There are ten different<unk>  layouts—indeed every technological method of capturing an optical image commonly used by human beings, with the exceptions of zoom and Fresnel lenses, occur in nature.<unk>  types can be categorised into "simple<unk> s", with one concave photoreceptive surface, and "compound<unk> s", 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<unk>  type can be adapted for almost any behaviour or environment. The only limitations specific to<unk>  types are that of resolution—the physics of compound<unk> s prevents them from achieving a resolution better than 1°. Also, superposition<unk> s can achieve greater sensitivity than apposition<unk> s, so are better suited to dark-dwelling creatures.<unk> s 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 gastropods, as well as some annelids possess both. Some organisms have photosensitive cells that do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. These are not considered<unk> s because they lack enough structure to be considered an organ, and do not produce an image. Non-compound<unk> s Simple<unk> s are rather ubiquitous, and lens-bearing<unk> s have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa. Pit<unk> s Pit<unk> s, also known as stemma, are<unk> -spots which may be set into a pit to reduce the angles of light that enters and affects the<unk> -spot, 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<unk> s". 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<unk> s by sensing thermal infra-red radiation, in addition to their optical wavelength<unk> s like those of other vertebrates (see infrared sensing in snakes). However, pit organs are fitted with receptors rather different from photoreceptors, namely a specific transient receptor potential channel (TRP channels) called TRPV1. The main difference is that photoreceptors are G-protein coupled receptors but TRP are ion channels. Spherical lens<unk>  The resolution of pit<unk> s 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 a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii. Heterogeneous<unk> s have evolved at least nine times: four or more times in gastropods, once in the copepods, once in the annelids, once in the cephalopods, and once in the chitons, which have aragonite lenses. No extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown". This<unk>  creates an image that is sharp enough that motion of the<unk>  can cause significant blurring. To minimise the effect of<unk>  motion while the animal moves, most such<unk> s have stabilising<unk>  muscles. The ocelli of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can't form a sharp image. Ocelli (pit-type<unk> s 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, has two lenses in each<unk> , arranged like those in a telescope. Such arrangements are rare and poorly understood, but represent an alternative construction. Multiple lenses are seen in some hunters such as eagles and jumping spiders, which have a refractive cornea: 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<unk> s 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. In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see Luneburg lens), or with an aspheric shape. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have<unk> s that make use of an inhomogeneous lens. 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<unk> s An alternative to a lens is to line the inside of the<unk>  with "mirrors", and reflect the image to focus at a central point. The nature of these<unk> s means that if one were to peer into the pupil of an<unk> , one would see the same image that the organism would see, reflected back out. Many small organisms such as rotifers, copepods and flatworms use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector<unk> s. The scallop Pecten has up to 100 millimetre-scale reflector<unk> s fringing the edge of its shell. It detects moving objects as they pass successive lenses. There is at least one vertebrate, the spookfish, whose<unk> s include reflective optics for focusing of light. Each of the two<unk> s 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<unk> s An image of a house fly compound<unk>  surface by using scanning electron microscope Anatomy of the compound<unk>  of an insect bluebottle fly have compound<unk> s A compound<unk>  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<unk> s, compound<unk> s possess a very large view angle, and can detect fast movement and, in some cases, the polarisation 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<unk> s, humans would require very large compound<unk> s, around in radius. Compound<unk> s fall into two groups: apposition<unk> s, which form multiple inverted images, and superposition<unk> s, which form a single erect image. Compound<unk> s are common in arthropods, annelids and some bivalved molluscs. Compound<unk> s in arthropods grow at their margins by the addition of new ommatidia. Apposition<unk> s Apposition<unk> s are the most common form of<unk> s and are presumably the ancestral form of compound<unk> s. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and
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