is anything that is seen with the
. Catch sight of; to
the eyes. The ability to see; the visual faculty. The range of
act of looking or seeing or observing
million people in the world are
, around one in every 200 people
on Earth. Another 246 million have low vision to degrees that impose
moderate or severe limits. Vision loss also affects hundreds of millions
more people. Often relatives, devoted to aiding
those who can’t see.
Many become blind after many years of slow and progressive retinal
Blind - Sight Problems
provides sustainable medical, surgical, and educational services through
volunteer ophthalmic surgeons with the objectives of restoring sight and
preventing blindness to disadvantaged individuals worldwide.
American Foundation for the Blind
is a decreased ability to see to a degree that
causes problems not fixable by usual means, such as glasses. Some also
include those who have a decreased ability to see because they do not have
access to glasses or contact lenses. Visual impairment is often defined as
a best corrected visual acuity of worse than either 20/40 or 20/60. The
term blindness is used for complete or nearly complete vision loss. Visual
impairment may cause people difficulties with normal daily activities such
as driving, reading, socializing, and walking.
Near-sightedness is a condition of the eye where light focuses in front
of, instead of on, the retina. This causes distant objects to be blurry
while close objects appear normal. Other symptoms may include headaches
and eye strain. Severe near-sightedness increases the risk of retinal
detachment, cataracts, and glaucoma.
clouding of the lens in the eye which leads to a decrease in vision.
Cataracts often develop slowly and can affect one or both eyes. Symptoms
may include faded colors, blurry vision, halos around light, trouble with
bright lights, and trouble seeing at night.
Cataract Diagnosis and Treatment
using LED technology to measure
cataracts at a molecular level.Blurring
is to make less clear and become vague or not clearly defined or easy to
perceive or understand.
Farsighted children struggle with Attention
is a group
of eye diseases which result in damage to the optic nerve and vision loss.
The most common type is open-angle glaucoma with less common types
including closed-angle glaucoma and normal-tension glaucoma. Open-angle
glaucoma develops slowly over time and there is no pain. Side vision may
begin to decrease followed by central vision resulting in blindness if not
treated. Closed-angle glaucoma can present gradually or suddenly. The
sudden presentation may involve severe eye pain, blurred vision,
mid-dilated pupil, redness of the eye, and nausea. Vision loss from
glaucoma, once it has occurred, is permanent.
is a medical condition which may result in
blurred or no vision in the center of the visual field. Early on there are
often no symptoms. Over time, however, some people experience a gradual
worsening of vision that may affect one or both eyes. While it does not
result in complete blindness, loss of central vision can make it hard to
recognize faces, drive, read, or perform other activities of daily life.
Visual hallucinations may also occur but these do not represent a mental
is the ability of people who are cortically blind
due to lesions in their striate cortex, also known as primary visual
cortex or V1, to respond to visual stimuli that they do not
is the total or partial loss of vision in a normal-appearing
eye caused by damage to the brain's occipital
. Cortical blindness can be acquired or congenital, and may also
be transient in certain instances.
deals with the diagnosis and characterization of
neoplastic and non-neoplastic diseases of the eyes. Ophthalmic
pathologists generally work closely with ophthalmologists.
The Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW)
is a condition in which the eyes do not properly align with
each other when looking at an object. Which eye is focused on the object
in question can switch. It may also be present occasionally or constantly.
If present during a large part of childhood, it may result in amblyopia or
loss of depth perception. Adults may have double vision.
is a form of strabismus in which one or both eyes turns
inward. The condition can be constantly present, or occur intermittently,
and can give the affected individual a "cross-eyed" appearance. It is the
opposite of Exotropia. Esotropia is sometimes erroneously called "lazy
eye", which describes the condition of amblyopia—a reduction in vision of
one or both eyes that is not the result of any pathology of the eye and
cannot be resolved by the use of corrective lenses. Amblyopia can,
however, arise as a result of esotropia occurring in childhood: In order
to relieve symptoms of diplopia or double vision, the child's brain will
ignore or "suppress" the image from the esotropic eye, which when allowed
to continue untreated will lead to the development of amblyopia. Treatment
options for esotropia include glasses to correct refractive errors (see
accommodative esotropia below), the use of prisms and/or orthoptic
exercises and/or eye muscle surgery. The term is from Greek eso meaning
"inward" and trope meaning "a turning".
