View source for Human eye

{{Short description|Sensory organ of vision}}
{{About|the eyes of humans|eyes in general|Eye|other uses|Eye (disambiguation)}}
{{Anatomy terms}}
{{Infobox anatomy
| Name        = The Human Eye
| Latin       = Oculus 
| Greek       =
| Image2       = Eye-diagram no circles border.svg
| Caption2     = 1. [[vitreous body]] 2. [[ora serrata]] 3. [[ciliary muscle]] 4. [[Zonule of Zinn|ciliary zonules]] 5. [[Schlemm's canal]] 6. [[pupil]] 7. [[Anterior chamber of eyeball|anterior chamber]] 8. [[cornea]] 9. [[iris (anatomy)|iris]] 10. [[lens cortex]] 11. [[lens nucleus]] 12. [[Ciliary processes|ciliary process]] 13. [[conjunctiva]] 14. [[inferior oblique muscle]] 15. [[inferior rectus muscle]] 16. [[medial rectus muscle]] 17. [[retinal arteries and veins]] 18. [[optic disc]] 19. [[dura mater]] 20. [[central retinal artery]] 21. [[central retinal vein]] 22. [[optic nerve]] 23. [[Vorticose veins|vorticose vein]] 24. [[Tenon's capsule|bulbar sheath]] 25. [[Macula of retina|macula]] 26. [[Fovea centralis|fovea]] 27. [[sclera]] 28. [[choroid]] 29. [[superior rectus muscle]] 30. [[retina]]
| Width       =
| Image      = Human eye, anterior view.jpg
| Caption    = The eye of the right side of the face, showing its visible components - a white [[sclera]], a light brown [[Iris (anatomy)|iris]], and the black [[pupil]], in its [[orbit (anatomy)|orbit]] surrounded by the [[eyelid|lids]] and [[eyelashes|lashes]]
| Precursor   =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
| System      = [[Visual system]]
}}
The '''human eye''' is a [[sensory organ]] in the [[visual system]] that reacts to [[light|visible light]] allowing [[eyesight]]. Other functions include maintaining the [[circadian rhythm]], and [[Balance (ability)|keeping balance]].
[[File:Arizona eye model.png|thumb|253x253px|Arizona Eye Model. "A" is accommodation in diopters.]]
The eye can be considered as a living [[optics|optical device]]. It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye (the [[sclera]]) and one of its inner layers (the pigmented [[choroid]]) keeping the eye essentially [[stray light|light tight]] except on the eye's [[optic axis]]. In order, along the optic axis, the optical components consist of a first lens (the [[cornea|cornea—the clear part of the eye]]) that accounts for most of the optical power of the eye and accomplishes most of the [[Focus (optics)|focusing of light]] from the outside world; then an [[aperture]] (the [[pupil]]) in a [[Diaphragm (optics)|diaphragm]] (the [[Iris (anatomy)|iris—the coloured part of the eye]]) that controls the amount of light entering the interior of the eye; then another lens (the [[Lens (anatomy)|crystalline lens]]) that accomplishes the remaining focusing of light into [[real image|images]]; and finally a light-sensitive part of the eye (the [[retina]]), where the images fall and are processed. The retina makes a connection to the [[Human brain|brain]] via the [[optic nerve]]. The remaining components of the eye keep it in its required shape, nourish and maintain it, and protect it.

Three types of cells in the retina convert light energy into electrical energy used by the [[nervous system]]: [[Rod cell|rods]] respond to low intensity light and contribute to perception of low-resolution, black-and-white images; [[Cone cell|cones]] respond to high intensity light and contribute to perception of high-resolution, coloured images; and the recently discovered [[photosensitive ganglion cell]]s respond to a full range of light intensities and contribute to adjusting the amount of light reaching the retina, to regulating and suppressing the hormone [[melatonin]], and to [[Entrainment (chronobiology)|entraining]] [[circadian rhythm]].<ref>{{cite magazine |last= Zimmer |first= Carl |date=February 2012 |title=Our Strange, Important, Subconscious Light Detectors |url=https://www.discovermagazine.com/mind/the-brain-our-strange-important-subconscious-light-detectors |magazine=[[Discover Magazine]] |location= |publisher=[[LabX Media Group]] |url-status=  |archive-url=https://archive.today/20250426013749/https://www.discovermagazine.com/mind/the-brain-our-strange-important-subconscious-light-detectors|archive-date=26 April 2025|access-date=2012-05-05}}</ref>
== Structure ==
[[File:3D Medical Animation Eye Structure.jpg|alt=A detailed depiction of eye using a 3D medical illustration|thumb|260x260px|A detailed medical illustration of the eye]]
[[File:MRI of human eye.jpg|thumb|upright|[[Magnetic resonance imaging|MRI]] scan of the human eye]]
Humans have two eyes, situated on the left and the right of the [[face]]. The eyes sit in bony cavities called the [[orbit (anatomy)|orbits]], in the [[Skull#Humans|skull]]. There are six [[extraocular muscles]] that control eye movements. The front visible part of the eye is made up of the whitish [[sclera]], a coloured [[Iris (anatomy)|iris]], and the [[pupil (eye)|pupil]]. A thin layer called the [[conjunctiva]] sits on top of this. The front part is also called the [[Anatomical terms of location#Anterior and posterior|anterior]] segment of the eye.

The eye is not shaped like a perfect sphere; rather it is a fused two-piece unit, composed of an [[Anatomical terms of location#anterior|anterior (front) segment and the posterior (back)]] segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about {{cvt|11.5|mm}} in diameter, and 0.5&nbsp;mm (500&nbsp;μm) in thickness near its centre. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about {{cvt|24|mm}}. An area termed the limbus connects the cornea and sclera. The iris is the pigmented circular structure concentrically surrounding the centre of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' [[iris dilator muscle|dilator]] and [[iris sphincter muscle|sphincter muscles]].

Light energy enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle. Between the two lenses (the cornea and the crystalline lens), there are four [[optical surfaces]] which each [[Refraction|refract]] light as it travels along the optical path. One basic model describing the geometry of the optical system is the Arizona Eye Model.<ref>{{Cite book |last=Schwiegerling |first=Jim |title=Field guide to visual and ophthalmic optics |date=2004 |publisher=SPIE Press |isbn=978-0-8194-5629-8 |series=SPIE FG |location=Bellingham, Wash}}</ref> This model describes the accommodation of the eye geometrically. Photons of light falling on the light-sensitive cells of the retina ([[Photoreceptor cell|photoreceptor cones and rods]]) are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision.

=== Development ===
The human eye primarily develops from the [[ectoderm]]. The lens and the [[epithelium]] of the cornea arise from the surface ectoderm directly; other structures come from either the neural ectoderm or the [[neural crest]], which itself arises from the ectoderm. The [[mesoderm]] has limited contributions: it is the origin of the [[vitreous body]], the blood vessels of the eye, and the extraocular muscles.<ref>{{Cite book |last=Riordan-Eva |first=Paul |title=Vaughan & Asbury's General Ophthalmology |publisher=McGraw-Hill Education |year=2017 |edition=19th |chapter=Chapter 1: Anatomy & Embryology of the Eye}}</ref>

=== Size ===
The size of the eye differs among adults by only 1-2 mm. The eyeball is generally less tall than it is wide. The sagittal vertical (height) of a human adult eye is approximately {{cvt|23.7|mm}}, the transverse horizontal diameter (width) is {{cvt|24.2|mm}} and the axial anteroposterior size (depth) averages {{cvt|22.0–24.8|mm}} with no significant difference between sexes and age groups.<ref name="pubfacts.com">{{cite web|url=http://www.pubfacts.com/detail/25431659/Variations-in-eyeball-diameters-of-the-healthy-adults |title=Variations in eyeball diameters of the healthy adults}}</ref> Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88).<ref name="pubfacts.com"/> The typical adult eye has an anterior to posterior diameter of {{cvt|24|mm}}, and a volume of {{convert|6|cm3}}.<ref name=GeneralOphthal>{{cite book|last1=Cunningham|first1=Emmett T.|last2=Riordan-Eva|first2=Paul|title=Vaughan & Asbury's General Ophthalmology|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-163420-5|url=http://accessmedicine.com/resourceTOC.aspx?resourceID=720|edition=18th|date=2011-05-17}}</ref>

The eyeball grows rapidly, increasing from about {{cvt|16–17|mm}} diameter at birth to {{cvt|22.5–23|mm}} by three years of age. By age 12, the eye attains its full size.

=== Components ===
[[File:Schematic diagram of the human eye en.svg|thumb|250px|Schematic diagram of the human eye. It shows a horizontal section through the right eye.]]
The eye is made up of three coats, or layers, enclosing various anatomical structures. The outermost layer, known as the [[Fibrous tunic of eyeball|fibrous tunic]], is composed of the [[cornea]] and [[sclera]], which provide shape to the eye and support the deeper structures. The middle layer, known as the [[Uvea|vascular tunic or uvea]], consists of the [[choroid]], [[ciliary body]], pigmented epithelium and [[Iris (anatomy)|iris]]. The innermost is the [[retina]], which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly).

