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Glossary of Optical Terms

AK (astigmatic keratotomy)
ALK (automated lamellar keratoplasty)
aqueous humor
barrel distortion
BVCA (best corrected visual acuity)
CE mark
chromatic aberration (color fringing)
ciliary muscles
convergence / divergence
crystalline lens
excimer laser
external ocular muscles
ICRS (Intrastromal Corneal Ring Segments)

LASIK (Laser Assisted In-Situ Keratomileusis)
modified monovision
near point of accommodation
PD (pupillary distance)
pincushion distortion
PRK (photorefractive keratotomy)
progressive lenses
refractive index
refractive error
RK (radial keratotomy)
saline solution
Snellen Line
Snellen visual acuity test
visual acuity
visual axis
vitreous humor

20/20 Term typically used in conjunction with a Snellen eye chart, indicating normal vision. A second number larger than twenty (i.e. 20/100) indicates diminished vision (i.e. nearsighted vision). For example, a person with 20/100 vision would see an object at 20 feet away with the same acuity that a person with 20/20 would see at 100 feet. In Japan, sometimes referred to as 6/6; in the U.K., 10/10. (top)

aberration A characteristic of a lens that prevents the formation of a perfect image. Aberrations affecting the quality of images produce degraded sharpness, lowered contrast, distorted shape, and color fringing. (top)

ablation Technique achieved with excimer laser in which tissue is removed from the central optical zone with the intent of reshaping the cornea's curvature; usually for correcting nearsightedness. (top)

accommodation The ability of the eye to change its focus from distant object to objects closer than optical infinity. The eye achieves this by altering the shape of the crystalline lens with the ciliary muscles.

acuity Clearness, as in visual acuity. The most common measure of visual acuity is the Snellen eye chart. Normal acuity is having 20/20 vision. 20/40 vision is the standard in most countries for obtaining a driver's license without corrective lenses. (top)

AK (astigmatic keratotomy) A surgical procedure to correct astigmatism. Microscopic incisions are placed in the peripheral cornea, similar to radial keratotomy. (top)

ALK (automated lamellar keratoplasty) A refractive surgery technique for low to moderate myopia. In the procedure, the refractive surgeon places an instrument called an automated microkeratome on the eye which removes, in a shaving motion, a thin layer of cornea. An even thinner layer of cornea underneath this top cap is removed, and the top cap is replaced. (top)

ametrope One who requires eyeglasses or contact lenses for in-focus vision. (top)

Any imperfection in refractive state of the eye. Examples would be one with hyperopia, myopia, or astigmatism. (top)

anamorphic A non-rotationally symmetrical lens element that distorts image size & shape in one axis more than the other, because of its barrel (cylindrical) shape. Panavision lenses used for wide-screen movies contain anamorphic elements to "squeeze" the image, later "de-squeezed" by the projection lens. (top)

anisometropia A difference in refractive power of the two eyes in which the variance is at least one diopter. (top)

aspheric Literally "not spherical." In optics, a lens that is rotationally symmetrical, but the front, back or both surfaces of the lens are not portions of a sphere, but rather a section from an ellipse. (top)

astigmatism A condition in which the surface of the cornea is not spherical, but bulges more in one axis than the other, like the back of a spoon. An astigmatic cornea causes light images to focus on two separate points in the eye, creating a distorted image and poor focus. Corrective lenses have a non-rotationally symmetrical shape, either a convex or concave portion of a cylinder, orientated to counteract the eye's distortion. Contact lenses designed to correct for astigmatism are called Toric-lenses. (top)

aqueous humor
Clear fluid that flows between and nourishes the crystalline lens and the cornea. It is secreted by the ciliary processes. (top)

barrel distortion A distortion in which straight lines not passing through the center of the image bend outward (away from the center of the image). The curvature becomes more pronounced further from the center. This is the opposite of pincushion distortion. (top)

BVCA (best corrected visual acuity) The best possible vision a person can achieve with corrective lenses measured in terms of Snellen lines on an eye chart. Contact lenses frequently achieve 20/15 vision, superior resolution to most surgeries and eyeglasses. (top)

CE mark When displayed, this mark indicates a device fulfills requirements for sale in the 15-nation European Union. (top)

chromatic aberration (same as color fringing) The failure of a lens to bring light of different colors to the same focus. (top)

ciliary muscles
The muscles that alter the shape of the crystalline lens, thereby focusing the eye. (top)