Testing Sight - Eye Exams
Stand 20 feet from the
at eye level
(cover one eye but don't close the eye or press on the eye).
means that the test subject sees the same line of letters at 20 feet that
person with normal vision sees at 20 feet. 20/40 vision means that the
test subject sees at 20 feet what a person with normal vision sees at 40
20/40 - Able to pass Driver's License Test in all 50 States. Most
printed material is at this level.
20/80 - Able to read alarm clock at
10 feet. News Headlines are this size.
20/200 - Legal blindness. Able
to see STOP sign letters.
numerator ( the top or first number 20 ) refers to a distance and the
Demoninator refers to a size. Since the standard distance of the chart is
20 feet which is equivalent to 6 meters the term 2020 Vision or Vision
2020 refers to the ability to see a certain size letter at a certain
distance from the eye. In this case size a size 20 Letter at 20 feet. The
term ""6/6 vision"" is used as the meter equivalent.
Eye Test Game
Eye Exam Seeing Test
Smartphone Apps for
20 20 20 Rule
is every 20 minutes, stare
and focus at an object 20 feet or more away for 20 seconds. This helps
Improve Vision Naturally
Glasses Off App
How to See Without Glasses
ZEISS Vision Care
= Right Eye
= Left Eye
is your prescription which corrects your
ability to see
things far away (distance vision).
means your prescription is
for reading only (near vision).
refers to the clarity of vision. Visual acuity is
dependent on optical and neural factors, i.e., (i) the sharpness of the
retinal focus within the eye, (ii) the health and functioning of the
retina, and (iii) the sensitivity of the interpretative faculty of the
brain. A common cause of low visual acuity is refractive error (ametropia),
or errors in how the light is refracted in the eyeball. Causes of
refractive errors include aberrations in the shape of the eyeball, the
shape of the cornea, and reduced flexibility of the lens. Too high or too
low refractive error (in relation to the length of the eyeball) is the
cause of nearsightedness (myopia) or farsightedness (hyperopia) (normal
refractive status is referred to as emmetropia). Other optical causes are
astigmatism or more complex corneal irregularities. These anomalies can
mostly be corrected by optical means (such as eyeglasses, contact lenses,
laser surgery, etc.).
Andrew Bastawrous: Get your next eye exam on a Smartphone
Personal Vision Tracker is
the affordable way to test your vision using your smartphone.
White Eye Detector App
is an eye examination that can detect dysfunction
in central and
which may be caused by various medical
conditions such as glaucoma, stroke, pituitary disease, brain tumours or
other neurological deficits. Visual field testing can be performed
clinically by keeping the subject's gaze fixed while presenting objects at
various places within their visual field. Simple manual equipment can be
used such as in the tangent screen test or the Amsler grid. When dedicated
machinery is used it is called a perimeter.
Visual Fields easy Eye Exam App
capture high-quality images
of the front and back of an eye.
is an organ which reacts to
and pressure. As a sense organ, the mammalian eye allows vision.
Human eyes help provide a three dimensional
, moving image, normally
in daylight. Rod and cone cells in the retina allow conscious
light perception and vision including color differentiation and the
perception of depth. The human eye can distinguish about 10 million
and is possibly capable of detecting a single photon
How Human Eye Works
Anatomy of the Eye
are organs of the
They provide organisms vision, the ability to process visual detail, as
well as enabling several photo response functions that are independent of
vision. Eyes detect light and convert it into electro-chemical impulses in
is the branch of medicine that deals with the anatomy, physiology and
diseases of the eyeball.
is a health care
which involves examining the eyes and applicable visual systems for
defects or abnormalities as well as the medical diagnosis and management
of eye disease.
is the branch of medicine that deals with the anatomy,
physiology and diseases of the eyeball and orbit. An ophthalmologist is a
specialist in medical and surgical eye problems. Their credentials include
an M.D. or D.O. degree, and an additional four years of residency.
