Glossary

The Prescription


 Vision begins when light rays enter the first surface of the eye called the cornea. The cornea is curved and bends or refracts light rays. The corneal curvature is usually between 40.00 to 47.00 diopters and is responsible for approximately 2/3rd of the eye’s refracting power.

Once light rays pass through the cornea they continue through the pupil opening to reach the crystalline lens, which is responsible for the remaining 1/3rd of the eyes refracting ability. The crystalline lens is held in place by ligaments that allow the lens to change shape. This change in lens shape leads to the eye’s ability to focus objects from distance to near and is referred to as accommodation.

Finally the light rays reach the light sensitive retina to receive images which are sent to the brain for interpretation. When light rays focus directly on the retina a person is considered emmetropic. Patients who are emmetropic do not require any corrective prescription lenses.

In addition to the cornea and crystalline lens, the length of the eye also affects how an image reaches the retina. Any alteration in the parts of the eye can cause light rays to be focused somewhere other than the retina to produce an unclear image. This is referred to as ametropia. There are several types of ametropia:

Hyperopia, also known as farsightedness occurs when the image is focused behind the retina, resulting in blurry vision at near. Hyperopia can result because the cornea is too flat, or because the eye is too short. Accommodation can make up for some of the refractive error in a hyperopic eye. Hyperopia is corrected with plus lenses, an example of a patient’s prescription would be +1.75 sphere.

Myopia, also known as nearsightedness occurs when the image is focused in front of the retina, resulting in blurry vision in the distance. Myopia can result because the cornea is too curved or because the eye is too long. Myopia is corrected with minus lenses, an example of a patient’s prescription would be -2.25 sphere.

Astigmatism occurs when the curvature of the corneal surface is longer in one meridian than the other. In this case images are not focused at one point but at two points, resulting in blurry vision at all distances. Astigmatism can also be located internally and linked to differences in the shape of the crystalline lens. To note the astigmatic orientation the prescription will include an axis. Astigmatism may be present in myopia or hyperopia, an example of a patient’s prescription would be -3.00 -1.00×180 or +3.00 -1.00×180

Presbyopia occurs when the crystalline lens inside the eye loses its ability to change shape resulting in blurry vision at near. For most patients symptoms begin in the early 40’s and continue until mid 60’s when the lens completely loses its ability to accommodate. Presbyopic patients will require an add power.

Keratoconus is a disease of the cornea that causes a cone-shaped protrusion of the center of the cornea.

GP Contact Lens Options

Gas Permeable or GP Contact lenses are firm lenses made from a combination of silicone and/or fluorine materials that allows oxygen transmission while maintaining their shape. Because of their firm shape they provide excellent vision, are easy to apply and remove from the eye, are easy to handle and care for, and can last a long time.


GP Lens Options Spherical contact lenses have a spherical surface where the curvature of the lens is equal in every meridian. For example, a baseball has a spherical shape where every curve on the ball is equal.

Aspheric contact lenses have a non-spherical surface where the lens curves and power changes gradually, from the center to the edge of the lens. Aspheric lenses can be designed for single vision and multifocal use.

Bifocal contact lenses have two separate areas of vision; one for distance and one for near. Bifocal contact lenses allow the patient to see distance objects through the top portion of the lens when looking straight ahead. As the patient looks down to read the lenses shift or translate up to view near vision through the lower portion of the lens. Due to the lens shift from distance to near vision, GP bifocal lenses are also referred to as alternating or translating designs. Bifocal lens designs generally have a straight-top or curved crescent segment shaped segment and are only available in GP lens materials.

Multifocal contact lenses provide vision at arm’s length in addition to near and distance focal lengths. The contact lenses usually have a center distance zone with near vision gradually progressing out into the periphery. Since light rays from near, intermediate and distance objects are focused on the retina at the same time this lens type is also referred to as a simultaneous vision design. They are referred to as aspheric designs. Multifocal lenses are an excellent choice for patients who need to see objects at all distances, as well as those who spend a great deal of time on a computer.