The spaces of the eye are filled with the [[aqueous humour]] anteriorly, between the cornea and lens, and the [[vitreous body]], a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid that is contained in two areas:  the [[Anterior chamber of eyeball|anterior chamber]] between the cornea and the iris, and the [[Posterior chamber of eyeball|posterior chamber]] between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament ([[zonule of Zinn]]), made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation (focusing). The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition.<ref>"eye, human."Encyclopædia Britannica from [[Encyclopædia Britannica Ultimate Reference Suite]] 2009</ref>

[[File:Gray892.png|thumb|The outer parts of the eye|208x208px]]

=== Extraocular muscles ===
{{Main|Extraocular muscles}}
Each eye has seven [[extraocular muscles]] located in its [[orbit (anatomy)|orbit]].<ref name="Haladaj">{{cite journal |last1=Haładaj |first1=R |title=Normal Anatomy and Anomalies of the Rectus Extraocular Muscles in Human: A Review of the Recent Data and Findings. |journal=BioMed Research International |date=2019 |volume=2019 |pages=8909162 |doi=10.1155/2019/8909162 |pmid=31976329|pmc=6954479 |doi-access=free }}</ref> Six of these muscles control the [[eye movements]], the seventh controls the movement of the upper [[eyelid]]. The six muscles are four recti muscles – the [[Lateral rectus muscle|lateral rectus]], the [[Medial rectus muscle|medial rectus]], the [[Inferior rectus muscle|inferior rectus]], and the [[Superior rectus muscle|superior rectus]], and two oblique muscles the [[Inferior oblique muscle|inferior oblique]], and the [[Superior oblique muscle|superior oblique]]. The seventh muscle is the [[levator palpebrae superioris muscle]]. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation.<ref>Carpenter, Roger H.S. (1988). ''Movements of the eyes (2nd ed.)''. London: Pion, Ltd {{ISBN|0-85086-109-8}}.</ref> Thus, the eye can be considered as undergoing rotations about a single point in the centre of the eye.

<gallery>
File:Eye orbit anterior.jpg|Eye and orbit anatomy with motor nerves
File:Lateral orbit nerves.jpg|Image showing orbita with eye and nerves visible (periocular fat removed)
File:Lateral orbit anatomy 2.jpg|Image showing orbita with eye and periocular fat
File:Eye orbit anatomy anterior2.jpg|Normal [[anatomy]] of the human eye and orbit, anterior view
</gallery>

== Vision ==
{{See also|Visual acuity|Eye#Visual acuity|Fovea centralis#Angular size of foveal cones|Color vision#Physiology of color perception}}

=== Field of view ===
[[File:Mairead cropped.png|thumb|Side-view of the human eye, viewed approximately 90° temporal, illustrating how the iris and pupil appear rotated towards the viewer due to the optical properties of the cornea and the aqueous humour]]
The approximate [[field of view]] of an individual human eye (measured from the fixation point, i.e., the point at which one's gaze is directed) varies by facial anatomy, but is typically 30° superior (up, limited by the brow), 45° nasal (limited by the nose), 70° inferior (down), and 100° temporal (towards the temple).<ref name="SavinoDanesh-Meyer2012">{{cite book|last1=Savino|first1=Peter J.|last2=Danesh-Meyer|first2=Helen V.|title=Colour Atlas and Synopsis of Clinical Ophthalmology – Wills Eye Institute – Neuro-Ophthalmology|url=https://books.google.com/books?id=6RgSZGWQZGIC&pg=PA12|date=2012|publisher=Lippincott Williams & Wilkins|isbn=978-1-60913-266-8|page=12}}</ref><ref name="RyanSchachat2012">{{cite book|last1=Ryan|first1=Stephen J.|last2=Schachat|first2=Andrew P.|last3=Wilkinson|first3=Charles P. |author4=David R. Hinton |author5=SriniVas R. Sadda |author6-link=Peter Wiedemann|author6=Peter Wiedemann|title=Retina|url=https://books.google.com/books?id=PdAsuzFRv5oC&pg=PT342|date=2012|publisher=Elsevier Health Sciences|isbn=978-1-4557-3780-2|page=342}}</ref><ref name="TrattlerKaiser2012">{{cite book|last1=Trattler|first1=William B.|last2=Kaiser|first2=Peter K.|last3=Friedman|first3=Neil J.|title=Review of Ophthalmology: Expert Consult – Online and Print|url=https://books.google.com/books?id=AazA_9TQnHYC&pg=PA255|date= 2012|publisher=Elsevier Health Sciences|isbn=978-1-4557-3773-4|page=255}}</ref> For both eyes, combined ([[binocular vision]]) visual field is approximately 100° vertical and a maximum 190° horizontal, approximately 120° of which makes up the binocular field of view (seen by both eyes) flanked by two uniocular fields (seen by only one eye) of approximately 40 degrees.<ref name="Dagnelie2011">{{cite book|last=Dagnelie|first=Gislin|title=Visual Prosthetics: Physiology, Bioengineering, Rehabilitation|url=https://archive.org/details/Gislin_Dagnelie_Visual_Prosthetics|date=2011|publisher=Springer Science & Business Media|isbn=978-1-4419-0754-7|page=[https://archive.org/details/Gislin_Dagnelie_Visual_Prosthetics/page/n401 398]}}</ref><ref name="Dohse2007">{{cite book|last=Dohse|first=K.C.|title=Effects of Field of View and Stereo Graphics on Memory in Immersive Command and Control|url=https://books.google.com/books?id=p2pSW2D4s7gC&pg=PA6|year=2007|isbn=978-0-549-33503-0|page=6}}{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> It is an area of 4.17 [[steradian]]s or 13700 [[square degree]]s for binocular vision.<ref name="Deering">{{cite book|last1=Deering|first1=Michael F.|title=The Limits of Human Vision|url=http://michaelfrankdeering.org/Projects/EyeModel/limits.pdf|date=1998}}</ref> When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle.<ref name="SpringStiles1948">{{cite journal|last1=Spring|first1=K. H.|last2=Stiles|first2=W. S.|title=Apparent shape and size of the pupil viewed obliquely|journal=British Journal of Ophthalmology|volume=32|issue=6|year=1948|pages=347–354|doi=10.1136/bjo.32.6.347|pmc=510837|pmid=18170457}}</ref><ref name="FedtkeManns2010">{{cite journal|last1=Fedtke|first1=Cathleen|last2=Manns|first2=Fabrice|last3=Ho|first3=Arthur|title=The entrance pupil of the human eye: a three-dimensional model as a function of viewing angle|journal=Optics Express|volume=18|issue=21|year=2010|pages=22364–22376|doi=10.1364/OE.18.022364|pmc=3408927|pmid=20941137|bibcode=2010OExpr..1822364F}}</ref><ref name="MathurGehrmann2013">{{cite journal|last1=Mathur|first1=A.|last2=Gehrmann|first2=J.|last3=Atchison|first3=D. A.|title=Pupil shape as viewed along the horizontal visual field|journal=Journal of Vision|volume=13|issue=6|year=2013|page=3|doi=10.1167/13.6.3|pmid=23648308|doi-access=free}}</ref>

About 15° temporal and 1.5° below the horizontal is the [[Blind spot (vision)|blind spot]] created by the optic nerve nasally, which is roughly 7.5° high and 5.5° wide.<ref>[http://www.everyspec.com/MIL-STD/MIL-STD-1400-1499/MIL-STD-1472F_208/ MIL-STD-1472F, Military Standard, Human Engineering, Design Criteria For Military Systems, Equipment, And Facilities]. everyspec.com (1999)</ref>

=== Dynamic range ===
The retina has a static [[contrast ratio]] of around 100:1 (about 6.5 [[F-number#Stops, f-stop conventions, and exposure|f-stops]]). As soon as the eye moves rapidly to acquire a target ([[saccade]]s), it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal rod photoreceptors is 80% complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again.[[File:Pupillary light reflex.jpg|thumb|300x300px|The pupil of the human eye can range in size from 2 mm to over 8 mm to [[Adaptation (eye)|adapt]] to the environment]]The human eye can detect a luminance from 10<sup>−6</sup> cd/m<sup>2</sup>, or one millionth (0.000001) of a [[Candela per square metre|candela per square meter]] to 10<sup>8</sup> cd/m<sup>2</sup> or one hundred million (100,000,000) candelas per square meter.<ref name="IvergardHunt2008">{{cite book |last1=Ivergard |first1=Toni |url=https://books.google.com/books?id=DR9UyqLkgH8C&pg=PT108 |title=Handbook of Control Room Design and Ergonomics: A Perspective for the Future, Second Edition |last2=Hunt |first2=Brian |date=2008 |publisher=CRC Press |isbn=978-1-4200-6434-6 |page=90}}</ref><ref name="KaschkeDonnerhacke2013">{{Cite journal |last1=Kaschke |first1=Michael |url=https://books.google.com/books?id=DPw8AgAAQBAJ&pg=PA26 |title=Optical Devices in Ophthalmology and Optometry: Technology, Design Principles and Clinical Applications |last2=Donnerhacke |first2=Karl-Heinz |last3=Rill |first3=Michael Stefan |date=2013 |journal=Journal of Biomedical Optics |isbn=978-3-527-64899-3 |volume=19 |page=26 |bibcode=2014JBO....19g9901M |doi=10.1117/1.JBO.19.7.079901 |issue=7 |s2cid=117946411|doi-access=free }}</ref><ref name="BanterleArtusi2011">{{cite book |last1=Banterle |first1=Francesco |url=https://books.google.com/books?id=LL5orppYlJsC&pg=PA7 |title=Advanced High Dynamic Range Imaging: Theory and Practice |last2=Artusi |first2=Alessandro |last3=Debattista |first3=Kurt |author4=Alan Chalmers |date=2011 |publisher=CRC Press |isbn=978-1-56881-719-4 |page=7}}</ref>  (that is it has a range of 10<sup>14</sup>, or one hundred trillion 100,000,000,000,000, about 46.5 f-stops). This range does not include looking at the midday sun (10<sup>9</sup> cd/m<sup>2</sup>)<ref name="PodeDiouf2011">{{cite book|last1=Pode|first1=Ramchandra|last2=Diouf|first2=Boucar|title=Solar Lighting|url=https://books.google.com/books?id=sqKUqfvDq_MC&pg=PA62|date=2011|publisher=Springer Science & Business Media|isbn=978-1-4471-2134-3|page=62}}</ref> or lightning discharge.

