Optical Glossary


20/20 -the expression for normal eyesight (or 6/6 in countries where metric measurements are used). This notation is expressed as a fraction. The numerator (1st number) refers to the distance you were from the test chart, which is usually 20 feet (6 meters). The denominator (2nd number) denotes the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read. For example, if your visual acuity is 20/100 that means that the line you correctly read at 20 feet could be read by a person with normal vision at 100 feet. The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight). A refraction test is used to determine the amount of correction needed for a prescription when treating refractive error such as astigmatism, myopia, or hyperopia. See “Refraction Test”.

Abbe Value: Is a measurement to the degree of which light is dispersed with an existing lens. The average range of an ophthalmic lense is between 30 to 60. The higher the abbe value the less chromatic aberration will be found in the lens, adversely the lower the abbe value there is a greater dispersion of light causing more chromatic aberration when viewing through the periphery of the lens.

ABOC Certification Certification is professional distinction — official and public recognition of your achievement that you have met a national set of standards as a qualified and competent optician or contact lens technician. Certification is official assurance to the public that you will handle their eye wear needs competently and carefully; certification is recognized by employers as a standard of competence.

AC/A Ratio- accommodative convergence / accommodative (measured in prism diopters/diopters). The convergence response of an individual (amount the eyes turn inward) in relation to the amount of stimulus of accommodation (eye focusing). The normal ratio is 4:1.

Accommodation- (eye focusing) the eye’s ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power. When this accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera. The ciliary muscles must contract to adjust for near vision, which causes the eye’s crystalline lens, which is flexible, to be squashed. For distant vision, the ciliary muscle must relax and the eye’s crystalline lens is stretched out.  The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia. (See “Presbyopia”).

Accommodative Fatigue– This clinical condition is also called Ill-Sustained Accommodation. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain), general fatigue, headaches and nausea, excess tearing, and an unusual sensitivity to light. Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses (glasses or contacts) and vision therapy are effective in treating this condition.

Accommodative Esotropia– (clinical condition) when an individual is focusing on a near object and his or her eyes are turning inward too much. It is caused by either uncorrected hyperopic refractive error and/or a high accommodative convergence/accommodation (AC/A) ratio. The average age of onset is 2 1/2 years. It is most noticeable when the child is tired or sick. This is treated with plus lenses (glasses or contacts) to help straighten the eyes. In some cases, vision therapy and corrective lenses are prescribed. (Please note that Accommodative Esophoria is a condition similar to accommodative esotropia but lesser in extent.)

Accommodative Excess (AE)–  This clinical condition is also called accommodative spasm. It is an over focusing, over stimulation of the focusing action of the crystalline lens causing an inability to relax the focusing system which may result in blurry vision when focusing at distance objects. Other symptoms include holding near work closer than normal, headaches with near work (such as reading or using a computer), eyestrain associated with near work, and possible double vision. Clinical signs include: patient accepts more minus on accommodative rock but blurs with plus lenses, lower NRA than PRA, dynamic retinoscopy findings indication of over accommodation and/or slow relaxation of accommodation, and reduced or erratic distance visual acuity. Treatment includes a low plus lens and/or vision therapy.

Accommodative Infacility– a clinical condition in which the individual has difficulty changing eye focus from distance to near. Symptoms include eyestrain associated with near work (such as reading or using a computer), periodic blurring of distance vision especially following sustained near visual work, tendency to hold near work closer than expected, headaches with near work, and possible double vision. Clinical signs include: patient will have difficulty with both the plus and the minus lens (fails +/- 2.00 D flipper test), low PRA and NRA, and poor recoveries on Bell Retinoscopy. Vision therapy is an effective treatment option.

Accommodative Insufficiency (AI)- This clinical condition is also called non-presbyopic accommodative insufficiency. It is an under focusing, a lack of focusing ability at a near distance. Symptoms include eyestrain, blurred vision, occasional or constant when doing near work (such as reading or using a computer), occasional unusual sensitivity to light, excess tearing, headaches, and general fatigue. Clinical signs include: patient will have difficulty with a minus lens, low amplitude of accommodation, low PRA and higher NRA.Vision therapy is an effective treatment option.

Accommodative Vergence- a convergence response (to turn the eyes inward) which occurs as a direct result of accommodation (eye focusing). (See “Vergence”)

Acuity- clearness of eyesight. Depends on the sharpness of images and the sensitivity of nerve elements in the retina. (See “Near Acuity” and “Distance Acuity”)

Add (or add power): If there is a value under the ‘add’ heading, then you have a bifocal (or Progressive) prescription.

After-image– the eye’s ability to still see an image during eye blinks and even after the viewed object is no longer present. The most common example is seeing light after the flash of a camera.

Albinism – pigmentation is deficient or absent. May occur in skin, hair, and eyes. Ocular albinism is a pigmentation deficiency occurring mainly in the eyes. Individuals with albinism including ocular albinism commonly have decreased visual acuity (20/70 -20/200), strabismus, photophobia, and nystagmus. There is no known treatment. Individuals may benefit from low-vision aids. Treatment options for strabismus and nystagmus does apply to these individuals.

Alignment- proper fusing (uniting) of images to each eye.

Amblyopia- (clinical condition) reduced visual acuity (poorer than 20/20) which is not correctable by glasses or contacts and is not caused by structural or pathological anomalies. This condition is often called “lazy eye” because it is typically the result of disuse. It is usually marked by blurred vision in one eye and favoring one eye over the other. About two percent of the population is affected.

Types of functional (reversible) amblyopia:

  • refractive- anisometropia (the two eyes have different refractive powers), or other amblyopiogenic refractive errors (hyperopia, myopia, or astigmatism)
  • strabismic- misalignment of the two eyes in which they point in different directions
  • form deprivation (may also be referred to as  amblyopia ex anopsia)- caused by conditions that prevent light from entering the eye. These may include congenital ptosis (droopy eyelid), corneal opacity, or cataract.

Ametropia- any optical error such as hyperopia, myopia, or astigmatism. Also called refractive error.

Amplitude of Accommodation (AA)- a measurement of the eye’s ability to focus clearly on objects at near distances. This eye focusing range for a child is usually about 2-3 inches. For a young adult, it is 4-6 inches. The focus range for a 45-year-old adult is about 20 inches. For an 80-year-old adult, it is 60 inches.

Aniseikonia- a difference in the size or shape of two visual images when the images should be the same size and/or shape.

Anomalous Retinal Correspondence (ARC)- a type of retinal projection, occurring frequently in strabismus, in which the foveae (center of the retina that produces the sharpest eyesight) of the two eyes do not facilitate a common visual direction; the fovea of one eye has the same functional direction with an extrafoveal (non-fovea) area of the other eye

Anisometropia- the condition in which the two eyes have different refractive

ANSI Standard Z87.1: The new standard is now called the ANSI Z87.1-2003 standard, and is performance driven with two levels of performance for non-plano (prescription) lenses.

– Basic Impact
– High Impact
• The new standard requires that prescription safety frames must meet the lens retention (High Velocity and High Mass) test requirement with 2.0mm lenses.
Please be aware that an employee subjected to High Impact may not be adequately protected if wearing lenses tested only for Basic Impact. Description of the new ANSI testing requirements:
High Impact Level Prescription Lenses
• Must not be less than 2.0mm thick.
• The lenses shall be capable of resisting impact from a 6.35mm (1/4 in.) steel ball traveling at 45.7 m/s (150 ft/s).
• Marking by manufacturer with “W” as a trademark would look
like this: W+ Basic Impact Level Prescription Lenses.
• Basic Impact lenses shall be a minimum 3.0mm thick except those lenses having a plus power of 3.00D or greater shall have a minimum thickness of 2.5mm (no change from the 1989 standard).
• Basic Impact lenses are not tested to stringent High Impact requirements.
• They will NOT be marked with a “+”
• Protectors with Basic Impact lenses will be delivered to the wearer bearing a Warning Label indicating that the protector only meets the Basic Impact Standard.

ANZI Prescription Frames
• All prescription safety frames must meet High Velocity and High Mass impact resistance tests while retaining the lenses.
• The frame will be marked with Z87-2.
• All frames marked with Z87-2 can be used for Basic Impact and High Impact protection.