When circles or spheres of different sizes share the same center point (i.e. a bulls-eye target). The twin-domes of the HydroOptix MEGA mask are concentric with the rotation point of each eye. (top)

cones One of two types of specialized light sensitive cells (photoreceptors) in the retina that provides the ability to see objects in color and at high resolution in the central field-of-view. See rods. (top)

convergence / divergence The turning of the eyes inward/outward so that they are both "aimed" toward the object being viewed. (top)

cornea The clear, dome-shaped "window" at the front of the eye that covers the iris and pupil. The cornea plays an important role in vision because it provides approximately 70 percent of the eye's light-focusing power. Contact lenses rest on the corneal surface. (top)

crystalline lens The natural lens of the eye, located behind the iris, which helps focus rays of light on the retina. The original state of the lens is transparent, but the lens may become cloudy with age (cataract). The lens has the ability to vary its shape, thereby focusing on objects closer than optical infinity. (top)

diopter A measurement of the refractive power of a lens element, as a portion of a meter. If a camera lens is focused on infinity, and a +1 diopter accessory lens is attached to the front, the camera will now have sharp focus at 1 meter, even though the barrel of the lens is still set to infinity. (top)

If a +1/2 diopter is used, the focus is shifted to 2 meters.
If a +1/4 diopter is used, the focus is shifted to 4 meters.
If a +2 diopter is used, the focus is shifted to 1/2 meter.
If a +3 diopter is used, the focus is shifted to 1/3 meter.

emmetrope One with 20/20 vision, without using corrective lenses. (top)

emmetropia A condition in which light rays focus correctly on the retina, without using corrective lenses; same as 20/20 vision. (top)

excimer laser
Used in PRK (photorefractive kerototomy) and LASIK (laser-assisted intrastromal keratoplasty) to reshape corneal curvature by ablating, or burning off, eye tissue. (top)

external ocular muscles The six muscles that turn the eyes to position them for viewing. (top)

farsightedness Common term for hyperopia. (top)

glaucoma An eye disease characterized by narrowing of one's field-of-view, caused by increased pressure within the eyeball. If not diagnosed and treated, glaucoma may lead to optic nerve damage, loss of visual field, gradual vision impairment, and sometimes blindness. (top)

hyperope One who is farsighted. (top)

hyperopia farsightedness (same as hypermetropia, called LONGSIGHTED in the U.K.); usually able to focus on distant objects, inability to focus on close objects. This occurs when one's eyeball is too small, short or flat for the focusing system of the eye, or when the eye's focusing mechanism is too weak (not enough positive diopter), thus causing light rays to focus behind the retina, making close objects appear blurry. A positive diopter lens is required to achieve normal vision (the naked eye has excess negative diopter). (top)

ICRS (Intrastromal Corneal Ring Segments) A clear, half-ring shaped plastic that is surgically inserted into the cornea, but outside of the visual axis. For example, for a nearsighted person, two ring segments are surgically inserted on opposite sides of the cornea, forming a hoop. This stretched-flat cornea now has less positive-diopter power (less refractive error). (top)

index see "refractive index." (top)

iris The tissue behind the cornea that gives color to the eye (blue, brown, hazel, etc.). It controls the amount of light that reaches the retina, and the depth of focus of the eye, by varying the size of the pupil. (top)

kera The Latin word for eye. (top)

keratometry The measurement of the curvature of the cornea. This process determines the best fitting contact lens for a given person; contact lenses of the same brand and power come in a choice of base curves. (top)

LASIK (Laser Assisted In-Situ Keratomileusis) A vision correction surgery procedure to treat hyperopia, myopia and astigmatism. The curvature of the cornea is altered by first raising a "flap" of cornea, ablating the tissue underneath with an excimer laser (PRK) machine, then closing the flap on top. Sometimes referred to as "flap-n-zap".(top)

modified monovision (first see monovision) Wearing one progressive ("bifocal") contact lens, usually only on the close-vision eye. The distance-vision eye wears a single power contact lens. Therefore distance vision receives in-focus images from both eyes, considered superior for low-light conditions. (top)

monovision For presbyopic individuals (need reading glasses) who wear contact lenses. For example, with one who is nearsighted, a stronger power contact lens (for distance vision) is worn on the dominant eye, while a weaker power contact lens (for close vision) is worn on the non-dominant eye. The brain ignores the fuzzy image from one eye, and concentrates on the high-resolution image from the other eye. (top)

myope One who is nearsighted. (top)

myopia Same as nearsightedness (called SHORTsighted in the U.K.); enhanced ability to see close objects, inability to focus on distant objects. This occurs when one's eyeball is too long or too big for the focusing system of the eye, or when the eye's focusing mechanism is too strong (excess positive diopter), thus causing light rays to focus in front of the retina, making far objects appear blurry. A negative diopter lens is required to achieve normal vision (the naked eye has excess positive diopter). (top)

near point of accommodation The closest distance to which the eyes can be clearly focused. (top)

nearsightedness Common term for myopia. (top)