Ophthalmologists are allowed to medically treat eye disease, implement
laser therapy, and perform incisional surgery when warranted.
Lateral Geniculate Nucleus
is a relay center in the thalamus
for the visual pathway. It receives a
major sensory input from the retina. The LGN is the main central
connection for the optic nerve to the occipital lobe. In humans, each LGN
has six layers of neurons (grey matter) alternating with optic fibers (white
is the third and inner coat of
the eye which is a light-sensitive layer of tissue. The optics of the eye
create an image of the visual world on the retina (through the cornea and
lens), which serves much the same function as the film in a camera. Light
striking the retina initiates a cascade of chemical and electrical events
that ultimately trigger nerve impulses. These are sent to various visual
centres of the brain through the fibres of the optic nerve. Neural retina
typically refers to three layers of neural cells (photo receptor cells,
bipolar cells, and ganglion cells) within the retina, while the entire
retina refers to these three layers plus a layer of pigmented epithelial
cells. The retina sits at the back of the human
, and contains protein cells that convert light into electrical
that travel through the nervous
, triggering a response from the brain, ultimately building a
picture of the scene being viewed. Just as photography depends on camera
pixels reacting to
, vision relies on the retina performing the same
is a paired nerve that transmits visual information from the
retina to the brain. The optic nerve is derived from optic stalks during
the seventh week of development and is composed of retinal ganglion cell
and glial cells
. In humans, the optic nerve extends from the optic
disc to the optic chiasm and continues as the optic tract to the lateral
geniculate nucleus, pretectal nuclei, and superior colliculus.
is a specialized type of cell found in the retina that is capable
, which is a process by which light is
electrical signals in the rod cells, cone cells and photosensitive
ganglion cells of the retina of the eye. The great biological importance
of photoreceptors is that they convert light (visible
) into signals that can stimulate biological
processes. To be more specific, photoreceptor proteins in the cell absorb
photons, triggering a change in the cell's membrane potential. There are
currently three known types of photoreceptor cells in mammalian eyes:
rods, cones, and photosensitive retinal ganglion cells. The two classic
photoreceptor cells are rods and cones, each contributing information used
by the visual system to form a representation of the visual world, sight.
The rods are narrower than the cones and distributed differently across
the retina, but the chemical process in each that supports
phototransduction is similar. A third class of mammalian photoreceptor
cell was discovered during the 1990s: the photosensitive
These cells do not contribute to sight directly, but are thought to
support circadian rhythms and pupillary reflex. There are major functional
differences between the rods and cones. Rods are extremely sensitive, and
can be triggered by a single photon. At very low light levels, visual
experience is based solely on the rod signal. This explains why colors
cannot be seen at low light levels: only one type of photoreceptor cell is
active. Cones require significantly brighter light (i.e., a larger numbers
of photons) in order to produce a signal. In humans, there are three
different types of cone cell, distinguished by their pattern of response
to different wavelengths of light. Color
experience is calculated from
these three distinct signals, perhaps via an opponent process. The three
types of cone cell respond (roughly) to light of short, medium, and long
wavelengths. Note that, due to the principle of univariance, the firing of
the cell depends upon only the number of photons absorbed. The different
responses of the three types of cone cells are determined by the
likelihoods that their respective photoreceptor proteins will absorb
photons of different wavelengths. So, for example, an L cone cell contains
a photoreceptor protein that more readily absorbs long wavelengths of
light (i.e., more "red"). Light of a shorter wavelength can also produce
the same response, but it must be much brighter to do so. The human retina
contains about 120 million rod cells, and 6 million cone cells. The number
and ratio of rods to cones varies among species, dependent on whether an
animal is primarily diurnal or nocturnal. Certain owls, such as the tawny
owl, have a tremendous number of rods in their retinae. In addition, there
are about 2.4 million to 3 million ganglion cells in the human visual
system, 1 to 2% of them photosensitive. The axons of ganglion cells form
the two optic nerves. The pineal and parapineal glands are photoreceptive
in non-mammalian vertebrates, but not in mammals. Birds have photoactive
cerebrospinal fluid (CSF)-contacting neurons
within the paraventricular
organ that respond to light in the absence of input from the eyes or
neurotransmitters. Invertebrate photoreceptors in organisms such as
insects and molluscs are different in both their morphological
organization and their underlying biochemical pathways. Described here are
are photoreceptor cells in the retina of the eye that can function in less
intense light than the other type of visual photoreceptor, cone cells.