ANZI Side shields
• Lateral protection shall be assessed using a rotation point 10mm behind the corneal vertex, which means that shields must now provide more coverage.

Aperture Rulea stick-like instrument used in vision therapy to develop convergence and divergence (eye teaming) skills.

Asthenopia eyestrain, symptoms include excessive tearing, itching, burning, visual fatigue, and headache. It can be caused from an uncorrected refractive error, accommodation (eye focusing) disorder, binocularity (eye teaming) disorder, or by extended, intense use of the eyes.

Astigmatism Astigmatism is one of a group of eye conditions known as refractive errors. Refractive errors cause a disturbance in the way that light rays are focused within the eye. Astigmatism often occurs with nearsightedness and farsightedness, conditions also resulting from refractive errors. Astigmatism is not a disease nor does it mean that you have “bad eyes.” It simply means that you have a variation or disturbance in the shape of your cornea. Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. Normally the cornea is smooth and equally curved in all directions and light entering the cornea is focused equally on all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in the other. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. The distortion results because of an inability of the eye to focus light rays to a point.

If the corneal surface has a high degree of variation in its curvature, light refraction may be impaired to the degree that corrective lenses are needed to help focus light rays better. At any time, only a small proportion of the rays are focused and the remainder are not, so that the image formed is always blurred. Usually, astigmatism causes blurred vision at all distances. Some people with very high degrees of astigmatism may have cornea problems such as keratoconus.

Astigmatism is very common. Some experts believe that almost everyone has a degree of astigmatism, often from birth, which may remain the same throughout life. The exact reason for differences in corneal shape remains unknown, but the tendency to develop astigmatism is inherited. For that reason, some people are more prone to develop astigmatism than others.

Automated refractor – also called auto refractor. This method determines the eye’s refractive error and the best corrective lenses to be prescribed by using a computerized device that varies its optical power mechanically and prints out the results.

Axis: As mentioned above, a special cylindrical lens is needed in order to correct astigmatism. Not only does the strength of the cylindrical lens need to be specified, but the lens itself must be rotated into a specific position in order to provide the proper vision correction. The axis represents the amount of rotation of the cylindrical lens in degrees ranging from 1 to 180.

Base-The base is to the prism what the axis is to the cylinder. As you know, a prism is shaped like a triangle. The thicker the triangle at its base, the stronger the power. Much like the axis specifies the direction of rotation of the cylindrical lens for astigmatism, the prism must also be rotated into a specific position. But the rotation of the prism is simply specified as ‘base in’ or ‘base out’ (where ‘in’ means towards the nose) or ‘base up’ or ‘base down’. Only these four positions exist as opposed to the 180 positions that can be specified for a cylindrical lens. However, orientations between these four positions can be specified by using combinations of horizontal and vertical prisms in the same lens.

Base Curve: The base curve is the front curve of any lens. The higher the base curve (8-12 base) the more curved the lenses is thus making the frame more curved.

Base-Down (BD) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned down. Prisms bend light (opposite direction from its thicker end) so the base-down prism turns the light upward thus causing the eye to also move up. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-In (BI) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned inward, closest to the nose. Prisms bend light (opposite direction from its thicker end) so the base-in prism turns the light outward (toward the ear) thus causing the eye to also move outward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Out (BO) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned outward, closest to the ear. Prisms bend light (opposite direction from its thicker end) so the base-out prism turns the light inward (toward the nose) thus causing the eye to also move inward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Up (BU) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned up. Prisms bend light (opposite direction from its thicker end (base)) so the base-up prism turns the light downward thus causing the eye to also move down. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Bridge Size: Is the distance between the lenses on any frame. Smaller bridge fits smaller noses.

Bifocal Glasses- used to correct vision at two distances, composed of two ophthalmic lenses such as a plus lens for near vision and a minus lens for distance vision.

Bi-lateral Integration/ Gross Motor Coordination- visual guidance of body movements and the coordination between both sides of the body.

Binocular Fusion Dysfunction- a clinical condition in which the eyes are not working as a team. Vision therapy is an effective treatment option. (See “General Binocular Vision Disorder”)

Binocular Vision- the simultaneous use of the two eyes.

Binocularity- the ability to use both eyes as a team and to be able to fuse (unite) two visual images into one, three-dimensional image (See “Convergence” and “Divergence”).

Bi –Ocularity– using both eyes, but not together as a team.

Blue Blur Is the condition of unclear vision due to the blue light waves being short and scattering easily in the visible light spectrum. A blue blocker lens is recommended to remedy this aversion in visual acuity.

Blurred Vision– lack of visual clarity or acuity.

Botulinum Toxin Type A (Oculinum, Botox®)- an injection of this poison has been used as an alternative to conventional surgery in selected strabismic patients. It causes a temporary paralysis of an extraocular muscle that leads to a change in eye position. This change has been reported to result in long-lasting and permanent alteration in eye alignment. Although one injection is often sufficient to produce positive results, one-third to one-half of patients may require additional injections. This technique has been most successful when used in adults with small-angle misalignments. It is not commonly used in children. This treatment is also used in patients who have blepharospam (an uncontrollable eye lid spasm).

Break Point– the point at which a person can no longer fuse (unite) two images into one. A blur point will occur before the this point.

Cataract: are the leading cause of blindness worldwide Approximately 20.5 million Americans age 40 and older have cataracts. Cataracts are the clouding of the eye’s clear lens-similar to a window that is “fogged” with steam. When the lens becomes cloudy, light rays cannot pass through it easily and vision becomes blurry. Cataracts are not a growth or a film over the eye. Cataracts start out small (mild) and have little effect on vision at first. But as the cataract grows (becomes denser), so does the impact on vision. How quickly the cataract develops varies among individuals, and may even be different between the two eyes. Most age-related cataracts progress gradually over a period of years. Other cataracts, especially in younger people and people with diabetes, may progress rapidly over a short time. It is not possible to predict exactly how fast cataracts will develop in any given person.

Cheiroscope– an instrument used in orthoptics/vision therapy to train binocular skills and accommodation skills. The Keystone Correct-Eye Scope is an example of a Cheiroscope.

Ciliary Body – a structure directly behind the iris of the eye and contains the ciliary muscle.

Ciliary Muscle– a band of muscle and fibers that are attached to the lens that controls the shape of the lens and allows the lens to accommodate (change focus).

Clip-On: Is a small device that holds colored lenses in front our your regular prescription eyewear. The normally come in grey amber and copper.

Comitant Strabismus– a condition in which the magnitude of deviation remains essentially the same in all positions of gaze and with either eye fixating.

Computer Vision Syndrome (CVS)– the complex of eye and vision problems related to near work that are experienced during or related to computer use. Its symptoms include eyestrain, dry or burning eyes, blurred vision, headaches, double vision, distorted color vision, and neck and backaches. The condition is caused by various internal and external factors. Treatment options may include prescription glasses and/or vision therapy.

Cone – a receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for color vision.

Conjunctivitis– an inflammation of the conjunctiva, the transparent layer covering the inner eyelid and the white portion (sclera) of the eyeball. Conjunctivitis can be caused by a virus, bacteria, or fungus (infectious conjunctivitis, or “pink eye”, may be contagious); by allergies to pollen, fabrics, animals, or cosmetics (allergic conjunctivitis); or by air pollution or noxious fumes such as swimming pool chorine (chemical conjunctivitis). Symptoms include red or watery eyes, blurred vision, inflamed inner eyelids, scratchiness in the eyes, or (with infectious conjunctivitis) a puss like or watery discharge and matted eyelids. Conjunctivitis is usually treated with antibiotic eye drops and/or ointment.

Convergence- the ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.

Convergence Excess (CE)– a clinical condition in which the eyes have a tendency to turn excessively inward when viewing an object at a near distance. Symptoms may include visual fatigue while reading or using a computer, occasional blurred or double vision, and inability to comprehend or concentrate while reading. Clinical signs include: greater esophoria at near than distance, high AC/A ratio, and a high lag of accommodation. Can be improved with vision therapy and/or glasses. (See “Esophoria”)

Convergence Insufficiency (CI)– (clinical condition) the inability of the eyes to turn inward and/or sustain an inward turn. Symptoms include eye strain with reading and using a computer, headaches, loss of comprehension, difficulty concentrating, blurred or double vision, and eye fatigue. Clinical signs include: near point of convergence of greater than 4 inches (10 cm), greater exophoria at near than at distance, and low AC/A ratio. Vision therapy is an effective treatment option.

