Accommodation

Adjustment by the eye for seeing at different distances, accomplished by changing the shape of the crystalline lens through action of the ciliary muscle.

Acuity

Clearness; visual acuity is measured by the smallest object that can be seen at a certain distance.

Adnexa Oculi

Accessory structures of the eye, such as the lacrimal apparatus and the eyelids.

Afterimage

Image of an object that persists when the lids are closed.

Annular Bifocal Contact

A lens with distance portion ground into the center of the lens and the near portion ground into the periphery.

Aphakic Lenses

Lenses designed for post cataract fitting.

Apical Zone of Cornea

The central portion of the cornea with a constant radius of curvature. Also called the corneal cap.

Artificial Tears

Wetting agent for the cornea to supplement the loss of tear formation.

Aspheric Lens

A lens which is not spherical in shape. The curvature gradually flattens as the periphery is approached.

Astigmatism

A refractive error that prevents the light rays from coming to a single focus on the retina because of different degrees of refraction in the various meridians of the eye.

Against-The-Rule Astigmatism

A condition in which the steepest corneal meridian is in the horizontal plane. An example of Keratometer readings for a patient exhibiting against-the-rule astigmatism would be: K’s 45.00@180/42.00@90.

Irregular Astigmatism

Astigmatism caused by an irregularly shaped cornea. This can occur in conditions such as Keratoconus or corneal scarring. This type of astigmatism cannot be corrected by cylinders.

Lenticular Astigmatism

Astigmatism that is found in the crystalline lens.

Lenticular astigmatism is present when there is significantly more astigmatism in the patient's refraction (cylinder power in Rx) than on their corneal surface (difference in K-readings).

Oblique Astigmatism

Regular astigmatism in which the principle meridians are other than 90 and 180.

Residual Astigmatism

Astigmatism remaining after the corneal astigmatism has been neutralized.

With-The-Rule Astigmatism

Condition in which the steepest corneal meridian is in the vertical plane. An example of Keratometer readings for a patient exhibiting with-the- rule astigmatism would be: K’s 42.00@180/45.00@90.

Bactericide

A chemical that disinfects and kills pathogenic organisms.

Benzalkonium Chloride

A preservative used in rigid contact lens solutions because of its germicidal qualities.

Biomicroscopy

A method of examining the cornea under a magnification of from 10 to 50 times utilizing a slit lamp.

Burton Lamp

An ultraviolet light used to illuminate the fluorescein dye that is used to analyze the fit of a rigid contact lens.

Canthus

The angle at either end of the slit between the eyelids; specified as outer (temporal) and inner (nasal).

Chord Length

The straight line measurement of the contact lens diameter from edge to edge.

Contact Lens Wetting Angle

The angle between the liquid and the lens surface.

Contour Lens

A tricurve lens designed to conform to the curvature of the cornea, which flattens as it extends in the periphery.

Copolymer

Two or more chemicals that are combined to form a new chemical compound.

Corneal Cap

The apical zone or central zone of the cornea that has a constant area of curvature.

Corneal Diameter

The diameter of the cornea, usually taken along the horizontal meridian with a ruler. Also called visible iris diameter.

Deturgence, Corneal

The state of relative dehydration maintained by the normal intact cornea that enables it to remain transparent.

Diagnostic Fitting Set

A set of trial lenses used to gain an overview of the fit of a contact lens.

Disinfection

Physical or chemical procedures that kill common pathogenic organisms but may permit some nonpathogenic organisms to survive.

DK Value

A measure of the oxygen permeability through a given material where D is the diffusion coefficient for oxygen movement on the lens material and K is the solubility of oxygen in this material.

Double Slab-Off Lenses

Sometimes called thick-thin lenses; the upper and lower portions of the lens are reduced in thickness so that when the lens is placed on the eye these portions lie under the upper and lower eyelids. The thin zones aid in stabilizing toric soft lenses.

Dry Spots

Areas of drying as noted by absent areas of fluorescein-stained tear film on the cornea when the patient stares.

Dyer Nomogram System of Lens Ordering

A simplified system of ordering rigid lenses based on clinical experience, corneal topogometry, and charts of associated lens parameters.

Enzyme Cleaner

A cleaning agent that acts on a soft lens by a digestion of protein.

Esthesiometer (Cochet-Bonnet)

A device used to evaluate corneal sensitivity, consisting essentially of a nylon thread mounted in a handle so that its length may be varied and calibrated in milligrams of weight necessary to bend a given length of the thread when pressed against the cornea.

Fissure

Elliptic space between the eyelids.

Finished Lens

A complete lens with anterior and posterior curves, a specified diameter, a designated peripheral curve, and edge design.

Fitting Set

A complete inventory of lenses of graduated powers and base curves.

Flare

Flutterings or fringing of lights caused by a lens with an optic zone too small as a decentered lens or an excessively loose lens.

Flat Cornea

A cornea with a K value less than 41.00 D.

Fluid Lens

Power created by having a very convex or concave tear film.