Rods are usually found concentrated at the outer edges of the retina and
are used in peripheral vision. On average, there are approximately 90
million rod cells in the human retina. Rod cells are more sensitive than
cone cells and are almost entirely responsible for night vision. However,
rods have little role in color vision, which is one of the main reasons
why colors are much less apparent in darkness.
are one of three types of photoreceptor cells in the retina of
mammalian eyes (e.g. the human eye). They are responsible for color vision
and function best in relatively bright light, as opposed to rod cells,
which work better in dim light. Cone cells are densely packed in the fovea
centralis, a 0.3 mm diameter rod-free area with very thin, densely packed
cones which quickly reduce in number towards the periphery of the retina.
There are about six to seven million cones in a human eye and are most
concentrated towards the macula. The commonly cited figure of six million
cone cells in the human eye was found by Osterberg in 1935. Oyster's
textbook (1999) cites work by Curcio et al. (1990) indicating an average
close to 4.5 million cone cells and 90 million rod cells in the human
retina. Cones are less sensitive to light than the rod cells in the retina
(which support vision at low light levels), but allow the perception of
colour. They are also able to perceive finer detail and more rapid changes
in images, because their response times to stimuli are faster than those
of rods. Cones are normally one of the three types, each with different
pigment, namely: S-cones, M-cones and L-cones. Each cone is therefore
sensitive to visible wavelengths of light that correspond to
short-wavelength, medium-wavelength and long-wavelength light. Because
humans usually have three kinds of cones with different photopsins, which
have different response curves and thus respond to variation in colour in
different ways, we have trichromatic vision. Being colour blind can change
this, and there have been some verified reports of people with four or
more types of cones, giving them tetrachromatic vision. The three pigments
responsible for detecting light have been shown to vary in their exact
chemical composition due to genetic mutation; different individuals will
have cones with different color sensitivity. Destruction of the cone cells
from disease would result in color blindness.
Wills Eye Hospital
A. Academy of Ophthalmology
Review of Ophthalmology
National Institutes of Health
is the dilation of the pupil, usually
defined as when having a non-physiological cause, but sometimes defined as
potentially being a physiological pupillary response. Non-physiological
causes of mydriasis include disease, trauma, or the use of drugs.
Normally, as part of the pupillary light reflex, the pupil dilates in the
dark and constricts in the light to respectively improve vividity at night
and to protect the retina from sunlight damage during the day. A mydriatic
pupil will remain excessively large even in a bright environment. The
excitation of the radial fibres of the iris which increases the pupillary
aperture is referred to as a mydriasis. More generally, mydriasis also
refers to the natural dilation of pupils, for instance in low light
conditions or under sympathetic stimulation.
is a physiological response that varies the size of the
pupil, via the optic and oculomotor cranial nerve. A constriction response
(miosis), is the narrowing the pupil, or which may be caused by scleral
buckles or drugs such as opiates/opioids or anti hypertension medications.
response (mydriasis), is the
widening the pupil and may be caused by anticholinergic agents or drugs
such as MDMA, cocaine and amphetamines. Dilation of the pupil occurs when
the smooth cells of the radial muscle, controlled by the sympathetic
nervous system (SNS), contract. Constriction of the pupil occurs when the
circular muscle, controlled by the parasympathetic nervous system (PSNS),
Animal Eyes Pupil Shapes
is vision in which
creatures having two eyes use them together.
is a Retinal prostheses for restoration of sight to
patients blinded by retinal degeneration meant to partially restore useful
vision to people who have lost their photoreceptors due to retinal
diseases such as retinitis pigmentosa.