Cornea– the transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye. Contact lenses are fitted over the cornea.

COVTT– Certified Optometric Vision Therapy Technician. To be certified an individual must be employed by a Fellow (FCOVD), provide documentation of 2000 hours or 2 years of direct clinical experience in vision therapy; or 1000 hours of clinical experience if the individual holds an AA degree or higher with emphasis in the behavioral sciences. Submit written answers to a series of Open Book Questions dealing with various aspects of vision function, testing and therapy. Pass an extensive written and oral examination evaluating the candidate’s knowledge and clinical abilities in behavioral vision, vision development and vision therapy. COVTTs must obtain at least 6 hours of continuing education annually in functional / developmental / behavioral vision care.

Contrast: Contrast is the difference in brightness between the light and dark areas of a picture, such as a photograph or video image. A high contrast lens gives you great visual acuity between light and dark areas in your view.

CR-39 (plastic) The advantages of plastic is that it is lightweight and easily tinted to just about any color. One of the disadvantages is that those lenses scratch easily.

Chromatic Aberration: is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). Longitudinal and lateral chromatic aberration of a lens is seen as “fringes” of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis. Since the focal lengthof a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength).

Crystalline Lens– transparent disc located behind the iris which changes shape to focus on objects at different distances from the eye.

Cycloplegic Refraction– one method available to eye doctors to determine the eye’s refractive error and the best corrective lenses to be prescribed if needed. The eye is dilated with the muscles of accommodation (eye focusing muscles) being temporarily paralyzed with specialized eye drops or spray (Atropine, Homatropine, Cyclogyl, or Mydriacyl). This is a good method for non-responsive or non-communicative patients such as young children. The technique of retinoscopy is used with this method. (See “Retinoscopy”)

Cylinder Lens– an ophthalmic lens that has at least one non-spherical surface. Used to correct astigmatism. The values are typically from -0.75 to -1.25. The cylinder measurement is given with a “-” sign. (Please note that the sign for myopia (nearsightedness) is also “-“.)

Cylinder: If there is no value under the cylinder heading, then you have a very simple prescription. If there is a value under this heading, then you have astigmatism. The majority of optometrists write the cylinder value with a minus sign in front while the majority of ophthalmologists (physicians who specialize in the eye) write the cylinder value with a plus sign in front. Regardless of which way this is written, your glasses will be made the exact same way — these are just two different ways to write the same spectacle prescription. Like sphere power, the cylinder power is also measured in diopters.

Day and Night (Photochromatic Lenses): New Photochromatic lenses are so smart they go from clear indoors to as dark as sunglasses outdoors. Their advanced technology adjusts to changing light, so you see clearly and more comfortably in virtually any light condition.Indoors, Photochromatic lenses are as clear as your regular eyeglasses. Outdoors, they quickly darken, getting as dark as sunglasses if necessary. They work so well, in a recent clinical study, four out of five eyeglass wearers preferred Photochromatic to their regular lenses, and photochromatic provide 100% UV protection.

Depth Perception- the ability to judge relative distances of objects.

Depth Perception Test- a test to measure the ability of the vision system to discern the relative distances of various objects. (Also called a “Stereopsis Test”)

Developmental Disorder- when a delay in an individual’s normal development has occurred.

Developmental Vision Analysis– more comprehensive than a routine eye exam, examination will evaluate all of the patient’s visual abilities such as visual acuity, eye focusing skills, eye teaming skills, eye tracking skills, visual motor skills, and visual perceptual skills.

Diopter: A unit of measurement of the optical power of lenses. For example; if a person has a prescription of -4.00, that person has 4 diopters of power in their lens. Diopters generally are measured in quarter steps like -4.00, -4.25, -4.50. Some doctors refine their measurements down to twelfth steps like -4.00, -4.12, -4.62. Diopters are also used to measure prism.

Diplopia– a single object is perceived as two rather than one; double vision.

Direct Occlusion– covering the non-amblyopic eye. (See “Inverse Occlusion” and “Occlusion”)

Directionality/Laterality– directionality relates to the awareness of the relationship of one object in space to another / laterality relates to the internal awareness of the two sides of the body. Directionality/Laterality can also be called spatial relations.

Directionality/Laterality Disorder– a condition in which an individual has poor development of left/right awareness. Symptoms of this disorder include confusion of right and left direction and letters and/or numbers reversals. Vision therapy is a helpful treatment option.

Distance Acuity- the eye’s ability to distinguish an object’s shape and details at a far distance such as 20 feet (6 meters).

Divergence– the ability to use both eyes as a team and be able to turn the eyes out toward a far object.

Divergence Excess (DE)– (clinical condition) the eye’s tendency to drift out relative to the direction of a distant object being viewed.Symptoms include: double vision at distance, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs include: exophoria greater at distance than near, high AC/A ratio, and reduced positive fusional vergence at distance. Can be improved with vision therapy.

Divergence Insufficiency (DI)– (clinical condition) the eye’s tendency to turn more inward than necessary when viewing a distant object. Symptoms include: double vision, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs: esophoria greater at distance than near, low AC/A ratio, and reduced negative fusional vergence at distance. Treated with corrective lenses and vision therapy.

Dominant Eye- the eye that “leads” it partner during eye movements. Humans also have dominant hand, foot, eye, and side of the brain (not necessarily all on the same side).

Duction Test– a test of the eye’s ability to turn inward or outward while maintaining single, binocular vision with the gradual introduction of progressively stronger base-in or base-out prisms.

Dysphoneidesia– inability to “sound out” words and poor sight recognition of words. Dysphoneidesia is a subtype of dyslexia. Its characteristics are a combination of the other two forms of dyslexia: Dysphonesia and Dyseidetic.

Dysphonesia– inability to “sound out” words. Dysphonesia is a subtype of dyslexia. Children with this form of dyslexia have difficulty sequentially analyzing and remembering what and where the sounds are in words. The resulting phonemic processing problems make it difficult to sound out new words, learn phonics, and make them dependent on their sight vocabulary. When they come to an unknown word they will often substitute a word using context clues. For example, “pony” for “horse”, even though the substituted word doesn’t look or sound anything like the original word. When spelling unknown words it is often difficult  to determine what the original word is. For example, they may write “fmlue” for “familiar” or “lap” for “lamp”. They cannot learn phonics because they cannot process where the sounds are. Their short term sequential auditory memory can be poor and result in repeating “8167” as “8671”, or remember to go to their room but forgetting to get the item requested.

poor sight recognition of words. Dyseidetic is a subtype of dyslexia. Children with this form of dyslexia have trouble analyzing and remembering written symbols. They continue to confuse the orientation. For example, they will write numbers and letters backwards long after other children have mastered these skills. They often confuse letter sequences in reading, and in spelling often get all the letters but in the wrong sequence (spelling “dose” for “does”,  “on ” for “no”, etc.). Their visual memory for words is poor, and after learning a new word they may fail to recognize that same new word later in the sentence. They have trouble learning to read and spell phonetically irregular words. For example, they may read ” laugh” as “log” and spell it as “laff”, both of which are phonetically consistent. Their spelling will have many mistakes, but will be phonetically consistent and one can usually tell what the word was they were trying to spell. When they are attempting to read an unknown word they will usually attempt to sound it out and do so very slowly.

– a specific language-based disorder. The individual has difficulty with letter or word recognition, spelling, reading, writing, and sometimes naming pictures of objects. Dyslexia varies in degree from mild to very sever. It is caused by an inability of the brain’s language centers to decode print or phonetically make the connection between the word’s written symbols and their appropriate sounds. Dyslexia is not caused by a vision disorder. Children often are of normal or above normal intelligence. Dyslexia cannot be cured and will never be outgrown. Appropriate teaching methods can be taught to help those with dyslexia overcome their weakness. The Dyslexia Determination test which is used by many optometrists who specialize in vision related vision problems investigates if the patient has one of the three forms of dyslexia: Dyseideticpoor sight recognition of words, Dysphonesia- inability to “sound out” words, and Dysphoneidesia – a combination of characteristics from both types. Vision therapy is NOT considered a direct treatment for dyslexia.

