Underwater vision

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Scuba diver with bifocal lenses fitted to a mask PBS bifocal mask P9251477.JPG
Scuba diver with bifocal lenses fitted to a mask

Underwater, things 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. [1]

Contents

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. [1]

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. [1]

Divers can to a large extent adapt to these effects over time and with practice. [1]

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 human eye is adapted for viewing in air. Water, however, has approximately the same refractive index as the cornea (both about 1.33), 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. [2]

In optics a ray is an idealized model of light, obtained by choosing a line that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very complex optical systems to be analyzed mathematically or simulated by computer. Ray tracing uses approximate solutions to Maxwell's equations that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength. Ray theory does not describe phenomena such as interference and diffraction, which require wave theory.

Refractive index dimensionless number that describes how fast light propagates through the material

In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light travels through the material. It is defined as

Cornea part of the eye

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK.

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.[ citation needed ]

Fish

The crystalline lenses of fishes' eyes are extremely convex, almost spherical, and their refractive indices are the highest of all the animals. These properties enable proper focusing of the light rays and in turn proper image formation on the retina. This convex lens gives the name to the fisheye lens in photography [3] .

Humans

Views through a flat mask, above and below water FlatMask above-below.jpg
Views through a flat mask, above and below water

By wearing a flat diving mask, humans can see clearly underwater. [2] [4] [5] The scuba mask's flat window separates the eyes from the surrounding water by a layer of air. Light rays entering from water into the flat parallel window change their direction minimally within the window material itself. [2] But when these rays exit the window into the air space between the flat window and the eye, the refraction is quite noticeable. The view paths refract (bend) in a manner similar to viewing fish kept in an aquarium. Linear polarizing filters decrease visibility underwater by limiting ambient light and dimming artificial light sources. [1]

While wearing a flat scuba mask or goggles, objects underwater will appear 33% bigger (34% bigger in salt water) and 25% closer than they actually are. [2] Also pincushion distortion and lateral chromatic aberration are noticeable. Double-dome masks restore natural sized underwater vision and field of view, with certain limitations. [2] [6]

Diving masks can be fitted with lenses for divers needing optical correction to improve vision. Corrective lenses are ground flat on one side and optically cemented to the inside face of the mask lens. This provides the same amount of correction above and below the surface of the water. Bifocal lenses are also available for this application. Some masks are made with removable lenses, and a range of standard corrective lenses are available which can be fitted. Plastic self-adhesive lenses that can be applied to the inside of the mask may fall off if the mask is flooded for a significant period. Contact lenses may be worn under a mask or helmet, but there is some risk of losing them if the mask floods. [6] [7]

Color vision

Comparison of penetration of light of different wavelengths in the open ocean and coastal waters NOAA Deep Light diagram3.jpg
Comparison of penetration of light of different wavelengths in the open ocean and coastal waters

Water attenuates light due to absorption [2] which varies as a function of frequency. In other words, as light passes through a greater distance of water color 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. [8] 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. [1]

Fluorescent paints absorb higher frequency light to which the human eye is relatively insensitive and emit lower frequencies, which are more easily detected. The emitted light and the reflected light combine and may be considerably more visible than the original light. The most visible frequencies are also those most rapidly attenuated in water, so the effect is for greatly increased colour contrast over a short range, until the longer wavelengths are attenuated by the water. [1]

Table of Light Absorption in pure water
ColorAverage wavelengthApproximate depth of total absorption
Ultraviolet300 nm25 m
Violet400 nm100 m
Blue475 nm275 m
Green525 nm110 m
Yellow575 nm50 m
Orange600 nm20 m
Red685 nm5 m
Infra-red800 nm3 m

The best colors to use for visibility in water was shown by Luria et al. and quoted from Adolfson and Berghage below: [2] [4]

A. For murky, turbid water of low visibility (rivers, harbors, etc.)

1. With natural illumination:
a. Fluorescent yellow, orange, and red.
b. Regular yellow, orange, and white.
2. With incandescent illumination:
a. Fluorescent and regular yellow, orange, red and white.
3. With a mercury light source:
a. Fluorescent yellow-green and yellow-orange.
b. Regular yellow and white.