At the low end of the range is the [[absolute threshold]] of vision for a steady light across a wide field of view, about 10<sup>−6</sup> cd/m<sup>2</sup> (0.000001 candela per square meter).<ref name="Davson2012">{{cite book|last=Davson|first=Hugh|author-link=Hugh Davson|title=The Physiology of The Eye|url=https://books.google.com/books?id=Q216982BQboC&pg=PA213|date=2012|publisher=Elsevier|isbn=978-0-323-14394-3|page=213}}</ref><ref name="name">{{Citation |last1=Denton|first1=E. J.|author-link = Eric James Denton|last2=Pirenne|first2=Maurice Henri|publication-date=Mar 29, 1954|journal=The Journal of Physiology|volume=123|issue=3|pages=417–442|pmc=1366217|title=The absolute sensitivity and functional stability of the human eye|doi=10.1113/jphysiol.1954.sp005062|pmid=13152690|year=1954}}</ref> The upper end of the range is given in terms of normal visual performance as 10<sup>8</sup> cd/m<sup>2</sup> (100,000,000 or one hundred million candelas per square meter).<ref name="NarisadaSchreuder2004">{{Cite book|last1=Narisada|first1=Kohei|last2=Schreuder|first2=Duco|series=Astrophysics and Space Science Library |title=Light Pollution Handbook|volume=322|url=https://books.google.com/books?id=61B_RV3EdIcC&pg=PA8|date=2004|isbn=978-1-4020-2665-2|page=8|bibcode=2004ASSL..322.....N|doi=10.1007/978-1-4020-2666-9}}</ref>
[[File:Voluntary pupil dilation.gif|thumb|Dilation and constriction of the pupil]]
The eye includes a [[Lens (anatomy)|lens]] similar to [[Lens (optics)|lenses]] found in optical instruments such as cameras and the same physics principles can be applied. The [[pupil]] of the human eye is its [[aperture]]; the iris is the diaphragm that serves as the aperture stop. Refraction in the [[cornea]] causes the effective aperture (the [[entrance pupil]]) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4&nbsp;mm in diameter, although it can range from 2&nbsp;mm ({{f/|8.3|}}) in a brightly lit place to 8&nbsp;mm ({{f/|2.1}}) in the dark. The latter value decreases slowly with age; older people's eyes sometimes dilate to not more than 5–6mm in the dark, and may be as small as 1mm in the light.<ref name="Timiras2007">{{cite book|last=Timiras|first=Paola S.|title=Physiological Basis of Aging and Geriatrics, Fourth Edition|url=https://books.google.com/books?id=zLm7sO1sZ6sC&pg=PA113|date=2007|publisher=CRC Press|isbn=978-1-4200-0709-1|page=113}}</ref><ref name="McGee2012">{{cite book|last=McGee|first=Steven R.|title=Evidence-based Physical Diagnosis|url=https://books.google.com/books?id=Xp8eZptLwX8C&pg=PA161|year=2012|publisher=Elsevier Health Sciences|isbn=978-1-4377-2207-9|page=161}}</ref>

== Movement ==
{{Main|Eye movement}}
[[File:Fundus photograph of normal right eye.jpg|thumb|The light circle is the [[optic disc]] where the optic nerve exits the retina]]

The visual system in the human brain is too slow to process information if images are slipping across the retina at more than a few degrees per second.<ref>{{cite journal | last1 = Westheimer | first1 = Gerald | last2 = McKee | first2 = Suzanne P | year = 1975 | title = Visual acuity in the presence of retinal-image motion | journal = [[Journal of the Optical Society of America]] | volume = 65 | issue = 7| pages = 847–850 | doi=10.1364/josa.65.000847 | pmid=1142031| bibcode = 1975JOSA...65..847W }}</ref> Thus, to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Frontal-eyed animals have a small area of the retina with very high visual acuity, the [[fovea centralis]]. It covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Any failure to make eye movements correctly can lead to serious visual degradation.

Having two eyes allows the brain to determine the depth and distance of an object, called stereovision, and gives the sense of three-dimensionality to the vision. Both eyes must point accurately enough that the object of regard falls on corresponding points of the two retinas to stimulate stereovision; otherwise, double vision might occur. Some persons with congenitally crossed eyes tend to ignore one eye's vision, thus do not suffer double vision, and do not have stereovision. The movements of the eye are controlled by six muscles attached to each eye, and allow the eye to elevate, depress, converge, diverge and roll. These muscles are both controlled voluntarily and involuntarily to track objects and correct for simultaneous head movements.

=== Rapid ===
{{Main|Rapid eye movement sleep}}
Rapid eye movement, REM, typically refers to the [[sleep]] stage during which the most vivid dreams occur. During this stage, the eyes move rapidly.

=== Saccadian ===
{{Main|Saccade}}
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain.

=== Fixational ===
{{Main|Fixation (visual)}}
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output.

Eye movements include drift, [[ocular tremor]], and microsaccades. Some irregular drifts, movements smaller than a saccade and larger than a microsaccade, subtend up to one tenth of a degree. Researchers vary in their definition of [[microsaccade]]s by amplitude. Martin Rolfs<ref>{{cite journal |doi=10.1016/j.visres.2009.08.010 |pmid=19683016 |title=Microsaccades: Small steps on a long way |year=2009 |last1=Rolfs |first1=Martin |journal=[[Vision Research]] |volume=49 |issue=20 |pages=2415–2441|doi-access=free }}</ref> states that 'the majority of microsaccades observed in a variety of tasks have amplitudes smaller than 30 min-arc'. However, others state that the "current consensus has largely consolidated around a definition of microsaccades that includes magnitudes up to 1°."<ref>{{cite book |last1=Alexander |first1=R. G. |last2=Martinez-Conde |first2=S |title=Eye Movement Research |date=2019 |publisher=Springer, Cham |chapter=Fixational eye movements |pages=78}}</ref>

=== Vestibulo-ocular ===
{{Main|Vestibulo-ocular reflex}}
The [[vestibulo-ocular reflex]] is a [[reflex]] eye movement that stabilizes images on the [[retina]] during head movement by producing an eye movement in the direction opposite to head movement in response to neural input from the vestibular system of the inner ear, thus maintaining the image in the centre of the visual field. For example, when the head moves to the right, the eyes move to the left.  This applies for head movements up and down, left and right, and tilt to the right and left, all of which give input to the ocular muscles to maintain visual stability.

=== Smooth pursuit ===
{{Main|Pursuit movement}}
Eyes can also follow a moving object around. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply [[feedback]]. Following an object moving at constant speed is relatively easy, though the eyes will often make saccades to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.

=== Optokinetic ===
{{Main |Optokinetic response}}

The optokinetic reflex (or optokinetic nystagmus) stabilizes the image on the retina through visual feedback. It is induced when the entire visual scene drifts across the retina, eliciting eye rotation in the same direction and at a velocity that minimizes the motion of the image on the retina. When the gaze direction deviates too far from the forward heading, a compensatory saccade is induced to reset the gaze to the centre of the visual field.<ref>{{cite journal|doi= 10.1371/journal.pone.0002055 |pmid=18446207|pmc=2323102|year=2008|last1=Cahill|first1=H|title=The Optokinetic Reflex as a Tool for Quantitative Analyses of Nervous System Function in Mice: Application to Genetic and Drug-Induced Variation|journal=PLOS ONE|volume=3|issue=4|page=e2055|last2=Nathans|first2=J|bibcode=2008PLoSO...3.2055C|doi-access=free}}</ref>

For example, when looking out of the window at a moving train, the eyes can focus on a moving train for a short moment (by stabilizing it on the retina), until the train moves out of the field of vision. At this point, the eye is moved back to the point where it first saw the train (through a saccade).

== Near response ==
The adjustment to close-range vision involves three processes to focus an image on the retina.

=== Vergence movement ===
{{Main|Vergence}}
[[File:Stereogram Tut Eye Convergence.png|thumb|The two eyes converge to point to the same object.]]
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at a nearby object, the eyes rotate 'towards each other' ([[convergence (eye)|convergence]]), while for an object farther away they rotate 'away from each other' ([[divergence (eye)|divergence]]).

=== Pupil constriction ===
Lenses cannot refract light rays at their edges as well as closer to the centre. The image produced by any lens is therefore somewhat blurry around the edges ([[spherical aberration]]). It can be minimized by screening out peripheral light rays and looking only at the better-focused centre. In the eye, the pupil serves this purpose by constricting while the eye is focused on nearby objects. Small apertures also give an increase in [[depth of field]], allowing a broader range of "in focus" vision. In this way the pupil has a dual purpose for near vision: to reduce spherical aberration and increase depth of field.<ref name=Saladin_A&Punity>{{cite book|last=Saladin|first=Kenneth S.|title=Anatomy & physiology : the unity of form and function|publisher=McGraw-Hill|location=New York|isbn=978-0-07-337825-1|pages=620–622|edition=6th|date=2011}}</ref>

=== Lens accommodation ===
{{Main|Lens (vertebrate anatomy)}}
Changing the curvature of the lens is carried out by the [[ciliary muscle]]s surrounding the lens; this process is known as "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and also allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus.<ref name=Saladin_A&Punity /><ref>{{cite web|title=Human eye|url=https://www.britannica.com/EBchecked/topic/1688997/human-eye|publisher=Encyclopædia Britannica}}</ref>

== Medicine ==
The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a [[general practitioner]]. These specialists, or [[eye care professional]]s, serve different functions in different countries. Eye care professionals can have overlap in their patient care privileges.  For example, both an [[ophthalmologist]] (M.D.) and [[optometrist]] (O.D.) are professionals who diagnose eye disease and can prescribe lenses to correct vision. Typically, only ophthalmologists are licensed to perform surgical procedures. Ophthalmologists may also specialize within a surgical area, such as [[Corneal transplantation|cornea]], [[Cataract surgery|cataracts]], [[Refractive surgery|laser]], [[Retinal detachment|retina]], or [[oculoplastics]].