Eccentric Fixation– the deviating eye does not use the central foveal (center of the retina that produces the sharpest eyesight) area for fixation. Commonly, individuals with amblyopia and some individuals with strabismus will have this visual adaptation. In esotropia, the eccentrically located retinal point used for fixation is usually in the nasal retina. In exotropia, the eccentrically located retinal point used for fixation is usually in the temporal retina. Vision therapy is a treatment option for those with amblyopia and/or strabismus. It is not a treatment option for an individual with a fovea that has been destroyed.

Emmetropia– normal vision, no correction needed.

Esophoria (Eso)– (clinical condition) a tendency of the eyes to want to turn more inward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic esophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic esophoria include: AC/A ratio is normal, equal esophoria at distance and near, and normal near point of convergence. Sometimes esophoria is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts can correct the problem alone. However, sometimes vision therapy is needed to to help re-train the eyes to function more appropriately. (See “Convergence Excess” and “Divergence Insufficiency”)

Esotropia (ET)– (clinical condition) a condition in which an eye is turned either constantly or intermittently inward toward the nose. Esotropia is a type of strabismus. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox®) injections. In some cases, esotropia is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts alone may allow the eyes to straighten. Vision therapy is most appropriate when there are small degrees of misalignment. Surgery, to re-position or shorten the eye muscles, may be required for high degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results.

Exophoria (Exo)– (clinical condition) a tendency of the eyes to want to turn more outward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic exophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic exophoria include: normal AC/A ratio, equal exophoria at distance and near, and decreased near point of convergence. Vision therapy is an effective treatment option. (See “Convergence Insufficiency” and “Divergence Excess”)

Exotropia (XT)– (clinical condition) a condition in which an eye is either constantly or intermittently turned outward toward the ear. Exotropia is a type of strabismus. It may also be called divergent strabismus, wandering eye, or wall eye(s). It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox®) injections. Vision therapy is most appropriate when there are small degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results.

Extraocular Muscles– the muscles attached to the outside of the eyeball which control eye movement. Each eye has six muscles (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, and inferior rectus) that are coordinated by the brain.

Eye Hand Coordination– the ability of our eyes to guide our hands, also called visual motor integration.

Eye Size: Is the horizontal measurement of the lens on any frame. Larger eye size fits bigger heads.

Eye Trac– (equipment) an electronic testing and recording system of eye movements as in reading.

Eye Tracking– the ability of the eyes to smoothly and effortlessly follow a moving target.

Facility of Accommodation- a measure of the ease and speed of the eye(s) to change focus.

Figure-Ground– the ability to recognize distinct shapes from their background, such as objects in a picture, or letters on a chalkboard.

Fine Motor Skills– the ability to coordinate hand and finger movements.

Fixation– the ability to direct and maintain steady visual attention on a target. Fixations are a form of pursuits.

Fixation Disparity (FD)– over-convergence or under-convergence, or vertical misalignment of the eyes under binocular (both eyes) viewing conditions small enough in magnitude so that fusion is present.

Flat Light: When light is “flat,” the slope looks like a white, empty canvas, and it’s impossible to read the snow surface clearly. Ruts, bumps, ice, even rocks and thin patches disappear. And if you’re out in an open expanse, it’s hard to determine the pitch of the slope as your depth perception shrinks to nil.

Floaters– also known as spots, are usually clouded or semi-opaque specks or particles within the eye that are seen in the field of vision. The eyes are filled with fluid which maintains the shape of the eye, supplies it with nutrition and aids in the focusing of light. Often, particles of protein or other natural materials are left floating or suspended in this fluid when the eye is formed before birth. If the particles are large or close together, they cast shadows which make them visible. This is particularly true when nearsightedness occurs or becomes more severe. In most cases this is normal but floaters can also be caused by certain injuries, eye disease or deterioration of eye fluid or its surrounding structures.

Form Constancy– the ability to recognize two objects that have the same shape but different size or position. This ability is needed to tell the difference between “b” and “d”, “p” and “q”, “m” and “w”.

Frame Measurements: What do the numbers located on the bridge and temples of the frames mean?


The numbers on the frame reflect the SIZE MEASUREMENTS in millimeters (mm).

1st Number (ex.54) = the width of the lenses
2nd Number (ex.38) = the distance between the Top of the lenses to the bottom.
3rd Number (ex.59) = the diagonal distance of the lens
4th Number (ex.18) = the distance of the bridge between the lenses
5th Number (ex.140) = the length of the temple arm including the portion going behind the ear

Fusional Vergence- a convergence response which serves to maintain (fusion) the union of images from each eye into a single image. The eyes will turn with a slow smooth tonic movement or a fast jumping movement called phasic.

Fusional Vergence Dysfunction– see “General Binocular Vision Disorder”.

Fusion– the union of images from each eye into a single image.  There are three degrees of fusion. 1st degree fusion is the superimposition of two dissimilar targets. 2nd degree fusion is flat fusion with a two-dimensional target. 3rd degree fusion is depth perception (stereopsis) with a three-dimensional target.

Fusion Test– determines the eyes ability to unite the images from each eye into a single image.

General Binocular Vision Disorder– inability to efficiently utilize and/or sustain binocular vision. Symptoms include eyestrain, headaches, decreased comprehension, inability to concentrate while reading, excessive tearing, and blurred vision. A patient will have difficulty with both base-in and base-out prisms. Vision therapy is an effective treatment option.

Glare: Reflected glare by light reflected off smooth, shiny surfaces blocking vision. The number one cause of automotive accidents is glare. A polarized lense is your best defense against blinding glare.

Grilamid Nylon Frames: No other material has the flexibility, remarkable toughness, and exceptional resistance to sun lotions. These elements allow our design team to combine the frame material’s light weight, impact resistance, flexibility in variable temperature and overall comfort to create models that are durable and surpass all expectations.

Heterophoria– tendency of the eyes to deviate from their normal position for visual alignment. This condition may be observed when one eye is covered.

Heterotropia- the eyes are abnormally turned.

High index 1.56: Thinner and lighter than regular plastic good for higher prescriptions. Can easily tinted into sunglasses.

High index 1.60: Thinner and lighter than 1.56 high index. These lenses already come with a scratch coat. Great choice for higher prescriptions.

High index 1.67: Thinner and lighter than 1.60. These lenses already come with scratch coating. Also a great choice for high prescriptions.

HTS Computerized Binocular Home Vision Therapy System – a computer program which is prescribed by an eye doctor. The computer program is for improving eye tracking, eye teaming, and/or eye focusing.

Hyperopia: Also known as farsightedness, is usually inherited. Children are often hyperopic which may lessen as an adult. Hyperopia is a refractive error, which results from a disorder rather than from disease. A refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image.

Hyperphoria– a condition in which one eye has a tendency to point higher than the other eye, causing eyestrain. Sometimes improved by prisms in glasses.

Hypertropia- strabismus,one eye turned in anupward direction.

Hypophoria– a condition in which one eye has a tendency to point lower than the other eye. This condition may be observed when one eye is covered.

Hypotropia– strabismus, one eye turned in a downward direction.

Hysterical Amblyopia- a non specific visual loss with an unknown cause. Upon examination the doctor is unable to find corroborating objective evidence of this abnormality. The most common symptom is an isolated visual acuity impairment, followed by combined visual acuity impairment and visual field constriction, and whereas an isolated visual field constriction occurred most infrequently. Thisvision loss may be due to anxiety or emotional repression. (See “Streff Syndrome”)

Ill-Sustained Accommodation– this clinical condition is also called Accommodative Fatigue. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain). Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses(glasses or contacts) and vision therapy are effective in treating this condition.

Impact Resistant: Resistant to shattering or splintering. impact resistant plastic is made so that it will not break into small pieces. Polycarbonate is impact resistant making it a very safe lens to wear.