B. For moderately turbid water (sounds, bays, coastal water).

1. With natural illumination or incandescent light source:
a. Any fluorescent in the yellows, oranges, and reds.
b. Regular yellow, orange, and white.
2. With a mercury light source:
a. Fluorescent yellow-green and yellow-orange.
b. Regular yellow and white.

C. For clear water (southern water,[ clarification needed ] deep water offshore, etc.).

1. With any type of illumination fluorescent paints are superior.
a. With long viewing distances, fluorescent green and yellow-green.
b. With short viewing distances, fluorescent orange is excellent.
2. With natural illumination:
a. Fluorescent paints.
b. Regular yellow, orange, and white.
3. With incandescent light source:
a. Fluorescent paints.
b. Regular yellow, orange, and white.
4. With a mercury light source:
a. Fluorescent paints.
b. Regular yellow, white.

The most difficult colors at the limits of visibility with a water background are dark colors such as gray or black.

Physiological variations

A very near-sighted person can see more or less normally underwater. Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions. [9]

The Moken people of South-East Asia are able to focus underwater to pick up tiny shellfish and other food items. [10] Gislén et al. have compared Moken and untrained European children and found that the underwater visual acuity of the Moken was twice that of their untrained European counterparts. [11] European children after 1 month of training also showed the same level of underwater visual acuity. [12] This is due to the contraction of the pupil, instead of the usual dilation (mydriasis) that is undergone when a normal, untrained eye, accustomed to viewing in air, is submerged. [13]

Visibility

Visibility is a measure of the distance at which an object or light can be discerned. The theoretical black body visibility of pure water based on the values for the optical properties of water for light of 550 nm has been estimated at 74 m. [14]

The standard measurement for underwater visibility is the distance at which a Secchi disc can be seen. The range of underwater vision is usually limited by turbidity. In very clear water visibility may extend as far as about 80m, [15] and a record Secchi depth of 79 m has been reported from a coastal polynya of the Eastern Weddell Sea, Antarctica. [15] In other sea waters, Secchi depths in the 50 to 70 m range have occasionally been recorded, including a 1985 record of 53 m in the Eastern and up to 62 m in the tropical Pacific Ocean. This level of visibility is seldom found in surface freshwater. [15] Crater Lake, Oregon, is often cited for clarity, but the maximum recorded Secchi depth using a 2 m disc is 44 m. [15] The McMurdo Dry Valleys of Antarctica and Silfra in Iceland have also been reported as exceptionally clear.[ citation needed ]

Factors affecting visibility include: particles in the water (turbidity), salinity gradients (haloclines), temperature gradients (thermoclines) and dissolved organic matter. [16]

Low visibility

Low visibility is defined by NOAA for operational purposes as: "When visual contact with the dive buddy can no longer be maintained." [17]

DAN-Southern Africa suggest that limited visibility is when a "buddy cannot be discerned at a distance greater than 3 metres." [18]

See also

Related Research Articles

Eye organ that detects light and converts it into electro-chemical impulses in neurons

Eyes are organs of the visual system. They provide animals with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.

Underwater photography Genre of photography

Underwater photography is the process of taking photographs while under water. It is usually done while scuba diving, but can be done while diving on surface supply, snorkeling, swimming, from a submersible or remotely operated underwater vehicle, or from automated cameras lowered from the surface.

Diving mask Watertight air-filled face cover with view-ports for improving underwater vision

A diving mask is an item of diving equipment that allows underwater divers, including scuba divers, free-divers, and snorkelers, to see clearly underwater. Surface supplied divers usually use a full face mask or diving helmet, but in some systems the half mask may be used. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light on the retina. By providing an air space in front of the eyes, the eye is able to focus nearly normally. The shape of the air space in the mask slightly affects the ability to focus. Corrective lenses can be fitted to the inside surface of the viewport or contact lenses may be worn inside the mask to allow normal vision for people with focusing defects.