Oxford Student Creates first Synthetic Soft Tissue Retina
is a experimental bionic eye visual device intended
to restore functional vision in those suffering from
partial or total
Argus Retinal Prosthesis
is an electronic retinal implant.
is sold under the trade name Avastin, is an
angiogenesis inhibitor, a drug that slows the growth of new
Diplopia is a Computer Game
that has been designed using the
latest research to help people with
(lazy eye) and
strabismus (crossed eyes).
Brain training video games help low-vision kids see better
Computer Vision Syndrome
Protecting your Vision
Why do we
Color-Binoculars App helps color blind individuals distinguish colors in
their everyday lives
vision science and optical technology. The only specialty
eyewear that alleviates red-green color blindness, enhancing colors
without the compromise of color accuracy.
Association for Research in
Vision and Ophthalmology
branch of medicine concerned with the eye and
is a miniature smart
camera that can attach to any pair of glasses. The device takes a photo of words
and reads them to the blind.
Brain-sensing technology developed by Stanford scientists allows typing at
rate of 12 words per minute
, could provide a way for people with
movement disorders to communicate.
VoiceOver Gesture Support, Text to Speech, Braille Display
Vision Correcting Light Field Display
Vision-impaired & Low Vision Tool
Daniel Kish: How I use Sonar to Navigate the World
is a technique that uses sound propagation to navigate, communicate with
or detect objects.
device is an electronic travel aid which fits on
the top fold of the white cane.
BuzzClip, wearable ultrasound for visually impaired people
How new technology helps blind people explore the world
(video and text)
Narrator Text–to–Speech Utility
- Speech Recognition
with Sound Augmented Reality
Eye Music: Hearing Colored Shapes
is the idea that vocal sounds or
phonemes carry meaning in and of themselves.
Sounds Create Shapes
The Read 'n Style Pen:
Turn any text to audio. The world's first
digital reading tool created by and for
Dizzy Head Spins
patterns of raised dots on a flat surface that represent
and numerals. A point
system of tactile writing
designed for the
sense of touch
SyncBraille is one of the smallest, lightest, and most
affordable, portable Braille Displays
Refreshable Braille Display
is an electro-mechanical device
for displaying braille characters, usually by means of round-tipped pins
raised through holes in a flat surface.
JAWS Screen Reader
is a computer screen reader program for
Microsoft Windows that allows blind and visually impaired users to read
the screen either with a text-to-speech output or by a refreshable Braille
Periscope Braille Tablet
The Elia Idea
reading system. An estimated 8.4 million individuals with visual
impairment in the US, it is estimated that
less than 60,000 can read
Braille. High schoolers with visual impairment have a dropout rate of
nearly 50%. The employment rate among individuals with sight is
approximately 84%. Individuals with visual impairment have an estimated
employment rate of 43%; but if the individual can use Braille, that rate
soars to 90%. It's estimated that the number of individuals with visual
impairment will double to 16 million by 2030.
is the ability of humans to detect
objects in their environment by sensing echoes
from those objects, by
actively creating sounds – for example, by tapping their canes, lightly
stomping their foot, snapping their fingers, or making clicking noises
with their mouths – people trained to orient by echolocation can interpret
the sound waves
reflected by nearby
objects, accurately identifying their location and size. This ability is
used by some blind people for acoustic wayfinding, or navigating within
their environment using auditory rather than visual cues. It is similar in
principle to active sonar and to animal echolocation, which is employed by
bats, dolphins and toothed whales to find prey.
Daniel Kish's echolocation in action
refers to the ways in which people and
other animals communicate
and interact via the
sense of touch
. As well as
about surfaces and textures, touch, or the haptic
sense, is a component of communication in
is nonverbal and nonvisual. Touch is extremely important for humans and is
vital in conveying physical intimacy
. Haptics is a branch of
Talking Barcode Scanners
Sheila Nirenberg: A Prosthetic Eye to treat Blindness
Can Blind People Draw?