Incomitant Strabismus – a condition also known as Noncomitant Strabismus. It occurs when the magnitude of deviation is not the same in the different positions of gaze or with either eye fixating.  There is an abnormal restriction to movement or an over-action of one or more of the extraocular muscles. Generally, the magnitude must change by at least 5 PD to be incomitant (nonconcomitant).

Inverse Occlusion- covering the amblyopic eye. (See “Direct Occlusion” and “Occlusion”)

Iris– the colored part of the eye located between the lens and cornea; it regulates the entrance of light. (See diagram of the eye)

Infrared (IR) Radiation (760 – 3000nm) Infrared Rays are radiant energy, or heat waves, not considered harmful under normal conditions. These heat rays cannot be seen but can be felt. If you are exposed to intense sunlight for a lengthy period of time (a day at the beach, for example) without infrared protection, you may experience a burning or stinging sensation in your eyes and a sense of fatigue. Infrared rays can be especially discomforting if you wear contact lenses. If your sunglasses fail to stop infrared light, it can be absorbed by your contacts, causing them to “warm up”.

Kinesthesia- the sensation of bodily position, presence, or movement resulting chiefly from stimulation of sensory nerve ending in muscles, tendons, and joints.

Lag of Accommodation- a measure of the eye’s ability to focus accurately on a given target. The dioptric difference between the eye’s focusing response and the stimulus to focus.

Latent Hyperopia– hyperopia (farsightedness) is compensated by accommodation and the tonicity (tension) of the ciliary muscle; identified by cycloplegic refraction. In mild cases of hyperopia (farsightedness), the eyes are able to compensate without corrective lenses; otherwise a plus lens (glasses or contacts) is prescribed. Vision therapy is not prescribed. (See “Hyperopia”)

Learning Disability (LD)- a disorder that affects people’s ability to either interpret what they see and hear or to link information from different parts of the brain. Learning disabilities can be divided into five broad categories: speech and language disorders, reading disorder, arithmetic disorder, writing disorder, and attention disorders. The term learning disability does not include children who have learning problems that are primarily the result of visual, hearing, or motor disorders.

Lensometer- also called Verometer, is a device used to measure the refractive power of eyeglasses and contact lenses.

Macula– the most sensitive part of the retina that is about the size of a pinhead and is where our most detailed vision occurs.

Macular Degeneration: is a degenerative disease that robs young and old of central vision.  As a result, children face a lifetime of uncertainty and elders risk the early loss of an independent life.  In addition to the support of long-term efforts in the fields of Stem Cell and Genetic research for an ultimate cure, we are particularly interested in promising near-term, scientific studies designed to inhibit the progression of macular degeneration and restore a measurable amount of vision to all.

Malingering– a voluntary or intentional reduction in visual acuity or other examination data.

Microstrabimus – also called microtropiamonofixation syndrome, and small angle strabismus. A small angle deviation (inward or outward, commonly inward) that is less than five degrees with some amount of stereopsis (depth perception) and anomalous retinal correspondence (ARC). Possible mild amblyopia, eccentric fixation, and/or anisometropia may also be present. It frequently results from the treatment of a larger-angle deviation (esotropia or exotropia) by optical correction, vision therapy, pharmacological agents, and/or extraocular muscle surgery. Treatment for microstrabimus consists mostly of correcting significant refractive errors and any coexisting amblyopia. The use of vision therapy and prisms to establish bifoveal fusion has been successful in selected cases.

Minus (-) Lens- concave lens, stimulates focusing and diverges light. The lens is thinner in the center than the edges. It is used in glasses or contact lenses for people who are nearsighted (myopia).

Monocular Vision- only one eye having useful vision.

Myopia– nearsightedness, an individual will have difficulty seeing clearly at distance. Light entering the eye focuses in front of the retina when the eye is at rest and is corrected with a minus lens. A condition known as high myopia occurs when myopia is greater than 6 diopters. Typically, vision therapy is not prescribed for myopia. For more information and a diagram, please

Nanometers: The measure of the length of a wave of light. One billionth (10-9) of a meter.

Nose pad: One of a pair of pads, usually clear, that rest on either side of your nose and help to support your glasses.

Near Point of Convergence (NPC)- the closest point at which the two eyes can maintain a single united image.

Near Point of Convergence Test– measures the patient’s ability to point the eyes at an approaching object and to keep them fixed on the object as it reaches the patient’s nose.  Normal range is 0 to 4 inches away from the nose.

Negative Relative Accommodation (NRA)- a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision.

Normal Retinal Correspondence (NRC)- the foveas of the two eyes are corresponding neural points in the visual cortex and binocular vision can occur.

Nystagmus– rhythmic oscillations or tremors of the eyes which occur independent of the normal eye movements. Generally nystagmus is not curable, but it is manageable. Treatments include prescription glasses or contact lenses, prisms, and vision therapy.

Occlusion– to block out light. An eye can be completely or partially blocked. This procedure is used to promote the use of one eye or both eyes. This therapy procedure may be used for people with amblyopia, strabismus, or closed head trauma. It may also be used in a vision therapy program for someone with amblyopia, eye focusing (accommodation) disorder, or poor eye tracking (oculomotor) skill.  An eye patch, black contact, or another device may be used to block out light from an eye. (See “Direct Occlusion” and “Inverse Occlusion”)


Ocular: Ocular is anything of or relating to the eye.

Ocular Motility- pertaining to binocular alignment and eye muscle movement. (See “Binocularity”, “Strabismus”)

Ocular Motor (OM)- general eye movement ability, which include pursuits (to visually track and/or follow moving objects) and saccades (to direct and coordinate eye movement as the eye quickly and voluntarily shift from one target to another).

Ocular Motor Dysfunction– poor eye movement skills. Vision therapy is an effective treatment option.

Oculomotor Skills- the ability to quickly and accurately move our eyes. These are sensory motor skills that allow us to move our eyes so we can fixate on objects (fixation), move our eyes smoothly from point to point as in reading (saccades), and to track a moving object (pursuits). (See “Fixation”, “Pursuits” and “Saccades”)

Oculus Dexter (OD)- right eye.O. D. O. D. simply means ‘right eye’. It is the short form of the latin term ‘oculus dextrous’.

Oculus Sinister (OS)O. S.: O. S. simply means — you guessed it — ‘left eye’. It is the short form of the latin term ‘oculus sinister’.

Oculus Uterque (OU)- both eyes.

O-Seg Bifocal: This lens is similar to the Round Seg, but uses new manufacturing techniques to allow us to put the reading area on the back of the lens, instead of the front, as was done before. This makes the lens more cosmetically appealing, and effectively increases the reading area to the wearer, as it’s closer to the eye.

Ophthalmologist – a physician (doctor of medicine (M.D.) or doctor of osteopathy (D.O.) who specializes in the comprehensive care of the eyes and visual system in the prevention of eye disease and injury. The ophthalmologist has completed four or more years of college premedical education, four or more years of medical school, one year of internship, and three or more years of specialized medical and surgical training and experience in eye care. The ophthalmologist is a physician who is qualified by lengthy medical education, training and experience to diagnose, treat and manage all eye and visual system problems, and is licensed by a state regulatory board to practice medicine and surgery. The ophthalmologist is the medically trained specialist who can deliver total eye care: primary, secondary and tertiary care services (i.e., vision services, contact lenses, eye examinations, medical eye care and surgical eye care), and diagnose general diseases of the body. An ophthalmologist is not trained to provide vision therapy.

Ophthalmoscope– a device used to illuminate the inside of the eye and enlarge the image for examining the retina, optic nerve entrance, arteries, and veins.

Optic Nerve– is a bundle of nerve fiber that connects each eye to the brain and transmits images from the retina to the brain. (Seediagram of the eye)

Optician– is a professional in the field of designing, finishing, fitting and dispensing of eyeglasses and contact lenses, based on an eye doctor’s prescription. The optician may also dispense colored and specialty lenses for particular needs as well as low-vision aids and artificial eyes.

Optometric Vision Therapy (VT)– as defined by the American Optometric Association: Optometric vision therapy is a treatment plan used to correct or improve specific dysfunctions of the vision system. It includes, but is not limited to, the treatment of strabismus (turned eye), other dysfunctions of binocularity (eye teaming), amblyopia (lazy eye), accommodation (eye focusing), ocular motor function (general eye movement ability), and visual-perception-motor abilities.