Eye care professionals include:
* [[Ocularist]]s
* [[Ophthalmology|Ophthalmologists]]
* [[Optometry|Optometrists]]
* [[Opticianry|Opticians]]
* [[Orthoptics|Orthoptists]] and [[Vision therapy|vision therapists]]

== Pigmentation ==
{{main|Eye colour}}
===Brown===
[[File:MyStrangeIris.JPG|thumb|upright|A brown iris]]
Almost all mammals have brown or darkly-pigmented irises.<ref>Brenda J Bradley, Anja Pedersen, Nicholas I Mundy: [https://pubmed.ncbi.nlm.nih.gov/19278018/ Blue eyes in lemurs and humans: same phenotype, different genetic mechanism]  Am J Phys Anthropol. 2009</ref> In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it.<ref name=":0">{{Cite web|last=Debrowski|first=Adam|title=Which Eye Colors Are the Rarest?|url=https://www.allaboutvision.com/eye-anatomy/rarest-eye-color/|access-date=2021-02-04|website=All About Vision|language=en-us}}</ref> Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.<ref name="Fox">{{cite book |last=Fox |first=Denis Llewellyn |title=Biochromy: Natural Coloration of Living Things |publisher=University of California Press |year=1979 |page=9 |url=https://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9|isbn=978-0-520-03699-4}}</ref>

[[File:Human_eye,_anterior_view.jpg|right|thumb|200px| A light brown iris with limbal ring]]

In many parts of the world, it is nearly the only iris color present.<ref>{{OMIM|227220|SKIN/HAIR/EYE PIGMENTATION, VARIATION IN, 1; SHEP1}}</ref> Brown eyes are common in [[Europe]], [[East Asia]], [[Southeast Asia]], [[Central Asia]], [[South Asia]], [[West Asia]], [[Oceania]], [[Africa]] and the [[Americas]].<ref name="Sulem">{{cite journal |title=Genetic determinants of hair, eye and skin pigmentation in Europeans |journal=Nat. Genet. |volume=39 |issue=12 |pages=1443–52 |year=2007 |pmid=17952075 |doi=10.1038/ng.2007.13 |last1=Sulem |first1=Patrick |last2=Gudbjartsson |first2=Daniel F |last3=Stacey |first3=Simon N |last4=Helgason |first4=Agnar |last5=Rafnar |first5=Thorunn |last6=Magnusson |first6=Kristinn P |last7=Manolescu |first7=Andrei |last8=Karason |first8=Ari |last9=Palsson |first9=Arnar |last10=Thorleifsson |first10=Gudmar |last11=Jakobsdottir |first11=Margret |last12=Steinberg |first12=Stacy |last13=Pálsson |first13=Snæbjörn |last14=Jonasson |first14=Fridbert |last15=Sigurgeirsson |first15=Bardur |last16=Thorisdottir |first16=Kristin |last17=Ragnarsson |first17=Rafn |last18=Benediktsdottir |first18=Kristrun R |last19=Aben |first19=Katja K |last20=Kiemeney |first20=Lambertus A |last21=Olafsson |first21=Jon H |last22=Gulcher |first22=Jeffrey |last23=Kong |first23=Augie |last24=Thorsteinsdottir |first24=Unnur |last25=Stefansson |first25=Kari |s2cid=19313549 |display-authors=8 }}</ref> Light or medium-pigmented brown eyes can also be commonly found in [[Europe]], among the [[Americas]], and parts of Central [[Asia]], [[West Asia]] and [[South Asia]]. Light brown eyes bordering amber and hazel coloration are common in Europe, but can also be observed in East Asia and Southeast Asia, though are uncommon in the region.

===Amber===
[[File:Amber-eyes.jpg|thumb|Amber eye]]

Amber eyes are a solid color with a strong yellowish/golden and russet/coppery tint, which may be due to the yellow pigment called [[lipochrome]]  (also found in green eyes).<ref>[http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Howard Hughes Medical Institute: Ask A Scientist] {{webarchive|url=https://web.archive.org/web/20100901150309/http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html |date=1 September 2010 }}. Hhmi.org. Retrieved on 23 December 2011.</ref><ref>Larry Bickford [http://www.eyecarecontacts.com/eyecolor.html Eye Color] {{webarchive|url=https://web.archive.org/web/20101023191830/http://www.eyecarecontacts.com/eyecolor.html |date=23 October 2010 }}. Eyecarecontacts.com. Retrieved on 23 December 2011.</ref> Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is similar to gold, some people have russet or copper colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray. People with that eye color are common in northern [[Europe]], and in fewer numbers in southern Europe, the [[Middle East]], [[North Africa]], and [[South America]].<ref>{{Cite journal |last1=Ruiz-Linares |first1=Andrés |last2=Adhikari |first2=Kaustubh |last3=Acuña-Alonzo |first3=Victor |last4=Quinto-Sanchez |first4=Mirsha |last5=Jaramillo |first5=Claudia |last6=Arias |first6=William |last7=Fuentes |first7=Macarena |last8=Pizarro |first8=María |last9=Everardo |first9=Paola |last10=de Avila |first10=Francisco |last11=Gómez-Valdés |first11=Jorge |last12=León-Mimila |first12=Paola |last13=Hunemeier |first13=Tábita |last14=Ramallo |first14=Virginia |last15=Silva de Cerqueira |first15=Caio C. |date=2014-09-25 |title=Admixture in Latin America: Geographic Structure, Phenotypic Diversity and Self-Perception of Ancestry Based on 7,342 Individuals |journal=PLOS Genetics |volume=10 |issue=9 |pages=e1004572 |doi=10.1371/journal.pgen.1004572 |issn=1553-7390 |pmc=4177621 |pmid=25254375 |doi-access=free |bibcode=2014PLOSG..10.4572R }}</ref>

===Hazel===
[[File:Hazel Eye HD.JPG|thumb|upright|right|Hazel eye]]

Hazel eyes are due to a combination of [[Rayleigh scattering]] and a moderate amount of melanin in the iris' anterior border layer.<ref name=Lefohn>{{cite journal |vauthors=Lefohn A, Budge B, Shirley P, Caruso R, Reinhard E |title=An Ocularist's Approach to Human Iris Synthesis |journal=IEEE Comput. Graph. Appl. |volume=23 |issue=6 |pages=70–5 |year=2003 |doi=10.1109/MCG.2003.1242384 |s2cid=537404 |url=https://escholarship.org/uc/item/8d88d3qc }}</ref> Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber and vice versa.<ref name=Zhu>{{cite journal |title=A genome scan for eye color in 502 twin families: most variation is due to a QTL on chromosome 15q |journal=Twin Res |volume=7 |issue=2 |pages=197–210 |year=2004 |pmid=15169604 |doi=10.1375/136905204323016186 |last1=Zhu |first1=Gu |last2=Evans |first2=David M. |last3=Duffy |first3=David L. |last4=Montgomery |first4=Grant W. |last5=Medland |first5=Sarah E.|author-link5=Sarah Medland |last6=Gillespie |first6=Nathan A. |last7=Ewen |first7=Kelly R. |last8=Jewell |first8=Mary |last9=Liew |first9=Yew Wah |first10=Nicholas K. |last10=Hayward |first11=Richard A. |last11=Sturma |first12=Jeffrey M. |last12=Trenta |first13=Nicholas G. |last13=Martina|doi-access=free }}</ref><ref>{{cite journal |last1=Albert |first1=Daniel M |last2=Green |first2=W Richard |last3=Zimbric |first3=Michele L |last4=Lo |first4=Cecilia |last5=Gangnon |first5=Ronald E |last6=Hope |first6=Kirsten L |last7=Gleiser |first7=Joel |title=Iris melanocyte numbers in Asian, African American, and Caucasian irides. |journal=Transactions of the American Ophthalmological Society |date=2003 |volume=101 |pages=217–222 |pmid=14971580 |pmc=1358991 }}</ref><ref name=Mitchell>{{cite journal |vauthors=Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P |title=Iris color and intraocular pressure: the Blue Mountains Eye Study |journal=Am. J. Ophthalmol. |volume=135 |issue=3 |pages=384–6 |year=2003 |pmid=12614760 |doi=10.1016/S0002-9394(02)01967-0 |url=https://ur.booksc.eu/dl/17418414/f57a1f |url-access=subscription }}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal |vauthors=Lindsey JD, Jones HL, Hewitt EG, Angert M, Weinreb RN |title=Induction of tyrosinase gene transcription in human iris organ cultures exposed to latanoprost |journal=Arch. Ophthalmol. |volume=119 |issue=6 |pages=853–60 |year=2001|pmid=11405836 |doi=10.1001/archopht.119.6.853|doi-access=free }}</ref><ref name=Frank>{{cite journal |vauthors=Frank RN, Puklin JE, Stock C, Canter LA |title=Race, iris color, and age-related macular degeneration |journal=Trans Am Ophthalmol Soc |volume=98 |pages=109–15; discussion 115–7 |year=2000 |pmid=11190014 |pmc=1298217 }}</ref><ref name=Regan>{{cite journal |vauthors=Regan S, Judge HE, Gragoudas ES, Egan KM |title=Iris color as a prognostic factor in ocular melanoma |journal=Arch. Ophthalmol. |volume=117 |issue=6 |pages=811–4 |year=1999 |pmid=10369595 |doi=10.1001/archopht.117.6.811|doi-access=free }}</ref><ref>{{cite journal |vauthors=Hawkins TA, Stewart WC, McMillan TA, Gwynn DR |title=Analysis of diode, argon, and Nd: YAG peripheral iridectomy in cadaver eyes |journal=Doc Ophthalmol |volume=87 |issue=4 |pages=367–76 |year=1994 |pmid=7851220 |doi=10.1007/BF01203345 |s2cid=30893783 }}</ref> The combination can sometimes produce a multicolored iris, i.e., an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color ''hazel'' vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a [[hazelnut]] shell.<ref name=Zhu /><ref name=Mitchell /><ref name=Regan /><ref name=Hammond>{{cite journal |vauthors=Hammond BR, Fuld K, Snodderly DM |title=Iris color and macular pigment optical density |journal=Exp. Eye Res. |volume=62 |issue=3 |pages=293–7 |year=1996 |pmid=8690039 |doi=10.1006/exer.1996.0035}}</ref>