Ophthalmologist A medical doctor who specializes in the diagnosis and medical or surgical treatment of eye diseases. To become an ophthalmologist one must first obtain a medical degree (M.D. or D.O. in North America) and then complete further specialty training. Ophthalmology is technically a "surgical" discipline. Some ophthalmologists choose not to perform surgery and can be called "medical ophthalmologists". An ophthalmologist may also prescribe eyeglasses and contact lenses. (top)

Optician An expert in the art and science of making and fitting glasses. The optician may also dispense and/or fit contact lenses, depending on local licensing practices. (top)

Optometrist A doctor of optometry is a non-medical primary eye health care provider who specializes in the examination, diagnosis, treatment, management, and prevention of diseases and disorders of the visual system. Optometrists today most often complete a bachelor of science degree (with very specific requirements) followed by an optometry degree program that requires four to eight years to complete. (top)

PD (pupillary distance) The distance between one's pupils, measured center-to-center. Measured in millimeters, average spacing is 64 mm; extreme ranges between 54 mm to 77 mm. (top)

pincushion distortion A distortion in which straight lines not passing through the center of the image bend inward (toward the center of the image). The curvature becomes more pronounced further from the center. This is the opposite of barrel distortion. (top)

presbyope One who requires reading glasses; see presbyopia. (top)

presbyopia With age, we all lose our ability to focus on nearby objects (accommodation). In our eyes, the crystalline lens loses flexibility and our ciliary muscles weaken. This limits our minimum focusing distance. Presbyopes with 20/20 vision need reading glasses; presbyopic myopes and hyperopes need bifocals. Contact lens users can choose between monovision, modified monovision or progressive contact lenses to achieve both near and far focusing ability. This chart graphs age vs. close-focusing ability. (top)

PRK (photorefractive keratotomy) A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma. (top)

progressive lenses Lenses of multiple zones of optical power that provide both near and far focusing ability for presbyopic vision. For spectacle lenses, this is an alternative to bifocals or trifocals (elimination of tell-tale "lines" that separate lenses of differing powers). For contact lenses, these are sometimes referred to as "bifocal" contacts. But they are actually concentric rings (think "bullseye target) of varying powers that provide simultaneous near and far focusing ability. The brain ignores the fuzzy part of the image and concentrates on the high-resolution image. (top)

pupil The black, circular opening in the center of the iris, which is the colored portion of the eye. (top)

refraction The bending of light rays by the use of a lens or other refractive material (i.e. water). (top)

refractive index a measure of a clear substance’s ability to slow photons, and thereby bend the direction of travel of off-axis rays of light. The denser the material, the more it will slow photons, and therefore bend the direction of travel. (top)

Material Index

Vacuum of space 1.00000
Air 1.00029
Distilled water 1.333
Seawater (Average) 1.341
Window Glass 1.49
Polycarbonate 1.586
Diamond 2.417

refractive error In human vision, a defect in the ability of the eye to focus an image accurately. Common errors are astigmatism, hyperopia and myopia; half the world's population requires some kind of vision correction. (top)

retina Like the film in a camera, the retina (made of rods and cones) at the back of the eye receives images formed by the eye's optical system, and sends impulses to the brain through the optic nerve. (top)

RK (radial keratotomy) Oldest vision correction surgery technique to treat nearsightedness. Flattens the curvature of the cornea by placing micro-incisions that resemble the spokes of a wheel in the tissue around the central optical zone. (top)

rods One of two types of specialized light sensitive cells (photoreceptors) in the retina that provides both the ability to see objects in dim light (night-vision), and peripheral vision. However rods provide monochromatic images; in very low light, humans see objects as shades of gray. See cones. (top)

saline solution Sterile salt solution used for cleaning, rinsing of contact lenses. Storing contact lenses requires a disinfecting solution. (top)

Snellen Line A line of same-sized letters on an eye chart that is used to test a patient's vision. (top)

Snellen visual acuity test The chart with the big E at the top and lines of letters that become progressively smaller. The Snellen test is one of many tests used to determine visual acuity. (top)

stroma The middle tissue layer of the cornea, comprising about 90 percent of corneal thickness. (top)

visual acuity Clearness of vision. The ability to distinguish details and shapes of objects; also called central vision. (top)

visual axis The central area of the cornea, pupil, and lens that light passes through to reach the retina and be "seen". (top)

vitreous humor
The transparent, colorless mass of gel that lies behind the lens and in front of the retina. (top)

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