A Contact Lens Glossary: Contact Lens & Vision Terms You’ve Always Wondered About


Suffering from a vision problem is difficult enough—and wearing contact lenses to correct that vision problem poses its own set of challenges. You’ll have to learn new terms, and since contact lens technology does evolve, you’ll continue to learn as you go along.

Whether you’re just starting your contact lens journey or you’ve been wearing contact lenses for years, bookmark this glossary to reference anytime you have a question about the world of contact lenses.

Types of vision problems

Here are the most common vision problems that contact lens wearers face:

  • myopia – Myopia refers to nearsightedness. It’s a condition wherein you can clearly see nearby objects, but objects that are farther away appear blurry.
  • hyperopia – Hyperopia refers to farsightedness. It’s a condition wherein you can clearly see objects in the distance, but nearby objects appear blurry.
  • astigmatism – Astigmatism refers to imperfections in the curvature of your cornea that cause blurred near vision and distance vision
  • Contact lens basics

Here is everything you need to understand about the basics of contact lenses and the materials they are made of.

  • contact lenses 

Contact lenses are thin, clear discs that you wear on your eye for vision correction. When worn, the contact lens sits on the tear film that covers your cornea. 

Soft contact lenses are made from hydrogel or silicone hydrogel while hard contact lenses are made from rigid plastic materials.

In the US, the FDA lists all contact lenses as medical devices. You’ll need a valid prescription before you can buy and wear any kind of contact lenses, even if they’re just colored Plano contact lenses or Halloween contact lenses.

  • hydrogel

Hydrogel is a plastic material infused with water. Contact lenses are made of hydrogel because it has a high degree of biocompatibility, which means the body doesn’t produce a negative immunological response when exposed to hydrogel.

Hydrogel is also thin and pliable, which allows it to easily conform to the shape of the human eye.

  • silicone hydrogel

Silicone hydrogel is plastic infused with both water and silicone. Silicone is a gel-like substance that makes silicone hydrogel contact lenses more porous and more flexible. 

Since silicone hydrogel is more porous, it has a high degree of oxygen permeability. Compared to hydrogel contact lenses, silicone hydrogel contact lenses allow up to five times more oxygen to pass through the lens and reach the cornea.

Types of contact lenses

Here are the types of contact lenses that people wear today.

  • soft contact lenses

Soft contact lenses are contact lenses made of either hydrogel or silicone hydrogel material. The US Food and Drug Administration (US FDA) approved the first use of hydrogel for contact lenses in 1971.

Soft contact lenses are the most popular and most commonly prescribed contact lenses today.

  • toric contact lenses

Toric contact lenses are soft contact lenses with a specific shape designed to correct astigmatism. Unlike standard soft contact lenses, toric lenses do not have a spherical shape. The corneas of astigmatism patients are irregularly shaped, so spherical contact lenses won’t help these patients.

The shape of toric contact lenses has different refractive powers on the vertical and horizontal orientations. This refractive power increases or decreases based on the position of the lens.

  • rigid gas-permeable contact lenses

Rigid gas-permeable (RGP) contact lenses are hard contact lenses made of porous plastic materials. These types of contact lenses are comparable to silicone hydrogel contact lenses in that they allow more oxygen to reach the eye than hydrogel contact lenses.

Although not as common as soft contact lenses, rigid gas-permeable contact lenses do have certain advantages over their hydrogel counterparts. Since they are rigid, RGPs retain their shape on the eye. Their rigidity allows RGPs to last longer than soft contact lenses and provide wearers with sharper vision. 

  • orthokeratology (ortho-k) contact lenses

Orthokeratology (or ortho-k) contact lenses are special rigid gas-permeable contact lenses to temporarily reshape the cornea and improve one’s vision. Orthokeratology refers to the lens fitting procedure that uses these lenses.

You wear the ortho-k contact lenses at night and the lenses reshape your cornea and improve your vision as you sleep. You remove the lenses the next morning and don’t have to wear them for the rest of the day.

Orthokeratology enables the wearer to go about their day without having to worry about contact lenses. However, orthokeratology provides only temporary vision correction. The cornea will gradually return to its original shape as the day passes. To maintain the vision correction, you’ll have to wear the ortho-k lenses each night.

  • colored contact lenses

Colored contact lenses function the same way that regular contact lenses do. The only difference is that colored contact lenses enable you to temporarily alter the color of your eyes.

There are three main types of colored contact lenses:

  • Visibility tint contact lenses – These lenses have a faint blue or green tint to help wearers see the lenses better during insertion or removal as well as in instances where the wearer drops the contact lens. The tints in these lenses do not affect eye color.
  • Enhancement tint contact lenses – These lenses have a darker tint than visibility tint contact lenses. As their name implies, enhancement tint contact lenses are designed to enhance the wearer’s natural eye color rather than alter it completely.
  • Opaque tint contact lenses – These lenses have the darkest tint among colored lenses. Opaque tint contact lenses are the ones that are designed to completely change the wearer’s eye color.