Around 18% of the US population and 5% of the world population have hazel eyes. Hazel eyes are found in [[Europe]], most commonly in the [[Netherlands]] and the [[United Kingdom]],<ref>{{cite web | url=https://www.coalitionbrewing.com/where-are-hazel-eyes-most-common/ | title=Where are hazel eyes most common? | date=26 February 2023 }}</ref> and have also been observed to be very common among the [[Low Saxon]] speaking populations of northern Germany.<ref>{{cite book |last1=Beddoe |first1=John |title=The Races of Britain. A contribution to the anthropology of Western Europe |date=1971 |publisher=Michigan State University |isbn=9780091013707 |page=39 |url=https://books.google.com/books?id=7z06AQAAMAAJ |language=en |quote=the Saxons...being very often hazel-eyed}}</ref>

===Green===

Green eyes are most common in [[Northern Europe|Northern]], [[Western Europe|Western]] and [[Central Europe]].<ref>[http://www.eyedoctorguide.com/eye_general/eye_color.html Blue Eyes Versus Brown Eyes: A Primer on Eye Color] {{Webarchive|url=https://web.archive.org/web/20081017164118/http://www.eyedoctorguide.com/eye_general/eye_color.html |date=17 October 2008 }}. Eyedoctorguide.com. Retrieved on 23 December 2011.</ref><ref>[http://cogweb.ucla.edu/ep/Frost_06.html Why Do Europeans Have So Many Hair and Eye Colors?]. Cogweb.ucla.edu. Retrieved on 23 December 2011.</ref> Around 8–10% of men and 18–21% of women in [[Iceland]] and 6% of men and 17% of women in the [[Netherlands]] have green eyes.<ref name="sulem">{{cite journal |last1=Sulem|first1=P.|last2= Gudbjartsson|first2=D. |display-authors=et al.|title=Genetic determinants of hair, eye and skin pigmentation in Europeans |journal=Nature Genetics |year=2007 |volume=39 |issue=12 |pages=1443–1452 |doi=10.1038/ng.2007.13 |pmid=17952075 |s2cid=19313549 |url=https://www.researchgate.net/publication/5893814 |access-date=21 February 2022}}</ref> Among [[European Americans]], green eyes are most common among those of recent [[Celts (modern)|Celtic]] and [[Germanic peoples|Germanic]] ancestry, with about 16%.<ref name=sulem/>

The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin) with: 2) a blue shade created by the Rayleigh scattering of reflected light.<ref name="Fox"/> Green eyes contain the yellowish pigment [[lipochrome]].<ref>[http://allaboutgenes.weebly.com/oca2.html "OCA2: The Gene for Color"] {{Webarchive|url=https://web.archive.org/web/20161006151409/http://allaboutgenes.weebly.com/oca2.html |date=6 October 2016 }}. allaboutgenes.weebly.com. Retrieved on 8 September 2016.</ref>

<gallery widths="200px" heights="200px">
File:Greeneyes.jpg|Green eyes
File:Hazel Eyes Ojos Avellana (Marron-Verde) Caracas Venezuela.jpg|Green-hazel eyes
</gallery>

===Blue===
[[File:Close up shot of the human eye, 9 August 2024.jpg|thumb|Blue eye]]
 
The inheritance pattern followed by blue eyes was previously assumed to be a [[Mendelian inheritance|mendelian recessive]] trait, however, eye color inheritance is now recognized as a [[polygenic trait]], meaning that it is controlled by the interactions of several genes.<ref>{{cite journal |title=A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation |journal=Am. J. Hum. Genet. |volume=80 |issue=2 |pages=241–52 |year=2007|pmid=17236130 |pmc=1785344 |doi=10.1086/510885 |last1=Duffy |first1=David L. |last2=Montgomery |first2=Grant W. |last3=Chen |first3=Wei |last4=Zhao |first4=Zhen Zhen |last5=Le |first5=Lien |last6=James |first6=Michael R. |last7=Hayward |first7=Nicholas K. |last8=Martin |first8=Nicholas G. |last9=Sturm |first9=Richard A. <!--exactly 9 authors--> }}</ref>

Blue eyes are predominant in northern and eastern Europe, particularly around the [[Baltic Sea]]. Blue eyes are also found in southern Europe, [[Central Asia]], [[South Asia]], [[North Africa]] and [[West Asia]].<ref name="Cavalli-Sforza, L. L. 1994">{{cite book |last1=Cavalli-Sforza |first1=Luigi Luca |last2=Cavalli-Sforza |first2=Luca |last3=Menozzi |first3=Paolo |last4=Piazza |first4=Alberto |title=The History and Geography of Human Genes |url=https://archive.org/details/historygeography0000cava_g9l7 |url-access=registration |date=1994 |publisher=Princeton University Press |isbn=978-0-691-08750-4 }}{{page needed|date=May 2020}}</ref><ref name="altervista1">{{cite web|url=http://carnby.altervista.org/troe/08-05.htm |title=Distribution of Bodily Characters. Pigmentation, the Pilous System, and Morphology of the Soft Parts |archive-url=https://web.archive.org/web/20110726095519/http://carnby.altervista.org/troe/08-05.htm |archive-date=26 July 2011 }}</ref>

Approximately 8% to 10% of the global population have blue eyes.<ref name=":0" /> A 2002 study found that the prevalence of blue eye color among the [[White Americans|white population]] in the United States to be 33.8% for those born from 1936 through 1951.

===Gray===
[[File:Gray eye, Northern Italy.jpg|thumb|Gray eye Northern Italy]]

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in the appearance of gray and blue eyes is that gray eyes have larger deposits of [[collagen]] in the stroma, so that the light that is reflected from the epithelium undergoes [[Mie scattering]] (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when the sky is cloudy.<ref name="Southworth">{{cite web |url=http://www.thetech.org/genetics/ask.php?id=232 |title=Are gray eyes the same as blue in terms of genetics? |work=Understanding Genetics: Human Health and the Genome |publisher=Stanford School of Medicine |author=Lucy Southworth |access-date=19 October 2011 |archive-url=https://web.archive.org/web/20110927085904/http://www.thetech.org/genetics/ask.php?id=232 |archive-date=27 September 2011 }}</ref> Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.<ref name="Southworth" />

Gray eyes can also be found among the [[Algeria]]n [[Shawia people]]<ref>{{in lang|fr}}  [https://books.google.com/books?id=7rwBAAAAYAAJ&pg=PA272 Provincia: bulletin trimestriel de la Société de Statistique ...], Volumes 16–17 By Société de statistique, d'histoire et d'archéologie de Marseille et de Provence p. 273 ''l'iris gris est celui des chaouias...''</ref> of the [[Aurès Mountains]] in Northwest Africa, in the [[Middle East]]/[[West Asia]], [[Central Asia]], and [[South Asia]].  Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

== Irritation ==
{{See also|Irritation#Eye irritation|Lacrimal gland}}
[[File:Bloodshot.jpg|thumb|Conjunctival infection, or redness of the sclera surrounding the iris and pupil]]
Eye irritation has been defined as "the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye".<ref name=Mendell_OfficeWorkers>{{cite journal|last=Mendell|first=Mark J.|title=Non-Specific Symptoms In Office Workers: A Review And Summary Of The Epidemiologic Literature|journal=Indoor Air|date=22 April 2004|volume=3|issue=4|pages=227–236|doi=10.1111/j.1600-0668.1993.00003.x|doi-access=free}}</ref> It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are discomfort, dryness, excess tearing, itchiness, grating, foreign body sensation, ocular fatigue, pain, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from mild to severe. It has been suggested that these eye symptoms are related to different causal mechanisms, and symptoms are related to the particular ocular anatomy involved.<ref name=Wolkoff>{{cite journal|last=Wolkoff|first=P|author2=Skov, P |author3=Franck, C |author4= Petersen, LN |title=Eye irritation and environmental factors in the office environment—hypotheses, causes and a physiological model|journal=Scandinavian Journal of Work, Environment & Health|date=December 2003|volume=29|issue=6|pages=411–430|pmid=14712848|doi=10.5271/sjweh.748|doi-access=free}}</ref>