Optometric vision therapy is based upon a medically necessary plan of treatment which is designed to improve specific vision dysfunctions determined by standardized diagnostic criteria. Treatment plans encompass lenses, prisms, occlusion (eye patching), and other appropriate materials, modalities, and equipment. (Vision therapy can also be called visual or vision training, orthoptics, eye training, or eye exercises.)

Please note that the definition above describes Optometric Vision Therapy, which has been clinically shown to improve certain eye disorders, which are described above. However Optometric Vision Therapy is NOT the same as the Bates Method, vision therapy using Bates, integrated vision therapy, or natural eye exercises. These holistic programs use some form of eye exercises associated with relaxation techniques, which claim to improve nearsightedness, farsightedness, astigmatism, decreasing vision with age, and other disorders. There is virtually no statistical studies/results indicating the success of these methods.

Optometrist – a health care professional who is state licensed to provide primary eye care service.  These services include comprehensive eye health and vision examinations; diagnosis and treatment of eye disease and vision disorders; the detection of general health problems; the prescribing of glasses, contact lenses, low vision rehabilitation, vision therapy, and medications; the performing of certain surgical procedures; and the counseling of patients regarding their surgical alternatives and vision needs as related to their occupations, avocations and lifestyle. The optometrist has completed pre-professional undergraduate education in a college or university and four years of professional education at a college of optometry, leading to the doctor of optometry (O.D.) degree. Some optometrists complete a residency.

Organic Amblyopia– gradual or sudden loss of central vision (partial loss) affecting visual acuity with no treatment options.

Types of organic (irreversible) amblyopia:

  • nutritional amblyopia- vision loss caused by low levels of vitamin B12 due to poor nutrition and poor absorption associated with drinking alcohol.
  • tobacco-alcohol amblyopia- clinical evidence exists that a nutritional deficiency is the underlying cause of this vision loss; however, many still believe that the toxic effects of alcohol and/or tobacco are contributing factors.
  • toxic amblyopia- caused by exposure to toxins such as ethambutol, methyl alcohol (moonshine), ethylene glycol (antifreeze), cyanide, lead, and carbon monoxide.

Orthophoria (ortho)– the absence of either esophoria or exophoria. The eyes do not have a tendency to want to turn more inward than necessary or want to turn more outward than necessary when pointed on an object.

Orthoptics– the science of correcting defects in binocular vision. The technique of eye exercises to correct strabismus (esotropia or exotropia), convergence insufficiency (exophoria), or convergence excess (esophoria), amblyopia, and ocular motility disorders. Orthoptics was pioneered by French ophthalmologist Javal in the mid to late 1800’s. Today ophthalmologists use specialty-trained healthcare professionals called orthoptists to evaluate patients and treat them with orthoptics. In America, the non-surgical technique of orthoptics is less commonly used by ophthalmologists compared to other countries. Orthoptics is a limited form of optometric vision therapy.

Paresis– a paralysis that when occurring in ocular muscles causes double vision when looking in some directions.

Penalization- to prevent sight out of the good eye and force the weaker, amblyopic eye, to function. A filter, eye patch, or eye drops such as atropin or miotics are used on the good eye.

Perceptual Skills– includes the identification, discrimination, spatial awareness, and visual-sensory integration. These are visual cognitive skills used to processes visual information to the brain to be organized and interpreted. (See “Visual Perceptual Disorder”)

Photochromic: Able to change lens color or darkness/density depending upon the degree of exposure to light.

PTS Computerized Perceptual Home Vision Therapy System– a home-based computerized perceptual therapy program, which was designed to enhance visual information processing. The therapy procedures address simultaneous processing, sequential processing and/or speed of information processing. This computer program contains 6 activities that are specifically for the following problems: a weakness with visual information processing skills such as figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization skills, slow speed of information processing, and acquired brain injury with perceptual-cognitive deficits. This program is available only from a licensed eye care practitioner.

Perimetry– the measurement of a visual field function (the total area that can be seen while looking straight ahead) using targets of different sizes and brightness (light levels). The visual field is measured in degrees. In a normal eye the peripheral field of vision is about 180 degrees. An instrument called a perimeter is used for mapping all areas of a person’s eyesight, including peripheral (side) vision. Visual field testing can help detect certain patterns of visual loss, indicating specific types of eye diseases or vision conditions. It is the single best test for diagnosing glaucoma.

Peripheral Vision- the ability to see or be aware of what is surrounding us, our side vision. (See “Visual Field”.)

Phasic– fast, jump movement. (See “Fusional Vergence”)

Photophobia– unusual sensitivity to light.

Physiological Diplopia- a normal diplopia (double vision) that occurs when an individual is not pointing his/her eyes on a certain object.

Plano Lens- a lens that has no prescription. No variance between the curvature of the front and back lens surfaces. It is a flat lens.

Pleoptics– a method of eye exercises created to stimulate and train an amblyopic eye. The goal is to have eyesight which is produced by the fovea. (See “Eccentric Fixation”)

Plus (+) Lens- convex lens (thicker in the middle) relaxes focusing and converges light. It is typically used in glasses or contact lenses for people who are farsighted (hyperopic). Although it may also be prescribed for other visual conditions as well.

Polarized Lenses: Light waves traveling freely can vibrate in any direction. When light strikes a horizontal reflecting surface such as water, sand or pavement, it vibrates horizontally creating glare. To the unprotected eye, glare can decrease depth perception., reduce visual acuity and create eye fatigue. These lenses are great by the water or for driving. A good example of how these lenses work. While driving in your car you often see the glare of the dashboard or if you have a white piece of paper on the dashboard for example you see its reflection off the windshield. Having a polarized lens virtually eliminates this reflection and all of this type of glare (horizontal glare). These lenses are laminated and have a polarizing film in side the lens they are available in gray or brown. They are also available in plastic, polycarbonate or glass.

Positive Relative Accommodation (PRA)- a measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision.

Polycarbonate- Polycarbonate lenses or “Poly” are “impact resistant” thinner and lighter in weight than traditional plastic eyeglass lenses, they also offer ultraviolet (UV) protection and scratch resistance. In addition, they are very impact resistant. This extra toughness makes them the lenses of choice for children’s glasses, sports eyewear and safety glasses.

Presbyopia: Inability of the eye lens to focus incoming light, resulting in blurred vision at a reading distance and eyestrain. Most people develop presbyopia in their 40s.

Prism This is a box on the prescription form that is rarely filled in. Occasionally, when the two eyes are not properly aligned and looking directly at the same thing, prism can be ground into the lenses in order to re-align them. This can occur with strabismus (i.e. – an eye turn) or in situations where the eyes are properly aligned but are under a tremendous amount of strain in order to keep them aligned. The value under the ‘prism’ heading denotes the strength of the prism.

Prismatic Effect By Lens– when light goes through a wedge shaped lens which is called a prism, it bends. Light is also bent when it does not go through the center of a lens. This is an undesirable effect that can occur in glasses. It commonly occurs when the pupillary distance (PD) is not measured or made correctly.

Progressive lenses: (also, progressive addition lenses or PALs) Multifocal lenses whose corrective powers change progressively throughout the lens. A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work. Each area is blended invisibly into the next, without the lines that traditional bifocals or trifocals have.

Pseudomyopia– the condition Accommodative Excess/Spasm causes an individual to experience blurry distance vision after prolonged near work such as reading or using a computer. The individual may appear to be nearsighted (myopia). Treatment options may include prescription lenses and/or vision therapy.

Ptosis– droopy upper eyelid, causing the eye to remain partially closed.

Pupil– the opening at the center of the iris of the eye. It contracts (dilates) in the dark and when the eye is focused on a distant object.

Pupillary Distance (PD): The distance between the center of your pupils is known as the Pupillary distance, this is measured . Before your prescription lenses are cut into the shape of your frame, the Pupillary distance needs to be measured. This measurement is then used so that the optical center of each lens can be lined up with your pupils in order to give optimal visual clarity. In general, PD measurements fall in the range of 48mm to 73mm. The most common measurements are between 58mm and 68mm.