There are colored contact lenses that provide vision correction and ones that don’t (Plano). Whether you have natural 20/20 vision and just want to change your eye color for cosmetic reasons or you have vision problems and want to wear something other than regular contact lenses, there are colored contact lenses available for you. 

  • decorative contact lenses

Decorative contact lenses are also known as Halloween contact lenses, special effect contact lenses, costume contact lenses, or novelty contact lenses. These contact lenses allow wearers to completely change the appearance of their eyes. 

People wear these types of lenses to enhance their looks on occasions that call for the wearing of costumes such as Halloween, comic book conventions, and costume parties.

Actors, actresses, and stunt performers also wear decorative contact lenses when playing roles such as aliens, vampires, werewolves, or other supernatural creatures in film and television productions.

Most decorative contact lenses are plano (no vision correction), so wearing them will not correct any vision issue.

Contact lenses according to wearing schedule

Here are the common wearing schedules of contact lenses available today.

  • daily disposable contact lenses (dailies)

Daily disposable contact lenses are contact lenses designed to be used once and then discarded. When using dailies, one wears a brand new pair of contact lenses every day. Dailies are meant to be inserted in the morning and taken out at night before sleeping.

  • bi-weekly and monthly daily wear contact lenses

Bi-weekly daily wear contact lenses are contact lenses designed to be used for 14 days and then discarded. Meanwhile, monthly daily wear contact lenses are soft contact lenses designed to be used for 30 days and then discarded.

When wearing either type of contact lenses, you insert them in the morning, take them out at night, and store them in a contact lens case so that you can wear them again the next day.

  • extended wear contact lenses

    Extended wear contact lenses are contact lenses designed for continuous wear for up to 30 nights, which is the maximum wearing time approved by the FDA.

When wearing these types of contact lenses, you don’t need to remove them at night. You can wear them in your sleep.

Technical contact lens terms

  • Oxygen permeability

Oxygen permeability is a contact lens parameter that measures the contact lens’s ability to let oxygen pass through the lens and reach the eye. In soft contact lenses, oxygen permeability depends on the material and thickness of the lens. Silicone hydrogel contact lenses have higher oxygen permeability compared to hydrogel contact lenses.

  • Dk, Dk/t

Dk is the numerical value of a contact lens’s oxygen permeability.

D stands for “diffusivity”, which measures how fast oxygen moves through the contact lens. Manufacturers measure diffusivity in cm2/sec, which is what you’ll see on contact lens boxes.

K stands for “solubility”, which measures the amount of oxygen in the material of the contact lens. Manufacturers measure solubility using the formula ml O2/ml of material x mm Hg.

Meanwhile, Dk/t is the numerical value of a contact lens’s oxygen permeability that is dependent on the thickness of the lens. The value “/t” stands for “per thickness”.

The higher the Dk (or Dk/t) of a contact lens, the higher its oxygen permeability. Hydrogel contact lenses usually have a Dk between 25 and 50 while silicone hydrogel contact lenses have a Dk of more than 100.

Contact lens prescription in detail

You will find the following items listed on your contact lens prescription. To help you make sense of your prescription, here are the definitions of each item.

  • OD – This stands for “Oculus Dexter”, which is Latin for “Right Eye”
  • OS – This stands for “Oculus Sinister”, which is Latin for “Left Eye”
  • OU – This stands for “Oculus Uterque”, which is Latin for “Both Eyes”
  • PWR/SPH – This stands for Power or Sphere, which is the overall power of the contact lenses. Power is measured in diopters and indicates the level of correction that the lenses need to provide to correct your vision.

If your power has a minus (-) sign, it means you have nearsightedness (myopia). On the other hand, if your power has a plus (+) sign, it means you have farsightedness (hyperopia).

The right power should give you 20/20 vision.

  • BC – This stands for Base Curve, which is the inside curve of the lens measured in millimeters. The base curve relates to the curvature of your cornea and is measured to ensure that the lens has an accurate fit. The lower the base curve, the steeper the corneal curve. 
  • DIA – This stands for Diameter, which is the width of the lens from edge to edge. Wearing contact lenses with the right diameter will allow the lens to sit perfectly on top of your cornea. Meanwhile, wearing contact lenses with the wrong diameter can cause eye pain and irritation.
  • CYL – This stands for Cylinder, which refers to your level of astigmatism. You will only see a value under “CYL” if your eye care professional (ECP) prescribed you toric lenses, and it will usually be a number between -4.00 and +4.00
  • AX – This stands for Axis, which is the orientation of your astigmatism measured in degrees. The axis indicates the position of the cylindrical power of your lens. Like “CYL”, you will only see a value under “AX” if you have astigmatism
  • ADD – This stands for Add Power, which indicates the magnifying power in a multifocal contact lens. Add Power is measured in diopters.

Hopefully, after reading this glossary, you’ll have a much better understanding of the vital information related to contact lenses—from what your vision problem really means, right down to the details listed on your contact lens prescription. For any other questions related to your vision problems, make sure that you consult your eye care professional (ECP) immediately.