Several suspected causal factors in our environment have been studied so far.<ref name=Mendell_OfficeWorkers /> One hypothesis is that [[indoor air quality|indoor air pollution]] may cause eye and airway irritation.<ref name=Norn_Pollution>{{cite journal|last=Norn|first=M|title=Pollution keratoconjunctivitis. A review|journal=Acta Ophthalmologica|date=April 1992|volume=70|issue=2|pages=269–273|pmid=1609579|doi=10.1111/j.1755-3768.1992.tb04136.x|s2cid=42248933}}</ref><ref>{{cite journal|last=Versura|first=P|author2=Profazio, V |author3=Cellini, M |author4=Torreggiani, A |author5= Caramazza, R |title=Eye discomfort and air pollution|journal=Ophthalmologica |year=1999|volume=213|issue=2|pages=103–109|pmid=9885386|doi=10.1159/000027401|s2cid=46791165}}</ref> Eye irritation depends somewhat on destabilization of the outer-eye tear film, i.e. the formation of dry spots on the cornea, resulting in ocular discomfort.<ref name=Norn_Pollution /><ref name=Lemp>{{cite journal|last=Lemp|first=MA|title=The 1998 Castroviejo Lecture. New strategies in the treatment of dry-eye states|journal=Cornea|date=November 1999|volume=18|issue=6|pages=625–632|pmid=10571289|doi=10.1097/00003226-199911000-00001}}</ref><ref>{{cite journal|last=Rolando|first=M|author2=Zierhut, M|title=The ocular surface and tear film and their dysfunction in dry eye disease|journal=Survey of Ophthalmology|date=March 2001|volume=45|issue=Suppl 2|pages=S203–210|pmid=11587144|doi=10.1016/S0039-6257(00)00203-4|hdl=11567/299358|hdl-access=free}}</ref> Occupational factors are also likely to influence the perception of eye irritation. Some of these are lighting (glare and poor contrast), gaze position, reduced blink rate, limited number of breaks from visual tasking, and a constant combination of accommodation, musculoskeletal burden, and impairment of the visual nervous system.<ref name=Murata_CNSeffects>{{cite journal|last=Murata|first=K|author2=Araki, S |author3=Kawakami, N |author4=Saito, Y |author5= Hino, E |title=Central nervous system effects and visual fatigue in VDT workers|journal=International Archives of Occupational and Environmental Health|year=1991|volume=63|issue=2|pages=109–113|pmid=1889879|doi=10.1007/BF00379073|bibcode=1991IAOEH..63..109M|s2cid=24238741}}</ref><ref name=Rossignol>{{cite journal|last=Rossignol|first=AM|author2=Morse, EP |author3=Summers, VM |author4= Pagnotto, LD |title=Video display terminal use and reported health symptoms among Massachusetts clerical workers|journal=Journal of Occupational Medicine |date=February 1987|volume=29|issue=2|pages=112–118|pmid=3819890}}</ref> Another factor that may be related is work stress.<ref name=Apter_SickBldg>{{cite journal|last=Apter|first=A|author2=Bracker, A |author3=Hodgson, M |author4=Sidman, J |author5= Leung, WY |title=Epidemiology of the sick building syndrome|journal=The Journal of Allergy and Clinical Immunology|date=August 1994|volume=94|issue=2 Pt 2|pages=277–288|pmid=8077580|doi=10.1053/ai.1994.v94.a56006}}</ref><ref name=Thomson_FactsandFallacies>{{cite journal|last=Thomson|first=W. David|title=Eye problems and visual display terminals—the facts and the fallacies|journal=Ophthalmic & Physiological Optics |date=March 1998|volume=18|issue=2|pages=111–119|doi=10.1046/j.1475-1313.1998.00323.x|pmid=9692030|s2cid=222083261}}</ref> In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among [[Visual display unit|VDU]] users.<ref name=Aronsson_VDTdiscomfort>{{cite journal|last=Aronsson|first=G|author2=Strömberg, A|title=Work Content and Eye Discomfort in VDT Work|journal=International Journal of Occupational Safety and Ergonomics |year=1995|volume=1|issue=1|pages=1–13|pmid=10603534|doi=10.1080/10803548.1995.11076300|doi-access=free}}</ref><ref name=Mocci>{{cite journal|last=Mocci|first=F|author2=Serra, A |author3=Corrias, GA |title=Psychological factors and visual fatigue in working with video display terminals|journal=Occupational and Environmental Medicine|date=April 2001|volume=58|issue=4|pages=267–271|doi=10.1136/oem.58.4.267|pmid=11245744|pmc=1740121}}</ref> Other risk factors, such as chemical toxins/irritants (e.g. [[amine]]s, [[formaldehyde]], [[acetaldehyde]], [[acrolein]], [[N-Decane]], [[Volatile organic compound|VOC]]s, [[ozone]], [[pesticide]]s and [[preservative]]s, [[allergen]]s, etc.) might cause eye irritation as well.

Certain [[volatile organic compound]]s that are both chemically reactive and airway irritants may cause eye irritation. Personal factors (e.g. use of contact lenses, eye make-up, and certain medications) may also affect destabilization of the tear film and possibly result in more eye symptoms.<ref name=Wolkoff /> Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high.<ref name=Wolkoff /> An integrated physiological risk model with [[blink]] frequency, destabilization, and break-up of the eye tear film as inseparable phenomena may explain eye irritation among office workers in terms of occupational, climate, and eye-related physiological risk factors.<ref name=Wolkoff />

There are two major measures of eye irritation. One is blink frequency, which can be observed by human behavior. The other measures are break up time, tear flow, hyperemia (redness, swelling), tear fluid cytology, and epithelial damage (vital stains) etc., which are human beings' physiological reactions. Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation. Blink frequencies are individual with mean frequencies of < 2–3 to 20–30 blinks/minute, and they depend on environmental factors including the use of [[contact lens]]es. Dehydration, mental activities, work conditions, room temperature, relative humidity, and illumination all influence blink frequency. Break-up time (BUT) is another major measure of eye irritation and tear film stability.<ref>{{cite book|last=Kjaergaard|first=SK|title=Indoor air quality handbook: Chapter 17, the Irritated Eye in the Indoor Environment|year=2001|publisher=McGraw-Hill|location=New York|isbn=978-0-07-445549-4}}</ref> It is defined as the time interval (in seconds) between blinking and rupture. BUT is considered to reflect the stability of the tear film as well. In normal persons, the break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained.<ref name=Wolkoff /> Studies have shown that blink frequency is correlated negatively with break-up time. This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch.<ref>{{cite book|last=Norn|first=Mogens S.|title=External eye : methods of examination|year=1974|publisher=Scriptor|location=Copenhagen|isbn=978-8787473033}}</ref><ref name="Sibony PA. et al. 1998, pp. 1509-1592">Sibony PA, Evinger C. "Anatomy and physiology of normal and abnormal eyelid position and movement". In: Miller NR, Newman NJ, editors. Walsh & Hoyt's ''Clinical Neuro-ophthalmology''. Baltimore, MD: Williams and Wilkins; 1998. pp. 1509–1592</ref> Other evaluating methods, such as hyperemia, cytology etc. have increasingly been used to assess eye irritation.

There are other factors that are related to eye irritation as well. Three major factors that influence the most are indoor air pollution, contact lenses and gender differences. Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparisons with random samples of the general population.<ref>{{cite journal|last=Franck|first=C|author2=Bach, E |author3=Skov, P |title=Prevalence of objective eye manifestations in people working in office buildings with different prevalences of the sick building syndrome compared with the general population|journal=International Archives of Occupational and Environmental Health|year=1993|volume=65|issue=1|pages=65–69|pmid=8354577|doi=10.1007/BF00586061|bibcode=1993IAOEH..65...65F|s2cid=42611161}}</ref><ref>{{cite journal|last=Franck|first=C|title=Fatty layer of the precorneal film in the 'office eye syndrome'|journal=Acta Ophthalmologica|date=December 1991|volume=69|issue=6|pages=737–743|pmid=1789088|doi=10.1111/j.1755-3768.1991.tb02052.x|s2cid=28011125}}</ref><ref>{{cite journal|last=Franck|first=C|author2=Skov, P|title=Foam at inner eye canthus in office workers, compared with an average Danish population as control group|journal=Acta Ophthalmologica|date=February 1989|volume=67|issue=1|pages=61–68|pmid=2773640|doi=10.1111/j.1755-3768.1989.tb00724.x|s2cid=21372866}}</ref><ref>{{cite journal|last=Franck|first=C|title=Eye symptoms and signs in buildings with indoor climate problems ('office eye syndrome')|journal=Acta Ophthalmologica|date=June 1986|volume=64|issue=3|pages=306–311|pmid=3751520|doi=10.1111/j.1755-3768.1986.tb06925.x|s2cid=28101689}}</ref> These research results might indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers.<ref>{{cite journal|last=Doughty|first=MJ|author2=Fonn, D |author3=Richter, D |author4=Simpson, T |author5=Caffery, B |author6= Gordon, K |title=A patient questionnaire approach to estimating the prevalence of dry eye symptoms in patients presenting to optometric practices across Canada|journal=Optometry and Vision Science |date=August 1997|volume=74|issue=8|pages=624–631|pmid=9323733|doi=10.1097/00006324-199708000-00023|s2cid=22062179}}</ref><ref>{{cite journal|last=Fonn|first=D|author2=Situ, P |author3=Simpson, T |title=Hydrogel lens dehydration and subjective comfort and dryness ratings in symptomatic and asymptomatic contact lens wearers|journal=Optometry and Vision Science |date=October 1999|volume=76|issue=10|pages=700–704|pmid=10524785|doi=10.1097/00006324-199910000-00021}}</ref><ref name=Vajdic>{{cite journal|last=Vajdic|first=C|author2=Holden, BA |author3=Sweeney, DF |author4= Cornish, RM |title=The frequency of ocular symptoms during spectacle and daily soft and rigid contact lens wear|journal=Optometry and Vision Science |date=October 1999|volume=76|issue=10|pages=705–711|pmid=10524786|doi=10.1097/00006324-199910000-00022}}</ref> Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms, dryness, redness, and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers.<ref name=Vajdic /> Studies have shown that incidence of dry eyes increases with age,<ref name="Seal, D.V. et al. 1986">Seal, D. V., and Mackie, I. A. (1986). "The questionable dry eye as a clinical and biochemical entity". In F. J. Holly (Ed.) ''The preocular tear film – In health, disease, and contact lens wear''. Dry Eye Institute, Lubbock, TX, pp. 41–51. {{ISBN|978-0961693800}}</ref><ref>{{cite journal|last=Hikichi|first=T|author2=Yoshida, A |author3=Fukui, Y |author4=Hamano, T |author5=Ri, M |author6=Araki, K |author7=Horimoto, K |author8=Takamura, E |author9=Kitagawa, K |author10= Oyama, M |title=Prevalence of dry eye in Japanese eye centres|journal=Graefe's Archive for Clinical and Experimental Ophthalmology |date=September 1995|volume=233|issue=9|pages=555–558|pmid=8543205|doi=10.1007/BF00404705|s2cid=20759190}}</ref> especially among women.<ref name=McCartyDryEyeAustralia>{{cite journal|last=McCarty|first=C|author2=Bansal, AK |author3=Livingston, PM |author4=Stanislavsky, YL |author5= Taylor, HR |title=The epidemiology of dry eye in Melbourne, Australia, Historical image|journal=Ophthalmology|date=June 1998|volume=105|issue=6|pages=1114–1119|doi=10.1016/S0161-6420(98)96016-X|pmid=9627665}}</ref> Tear film stability (e.g. [[tear break-up time]]) is significantly lower among women than among men. In addition, women have a higher blink frequency while reading.<ref name="Bentivoglio AR. et al. 1997, pp.1028-1034">{{cite journal | last1 = Bentivoglio | first1 = AR | last2 = Bressman | first2 = SB | last3 = Cassetta | first3 = E. Caretta D | last4 = Tonali | first4 = P | last5 = Albanese | first5 = A. | year = 1997 | title = Analysis of blink rate patterns in normal subjects | journal = Mov Disord | volume = 12| issue = 6| pages = 1028–1034 | doi = 10.1002/mds.870120629 | pmid = 9399231 | s2cid = 12607655 }}</ref> Several factors may contribute to gender differences. One is the use of eye make-up. Another reason could be that the women in the reported studies have done more VDU work than the men, including lower grade work. A third often-quoted explanation is related to the age-dependent decrease of tear secretion, particularly among women after 40 years of age.<ref name=McCartyDryEyeAustralia /><ref>{{cite journal|last=Mathers|first=WD|author2=Lane, JA |author3=Zimmerman, MB |title=Tear film changes associated with normal aging|journal=Cornea|date=May 1996|volume=15|issue=3|pages=229–234|pmid=8713923|doi=10.1097/00003226-199605000-00001|s2cid=32866587 }}</ref><ref>{{cite journal|last=Mathers|first=WD|author2=Stovall, D |author3=Lane, JA |author4=Zimmerman, MB |author5= Johnson, S |title=Menopause and tear function: the influence of prolactin and sex hormones on human tear production|journal=Cornea|date=July 1998|volume=17|issue=4|pages=353–358|pmid=9676904|doi=10.1097/00003226-199807000-00002}}</ref>

