Underwater vision
Underwater vision is the ability to see objects underwater, and this is significantly affected by several factors. Underwater, objects are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water. They are also blurred by scattering of light between the object and the viewer, also resulting in lower contrast. These effects vary with wavelength of the light, and color and turbidity of the water. The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact. Provision of an airspace between the cornea and the water can compensate, but has the side effect of scale and distance distortion. The diver learns to compensate for these distortions. Artificial illumination is effective to improve illumination at short range.
Stereoscopic acuity, the ability to judge relative distances of different objects, is considerably reduced underwater, and this is affected by the field of vision. A narrow field of vision caused by a small viewport in a helmet results in greatly reduced stereoacuity, and associated loss of hand-eye coordination. At very short range in clear water distance is underestimated, in accordance with magnification due to refraction through the flat lens of the mask, but at greater distances - greater than arm's reach, the distance tends to be overestimated to a degree influenced by turbidity. Both relative and absolute depth perception are reduced underwater. Loss of contrast results in overestimation, and magnification effects account for underestimation at short range. Divers can to a large extent adapt to these effects over time and with practice.
Light rays bend when they travel from one medium to another; the amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses images on the retina. The eye of most land animals is adapted for viewing in air. Water, however, has approximately the same refractive index as the cornea, effectively eliminating the cornea's focusing properties. When immersed in water, instead of focusing images on the retina, they are focused behind the retina, resulting in an extremely blurred image from hypermetropia. This is largely avoided by having an air space between the water and the cornea, trapped inside the mask or helmet.
Water attenuates light due to absorption and as light passes through water colour is selectively absorbed by the water. Color absorption is also affected by turbidity of the water and dissolved material. Water preferentially absorbs red light, and to a lesser extent, yellow, green and violet light, so the color that is least absorbed by water is blue light. Particulates and dissolved materials may absorb different frequencies, and this will affect the color at depth, with results such as the typically green color in many coastal waters, and the dark red-brown color of many freshwater rivers and lakes due to dissolved organic matter.
Visibility is a term which generally predicts the ability of some human, animal, or instrument to optically detect an object in the given environment, and may be expressed as a measure of the distance at which an object or light can be discerned. Factors affecting visibility include illumination, length of the light path, particles which cause scattering, dissolved pigments which absorb specific colours, and salinity and temperature gradients which affect refractive index. Visibility can be measured in any arbitrary direction, and for various colour targets, but horizontal visibility of a black target reduces the variables and meets the requirements for a straight-forward and robust parameter for underwater visibility. Instruments are available for field estimates of visibility from the surface, which can inform the dive team on probable complications.
Illumination
Illumination of underwater environments is limited by the characteristics of the water. Light absorption by water is variable and depends on the temperature of the water and concentration of ions. Accurate values for the absorption coefficient and the temperature and salinity coefficients are available for specific ranges and values of wavelength from 400nm to 14000nm.There are three dominant molrcular vibration modes but the absorption spectrum in liquid water is a continuum. Light scattering is also variable depending on temperature and salinity.Natural Illumination
Natural illumination underwater comes primarily from sunlight during the day and moonlight at night in the uppermost layer. In deeper regions, where solar light does not penetrate, bioluminescence—light produced by living organisms—provides the primary source of natural illumination. In marine environments, light availability defines five major zones: the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadalpelagic zones, in order of least to greatest depth.- Epipelagic zone: Extending to a depth of about 200 meters, the epipelagic is where most natural light exists. The epipelagic zone is lit up by rays of sunlight that can penetrate roughly 200 meters of depth.
- Mesopelagic zone: Extending from a depth of about 200 meters to 1,000 meters, is just beyond the reach of most sunlight. The mesopelagic zone receives faint sunlight but is home to bioluminescence—light producing organisms.
- Bathypelagic zone : Extending from a depth of about 1,000-4,000 meters, the bathypelagic zone is well beyond the range of sunlight. It is characterized by almost complete darkness broken only by light from bioluminescent organisms.
- Abyssopelagic zone : Extending from a depth of about 4,000-6,000 meters, the abyssopelagic zone is pitch-black but inhabited by bioluminescent organisms. The ocean floor usually lies in this zone.
- Hadalpelagic zone : At a depth of 6,000 meters and greater, the hadalpelagic zone is the deepest zone of the ocean, and exists only in trenches such as the Mariana Trench. Similar to the abyssopelagic zone, it is pitch-black and receives light only from bioluminescent organisms.
Artificial illumination
Light absorption and scattering
- backscatter has a greater effect when from artificial illumination as the light source is more likely to be close to the viewer than for natural light.
Evolution of the eye
Types of eye
There are several types of eye, comprising simple eyes, with one concave photoreceptive surface, and compound eyes which include a group of individual lenses laid out over a convex surface. Each of these major types has several lesser variations, with about 10 significant types recognised.All of these originated in aquatic organisms, and therefore have, at some stages of their evolution, been adapted primarily for underwater vision. Some lineages took to terrestrial life, and their eyes evolved further in that environment, and of those, a few returned to an amphibious or aquatic lifestyle, with further adaptation in some cases.
Photosensitivity
Focus
Water has a significantly different refractive index to air, and this affects the focusing of the eye. Most animals' eyes are adapted to either underwater or air vision, and do not focus properly when in the other environment.Variations by taxa
Invertebrates have a large variety of eye structures. Most, possibly all, originated in an aquatic environment, but some have later adapted to a terrestrial environment, and later re-adapted to an aquatic environment. Vertebrates all evolved from a common marine vertebrate ancestor, which already had well developed underwater vision and a specific eye structure, which has been conserved, or in some cases atrophied in animals living in the lightless cave environment.Arthropods
Most arthropods have at least one of two types of eye: lateral compound eyes, and smaller median ocelli, which are simple eyes. When both are present, the two eye types are used in concert because each has its own advantage. Ocelli can detect lower light levels, and have a faster response time, while compound eyes are better at detecting edges and are capable of forming images.Molluscs
The molluscs have the widest variety of eye morphologies of any phylum, and a large degree of variation in their function. Cephalopods such as octopus, squid, and cuttlefish have paired eyes on their heads as complex as those of vertebrates, while scallops have large numbers of simple eyes along the edges of the shell opening, and chitons have a dispersed network of tiny eyes over the surface of their shells which may act together as a compound eye. Many gastropods have stalked eyes which can be retracted in the presence of danger.There are between seven and eleven distinct eye types in molluscs, of all levels of complexity, from the pit eyes of many gastropods, to the pinhole eyes of the Nautilus, to the lensed eyes of the other cephalopods. Compound eyes are present in some bivalves, and reflective 'mirrors' have been innovated by other lineages such as scallops. The eyes of molluscs also span a large range in size, from to across.