Monovision is a method of correcting presbyopia by fitting a single vision contact lens on one eye to see at distance and the other for near.

Toric contact lenses are used to correct astigmatism. These lenses have a different curvature in the principle meridians that coincides with the location of the astigmatism. There are different types of toric GP lenses. A front surface toric lens has a toric front surface and a spherical back surface. These lenses usually have a prism ballast to orient the lens in the proper position. A back surface toric lens has a spherical front surface and a toric back surface. A bitoric lens has a toric front and back surface.

Reverse geometry lenses have a central base curve zone that is flatter than the adjacent secondary curve. This differs from conventional designs that have a steeper base curve. These lenses are used for orthokeratology and post-surgical fitting.

GP Contact Lens Parameters


Lens Parameters Base curve radius (BCR) or base curve (BC) of a contact lens is the primary curve on the backside of the lens. The base curve is expressed in millimeters of radius of curvature or dipoters. For example, a 7.5 millimeter base curve is the same as a 45.00 diopter base curve.

Lens power is the prescription strength of the contact lens. The lens power of a contact lens is often different from the spectacle lens power because the contact lens fits on the eye and not in front of the eye as spectacles do.

Overall diameter (OAD) is the size of the lens when measured from edge to edge at the widest point. The lens diameter is specified in millimeters.

Optical zone diameter (OZD) or optical zone (OZ) is the central area or zone that provides optical correction for the patient’s vision. The optic zone diameter varies depending on the pupil size of the patient, the overall lens diameter and peripheral curve width. The optic zone is specified in millimeters.

Peripheral curves are additional curves on the back surface of the lens that surround the optical zone to help tailor the contact lens fit to the individual patient. Peripheral curves allow the back surface of the contact lens to follow the gradual sloping of the cornea which is steeper in the center and flattens towards the periphery. Peripheral curves have a width and radius of curvature. The secondary curve is the curve immediately surrounding the optical zone. The peripheral curve is the outermost curve.

Center thickness refers to the thickness of the center of the lens.

Tints are available in a variety of colors for GP lenses. These tints do not change the eye color.

GP lens materials are available in a variety of Silicon Acrylate and Fluoro Silicon Acrylate options. Generally the lenses are ordered by their brand name.

Identification dots are commonly used to differentiate between the right and left lens.

Prism ballast is the addition of a prism shaped wedge used to weigh down the lens and achieve the proper orientation in toric and bifocal contact lens fitting.

Truncation is the removal of a section of the lower, or upper and lower portion of a lens to achieve proper orientation in toric and bifocal contact lens fitting.

Blend is the treatment given to the junctions where the optical zone and peripheral curves meet to ensure smoothness between the curves.

GP Contact Lens Solutions

Cleaning a GP lens is done immediately after lens removal to get rid of debris and bacteria from the surface of the lens. When cleaning lenses, apply a few drops of cleaning solution to the lens surface and rub gently as recommended by the solution manufacturer. Lens cleaning is a necessary step to ensure the lenses remain comfortable and safe to wear. The cleaning step can be done with a separate surfactant cleaner or a multipurpose all-in-one solution.


GP Lens Solutions Wetting a GP lens is necessary to convert the dry plastic into a water-loving surface so the tear film spreads more easily and evenly over the lens. This improves lens comfort by acting as a cushion, and improves vision by providing a clear refracting surface. A wetting solution also tends to help maintain the cleanliness of the lens by preventing smudging from fingers.

Disinfecting and storing lenses is done with a soaking solution. The chemicals in the soaking solution disinfect the lenses over a period of time as specified by the manufacturer.

Enzyme cleaning removes the protein from the lens surface to improve comfort and vision.

Multipurpose or one-bottle solutions are formulated to perform multiple tasks of cleaning, wetting and disinfecting.