In a study conducted by [[UCLA]], the frequency of reported symptoms in industrial buildings was investigated.<ref>{{cite book|last=Heating|first=American Society of|title=Managing indoor air for health and energy conservation : proceedings of the ASHRAE conference IAQ '86, April 20–23, 1986, Atlanta, GA|year=1986|publisher=American Society of Heating, Refrigerating and Air-Conditioning Engineers|location=Atlanta, GA|isbn=978-0-910110-48-8|page=448|author2=Refrigerating  |author3=Engineers, Air-Conditioning }}</ref> The study's results were that eye irritation was the most frequent symptom in industrial building spaces, at 81%. Modern office work with use of office equipment has raised concerns about possible adverse health effects.<ref>{{cite journal|last=Jaakkola|first=MS|author2=Jaakkola, JJ|title=Office equipment and supplies: a modern occupational health concern?|journal=American Journal of Epidemiology|date=1 December 1999|volume=150|issue=11|pages=1223–1228|pmid=10588083|doi=10.1093/oxfordjournals.aje.a009949|doi-access=free}}</ref> Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying paper. Emission of various particulate and volatile substances has been suggested as specific causes. These symptoms have been related to [[sick building syndrome]] (SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.<ref>{{cite journal|last=Nordström|first=K|author2=Norbäck, D |author3=Akselsson, R |title=Influence of indoor air quality and personal factors on the sick building syndrome (SBS) in Swedish geriatric hospitals|journal=Occupational and Environmental Medicine|date=March 1995|volume=52|issue=3|pages=170–176|pmid=7735389|pmc=1128182|doi=10.1136/oem.52.3.170}}</ref>

Many of the symptoms described in SBS and [[multiple chemical sensitivity]] (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals.<ref>{{cite journal|last=Anderson|first=RC|author2=Anderson, JH|title=Sensory irritation and multiple chemical sensitivity|journal=Toxicology and Industrial Health|year=1999 |volume=15|issue=3–4|pages=339–345|pmid=10416286|doi=10.1177/074823379901500308|bibcode=1999ToxIH..15..339A }}</ref> A repeated measurement design was employed in the study of acute symptoms of eye and respiratory tract irritation resulting from occupational exposure to sodium borate dusts.<ref name=Wegman_DoseRelatedAcuteIrritants>{{cite journal|last=Hu|first=X|author2=Wegman, DH |author3=Eisen, EA |author4=Woskie, SR |author5= Smith, RG |title=Dose related acute irritant symptom responses to occupational exposure to sodium borate dusts|journal=British Journal of Industrial Medicine|date=October 1992|volume=49|issue=10|pages=706–713|pmid=1419859|pmc=1012146 |doi=10.1136/oem.49.10.706}}</ref> The symptom assessment of the 79 exposed and 27 unexposed subjects comprised interviews before the shift began and then at regular hourly intervals for the next six hours of the shift, four days in a row.<ref name=Wegman_DoseRelatedAcuteIrritants /> Exposures were monitored concurrently with a personal real time aerosol monitor. Two different exposure profiles, a daily average and short term (15 minute) average, were used in the analysis. Exposure-response relations were evaluated by linking incidence rates for each symptom with categories of exposure.<ref name=Wegman_DoseRelatedAcuteIrritants />

Acute incidence rates for nasal, eye, and [[throat irritation]], and coughing and breathlessness were found to be associated with increased exposure levels of both exposure indices. Steeper exposure-response slopes were seen when short term exposure concentrations were used. Results from multivariate logistic regression analysis suggest that current smokers tended to be less sensitive to the exposure to airborne sodium borate dust.<ref name=Wegman_DoseRelatedAcuteIrritants />

Several actions can be taken to prevent eye irritation—
* trying to maintain normal blinking by avoiding room temperatures that are too high; avoiding relative humidities that are too high or too low, because they reduce blink frequency or may increase water evaporation.<ref name=Wolkoff />
* trying to maintain an intact film of [[tears]] by the following actions:
:# Blinking and short breaks may be beneficial for VDU users.<ref>{{cite journal|last=Carney|first=LG|author2=Hill, RM|title=The nature of normal blinking patterns|journal=Acta Ophthalmologica|date=June 1982|volume=60|issue=3|pages=427–433|pmid=7136554|doi=10.1111/j.1755-3768.1982.tb03034.x|s2cid=22362219}}</ref><ref>{{cite journal |title=Frequent short rest breaks from computer work: effects on productivity and well-being at two field sites|journal=Ergonomics|date=January 1997|volume=40|issue=1|pages=78–91|doi=10.1080/001401397188396|pmid=8995049|last1=Henning|first1=R. A|last2=Jacques|first2=P|last3=Kissel|first3=G. V|last4=Sullivan|first4=A. B|last5=Alteras-Webb|first5=S. M}}</ref> Increasing these two actions might help maintain the tear film.
:# Downward gazing is recommended to reduce ocular surface area and water evaporation.<ref>{{cite journal|last=Nakamori|first=K|author2=Odawara, M |author3=Nakajima, T |author4=Mizutani, T |author5= Tsubota, K |title=Blinking is controlled primarily by ocular surface conditions|journal=American Journal of Ophthalmology|date=July 1997|volume=124|issue=1|pages=24–30|pmid=9222228 |doi=10.1016/s0002-9394(14)71639-3}}</ref><ref>{{cite journal|last=Barbato|first=G|author2=Ficca, G |author3=Muscettola, G |author4=Fichele, M |author5=Beatrice, M |author6= Rinaldi, F |title=Diurnal variation in spontaneous eye-blink rate|journal=Psychiatry Research|date=Mar 6, 2000|volume=93|issue=2|pages=145–151|pmid=10725531|doi=10.1016/S0165-1781(00)00108-6|s2cid=35982831}}</ref><ref>{{cite journal|last=Sotoyama|first=M|author2=Villanueva, MB |author3=Jonai, H |author4= Saito, S |title=Ocular surface area as an informative index of visual ergonomics|journal=Industrial Health|year=1995|volume=33|issue=2|pages=43–55|pmid=7493821|doi=10.2486/indhealth.33.43|doi-access=free|bibcode=1995IndHe..33...43S}}</ref>
:# The distance between the VDU and keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by a low direction of the gaze,<ref>{{cite journal|last=Sotoyama|first=Midori|author2=Jonai, H |author3=Saito, S |author4= Villanueva, MB |title=Analysis of ocular surface area for comfortable VDT workstation layout|journal=Ergonomics|date=June 1996|volume=39|issue=6|pages=877–884|doi=10.1080/00140139608964508|pmid=8681929}}</ref> and
:#Blink training can be beneficial.<ref>{{cite journal|last=Collins|first=M|author2=Heron, H |author3=Larsen, R |author4= Lindner, R |title=Blinking patterns in soft contact lens wearers can be altered with training|journal=American Journal of Optometry and Physiological Optics|date=February 1987|volume=64|issue=2|pages=100–103|pmid=3826282|doi=10.1097/00006324-198702000-00004|s2cid=11828508|url=https://eprints.qut.edu.au/1458/1/1458.pdf}}</ref>

In addition, other measures are proper lid hygiene, avoidance of eye rubbing,<ref>{{cite journal|last=Piccoli|first=B|author2=Assini, R |author3=Gambaro, S |author4=Pastoni, F |author5=D'Orso, M |author6=Franceschin, S |author7=Zampollo, F |author8= De Vito, G |title=Microbiological pollution and ocular infection in CAD operators: an on-site investigation|journal=Ergonomics|date=May 15, 2001|volume=44|issue=6|pages=658–667|doi=10.1080/00140130117916|pmid=11373026|s2cid=37127979}}</ref> and proper use of personal products and medication. Eye make-up should be used with care.<ref>{{cite journal|last=Lozato|first=PA|author2=Pisella, PJ |author3=Baudouin, C |title=The lipid layer of the lacrimal tear film: physiology and pathology|journal=Journal Français d'Ophtalmologie|date=June 2001|volume=24|issue=6|pages=643–658|pmid=11460063}}</ref>

== Disease ==
{{Full explanation diagram of a human eye section}}
There are many [[Eye disease|diseases]], disorders, and [[Aging-associated diseases|age-related]] changes that may affect the eyes and surrounding structures.

