આંખ

આંખ
Schematic diagram of the vertebrate eye.
Compound eye of Antarctic krill

આંખ એ સજીવનું જોવા માટેનું અંગ છે. આંખને નેત્ર, નયન, નેણ, લોચન તરીકે પણ ઓળખવામાં આવે છે. આંખો પ્રકાશને ઓળખી તેનું ન્યુરોન્સમાં થતા ઈલેક્ટ્રોકેમિકલ ઈમ્પલ્સમાં રૂપાંતર કરે છે. ખૂબ જ સાદા ફોટોરિસેપ્ટર્સ પ્રકાશને જાગૃત દ્રશ્યમાં થતા હલનચલન સાથે જોડે છે. ઉચ્ચ કક્ષાનાં ઓર્ગેનિઝમ્સમાં આંખ એક કોમ્પલેક્ષ સિસ્ટમ છે જે આસપાસથી પ્રકાશ ભેગો કરી તેની તીવ્રતાને રેગ્યુલેટ કરી; સ્થિતિસ્થાપક લેન્સીસનાં આયોજન વડે કેન્દ્રિત કરી ચિત્ર બનાવે છે; આ ચિત્રનું ઈલેક્ટ્રિકલ સિગ્નલમાં રૂપાંતર કરી મગજને આંખની ઓપ્ટીક નર્વ સાથે જોડતાં કોમ્પલેક્ષ ન્યુરલ માર્ગો દ્વારા મગજનાં વિઝ્યુઅલ કોર્ટેક્ષ અને બીજા ભાગોમાં મોકલાવે છે. પ્રકાશનાં ચિત્રીકરણની ક્ષમતા ધરાવતી આંખો મુખ્યત્વે દસ પ્રકારની છે અને ૯૬% પ્રાણીસૃષ્ટિ સંકુલિત દ્રષ્ટિ યોજના ધરાવે છે. મોલ્યુસ્ક, કોર્ડેટ અને આર્થરોપોડ પ્રાણીઓ ચિત્રીકરણ કરતી આંખો ધરાવે છે.

સાદામાં સાદી આંખો, જેમ કે સુક્ષ્મજીવોની આંખો પ્રકાશની હાજરી અથવા અંધકાર આ બે જ સ્થિતિ ઓળખી શકે છે. આટલી માહિતી તેમની રાત અને દિવસ સાથે જીવનનો તાલ મેળવવા માટે પૂરતી છે. વધુ કોમ્પલેક્ષ આંખો તેમનાં રેટીનલ ફોટોસેન્સિટીવ ગેન્ગલીયન કોષો દ્વારા રેટીનોહાઈપોથાલ્મીક માર્ગનો ઉપયોગ કરી સુપ્રાકીયાસ્માટીક ન્યુકલી ને રાત અને દિવસનાં ચક્ર વિષે માહિતી પહોંચાડે છે.

Overview

Eye of the wisent,
the European bison

કોમ્પલેક્ષ આંખો આકાર અને રંગનો તફાવત પારખે છે. ઘણાં સજીવોના, ખાસ કરીને શિકારી જીવોના, દૃષ્ટિક્ષેત્રોમાં ઊંડાણની પરખ માટે વિસ્તૃત બાઈનોક્યુલર દૃષ્ટિ સંકળાયેલી હોય છે, જ્યારે બીજા જીવોમા આંખોનું સ્થન એવુ હોય છે જેથી ખૂબ જ મોટું દૃષ્ટિક્ષેત્ર મળે, જેમ કે સસલાં અને ઘોડા, જેમની મોનોક્યુલર દૃષ્ટિ છે.

