Contact lens basics: Types of contact lenses and more

By Liz Segre

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Contact lenses are an excellent choice for nearly anyone who needs vision correction and doesn't want to wear eyeglasses full time or undergo LASIK surgery.

Unsure about contact lenses? This article will detail contact lens materials, contact lens designs and features and even new contact lens formats.

For instance, the first light-adaptive contact lenses, Acuvue Oasys with Transitions, debuted in the United States in , and contact lenses embedded with antibiotics are in the works. (See our contact lens news page for the latest in contact lenses.)

Here are the basics you should know about contact lenses before seeing your eye doctor if you are interested in wearing contacts.

Contact Lens Materials

The first choice when considering contact lenses is which lens material will best satisfy your needs. There are five types of contact lenses, based on type of lens material they are made of:

Contact Lens Material

• Soft lenses are made from gel-like, water-containing plastics called hydrogels. These lenses are very thin and pliable and conform to the front surface of the eye. Introduced in the early s, hydrogel lenses made contact lens wear much more popular because they typically are immediately comfortable. The only alternative at the time was hard contact lenses made of PMMA plastic (see below). PMMA lenses typically took weeks to adapt to and many people couldn't wear them successfully.

• Silicone hydrogel lenses are an advanced type of soft contact lenses that are more porous than regular hydrogel lenses and allow even more oxygen to reach the cornea. Introduced in , silicone hydrogel contact lenses are now the most popular lenses prescribed in the United States.

• Gas permeable lenses &#; also called GP or RGP lenses &#; are rigid contact lenses that look and feel like PMMA lenses (see below) but are porous and allow oxygen to pass through them. Because they are permeable to oxygen, GP lenses can be fit closer to the eye than PMMA lenses, making them more comfortable than conventional hard lenses. Since their introduction in , gas permeable contact lenses have essentially replaced nonporous PMMA contact lenses. GP contacts often provide sharper vision than soft and silicone hydrogel contacts &#; especially if you have astigmatism. It usually takes some time for your eyes to adjust to gas permeable lenses when you first start wearing them, but after this initial adaptation period, most people find GP lenses are as comfortable as hydrogel lenses.

• Hybrid contact lenses are designed to provide wearing comfort that rivals soft or silicone hydrogel lenses, combined with the crystal-clear optics of gas permeable lenses. Hybrid lenses have a rigid gas permeable central zone, surrounded by a "skirt" of hydrogel or silicone hydrogel material. Despite these features, only a small percentage of people in the U.S. wear hybrid contact lenses, perhaps because these lenses are more difficult to fit and are more expensive to replace than soft and silicone hydrogel lenses.

• PMMA lenses are made from a transparent rigid plastic material called polymethyl methacrylate (PMMA), which also is used as a substitute for glass in shatterproof windows and is sold under the trademarks Lucite, Perspex and Plexiglas. PMMA lenses have excellent optics, but they do not transmit oxygen to the eye and can be difficult to adapt to. These (now old-fashioned) "hard contacts" have virtually been replaced by GP lenses and are rarely prescribed today.

In , 64 percent of contact lenses prescribed in the U.S. were silicone hydrogel lenses, followed by soft (hydrogel) lenses (22 percent), gas permeable lenses (11 percent), hybrid lenses (2 percent) and PMMA lenses (1 percent).

Contact Lens Wearing Time

Until , everyone who wore contact lenses removed and cleaned them nightly. The introduction of "extended wear" enabled wearers to sleep in their contacts. Now, two types of lenses are classified by wearing time:

• Daily wear &#; must be removed nightly

• Extended wear &#; can be worn overnight, usually for seven days consecutively without removal

"Continuous wear" is a term that's sometimes used to describe 30 consecutive nights of lens wear &#; the maximum wearing time approved by the FDA for certain brands of extended wear lenses.

When To Replace Your Contact Lenses

Even with proper care, contact lenses (especially soft contacts) should be replaced frequently to prevent the build-up of lens deposits and contamination that increase the risk of eye infections.

Soft lenses have these general classifications, based on how frequently they should be discarded:

Lens Replacement Frequency

• Daily disposable lenses &#; Discard after a single day of wear

• Disposable lenses &#; Discard every two weeks, or sooner

• Frequent replacement lenses &#; Discard monthly or quarterly

• Traditional (reusable) lenses &#; Discard every six months or longer

Gas permeable contact lenses are more resistant to lens deposits and don't need to be discarded as frequently as soft lenses. Often, GP lenses can last a year or longer before they need to be replaced.