As the eye ages, certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of diseases of the aging eye. While there are many changes of significance in the non-diseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability ([[presbyopia]]). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from [[sexually transmitted infection]]s such as herpes and genital warts. If contact between the eye and area of infection occurs, the STI can be transmitted to the eye.<ref>{{cite web|first1=Laurie Gray|last1=Barber|first2=Dan T.|last2=Gudgel|url=https://www.aao.org/eye-health/tips-prevention/sex-stds-and-eye-health|title=How Sexual Activity Can Affect Your Vision|publisher=American Academy of Ophthalmology|date=March 2, 2018|access-date=November 28, 2020}}</ref>

With aging, a prominent white ring develops in the periphery of the cornea called [[arcus senilis]]. Aging causes laxity, downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as [[ectropion]], [[entropion]], [[dermatochalasis]], and [[ptosis (eyelid)|ptosis]]. The vitreous gel undergoes liquefaction ([[posterior vitreous detachment]] or PVD) and its opacities — visible as [[floater]]s — gradually increase in number.

[[Eye care professional]]s, including [[ophthalmologist]]s and [[optometrist]]s, are involved in the treatment and management of ocular and vision disorders. A [[Snellen chart]] is one type of [[eye chart]] used to measure [[visual acuity]]. At the conclusion of a complete [[eye examination]], the eye doctor might provide the patient with an [[eyeglass prescription]] for [[corrective lens]]es. Some disorders of the eyes for which corrective lenses are prescribed include myopia ([[near-sightedness]]), [[hyperopia]] (far-sightedness), [[astigmatism]], and [[presbyopia]] (the loss of focusing range during aging).

=== Macular degeneration ===
{{Main|Macular degeneration}}
Macular degeneration is especially prevalent in the U.S. and affects roughly 1.75 million Americans each year.<ref>{{cite journal|doi=10.1001/archopht.122.4.564|pmid=15078675|title=Prevalence of Age-Related Macular Degeneration in the United States|journal=Archives of Ophthalmology|volume=122|issue=4|pages=564–572|year=2004|last1=Friedman|first1=D. S|last2=O'Colmain|first2=B. J|last3=Muñoz|first3=B|last4=Tomany|first4=S. C|last5=McCarty|first5=C|last6=De Jong|first6=P. T|last7=Nemesure|first7=B|last8=Mitchell|first8=P|last9=Kempen|first9=J|author10=Eye Diseases Prevalence Research Group|doi-access=free}}</ref> Having lower levels of lutein and zeaxanthin within the macula may be associated with an increase in the risk of age-related macular degeneration.<ref>{{cite journal|pmid=10973733|year=2000|last1=Bone|first1=R. A|title=Lutein and zeaxanthin in the eyes, serum and diet of human subjects|journal=Experimental Eye Research|volume=71|issue=3|pages=239–245|last2=Landrum|first2=J. T|last3=Dixon|first3=Z|last4=Chen|first4=Y|last5=Llerena|first5=C. M|doi=10.1006/exer.2000.0870}}</ref> Lutein and zeaxanthin act as [[antioxidant]]s that protect the retina and macula from oxidative damage from high-energy light waves.<ref>{{cite journal|pmid=10837298|year=2000|last1=Johnson|first1=E. J|title=Relation among serum and tissue concentrations of lutein and zeaxanthin and macular pigment density|journal=The American Journal of Clinical Nutrition|volume=71|issue=6|pages=1555–1562|last2=Hammond|first2=B. R|last3=Yeum|first3=K. J|last4=Qin|first4=J|last5=Wang|first5=X. D|last6=Castaneda|first6=C|last7=Snodderly|first7=D. M|last8=Russell|first8=R. M|doi=10.1093/ajcn/71.6.1555|doi-access=free}}</ref> As the light waves enter the eye, they excite electrons that can cause harm to the cells in the eye, but they can cause oxidative damage that may lead to macular degeneration or cataracts. Lutein and zeaxanthin bind to the electron free radical and are reduced rendering the electron safe. There are many ways to ensure a diet rich in lutein and zeaxanthin, the best of which is to eat dark green vegetables including kale, spinach, broccoli and turnip greens. Nutrition is an important aspect of the ability to achieve and maintain proper eye health. [[Lutein]] and [[zeaxanthin]] are two major carotenoids, found in the macula of the eye, that are being researched to identify their role in the pathogenesis of eye disorders such as age-related [[macular degeneration]] and [[cataracts]].<ref>American Optometric Association (2013). [http://www.aoa.org/patients-and-public/caring-for-your-vision/diet-and-nutrition/lutein "Lutein and zeaxanthin"]</ref>

== Sexuality ==
Human eyes (particularly the [[Iris (anatomy)|iris]] and its [[Eye color|color]]) and the area surrounding the eye ([[Eyelid|lids]], [[Eyelash|lashes]], [[Eyebrow|brows]]) have long been a key component of [[physical attractiveness]]. [[Eye contact]] plays a significant role in human nonverbal communication. A prominent [[limbal ring]] (dark ring around the [[Iris (anatomy)|iris]] of the eye) is considered attractive.<ref>{{Cite journal |last1=Peshek |first1=Darren |last2=Semmaknejad |first2=Negar |last3=Hoffman |first3=Donald |last4=Foley |first4=Pete |date=2011-04-01 |title=Preliminary Evidence that the Limbal Ring Influences Facial Attractiveness |journal=Evolutionary Psychology |language=en |volume=9 |issue=2 |pages=137–146 |doi=10.1177/147470491100900201 |issn=1474-7049|doi-access=free |pmid=22947961 |pmc=10519137 }}</ref> Additionally, long and full [[Eyelash|eyelashes]] are coveted as a sign of beauty and are considered an attractive [[Face|facial feature]].<ref>{{Cite journal |last1=Aguinaldo |first1=Erick |last2=Mousavi |first2=Maedeh |last3=Peissig |first3=Jessie |date=2018-09-01 |title=Eyelashes and Attraction: Eyelash Length and Fullness are Significantly Correlated with Facial Attractiveness |journal=Journal of Vision |language=en |volume=18 |issue=10 |pages=1338 |doi=10.1167/18.10.1338 |issn=1534-7362|doi-access=free }}</ref> Pupil size has also been shown to play an influential role in attraction and nonverbal communication, with [[Mydriasis|dilated (larger) pupils]] perceived to be more attractive.<ref>{{Cite journal |last1=Tombs |first1=Selina |last2=Silverman |first2=Irwin |date=2004-07-01 |title=Pupillometry: A sexual selection approach |url=https://www.sciencedirect.com/science/article/pii/S1090513804000261 |journal=Evolution and Human Behavior |language=en |volume=25 |issue=4 |pages=221–228 |doi=10.1016/j.evolhumbehav.2004.05.001 |bibcode=2004EHumB..25..221T |issn=1090-5138|url-access=subscription }}</ref> It should also be noted that dilated pupils are a response to sexual arousal and stimuli.<ref>{{Cite journal |last1=Hess |first1=Eckhard H. |last2=Polt |first2=James M. |date=1960-08-05 |title=Pupil Size as Related to Interest Value of Visual Stimuli |url=https://www.science.org/doi/10.1126/science.132.3423.349 |journal=Science |language=en |volume=132 |issue=3423 |pages=349–350 |doi=10.1126/science.132.3423.349 |pmid=14401489 |bibcode=1960Sci...132..349H |s2cid=12857616 |issn=0036-8075|url-access=subscription }}</ref> In the [[Renaissance]], women used the juice of the berries of the [[Atropa belladonna|belladonna]] plant in eyedrops to [[Pupillary response|dilate the pupils]] and make the eyes appear more seductive.<gallery mode="packed" heights="160">
File:Human eyelashes.jpg|Long, thick, and dark eyelashes are considered an attractive facial feature as they draw attention to the eyes. Subject exhibits [[trichomegaly]] (exceptionally long lashes)
File:Human eye with limbal ring, anterior view.jpg|A thick, dark, limbal ring is seen as an attractive feature
File:Human eye in dim light.jpg|Pupils dilate in response to sexual arousal and larger pupils are perceived to be more attractive
</gallery>

== Images ==
<gallery>
File:Diagram of human eye without labels.svg|Right eye without labels (horizontal section)
File:Blausen 0388 EyeAnatomy 01.png
File:Blausen 0389 EyeAnatomy 02.png
File:Three Main Layers of the Eye.png|The structures of the eye labeled
File:Three Internal chambers of the Eye.svg|Another view of the eye and the structures of the eye labeled
</gallery>

== See also ==
{{Portal|Anatomy}}
{{div col|colwidth=30em}}
* [[Eye colour]]
* [[Eye development]]
* [[Eye disease]]
* [[Eye strain]]
* [[Hyaloid canal]]
* [[Iris recognition]]
* [[Knobloch syndrome]]
* [[Lacrimal caruncle]]
* [[Rheum]]
* [[Spectral sensitivity]]
* [https://optiflexlens.com/ Eye Lens]
{{div col end}}

== References ==
{{Reflist}}

== External links ==

* {{usurped|1=[https://web.archive.org/web/20150523224359/http://hilzbook.com/organs/head/eye/ Eye – Hilzbook]}}
* {{usurped|1=[https://web.archive.org/web/20150524023842/http://hilzbook.com/organs/head/eye/retina/ Retina – Hilzbook]}}
* [https://www.visiondirect.co.uk/the-human-eye Interactive Tool to explore the Human Eye]
* {{Commons category-inline|Human eyes}}
* {{Wikiquote-inline|eyes}}
{{Accessory organs of the eye}}
{{Eye proteins}}
{{Human regional anatomy}}
{{Human systems and organs}}
{{Eye anatomy}}
{{Authority control}}

[[Category:Human eye| ]]
[[Category:Facial features]]
[[Category:Sensory organs]]
[[Category:Ophthalmology]]
[[Category:Vision by taxon]]
[[Category:Visual system]]