૬૦ અબજ (૬૦૦ મિલિયન) વર્ષો પહેલા, કેમ્બ્રિયન વિસ્ફોટના ગાળામાં પ્રથમ પ્રોટોચક્ષુ ઉદભવ્યા હતા. પ્રાણીઓનાં સૌથી જૂનાં પૂર્વજો જરૂરી દૃષ્ટિ માટે બાયોકેમિકલ સાધનો ધરાવતાં હતાં અને વધુ અગ્ર કક્ષાની આંખો ૯૬% પ્રાણીની ત્રીસથી વધુ પ્રજાતિઓમાંથી છ મુખ્ય ફાયલામાં ઉદભવી હતી. મોટા ભાગના વર્ટીબ્રેટ અને કેટલાક મોલ્યુસ્કમાં આંખો પ્રકાશને અંદર આવવા દઈ તેને આંખના પાછળના ભાગમાં આવેલી રેટીના નામની પ્રકાશ-સંવેદનશીલ કોષોની બનેલી પેનલ પર દર્શાવે છે. રેટીનાનાં શંકુ કોષો (રંગ માટે) અને દંડ કોષો (આછા પ્રકાશનાં કોન્ટ્રાસ્ટ માટે) પ્રકાશને પારખી તેનું દૃષ્ય માટેનાં ન્યુરલ સંદેશાઓમાં રૂપાંતર કરે છે. ત્યાર બાદ આ દૃષ્ય સંદેશાઓ ઓપ્ટીકલ નર્વ દ્વારા મગજને પહોંચાડવામાં આવે છે. આવી આંખો ગોળાકાર હોય છે જે વિટરસ હ્યુમર નામનાં પારદર્શક ચીકણા પદાર્થથી ભરેલી, પ્રકાશને કેન્દ્રિત કરતા લેન્સ ધરાવતી, ક્યારેક આઈરિસવાળી, આઈરિસની આજુબાજુનાં સ્નાયુઓને આકુંચિત કે શિથિલ કરીને પ્યુપિલની સાઈઝ બદલી આંખમાં પ્રવેશતા પ્રકાશની માત્રા રેગ્ય્લેટ કરે છે, અને જ્યારે પૂરતો પ્રકાશ હોય ત્યારે એબરેશન ઘટાડે છે.

મોટાં ભાગનાં સેફલોપોડ પ્રાણીઓ, માછલીઓ, દ્વિચર પ્રાણીઓ અને સર્પોની આંખોનાં લેન્સ ફીક્સ આકારનાં હોય છે અને લેન્સને કેમેરાની જેમ આગળ-પાછળ કરી દૃષ્યને કેન્દ્રિત કરે છે.

આર્થરોપોડ પ્રાણીઓમાં જોવા મળતી કમ્પાઉન્ડ આંખો અનેક સાદાં પાસાંઓની બનેલી હોય છે જેની સંખ્યા શરીરની રચના પ્રમાણે નક્કી થયેલી હોય છે. આ કમ્પાઉન્ડ આંખો અનેક ટપકાંવાળી એક જ છબી બનાવે છે અથવા એક આંખની એક તે મુજબ અનેક આંખોની અનેક છબીઓ બનાવે છે. આંખનાં દરેક પાસાંનો પોતાનો લેન્સ અને પોતાનાં પ્રકાશસંવેદનશીલ કોષો હોય છે. કેટલીક આંખો ૨૮,૦૦૦ સુધીનાં ષટકોણાકાર માં ગોઠવાયેલાં પાસાં ધરાવે છે અને ૩૬૦ અંશનો દૃષ્ટિવ્યાપ ધરાવે છે. કમ્પાઉન્ડ આંખો દૃષ્યમાં થતા નાજુક ફેરફારને પણ પારખે છે. અનેક આંખો દ્વારા મળતી દૃષ્યની માહિતી મગજમાં જોડાઈને જૂદી જાતનું અત્યંત સ્પષ્ટ ચિત્ર બનાવે છે.

મેન્ટીસ નામ્ના ઝીંગા સોથી જટીલ રંગ-દ્રશ્ય રચના ધારાવે છે. તે ઝીણવટ ભરેલી આત્યંતિક રંગપટલ ભેદની માહિતીનું વિશ્લેષણ કરે છે. હાલના સમયમાં લુપ્ત એવા ટ્રાયલોબાઈટ પણ વિશિષ્ટ પ્રકારની સંયુક્ત આંખો ધરાવતા હતા. તેઓ તેમની આંખના લેન્સના નિર્માણ મટે સાફ કેલ્સાઈટાના સ્ફટિકો વાપરતા હતા. અહીં તેઓ મૃદુ આંખો ધરાવતા અન્ય સન્ધિપાદ વર્ગના પ્રાણીઓથી જુદા પડતા હતા. આ પ્રાણીઓમાં લેન્સની સંખ્યા ભિન્ન હતી. અમુક જાતિમાં એક લેન્સ હતું, જ્યારે અમુકમાં એક આંખમાં હજારો લેન્સ હોઈ શકતાં.