The most frequently prescribed contact lens replacement schedule in the U.S. in was monthly (40 percent), followed by daily (35 percent), every one to two weeks (24 percent) and annually (1 percent).

Contact Lens Designs

Soft contact lenses (both standard hydrogel and silicone hydrogel lenses) are available in a variety of designs, depending on their intended purpose:

Soft contact lens designs

• Spherical contact lenses have the same lens power throughout the entire optical part of the lens to correct myopia (nearsightedness) or hyperopia (farsightedness).

• Toric soft contact lenses have different powers in different meridians of the lens to correct astigmatism as well as nearsightedness or farsightedness. [Read more about toric contact lenses.]

• Multifocal contact lenses (including bifocal contacts) contain different power zones for near and far vision to correct presbyopia as well as nearsightedness or farsightedness. Some multifocal lenses also can correct astigmatism. [Read more about bifocal contacts.]

• Cosmetic contact lenses include color contacts designed to change or intensify your eye color. Halloween, theatrical and other special-effect contacts also are considered cosmetic lenses. A contact lens prescription is required for cosmetic contacts even if you have no refractive errors that need correction.

All of these lenses can be custom made for hard-to-fit eyes. Other lens designs also are available &#; including lenses fabricated for use in special situations, such as correcting for keratoconus.

More Contact Lens Features

Bifocal contacts for astigmatism. These are advanced soft contacts that correct both presbyopia and astigmatism, so you can remain glasses-free after age 40 even if you have astigmatism. [More about bifocal contact lenses for astigmatism.]

Contacts for dry eyes. Are your contacts uncomfortably dry? Certain soft contact lenses are specially made to reduce the risk of contact lens-related dry eye symptoms. [More about contact lenses for dry eyes.]

Colored lenses. Many of the types of lenses described above also come in colors that can enhance the natural color of your eyes &#; that is, make your green eyes even greener, for example. Other colored lenses can totally change the color of your eyes, as in from brown to blue.

Special-effect lenses. Also called theatrical, novelty, or costume lenses, special-effect contacts take coloration one step further to make you look like a cat, a vampire, or another alter-ego of your choice.

Prosthetic lenses. Colored contact lenses also can be used for more medically oriented purposes. Opaque soft lenses called prosthetic contacts can be custom-designed for an eye that has been disfigured by injury or disease to mask the disfigurement and match the appearance of the other, unaffected eye.

Custom lenses. If conventional contact lenses don't seem to work for you, you might be a candidate for custom contact lenses that are made-to-order for your individual eye shape and visual needs.

UV-inhibiting lenses. Some soft contact lenses help protect your eyes from the sun's ultraviolet rays that can cause cataracts and other eye problems. But because contacts don't cover your entire eye, you still should wear UV-blocking sunglasses outdoors for the best protection from the sun.

Scleral lenses. Large-diameter gas permeable lenses called scleral contacts are specially designed to treat keratoconus and other corneal irregularities, as well as presbyopia.

Link to optec

Myopia control contacts. Special contact lenses are being developed to slow or stop the progression of nearsightedness in children. [More about myopia control.]

Which Contact Lens Is Right for You?

First, your contacts must address the problem that is prompting you to wear lenses in the first place. Your contact lenses must provide good vision by correcting your myopia, hyperopia, astigmatism, or some combination of those vision problems.

Second, the lens must fit your eye. To do that, lenses come in tens of thousands of combinations of diameter and curvature. Of course, not every lens brand comes in every "size."

Your ECP is skilled in evaluating your eye's physiology, and your eyesight, to determine which lens best satisfies the two criteria above.

Third, you may have another medical need that drives the choice of lens. For example, your ECP might pick a particular lens if your eyes tend to be dry.

Finally, consider your "wish list" of contact lens features &#; colors, for example, or overnight wear.

When you and your ECP decide on the right lens for you, you'll be given a contact lens prescription. You'll be able to buy a supply of lenses from your ECP or from the many other outlets that sell contact lenses.

SEE RELATED: Contact solution as eye drops?

Contact Lens Wear and Care

Caring for your contact lenses &#; cleaning, disinfecting and storing them &#; is much easier than it used to be.