Ron Mccallum: How Technology allowed me to Read
The Blind Woman Who Saw Rain
Suppression of an eye
is a subconscious adaptation by a person's brain
to eliminate the symptoms of disorders of
. The brain can eliminate double vision by ignoring all or
part of the image of one of the eyes. The area of a person's visual field
that is suppressed is called the suppression scotoma (with a
meaning, more generally, an area of partial alteration in the visual
field). Suppression can lead to
Aphantasia: How It Feels To Be Blind In Your Mind
voice activated search and information.
Be My Eyes
Smartphone App that allows a blind person to use friends and
family to help identify things using video.
How Do A Blind Person & A Deaf Person Communicate? (with Rikki Poynter)
Lechal Haptic Footwear
Injecting a benign virus that carries good copies of a malformed gene in the
Visual Cortex Activity in Early and Late Blind People
Leber Congenital Amaurosis
is an eye disorder that primarily affects
the retina, which is the specialized tissue at the back of the eye that
detects light and color. People with this disorder typically have severe
visual impairment beginning in infancy. The visual impairment tends to be
stable, although it may worsen very slowly over time.
Website Tools for
refers to the accessibility of a computer
system to all people, regardless of disability type or severity of
impairment. The term "accessibility" is most often used in reference to
specialized hardware or software, or a combination of both, designed to
enable use of a computer by a person with a
. Specific technologies may be referred to as assistive
promotes greater independence by enabling people
to perform tasks that they were formerly unable to accomplish, or had
great difficulty accomplishing, by providing enhancements to, or changing
methods of interacting with, the technology needed to accomplish such
tasks. Due to assistive technology, people with disabilities have an
opportunity of a more positive and easygoing lifestyle, with an increase
in "social participation," "security and control," and a greater chance to
"reduce institutional costs without significantly increasing household
expenses. There are many disabilities or impairments that can be a barrier
to effective computer use. These impairments, which can be acquired from
disease, trauma, or may be congenital, include but are not limited to:
Cognitive impairments (head injury, autism, developmental disabilities),
and learning disabilities, such as dyslexia, dyscalculia or ADHD. Visual
impairment such as low-vision, complete or partial blindness, and color
blindness. Hearing-related disabilities including deafness, being hard of
hearing, or hyperacusis. Motor or dexterity impairment such as paralysis,
cerebral palsy, dyspraxia, carpal tunnel syndrome and repetitive strain
injury. Accessibility is often abbreviated as the numeronym a11y, where
the number 11 refers to the number of letters omitted. This parallels the
abbreviations of internationalization and localization as i18n and l10n
makes digital content more accessible, and more usable, for more people.
Wave Web Aim
tool that determines
the accessibility of web content for sight impaired.
- W3C Web
Accessibility Initiative (WAI).
is a form of assistive technology (AT) which is
essential to people who are blind, as well as useful to people who are
visually impaired, illiterate, or have a
Screen readers are software applications that attempt to convey what
people with normal eyesight see on a display to their users via non-visual
means, like text-to-speech, sound icons, or a Braille device. They do this
by applying a wide variety of techniques that include for example
interacting with dedicated accessibility APIs, using various operating
system features (like inter-process communication and querying user
interface properties) and employing hooking techniques.
