Glossary
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accommodation
Changing the shape of the eye's lens to focus on near and far objects.
acuity
The
capacity to discriminate fine details of objects.
amblyopia
See: lazy eye
angle-closure
glaucoma
Type
of glaucoma in which the aqueous is trapped by the iris bending too close
to the cornea. If this occurs suddenly, it may be a medical emergency.
antireflective
coatings
Chemical
coatings applied to lenses to reduce glare inside and outside eyeglasses.
Reduced glare improves both day and night vision and reaction time, thereby
enhancing safety. Coatings also make glasses more attractive to look at
by reducing their "shop window" appearance.
aperture
The
diameter (mm) of the opening of an optical system that determines the
size of the shaft of light that travels through the instrument; the diameter
of an objective lens of a telescope or microscope.
aqueous
humor
A
watery liquid between the lens and cornea, which bathes and feeds the
cornea, lens and iris.
aspheric
A
lens with a front surface that is not curved like a sphere and that has
a relatively flatter outside edge. Useful for correction of high refractive
errors.
astigmatism
A
common vision defect, caused by the irregular shape of the cornea, which
blurs and distorts eyesight.
background
retinopathy
The
first stage of diabetic retinopathy. Although vision is rarely affected
at this stage, there is blood-vessel damage.
bifocal
lenses
Glasses
with two vision-correction zones: one for seeing near objects and one
for distance vision. Bifocal lenses are really two lenses, with an obvious
line separating the two viewing zones.
binocular
vision
The
brain's ability to combine the images received in both eyes to form a
single, sharp image. Convergence and divergence of both eyes are necessary
to achieve binocular vision at all distances.
blind
spot
The
connection point of the optic nerve from the retina to the brain. There
are no rods or cones around this area of the retina. Therefore, this is
the one part of the eye that does not sense light, or "see."
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cataract
Cloudiness
of the eye’s crystalline lens, which may prevent a clear image from forming
on the retina.
central
vision
The
central area of the field of view in which vision is strongest and sharpest
in humans.
chalazion
A
harmless bump caused by a blocked oil-gland duct in the upper or lower
eyelid.
choroid
The
choroid — the middle membrane of the eye — is part of the uveal tunic,
between the retina and outer coating. It supplies blood to the eye. See:
uvea.
chronic
angle-closure glaucoma
The
form of angle-closure glaucoma in which the pressure build-up of liquid
in the front part of the eye acts over time and not suddenly.
ciliary
muscle
Also
called the ciliary body. A tiny muscle that changes the thickness of the
lens to achieve close and distant vision.
ciliary
process
Secretes
the liquid from behind the iris that feeds and bathes the middle and front
eye structures.
color
blindness
A
vision defect in which colors are not seen normally. A term covering a
broad range of conditions in which color-sensing cone cells of the eye
are either absent or do not function properly.
computer
vision syndrome (CVS)
Defined
by the American Academy of Optometry as "the complex of eye and vision
problems related to near work, which are experienced during or related
to computer use."
cones
Cone-shaped
photoreceptors located in the retina that sense color and detail in bright
lighting conditions. Three pigments of cones sense the three colors of
light (red, green and blue) to provide color vision.
conjunctiva
The
clear, protective membrane that lines the eyelids and covers exposed areas
of the sclera.
convergence
The
ability of both eyes to converge, or move closer together, in order to
focus on near objects.
cornea
The
eye's outer window. Sensitive, protective and transparent, the cornea's
unchanging, curved surface provides 75 percent of the eye's focusing ability,
or refractive index.
corneal
topography
A
relief map that reveals corneal contours and shows any variations in corneal
curvature.
corneal
transplantation
An
operation that removes a defective cornea and replaces it with healthy
corneal tissue from a donor. When used for keratoconus, it has a 90 percent
success rate.
cortical
cataract
With
this type of cataract, wedge-shaped spokes extend from the outer rim of
the lens to the central core (nucleus). The spokes block light, causing
glare and loss of contrast.
crypts
of Henle
Microscopic
pockets, located in the loose sections of conjunctiva around the eyeball,
that secrete mucin into tears.
diabetic
retinopathy
A
retinal blood-vessel disease that affects people with diabetes.
dichromatism
Sensitivity
to two colors rather than the three needed to perceive all the colors
in the spectrum by combining primary colors.
divergence
The
ability of both eyes to diverge, or move further apart, in order to focus
on distant objects.
dry eye
Dryness
of the eye’s surface caused by too little tear production, too much tear
drainage, changes in tear quality and a host of other problems. Symptoms
include red, burning eyes and foreign body sensations.