Presbyopia medical condition associated with aging of the eye

Presbyopia is a condition associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold reading material farther away, headaches, and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia.

Full face diving mask Diving mask that covers the mouth as well as the eyes and nose

A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece, demand valve or constant flow gas supply that provides the diver with breathing gas. The full face mask has several functions: it lets the diver see clearly underwater, it provides the diver's face with some protection from cold and polluted water and from stings, such as from jellyfish or coral. It increases breathing security and provides a space for equipment that lets the diver communicate with the surface support team.

Visual acuity clarity of vision

Visual acuity (VA) commonly refers to the clarity of vision. Visual acuity is dependent on optical and neural factors, i.e., (i) the sharpness of the retinal focus within the eye, (ii) the health and functioning of the retina, and (iii) the sensitivity of the interpretative faculty of the brain.

Radial keratotomy

Radial keratotomy (RK) is a refractive surgical procedure to correct myopia (nearsightedness) that was developed in 1974, by Svyatoslav Fyodorov, a Russian ophthalmologist. It has been largely supplanted by newer operations, such as photorefractive keratectomy, LASIK, Epi-LASIK and the phakic intraocular lens.

Eye examination a series of tests assessing vision and pertaining to the eyes

An eye examination is a series of tests performed by an ophthalmologist, optometrist, or orthoptist, optician (UK), assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

Scuba diving Using bottled air to swim underwater

Scuba diving is a mode of underwater diving where the diver uses a self-contained underwater breathing apparatus (scuba), which is completely independent of surface supply, to breathe underwater. Scuba divers carry their own source of breathing gas, usually compressed air, allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers. Although the use of compressed air is common, a new mixture called enriched air (Nitrox) has been gaining popularity due to its benefit of reduced nitrogen intake during repetitive dives. Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a regulator. They may include additional cylinders for range extension, decompression gas or emergency breathing gas. Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open circuit, so a smaller cylinder or cylinders may be used for an equivalent dive duration. Rebreathers extend the time spent underwater compared to open circuit for the same gas consumption; they produce fewer bubbles and less noise than open circuit scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.

Moken ethnic group

The Moken are an Austronesian people of the Mergui Archipelago, a group of approximately 800 islands claimed by both Burma and Thailand. Most of the 2,000 to 3,000 Moken live a semi-nomadic hunter-gatherer lifestyle heavily based on the sea, though this is increasingly under threat.

Underwater diving Descending below the surface of the water to interact with the environment

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving. Humans are not physiologically and anatomically well adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.

Defocus aberration

In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the image. What should be sharp, high-contrast edges in a scene become gradual transitions. Fine detail in the scene is blurred or even becomes invisible. Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.

Astigmatism Type of eye defect

Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at all distances. Other symptoms can include eyestrain, headaches, and trouble driving at night. If it occurs early in life it can result in amblyopia.

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfect vision. A human eye in a state of emmetropia requires no corrective lenses; the vision scores well on a visual acuity test.

A silt out is a situation when underwater visibility is rapidly reduced to functional zero by disturbing fine particulate deposits on the bottom or other solid surfaces. This can happen in scuba and surface supplied diving, or in ROV and submersible operations, and is a more serious hazard for scuba diving in penetration situations where the route to the surface may be obscured.

Bird vision

Vision is the most important sense for birds, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". The avian eye resembles that of a reptile, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

Diving equipment Equipment used to facilitate underwater diving

Diving equipment is equipment used by underwater divers to make diving activities possible, easier, safer and/or more comfortable. This may be equipment primarily intended for this purpose, or equipment intended for other purposes which is found to be suitable for diving use.

Dive light Light used underwater by a diver

A dive light is a light source carried by an underwater diver to illuminate the underwater environment. Scuba divers generally carry self-contained lights, but surface supplied divers may carry lights powered by cable supply.

Outline of underwater diving Hierarchical outline list of articles related to underwater diving

The following outline is provided as an overview of and topical guide to underwater diving:

References

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Further reading