સંયુક્ત આંખની સરખામણીમાં સાદી આંખો માત્ર એક જ લેન્સ ધરાવે છે. દા. ત. ઉછળતા કરોળિયા માર્યાદિત દ્રષ્ટિ પટલ ધરાવતી મોટા આકાર સાદી આંખો ની એક જોડ ધરાવે છે. આ સાથે પરિમિતિય દ્રષ્ટિ માટે નાની નાની આંખોની અન્ય જોડીઓ ધરાવટ છે

For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larva]]e, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out ofજ direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents–in this way the bearers can spot hot springs and avoid being boiled alive.

Evolution

મુખ્ય લેખ: Evolution of the eye

Evolution of the eye

Photoreception is phylogenetically very old, with various theories of phylogenesis. The common origin (monophyly) of all animal eyes is now widely accepted as fact. This is based upon the shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race". Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel.

Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry.

The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource.

Through gradual change, as the eyespot depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.

The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.

Types of eye

There are ten different eye layouts—indeed every way of capturing an optical image commonly used by man, with the exceptions of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behavior or environment. The only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures. Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. These two groups are not monophyletic; the cnidaria also possess cilliated cells, and some annelids possess both.

Non-compound eyes

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustacea and cubozoa.

Pit eyes

Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.

Pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates.

Spherical lensed eye

The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.

Heterogeneous eyes have evolved at least eight times: four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods. No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.

The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).

Multiple lenses

Some marine organisms bear more than one lens; for instance the copepod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilia's eyes have two lenses, arranged like those in a telescope. Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.

Refractive cornea

In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained. As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material (See Luneburg lens.), or by flattening the lens. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead.

As mentioned above, a refractive cornea is only useful out of water; in water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.

Reflector eyes

An alternative to a lens is to line the inside of the eye with " mirrors", and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.

Many small organisms such as rotifers, copeopods and platyhelminths use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.

There is at least one vertebrate, the spookfish, whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of guanine crystals.

Compound eyes

An image of a house fly compound eye surface by using Scanning Electron Microscope

ચિત્ર:Compound eye1.jpg

Anatomy of the compound eye of an insect

Arthropods such as this Calliphora vomitoria fly have compound eyes

A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarization of light. Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head.

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.

Apposition eyes

Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information.

The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels.

Superposition eyes

The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also have a transparent gap but use corner mirrors instead of lenses.

Parabolic superposition

This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.

Other

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives acute vision. In the acute zone the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution.

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialized retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye.

The body of Ophiocoma wendtii, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons.

Nutrients of the eye

The ciliary body is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The vitreous is the transparent, colorless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibers with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.

Relationship to life requirements

Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition. Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia. Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope. An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey. In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether. Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.

It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.

Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.

Visual acuity

A hawk's eye

Visual acuity, or resolving power, is "the ability to distinguish fine detail" and is the property of cones. It is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.

Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.

Perception of colours

"Colour vision is the faculty of the organism to distinguish lights of different spectral qualities." All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm. This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.

The most sensitive pigment, rhodopsin, has a peak response at 500 nm. Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also filter incoming light, changing the peak response. Many organisms are unable to discriminate between colours, seeing instead in shades of grey; color vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved. Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.

Most organisms with colour vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.

Rods and cones

The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones.

Rods cannot distinguish colours, but are responsible for low-light (scotopic) monochrome (black-and-white) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for colour vision. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibres to the brain.

Pigmentation

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.

Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.

There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells. The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes. Likewise, c-opsins have been found to be expressed in the brain of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form. C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins. Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.

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