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epikeratophakia
In
this operation, an eye surgeon grafts a layer of corneal epithelial cells
("skin" cells from the cornea’s upper layers) around the central cone
of someone with keratoconus. The grafted cells flatten the cornea, improving
nearsightedness and blurred vision. This operation has a success rate
to similar corneal transplantation.
exit pupil
The
size of a shaft of light transmitted to the eye through a telescope or
pair of binocular lenses; an important indicator of low-light performance;
usually measured in millimeters (diameter).
extracapsular
cataract surgery
The
surgical removal of a cloudy lens that opens the front but leaves the
back of its surrounding capsule intact.
eye
The organ
of sight. The key parts of the eye are as follows: the sclera, the choroid,
the cornea, the aqueous humor, the iris, the pupil, the lens, the vitreous
humor, the retina, the macula, the fovea and the optic nerve.
eye relief
The
distance between the eye and an eyepiece of a telescope, set of binoculars
or microscope.
eyedrops
The
liquid form of any active medication (in various strengths) that is applied
by letting a drop glide under the lower eyelid. Some conditions require
taking color-coded types of eye drops at specific moments in a day.
farsightedness
A
common name for hyperopia, a widespread vision defect in which distant
objects are seen clearly and close objects are seen less clearly. Farsightedness
occurs when the eye is too short and/or the cornea too flat.
field
of view
The
width, measured in degrees, feet or millimeters, of the viewing area visible
through a pair of binoculars, a telescope or a microscope at a specified
distance.
flashes
Stimulation
of the retina and/or optic nerve that creates the illusion that we are
seeing "stars" or streaks of light. Continuous flashes persisting for
more than 20 minutes are potentially dangerous and require immediate medical
attention.
fluorescein
angiography
A
test to detect leaky and abnormal blood vessels in the eye. In hospital,
an eye specialist injects a yellow or red dye into an arm vein. Exposed
to ultraviolet rays, the dye glows yellow-green. The eye specialist photographs
the fluorescent dye, as it travels through blood vessels in the eye.
fovea
A
small depression in the center of the macula that contains only cones.
The fovea is the area of sharpest vision, or acuity, in the eye.
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ganglion
cells
A
layer of the retina, which transmits signals to the optic nerve.
glands
of Manz
Microscopic
glands within the bulbar conjunctiva, arranged in a ring around the cornea,
near the scleral junction. Secrete mucin, a proteinous substance that
makes up the inner layer of tears.
glands
of Zeis
Oil-producing
glands that surround the eyelashes. The oil forms the outer layer of tears.
glare
A
harsh, uncomfortably bright light or reflection.
glass
Original
material used for eyeglasses. Made mainly of sand (silicon dioxide) plus
various elements according to special needs (titanium dioxide for thinness).
Glass lenses are long lasting and naturally scratch-resistant. But like
a regular drinking glass, they may break if you drop them.
glaucoma
Condition
with moderate to severe vision loss due to areas of failing optic nerves.
Excess pressure exerted by the liquid in the front part of the eye (the
aqueous) damages the nerves.
goblet
cells
Large
glands in the conjunctiva that secrete mucin, which forms the inner layer
of tear film. The mucin layer enables tears to glide across the eye’s
surface. Missing or damaged in people with dry eye.
high-index
lenses
Advanced
plastic lenses that are thinner and flatter than conventional plastic
or glass lenses.
high-index
plastic
The
third wave of plastic materials for those with moderate to severe correction
needs. Effectively hides the level of correction.
hyperopia
See: farsightedness.
intracapsular
cataract surgery
The
surgical removal of a cloudy lens and its surrounding capsule.
intraocular
pressure (IOP)
A
measure of how much the liquid contained in front of and behind the iris
pushes in all directions (particularly affecting the retina in the back
of the eye).
IOP
See: intraocular pressure.
iris
The
colored or pigmented eye tissue behind the cornea. The iris acts like
a muscular diaphragm, regulating the amount of light that enters the eye
through the pupil
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keratoconus
A
condition in which the center of the cornea thins and develops a cone-shaped
bulge that blurs and distorts vision.
keratometry
A
mechanical means of measuring the curvature of the cornea. A series of
concentric circles is projected on a 3-mm wide spot in the central cornea
by a keratometer. If wavy lines appear in the circles, the person is nearsighted.
By using these lines as a guide, the eye doctor can measure the amount
of astigmatism (blurred vision). Can also be used to detect and monitor
keratoconus.
Krause
glands
Accessory
tear glands located under the eyelids where the upper and lower conjunctiva
meet.
lacrimal
gland
An
almond-shaped gland that produces tears. It is located above the eyeball,
under a bone near the eyebrow.