refers to the inclusive practice of removing barriers
that prevent interaction
with, or access to websites, by people with disabilities. When sites are
correctly designed, developed and edited, all users have equal access to
information and functionality. For example, when a site is
coded with semantically meaningful HTML
with textual equivalents provided for images and with links named
meaningfully, this helps blind users using text-to-speech software and/or
text-to-Braille hardware. When text and images are large and/or
enlargeable, it is easier for users with poor sight to read and understand
the content. When links are underlined (or otherwise differentiated) as
well as colored, this ensures that color blind users will be able to
notice them. When clickable links and areas are large, this helps users
who cannot control a mouse with precision. When pages are coded so that
users can navigate by means of the keyboard alone, or a single switch
access device alone, this helps users who cannot use a mouse or even a
standard keyboard. When videos are closed captioned or a sign language
version is available, deaf and hard-of-hearing users can understand the
video. When flashing effects are avoided or made optional, users prone to
seizures caused by these effects are not put at risk. And when content is
written in plain language and illustrated with instructional diagrams and
animations, users with dyslexia and learning difficulties are better able
to understand the content. When sites are correctly built and maintained,
all of these users can be accommodated without decreasing the usability of
the site for non-disabled users. The needs that Web accessibility aims to
address include: Visual: Visual impairments including blindness, various
common types of low vision and poor eyesight, various types of color
blindness; Motor/mobility: e.g. difficulty or inability to use the hands,
including tremors, muscle slowness, loss of fine muscle control, etc., due
to conditions such as Parkinson's Disease, muscular dystrophy, cerebral
palsy, stroke; Auditory: Deafness or hearing
, including individuals who are hard of hearing; Seizures:
Photo epileptic seizures caused by visual strobe or flashing effects.
Cognitive/Intellectual: Developmental disabilities, learning disabilities
, dyscalculia, etc.), and
cognitive disabilities of various origins, affecting memory, attention,
developmental "maturity," problem-solving and logic skills, etc.
solutions for people with disabilities to help them read and learn.
People with Disabilities are Gifts from
a polygenic phenotypic character
determined by two distinct factors:
of the eye's iris and the frequency-dependence of the
scattering of light by the turbid medium in the stroma of the iris.
of the iris is a fibrovascular layer of tissue (vessels and
Fibers). It is the upper layer of two in the iris. The stroma is a
delicate interlacement of fibres. Some circle the circumference of the
iris and the majority radiate toward the pupil. Blood vessels and nerves
intersperse this mesh. In dark eyes, the stroma often contains
granules. Blue eyes and the
eyes of albinos, however, lack pigment. The stroma connects to a sphincter
muscle (sphincter pupillae), which contracts the pupil in a circular
motion, and a set of dilator muscles (dilator pupillae) which pull the
iris radially to enlarge the pupil, pulling it in folds. The back surface
is covered by a commonly, heavily pigmented epithelial layer that is two
cells thick (the iris pigment epithelium), but the front surface has no
epithelium. This anterior surface projects as the muscles dilate.
is a thin, circular structure in the eye, responsible for
controlling the diameter and size of the pupil and thus the amount of
light reaching the retina. Eye color is defined by that of the iris. In
optical terms, the pupil is the eye's aperture, while the iris is the
diaphragm that serves as the aperture stop.
Prosthetic Iris Devices
a hole located in the centre of the iris of the eye that allows light to
strike the retina. It appears black because light rays entering the pupil
are either absorbed by the tissues inside the eye directly, or absorbed
after diffuse reflections within the eye that mostly miss exiting the
are the most common eye color in the world with over 55% of the
world's population having brown eyes.
are around 5-8% of the world's population. Hazel eyes are
similar to brown eyes, although they are typically lighter in
color, and have more of a green-yellow tint. Hazel eyes have a
higher concentration of melanin (pigment) around the eye's
border, which can result in a multi-colored appearance that
varies between copper and green depending on the lighting.
are around 8% of the world's population.
are often confused with hazel eye color, yet is entirely
separate and distinct. Green eye color is the rarest color found
around the world, and it is estimated that only around 2% of the
world's population has green colored eyes. Green eye color is
a result of a mild amount of pigmentation in the eye with a
golden tint. When combined with the natural blue scattering of
the eye, the colors mix to give a green appearance. Green eye
color is most common in northern and central Europe, but can
also be found in western Asian cultures on rare occasion.
are also quite rare. Silver eyes are the result of a very low
amount of pigmentation in the eye, which reflects a gray-silver
appearance. Amber eyes show off a yellow-copper tone, which
results from the yellow colored pigment lipochrome.
are very rare worldwide, and are most common in Asia and South
American countries. Amber eye color can range from golden yellow
to a more copper tone.
History of Eye Color