to the edge for a smoother and more comfortable feel. Since the contact lens does not require power away from the optical zone, this area of the lens can be designed to make every necessary adjustment to make the contact lens fit and feel just right.
The cornea is about 12 millimeters wide. Soft contact lenses are made so that they have about a 1 millimeter overlap all the way around, therefore a size of 14 millimeters.
This 14 millimeters is an example of the diameter parameter required within a contact lens prescription. The dome of the cornea has a simplified base curve (which is the radius of curvature of the dome itself) around 7.7 millimeters. Since a soft lens sits so wide and we want the lens to fit comfortably and have proper tear movement, the lens is fit with a fitting curve, such as 8.4 millimeters, that is much flatter than the cornea itself. This 8.4 millimeters is an example of the base curve parameter required for a contact lens prescription.
Contact Lens Longevity & Durability
Buildup occurs the longer that a contact lens is worn and can disrupt the tear function, comfort, and health of an eye. Along with establishing a fit with components that function to maintain health and comfort, certain contact lens designs have a limit to their longevity. In fact, some designs are made to withstand longer modes of wear by limiting the amount of build up occurring over time. Contacts should feel just as good on the first day of wear as the last day of wear, and not all designs are equal.
Daily Disposable Lenses
One-day disposable lenses have excellent designs for the least amount of build up with only a single day’s wear, but the components will not maintain integrity for longer periods, and, therefore, should only be worn for one day.
Two-week design lenses will have more opportunity for build up than a one-day, but, again, have a limited life expectancy of two weeks.
One-month lenses have the greatest exposure to build up, but are designed to last for a whole month.
These three modes of wear options are available for all three optical vision correction designs: spherical, astigmatism, and multifocal lenses.
Adjusting Focus for Different Vision Problems
Spherical Lenses (Myopia & Hyperopia)
Spherical lenses are for people who have spherical corrections or low amounts of astigmatism. Spherical lenses generally do not have the modifier “spherical” attached to their name.
For example, ACUVUE has a spherical lens called, “Oasys,” and it is not termed “Oasys Spherical.” A spherical lens moves light one distance to correct myopia (nearsightedness) or hyperopia (farsightedness).
For example, this lens may say, “-1.50,” to indicate that the point of focus will be moved backward 1.50 diopters for someone who is nearsighted. These lenses are the same power regardless of how they sit on the eye, and can spin around without affecting vision quality.
Toric Lenses (Astigmatism)
Toric lenses are for people who have astigmatism and require the refractive correction of two separate points back to the fovea. These lenses are designated with the words “for Astigmatism” or “Toric” attached to their name.
For example, ACUVUE also has a lens called, “Oasys for Astigmatism,” and CooperVision has an astigmatism lens called, “Biofinity Toric.”
This lens may have the designation of “-2.00-1.25X180” which means that one point will be moved backward 2.00 diopters, and the other point will be moved back 3.25 diopters (the 1.25 is the width of the second point from the first point, so (-2.00)+(-1.25)=(-3.25)), and they are oriented across the 180 degree axis.
These lenses need to hold the powers in a specific position or vision will become distorted. To do so, manufacturer astigmatism designs vary to create comfort that include stabilization factors and tend to have a wider diameter parameter.
Multifocal Lenses (Presbyopia)
Multifocal lenses are for people over forty (with presbyopia) who require additional help for up close when they are corrected in the distance.
These lenses are designated with the attached words “Multifocal” or “for Presbyopia”, as in ACUVUE’s “Oasys Multifocal,” or Bausch & Lomb’s “BioTrue ONEday for Presbyopia.”
An example of power would be -4.75 High, which would mean that one point needs to be moved backward 4.75 diopters to see far away, and that the eye needs additional help to focus up close with a “High” ADD power (additional power). “High Add” is comparable to a glasses equivalent power of +2.00, +2.25, or +2.50.
Also, both the near and far power of these lenses are aligned in the “optical zone” over the pupil, so the pupil “shares” far and near at the same time. Different designs of Multifocal lenses achieve this correction via different means, with variable amounts of success.
The Mechanics of Contact Lenses
Currently, the goal of soft contact lenses is to optimally correct vision while maintaining health and comfort with minimal risks. In fact, we are forever stepping closer to achieving this goal. Soft contact lenses truly are a safe form of vision correction if worn correctly. So, listen to your doctor and don’t test the limits of your eyes or your contacts.
And be sure to “keep a weather eye on the horizon”. There are already contact lens designs for slowing the progression of myopia, allergy control, glaucoma treatment, and for drug installation. There are even designs for managing other health problems like high blood pressure and diabetes.
And, with our greater understanding of the eye, continual growth in technology, and relentless drive toward advancements, it’s not a far stretch to imagine what possibilities contact lenses will likely have in store for us. Perhaps the Bionic Man’s telescopic vision?… or Iron Man’s Heads-Up Display? Or Superman’s… nah… we can’t match Superman… Or can we?… I guess we shall see.
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