LASIK
Laser
in-situ keratomileusis, a surgical procedure to reshape the cornea using
an excimer laser to correct myopia, hyperopia and astigmatism.
lazy
eye
A
common name for amblyopia, which is the loss or lack of development of
vision (usually in one eye) during childhood.
lens
A
transparent lentil-shaped body behind the iris that is controlled by the
ciliary muscle. The lens provides 25 percent of the eye's focusing power.
To focus on close-up objects, the ciliary muscle squeezes the lens to
make it thicker. For far-away objects, it flattens the lens to make it
thinner.
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macula
A
Latin word, meaning spot, which is used to describe the yellow central
area of the retina, where vision is sharpest.
macular
degeneration
Deterioration
of the macula (central spot) of the retina that eventually causes the
permanent loss of central vision. There are two types of macular degeneration:
dry and wet. The most common cause of vision loss in people older than
60 years of age.
macular
edema
Swelling
of the central spot (macula) in the retina, often caused by blood vessel
leakage in certain eye diseases, such as diabetic retinopathy.
meibomian
glands
Oil-producing
glands located within the eyelid. The oil, which forms the outer layer
of tears, drains through ducts that open on the eyelid margins.
mid-index
plastic
A
new generation of plastic materials that reduces lens thickness while
providing the same optical performance as earlier plastics. Recommended
for light to moderate correction.
misaligned
eyes
The
medical term for misaligned eyes. Eye misalignment is a common vision
defect in which one or both eyes are turned inward, outward, upward or
downward. One or both eyes may move irregularly.
monovision
Vision
therapy in which a contact lens with close-up correction is worn in one
eye, and (if necessary) a contact for distance correction is worn in the
other eye.
movement
parallax
The
perceived motion of near and far objects when either the object or the
observer moves.
myopia
See: nearsightedness.
nearsightedness
A
common name for myopia, a widespread vision defect in which close objects
are seen clearly and distant objects are seen less clearly. Nearsightedness
generally occurs because the eyeball is too long or the cornea too curved.
Correctable with single-vision lenses.
nuclear
cataract
This
type of cataract clouds the central part (nucleus) of the eye’s lens.
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objective
lens
A
lens or system of lenses that creates an image of an object.
open-angle
glaucoma
The
most common type of glaucoma. Caused by high intraocular pressure resulting
from clogging of the filter mechanism that returns liquid to the bloodstream
from the front chambers of the eye.
ophthalmologist
A
medical doctor who specializes in the eye and has completed medical school.
Licensed to examine eyes, treat eye diseases and perform eye surgery.
optic
nerve
The
bundle of nerve fibers that transmits light-generated electrical impulses
from the retina to the brain.
optic
nerve head
Also
called the optic disk. The area where the optic nerve is attached to the
retina and the location of the blind spot.
optometrist
Doctors
of optometry are independent primary healthcare providers who examine,
diagnose, treat and manage diseases and disorders of the visual system,
the eye and associated structures, as well as diagnose related systemic
conditions.
orthoptics
Vision
therapy that can strengthen, coordinate and improve the functions of both
eyes, especially in the early years of life.
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phacoemulsification
A
technique for removing a cloudy lens during cataract surgery. Sound waves
are used to shatter the cataract. The broken pieces are vacuumed from
the lens capsule using a long, narrow tube.
photochromic
lenses
Lenses
that darken in bright sunlight and clear in dim light.
photoreceptors
Cells at the back of the retina that contain light-sensitive pigments.
When light hits these pigments, they trigger nerve impulses, resulting
in vision (see: cones; rods).
polarization
A
manufacturing process that adds certain materials to optical lenses which
separate light into beams that vibrate in one plane only instead of all
planes (like normal light); cuts glare.
polycarbonate
Plastic
lenses designed for impact resistance and UV protection. Increasingly
popular for sport/sunglasses and prescription lenses.
power
The
degree to which an object is enlarged. Also called magnification.
presbyopia
A
common vision defect associated with the aging eye’s diminishing ability
to focus on close objects. Correctable with bifocals or progressive addition
lenses.
PRK
Surgical
procedure to reshape the cornea with excimer laser; approved for use in
correcting myopia; also known as laser vision correction.
progressive-addition
lenses
Lenses
that have progressively more reading power from top to bottom. They are
used for correcting presbyopia.
proliferative
retinopathy
Severe
retinal blood-vessel disease that occurs in people with diabetes. Fragile,
new blood vessels and fibrous tissues grow on the retina and extend into
the vitreous. These blood vessels may leak, leading to macular edema,
retinal detachment and vision loss.
punctum
One
entrance to the eye’s drainpipe. The eye has two drains or puncta — in
the upper and lower corners of the eyelid margins near the nose. Pulling
back the eyelid will reveal these tiny holes.
pupil
The
space in the center of the iris where light enters the inner eye. The
widening, or dilation, of the pupil is controlled by the iris.