Pupillary Reflex the automatic contraction or enlargement of the pupil when confronted with the presence or absence of light, accommodation, or emotional change.

Pupillometer a device used to measure the distance between the pupils of the eyes, in millimeters, which is a necessary measurement for proper lens prescription. It also measures the diameter of the pupil.

Pursuit Dysfunction a condition in which the individual’s ability to follow a moving target is inadequate. Vision therapy is an effective treatment option. T

Pursuit Testmeasures the eyes ability to follow a moving target.

Pursuits– the eye’s ability to smoothly follow a moving target.

Reading requires the use of good visual skills, which are distance and near acuity, accommodation skills, binocularity skills (convergence), oculomotor skills (saccadic), peripheral vision, figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.

Refraction Test– determines the eye’s refractive error and the best corrective lenses to be prescribed. There are several methods of performing refraction: Retinoscopy, Automated Refractor, and Subjective Refraction.

Refractive Error– defects in vision caused by the eye’s inability to bend, or refract light and focus it clearly on the retina. Astigmatism, hyperopia, and myopia are common conditions of refractive error, also called ametropia.

Refractive Power- a lens’ ability to bend parallel light rays into focus, as measured by power diopters. In general, the greater the curvature of a lens and the greater the difference between center thickness and edge thickness, the higher the index of refraction and the greater its refractive power. Refractive power can also refer the strength of a person’s contact lenses or glasses.

Refractive Media– the parts of the eye that light travels through before being focused on the retina includes the cornea, crystalline lens, aqueous, and vitreous.

Relative Amblyopia functional amblyopia can co-exit with a pathology abnormality. Treatment is possible.

Retina the innermost layer of the eye, a neurological tissue, which receives light rays focused on it by the lens. This tissue contains receptor cells (rods and cones) that send electrical impulses to the brain via the optic nerve when the light rays are present. (Seediagram of the eye)

Retinoscopy this technique determines the eye’s refractive error and the best corrective lenses to be prescribed. An instrument called a retinoscope which consists of a light, lens, mirror, and handle, is used to shine light into a patient’s eye. There are two types of retinoscope: streak and spot retinoscope. When light is shone into patient’s eye, the light is reflected back (“reflex”). If the reflection is in the same direction (“with movement”) of the retinoscope then the refractive error is hyperopia (farsightedness) and a plus lens is prescribed. If the reflection is in the opposite direction (“against movement”) of the retinoscope then the refractive error is myopia (nearsightedness) and a minus lens is prescribed. The strength of the prescription is determined when the pupil is suddenly filled with light (“neutralized”) with the appropriate lens powers (strength).

Rod- a receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for night vision (non-color vision in low level light).

Saccades– the eye’s ability to direct and coordinate movement as it quickly and voluntarily shift from one target to another.

Saccades Dysfunction– a condition in which the individual’s ability to scan along a printed page and move his eyes from point to point is inadequate. Symptoms include frequent loss of place while reading, skip or transpose words, and have difficulty comprehending because of an inaccurate eye movement. Vision therapy is an effective treatment option.

Saccadic Test– measures the eyes ability to move quickly and precisely from point to point.

Sclera– the white protective covering of the eye.

Round Seg Bifocal: Unlike the traditional “flat top” or “D” segment bifocal, this lens uses a round reading area, instead of the traditional lined segment. This allows us to rotate the lens to place the reading area in the proper position, giving us more freedom with regard to frame sizes.

Selective Light Filtration: The act of filtering a certain wave of light. An amber or yellow lens often filters the blue wave of light giving a sharper high contrast view of the world.

Slit Lamp (Biomicroscope) – this instrument can examine ocular tissue from the front of the cornea to the back of the lens. A narrow “slit” beam of very bright light produced by a lamp. This beam is focused on to the eye which is then viewed under magnification with amicroscope. A joystick control is employed to enable instrument to be moved left-right and up-down. A chin rest, head rest and fixation target is also required. Some slit lamps have a tilting mechanism to enable the lamp to be directed from different angles.

Spatial Relation- the ability to judge the relative position of one object to another and the internal awareness of the two sides of the body. These skills allow the individual to develop the concepts of right, left, front, back, up, and down. This ability is needed in reading and math. (See “Directionality/Laterality”)

Sphere: The number under the heading ‘sphere’ is the main part of your prescription. The number itself denotes the strength of the lens as measured in diopters. A diopter is a unit of measurement that is simply the inverse of the focal distance of the lens as measured in meters. For example, if a lens has a strength of 2 diopters, then parallel light rays that pass through this lens will focus together at a distance of 1/2 meter (50 cm) away from the lens. If you are near-sighted (i.e. – you have trouble seeing far away but can see fine up close), then you can make a rough calculation of the strength of your glasses.

Squint– to be unable to direct both eyes simultaneously toward a point. Also known as strabismus (turned eye).

Stereopsis- the ability to perceive a three dimensional depth which requires adequate fusion (union) of the images from each eye.

Stereopsis Test- measures depth perception that is dependent on the accuracy of eye teaming.

Strabismus– (clinical condition) turned eye (s), the eyes are misaligned. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. In strabismus, the eyes send conflicting images to the brain, and the brain cannot combine these images as it would in normal vision. The brain compensates by ignoring one image in favor of the other, causing a loss of depth perception. Strabismus in more common in children, and affects four percent of all children (although it may also appear later in life).

It is characterized by using the following categories:

  • Strabismus in which only one eye deviates.
  • Strabismus in which the deviating eye can change.
  • Strabismus which is not present at all times.
  • Strabismus which is present at all times.
  • Strabismus which occurs at one testing distance but not at another.

Strabismus is also known as squint. It may also be referred to as cross-eyes (convergent- turning inward) or wall eyes (divergent- turning outward). Treatment options may include one or more of the following: optical lenses, bi-focal lenses, prisms, surgery, vision therapy, or Botulinum Toxin Type A (Oculinum, Botox®) injections.

Streff Syndrome– named after the optometrist who originally described it, Dr. John Streff. This functional vision loss is also known as Non-Malingering Syndrome. Signs include reduced visual acuity in both eyes at distance and near. The visual acuity at near is more reduced than the distance acuity. Frequently patients will have reduced stereopsis, large accommodative lag on dynamic retinoscopy, and a reduced visual field (tubular or spiral field). The syndrome is associated with a visual or emotional stress occurring in the child’s life. It is more prominent in girls (ages 7-13) than boys. Treatment includes a low plus lens and/or vision therapy. This condition is sometimes incorrectly diagnosed by doctors as hysterical amblyopia.

Subconjunctival Hemorrhage – a blood spot on the eye. It occurs when a small blood vessel under the conjunctiva (the transparent coating that covers the inner eyelid and the white of the eye) breaks and bleeds. A common condition caused spontaneously from coughing, heavy lifting, or vomiting. In some cases, it may develop following eye surgery or trauma. It tends to be more common among those with diabetes, hypertension, and taking blood thinners (including aspirin). A subconjunctival hemorrhage is essentially harmless. The blood naturally absorbs within one to three weeks and no treatment is required. If a mild irritation is present, artificial tear drops can be used. You can speed up the healing process by applying cool compresses for the first two days and then warm compresses in the following days.

Subjective Refraction – the procedure in which the patient is asked to report on which lens combination provides the clearest vision. While this is the method of choice for determining prescription in those able to understand the task and respond to the examiner, it is less reliable in children.

Suppression of Binocular Vision- when the brain ignores the image that is seen by one eye. It is the result of weak eye teaming skills (binocularity).

Suppression Test- determines if there is any tendency for the visual processing center of the brain to ignore or suppress visual data from one eye.

Tactile– pertaining to the sense of touch.

Temple Length: Is the length of the “arm” of a pair of glasses, running from the ear to the lens area in Millimeters.

Tonic– slow, smooth tension. (See “Fusional Vergence”)

Tonic Vergence– convergence due to the basic tonicity (tension) of the extraocular muscles, which are responsible, in part, for the distance phoria.Deficient tonic vergence would result in exophoria and excessive tonic vergence results in esophoria. (See “Vergence”)

Tonometry- an instrument that measures the pressure within the eye, which is known as intraocular pressure (IOP).