A few years ago, you would have needed several bottles of cleaning products, and perhaps enzyme tablets, for proper care. Today, most people can use "multipurpose" solutions &#; meaning that one product both cleans and disinfects, and is used for storage.

People who are sensitive to the preservatives in multipurpose solutions might need preservative-free systems, such as those containing hydrogen peroxide. These do an excellent job of cleaning contacts, but it's very important to follow the directions for using them. The solution should not come into contact with your eyes until soaking is complete and the solution is neutralized.

Of course, you can avoid lens care altogether by wearing daily disposable contact lenses.

Contact Lens Problems

Trial and error often is involved in finding the perfect lens for you. People react differently to various lens materials and cleaning solutions.

Also, the correct "parameters" of your lens &#; that is, power, diameter, and curvature &#; can be finalized only after you've successfully worn the lens. This is especially true for more complex fits involving extra parameters, such as with bifocals or toric contact lenses for astigmatism.

If you experience discomfort or poor vision when wearing contact lenses, chances are that an adjustment or change of lens can help.

Today, more contact lens choices than ever are available to provide comfort, good vision, and healthy eyes. If your eyes or lenses are uncomfortable or you are not seeing well, remove your lenses and visit your eye care professional to explore available remedies for contact lens discomfort.

Buying Contact Lenses

You can buy replacement contact lenses at many places, and some offer a better value than others. Find out more about where to buy contacts and buying contact lenses online.

More Information About Contacts

For more information about contacts, visit our contact lens FAQ, eye doctor Q&A and contact lens news pages.

READ MORE: The best contact solution products

Notes and References

Contact Lenses . Contact Lens Spectrum. January .

We continue our discussion through the basic concepts of light and optics by making a detour away from the fundamentals and instead focusing on an often undiscussed aspect of our field &#; how to request an optic. This article is directed to the procurement specialist, college student and any individual who needs an optical component but has very little experience in contacting an optical supplier for a specific need. Perhaps you were tasked with finding a window or directed to purchase a lens with a 100mm focal length and simply don&#;t know where to begin. Our goal is to offer a reference to assist you in finding the most appropriate optic for your application in as little as time as possible.

Often times at Esco we receive inquiries that generate more questions than answers. In fact, this is a near daily occurrence. Through repetition, we&#;ve found that in order to understand a customer&#;s requirements, we generally ask many of the same questions and, by simply offering a few additional details up front, a solution is almost always readily available. For example, as noted above, it is not uncommon for us to receive a request for a lens with a defined focal length but with no further information. While this is a good starting point, it presents a vast array of choices that may be fulfilled from one of our stock product lines (or perhaps move into the realm of a custom optic) but we simply don&#;t know since we have no idea how the optic is being used or how it is installed in your system. By providing details that address the optic&#;s dimensions, spectrum of use, application and environment, as we as, expected performance including the possible addition of thin film coatings, you can avoid the back and forth dialogue that may take days to resolve, delaying your procurement and schedule.

Dimensions

Surprising to us is the number of inquiries we receive that lack information regarding the size of a customer&#;s required optic. Whether a diameter or rectangle, without the basics of size and tolerances, determining any further details, let alone cost, is impossible. If the optic in questions is a lens, information on any restrictions in center or edge thickness is important to begin the process of finding the appropriate match. If unknown, refer back to how the optic is mounted in the system to better understand the required final configuration.

Material & Spectrum

We tie these two aspects of optics together because not all materials are created equal, both in performance and cost. If you are unfamiliar with the electromagnetic spectrum, we encourage you to read our article Essential Concepts in Light and Optics for a better understanding of how light works. That said, if you&#;re pressed for time, we can break down the spectrum into three simple optical regions and letting us know your areas of interest will help narrow down your choices of material considerably. Your regions may overlap somewhat, but don&#;t worry, a lot of materials do as well so we can recommend what may work best for you. What follows are generalities and not considered true scientific definitions &#; consider them a guideline.

Ultraviolet:  250nm &#; 400nm 

These are wavelength below the visible spectrum where fused silica reigns supreme. If you are working below 250nm, still considered ultraviolet, we will need to know more about your application (see section below) as this region moves into some highly specific optical materials requiring special considerations.