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refraction
The
bending of light rays as they pass through a substance. In the eyes, light
is refracted by the cornea and lens, and focused on each retina.
refractive
index
A
measure of how much light rays are bent when they pass from one medium
to another. Water, air, glass and plastic each have a specific refraction
index.
retina
The
inner layer at the back of the eye, where light-sensitive rods and cones
are located. Chemical changes in the retina transmit electrical signals
through the optic nerve to the brain to produce sight.
retinal
detachment
Separation
of the light-sensitive retina from the back of the eye, which may cause
vision loss. Laser surgery is required as soon as possible after detachment
is detected to secure the retina to the inner eye.
retinitis
pigmentosa (RP)
A
group of rare, inherited degenerative diseases that affect the retina
and can lead to various degrees of vision loss or blindness.
rhodopsin
Also
called visual purple, a light-sensitive pigment in the rods. It bleaches
in light and regenerates in the dark using vitamin A compounds.
rigid
gas-permeable (RGP) lenses
Hard
plastic contact lenses that allow oxygen and carbon dioxide to pass.
risk factors
Identifiable
characteristics or life history for which there is an observable link
to a problem, disease or predisposition toward a health condition.
rods
The
cylindrical photoreceptors located on the retina that do not sense color
but enhance peripheral vision and vision in dim light.
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sclera
The
tough white part of the eye, or outermost shell, that contains the eyeball.
seborrhea
The
over-production of sebum, the oily secretion from the sebaceous gland.
Characterized by oily skin and flaky crust along the edges of the eyelids.
If left untreated, seborrhea can cause inflammation or infection.
scratch-resistant
coatings
Hardened
coatings that protect plastic lenses from being scratched.
single-vision
lenses
Eyeglass
lenses that correct a single refractive need, such as myopia or hyperopia.
Sjogren’s
syndrome
An
immune disorder that causes mild to extreme dryness in the eyes and mouth.
It dries mucous membranes and affects the lacrimal glands, which make
tears. About 3 million Americans suffer from this disorder, which may
be related to arthritis.
slit lamp
A
biomicroscope that helps eye doctors examine the various parts of the
eye under high magnification. This special microscope has a chin and headrest.
Colored filters can enhance the doctor’s view.
spheric
A
lens with a smooth, sphere-shaped or rounded surface that bends light
rays equally in all directions.
spots
and floaters
Pieces
of embryonic blood vessels, specks of pigment on the muscle fibers attached
to the iris, particles caught in the tear layer in front of the eye, or
clumps of collagen in the vitreous humor. Usually harmless, these tiny
objects drift freely in the eye and cast shadows across our line of vision.
standard
plastic
A
hard synthetic resin that is light and impact-resistant, but thick and
damage-prone (scratches) unless coated with protective varnish.
stereoscopic
vision
The ability
to judge depth accurately or see in three dimensions. The brain achieves
stereoscopic vision by merging the images it receives from the slightly
different angles provided by each eye.
sties
Infections
of the glands around the eyelashes and under the upper and lower eyelids
that may be associated with stress or other eye conditions.
strabismus
See: misaligned eyes
subcapsular
cataract
This
type of cataract affects the back of the eye’s lens, causing blurriness
and glare.
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topical
eye ointment
Thick
paste of any active medication (in various strengths) that is applied
by placing it along the lower eyelid and gently applying it around the
tear duct.
toric
A
lens with a cylindrical as well as rounded shape; used to correct astigmatism.
trabecular
meshwork
Tissue
that filters the liquid aqueous after it has circulated toward the outer
front edge of the iris. The trabecular meshwork drains back into the bloodstream.
trifocals
Eyeglasses
with three different viewing areas to provide clear vision at close distances,
mid-range and far away. Trifocals have two well-defined visible lines
separating the viewing zones.
ultraviolet
rays
Wavelengths
of light below the color blue on the color spectrum. Invisible radiation
from sunlight, sunlamps and video terminals. Ultraviolet rays are potentially
harmful to the eyes and are strongly linked to the development of cataracts
and macular degeneration.
uvea
The
middle layer of the eye, which contains the iris, the ciliary body and
blood vessels that supply the retina.
uveitis
Infection
or inflammation of the uvea.
visual
cortex
The
part of the cerebral cortex of the brain that is primarily responsible
for interpreting signals from the eye.
visual
field
What
you see, particularly the area surrounding the center of vision. Each
eye has a visual field.
vitrectomy
The
replacement of clouded vitreous humor (the gel-like substance that fills
the eye) with clear saline solution.
vitreous
humor
A
thick, transparent, gel-like substance which fills the eye, located between
the lens and retina
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wavelength
The
distance between two similar points of a given wave. UV light has a shorter
wavelength than the light the eye can see.
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