Fluorescein

An organic compound that is inert and used to stain the tear film for contact lens fitting and to assess the integrity of the cornea.
Fluorescein is a valuable tool in evaluating the lens to cornea relationship of a rigid gas permeable lens. A
special type of fluorescein (fluorosoft) should be used to evaluate the fitting relationship of a soft contact
lens but will not be useful in evaluating the parameters of that lens.

Haptic

The part of a contact or intraocular lens that supports the optic portion and touches the peripheral or nonoptic portion of the cornea: the word indicates “fastening, contact, sense of touch.”

Hydrogen Peroxide

A bactericide used for soft lenses.

Keratoplasty

A corneal transplant.

Lenticular Lens

Relatively large lens most suitable for large, flat eyes; consists of a central optic zone and a surrounding nonoptic flange.

Limbal Zone

Junction between the periphery of the cornea with the sclera.

Loose Lens

A contact lens with excessive movement; it can be caused by a lens that is too small in diameter, too thick or too flat.

Meibomian Glands

Sebaceous glands of the eyelid.

Microthin Lens

A lens less than 0.10 mm in thickness.

Minus Carrier

A lens designed with an edge configuration similar to that of a minus lens that is thicker at its periphery; often used with high-plus lenses such as aphakic lenses.

Monovision

Single-vision contact lenses used for presbyopes for whom the power of the lenses is such that one eye is used for distance vision and the other is used for near vision.

Mydriatic Agent

A drug that causes the pupil to contract.

Nomogram

A table of precalculated mathematical values used to arrive at the specifications of a rigid lens design.

Orthokeratology

The technique of flattening the cornea and thus correcting refractive errors by the use of a series of progressively flatter contact lenses.

Oxygen Flux

A measure of the amount of oxygen that will pass through a given area of material in a given unit of time.

Oxygen Permeability

The degree to which a lens permits the passage of oxygen across it. It depends on the composition of the plastic (that is, silicone has excellent permeability, whereas PMMA has no permeability), the thickness of the lens, and its water content. It is often expressed as the DK value.

Pachometer

An instrument used to measure the thickness of the cornea and depth of the anterior chamber.

Photokeratoscope

An instrument designed to photograph annular rings of the cornea and to aid in making a contact lens that will contour to the cornea.

Placido’s Disc

A disc with concentric rings to determine the regularity of the cornea when its reflection is revealed on the corneal surface.

Plano Lens

A lens with zero power.

Polymer

A chain of linked molecular unity of dimension greater than 5 monomer unity.

Polymerization

The union of molecules of a compound to form larger molecules and a new compound.

Polyvinyl Alcohol

A wetting agent.

Prism Ballast Lens

Contact lens with base-down prism added inferiorly to improve the stability of as soft toric or rigid lens. Usually 1 to 1.5 D of prism is added.

Radial Keratotomy

A surgical procedure in which clock like incisions are made into the cornea to flatten the cornea and correct myopic refractive errors.

Residual Astigmatism

The astigmatism present after the corneal astigmatism has been nullified by a contact lens.

Retroillumination

Light is focused on deeper structures such as the iris, while the microscope is adjusted to study the cornea.

Schirmer Test

Measures normal tear secretion; the ability of the eye to wet in 5 minutes 15 mm of a 5 x 35 mm strip of filter paper.

Semifinished Blank

A contact lens blank in which the posterior curve of the contact lens has been fabricated.

Soaking Solution

A solution designed to keep a lens moist and free from contamination.

Soper Lens

A rigid lens designed by Joseph Soper with a steep central posterior curve to accommodate large cones of keratoconus.

Specular Reflection

A reflection from a mirror surface, such as the back of the cornea.

Spheric Equivalent

It is the spheric power of the lens plus half the cylindric power.

Sterilization

A method to ensure the complete death of all forms of bacteria fungi, and spores.

Surfactant

A cleanser that acts on the surface of a contact lens.

Tear Film Breakup Time (BUT)

An evaluation of tear quality; the tear film will normally break up in 10 to 30 seconds and show dry spots. Any dry spot that appears in less than 10 seconds is pathologic.

Thermal Disinfection

Disinfection of a lens by heat.

Thickness of a Lens

The measurement of the center of the lens.

Thimerosol (Merthiolate)

A mercurial agent used for disinfection. Has a high ocular sensitivity rate.

Three-Point Touch

That area of the cornea between the apical zone and the limbal zone.

Truncation

A design feature used in toric lenses to reduce lens rotation by cutting off a peripheral portion of the lens to conform with the lower lid border.

Can also be used to help the positioning of a rigid prism ballast lens.

V-Groove Gauge

A ruler measure with a groove to measure the diameter of rigid lens.

Visible Iris Diameter (VID)

A term that represents the iris diameter and aids in selecting the initial lens; often used in place of the corneal diameter.

Warpage

A permanent bending of a rigid lens. May also refer to a semipermanent altering of the corneal curvature.

Wet Storage

The use of soaking solution to store rigid contact lenses.

Wetting Solution

Solutions that increase the spreading or wettability of liquids in the plastic contact lens by converting the surface of a lens from a hydrophobic to a hydrophilic surface.

Chlorbutanol

An antimicrobial agent used in rigid lens disinfectants.

Ascorbic Acid

One of the most common preservatives in soft contact lens disinfecting solutions.