Visible:  400nm &#; 750nm

There are literally hundreds of materials to choose from in this range, however, there is also a reason why Esco&#;s stock product line of windows is kept simple with the choice of standard optical glass (BK7), fused silica/quartz, low expansion (Borofloat) and sapphire. These four material variations cover the vast majority of applications and optical environments that may be encountered when working in the visible region. Usually it simply comes down to the consideration of cost in relation to performance and durability.

Near Infrared / Infrared: >750nm

This is the region above the visible spectrum most commonly associated with the detection and imaging of heat signatures. It is important to specify because many of the materials that work in the visible and ultraviolet regions no longer transmit light above a certain defined wavelength. They essentially &#;shut off&#; and offer no functionality.

Application & Environment

This section could literally become a full article in its own right, but our goal is to keep things simple. In short, when requesting an optic, offering a few details on how it will be used will provide us almost all the information we need to present a solution. Is it an imaging application and, if so, what are you imaging? Is it a basic viewing window that only requires transparency? Will you be directing a laser through the optic (see section on lasers below)? Having covered hundreds of different applications over Esco&#;s 60+ years in business, we&#;ve probably come across a similar, if not exact request for optics used in your setup. Let our expertise be your guide by letting us know the basics of how it&#;s used.

Environment is also important because not all materials withstand the rigors of exposure to harsh conditions. Are there temperature considerations? Will the optic be exposed to moisture, salt, caustic chemicals or excessive handling? Perhaps the optic is used in environments where extremely low or high pressure is a concern. Again, providing a few details on these type of factors will help us narrow down the available choices of materials and specifications.

Optical Specifications & Performance

Esco&#;s commercial optics are specified to approximately 5 waves in surface form and for most basic applications, this is sufficient for functionality. For more complex systems, determining functionality without the assistance of an optical designer or engineer is difficult and we always prefer a drawing of the optic to assist in providing a quotation, especially if it is a custom configuration. Unfortunately, optical specifications can be quite complex and if you are uncertain of how to specify your optic we recommend reading our articles on how optical surfaces are characterized (Lenses Part 2 & Plano Optics). If you still are unsure how to proceed, an Esco sales representative can assist in determining how precise your optics may need to be by understanding the details of dimensions, application/environment and the spectrum in which it will be used. Please contact us directly for further assistance and we can take you through the steps of characterizing your optic.

Lasers

Lasers deserve their own consideration because not all optics are made to withstand the level of power density some lasers can generate. You need not understand how lasers work but there is no guarantee of functionality or longevity without the following details:

Type/Wavelength: What kind of laser are you using and what portion of the spectrum does it operate? For example, a common laser type is the Nd/YAG (Neodymium-doped / Yttrium Aluminum Garnet) which produces a beam of light at nm. Understanding the type of laser is very important in determining the appropriate material. Also of interest is the power rating of the laser, specifically whether the laser is pulsed or continuous.

Pulsed Lasers: These laser can generate an incredible amount of power and cycle billions, if not more times per second. The output power is specified in joules or millijoules per square centimeter, written mJ/cm^2 or J/cm^2 and this information is important when requesting an optic for use in the lasers path. The second characteristic is the physical spot size or, more simply, the actual diameter of the laser beam.

Continuous Lasers: Continuous lasers, as the name implies, are laser that have a continuous output of power. Their power rating is specified in watts or milliwatts per square centimeter, written mW/cm^2 or W/cm^2. Please specify the power rating when using a continuous laser.

Coatings

Optical thin film coatings, like application and environment, could warrant its own article, but for the novice, we can breakdown the requirements one simple parameter &#; spectrum. Coatings are designed to function over specific regions of the electromagnetic spectrum and we need to know where to optimize the design.  Coatings used with lasers are also specially designed to withstand the intense power in order not to fail in the system. By specifying application, environment and the possible use of lasers, you&#;ve already covered a large portion of the information we need to determine what type of coating works best.

Summation

As we&#;ve described above, a request that does not specify the dimensions, application/environment, spectrum or performance expectations leaves us without the appropriate tools to determine which optical configuration is required. By adding a few extra details to your inquiry, you will save valuable time and assist us in providing a solution that places an optic in your hands as fast as possible. And to make your procurement even easier, we&#;ve added a new online inquiry page that covers all of the aspects listed above. With a few clicks of your mouse, you can send a request with all required info directly to the Esco sales team where we&#;ll be happy to locate the most appropriate optic for your application. We look forward to hearing from you.

If you want to learn more, please visit our website contact us for custom optics.