Tranaglyph- red/green targets used with red/green glasses to develop eye teaming skills.

Tunnel Vision- a constriction of the visual field that is commonly caused by chronic glaucoma, retinal degeneration, a tumor, or a brain disorder that interferes with the fibers that connect the optic nerve to the brain. (Please note that a visual stress,  emotional stress, or emotional trauma can also cause a constriction of the visual field.) (See “Streff Syndrome”.)

Ultraviolet (UV) Radiation
Solar radiation is formed by visible and invisible light waves. Measured in nanometers, it has three wave lengths that reach and affect your eyes: Ultraviolet, Visible Lights, and Infrared Radiation. The shorter the wavelength, the more damage to your eyes.

Ultraviolet Radiation (200 – 380 nm)
Studies show that long term exposure to UV rays may cause eye diseases, including cataracts. In addition, short-term exposure can cause temporary conditions such as “snow blindness”, Ultraviolet radiation can be divided into three categories.

UVC (200 – 290 nm)
UVC is absorbed by the atmospheric ozone layer and never reaches your eyes.

UVB (290 – 320 nm)
UVB is a physically painful form of ultraviolet radiation. These are the sun’s “tanning rays” which are absorbed by the cornea. Exposure to UVB can temporarily damage the cornea. This band of radiation causes cancer and burning of the eye and has been linked to damage of the lens inside the eye.

UVA (320 – 380 nm)
UVA rays are absorbed by the lens of your eye. The resulting damage heals slowly, if at all. About one percent of all UVA rays penetrate the retina, which cannot repair itself. These rays, which cause sun tanning and wrinkles, pass through the outer structure of the eye and are absorbed by your eye’s lens. The resulting damage heals slowly, it at all. Extended exposure to UVA rays can lead to the formation of cataracts.

Protection from harmful UV radiation is becoming more and more important as the earth’s ozone layer continues to be depleted. Contrary to what you might think, clouds do not block out all UV rays. Though thick, heavy clouds absorb most UV radiation, UV will pass through thin clouds, even when the sun’s rays don’t feel hot. And watch out for haze – it doesn’t’t block UV radiation at all. You get higher exposure to UV radiation on snow, sand, water, or concrete, since these surfaces reflect the sun’s rays. You will also get higher UV radiation levels at higher altitudes and at areas closer to the equator. To make matters worse, the damaging effects of long-term UV radiation exposure are cumulative with the consequences not immediately apparent. So beware: excessive exposure to UV radiation today can cause eye problems tomorrow. Therefore, a good pair of sunglasses is essential for enjoying the sun. The filtering qualities of the lenses must be considered so that you have the correct type of lens for your intended use. There is no point in buying lenses designed for high altitude if you use them for everyday urban life.

UV Filter (coating): This coating is a filter for the harmful UV radiation. The coating is either put on the lens or is imbedded into the lens to protect your eyes for the harmful effect of the sun’s UV radiation.

Vectogram– a three-dimensional picture that is used to strengthen the binocularity system. Available in fixed and variable styles to provide base-in and/or base-out training. 3D glasses are used to view the picture.

Vergence– to turn the eyes horizontally (convergence- inward or divergence- outward). Accommodative vergence, fusional vergence,proximal vergence, and tonic vergence are needed to maintain single vision.

Vergence Facility- a measure of the ease and speed of the eyes to change from a converging to diverging position.

Vertigo– a disordered state in which the individual is dizzy or feels that the surrounding environment is whirling.

Visagraph Eye-Movement Recording System – records and measures eye movements while an individual reads. The system also measures reading efficiency. Specially created goggles and a computer program are used.

Visible Light (380 – 760nm) Visible light is that portion of the light spectrum that the eye perceives as color. In this spectrum, protective eyewear deals with glare or the brightness of the sun. Excessive amounts of visible light are irritating to unprotected eyes and can reduce your visual perception by as much as 50%. It can also hinder your eye’s ability to adjust to darkness (night blindness) and cause difficulty in depth perception. The unprotected eye strains to focus (squint), which can cause wrinkles and chromatic aberration. Athletic Optics offers a variety of lens colors to enhance your vision in various lighting conditions. Blue Light (380 – 480nm): Closest to the high energy, UV portion of the visible light spectrum, it focuses in front of the retina rather than on it, which blurs vision, reduces contrast and hinders depth perception.

Vision- the ability to take in information through our eyes and process the information so that it has meaning.

Vision Therapy (VT) – see “Optometric Vision Therapy”

Vision Therapy Technician- one who works under the supervision of an optometrist in evaluating clients and in planning and implementing vision therapy programs. (See “COVTT”)

Vision Therapist– Optometrist or an optometric vision therapy technician who develops and administers vision therapy programs. (Typically this term is referring to a vision therapy technician rather than an optometrist.)

Visual Acuity- sharpness or clearness of eyesight. For more information, please click here. (See “Near Acuity” and “Distance Acuity”, “20/20”)

Visual Analysis- refers to figure-ground, form constancy, spatial relation, visual closure, visual discrimination, visual memory, and visualization.

Visual Closure– the ability to identify or recognize a symbol or object when the entire object is not visible.

Visual Discrimination– the ability to discriminate between visible likeness and differences in size, shape, pattern, form, position, and color. Such as the ability to distinguish between similar words like “ran” and “run”.

Visual Field– the total area that can be seen while looking straight ahead. (See “Tunnel Vision”.) (Note: Perimetry is the method of testing an eye’s field of vision. For more information, please see “Perimetry”.)

Visual Form Dysfunction– difficulty with figure-ground, form constancy, visual closure, and visual discrimination. Symptoms include confusion with similar objects, words, or colors. Vision therapy is a treatment option.

Visual Memory– the ability to recall and use visual information from the past.  (See “Visual Sequential Memory”)

Visual Memory Dysfunction– difficulty with retention, recall, or recognition of things seen. Symptoms can include poor spelling and poor recall of visual information. Vision therapy is a treatment option.

Visual-Motor Dysfunction– the inability to process and reproduce visual images by writing or drawing. Symptoms can include poor pencil grip/writing, poor organization on written page, poor copying/spacing, and excessive erasing. Vision therapy is a treatment option.

Visual-Motor Skills- the ability of our eyes to guide our hands (eye hand coordination, visual-motor integration).

Visual Pathway- route of the nerve impulses from the retina along the optic nerve, and optic nerve radiations to the brain’s sensory cortex that is located at the base of the skull.

Visual Perceptual Disorders– information processing dysfunctions of the visual system. These dysfunctions can be a directionality/laterality disorder, visual form dysfunction, visual memory dysfunction, and visual-motor dysfunction. Vision therapy is a treatment option. (Also see “Perceptual Skills”)

Visual Perceptual Skills- the ability to organize and interpret information that is seen and give it meaning. These information-processing skills include figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.

Visual-Sensory Integration– after visual data is gathered, it is processed and combined in the brain with information from hearing (auditory-visual integration), balance (gross-motor/bilateral integration), posture, and movement (visual-motor integration).

Visual Sequential Memory- ability to recall a sequence of numbers, letters or objects in the order they were originally given.

Visual Skills– are accommodation (eye focusing), binocularity (eye teaming), and oculomotor skills (eye movement), which are neuro-muscular abilities that are controlled by muscles inside and outside of the eye and are networked with the brain.

Visualization- the ability to crate and manipulate mental pictures of an object or concept on the basis of past visual experience and memory. Essential in reading and playing sports.

Wandering Eye(s)– see “Exotropia”.

Wheatstone Stereoscope -an instrument designed to present separate images to each eye. Each eye can see independently. Two plane mirrors are joined at one edge at a 90-degree angle and two target holders, one opposite one mirror and the other, mounted on a screw base which, when turned, synchronously moves the targets toward or away from each other. The Amblyoscope, Troposcope, Synoptophore, and the Bernell Mirror Stereoscope are examples of this design.

Yoked Prisms– a wedge-shaped lens which is thicker on one edge than the other. The prism bases (thicker end) are in the same direction for both eyes (up, down, left, or right). Yoked prisms are used to train or compensate for a binocular dysfunction (eye teaming problem) or a visual field loss. Sometimes used in optometric vision therapy programs.

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