Underwater vision

src: webee.technion.ac.il

Underwater, things are less visible due to lower natural lighting levels caused by the rapid damping of light with distances that pass through water. They are also blurred by light scattering between the object and the viewer, also resulting in lower contrast. This effect varies with the wavelength of light, and the color and turbidity of the water. The eyes of vertebrates are usually optimized for underwater or aerial vision, as is the case with the human eye. Air maximized visual acuity is strongly influenced by the difference in refractive index between air and water when immersed in direct contact. The provision of air space between the cornea and water can compensate, but has side effects of scale and distance distortion. Divers learn to compensate for this distortion. Artificial lighting is effective for improving lighting at short distances.

Stereoscopic acuity, the ability to assess the relative distance of different objects, is greatly reduced underwater, and this is influenced by the field of vision. The narrow field of vision caused by a small viewport on the helmet results in reduced stereocarity, and loss of hand-eye coordination.

At a very short distance in clear water distance is underestimated, according to the enlargement due to refraction through the flat lens of the mask, but at a greater distance - greater than arm range, the distance tends to be too high to the level affected by the turbidity. Relative and absolute depth perception is reduced underwater. The loss of contrast causes overestimation, and the enlargement effect contributes too low at short distances.

Divers can adjust to these effects over time and with practice.

Light rays bend as they travel from one medium to another; the amount of bending is determined by the refractive index of the two media. If one medium has a particular curved shape, it acts as a lens. The cornea, humor, and eye crystal lenses together form the lens that focuses the image on the retina. The human eye is adjusted for viewing in the air. Water, however, has approximately the same refractive index as the cornea (both around 1.33), which effectively eliminates the corneal focusing properties. When immersed in water, instead of focusing the image on the retina, they focus behind the retina, producing a very blurry image of hypermetropia.

Video Underwater vision

Focus

Water has a significantly different refractive index of air, and this affects the focusing of the eye. Most animal eyes are adapted to underwater or aerial vision, and do not focus well when in another environment.

Fish

The fish crystal lenses are very convex, almost spherical, and their refractive index is the highest of all animals. These properties enable the focusing of the right light and, in turn, the proper formation of the image on the retina. This convex lens gives the name of a fish eye lens in photography.

Man

By wearing flat diving masks, humans can see clearly under the water. The flat scuba mask window separates the eyes from the surrounding water with a layer of air. The incoming light from the water to the flat parallel window changes its direction minimally within the window material itself. But when these rays come out the window into the air space between the flat window and the eye, the refraction is quite visible. The outline view refracts (bent) in a similar way to seeing the fish stored in the aquarium. Linear polarization filters decrease visibility under water by limiting ambient light and dimming artificial light sources.

When wearing flat scuba masks or glasses, underwater objects will appear 33% larger (34% larger in saltwater) and 25% closer than they really are. Also distorted bearings and lateral chromatic aberrations are seen. The double-dome mask restores the lower visibility of nature and field of view in size, with certain limitations.

The diving mask can be equipped with a dive lens that requires optical correction to improve vision. Correct ground lens is flat on one side and cemented optically to the inside of the face of the lens mask. This gives the same amount of correction above and below the water level. Bifocal lenses are also available for this application. Some masks are made with removable lenses, and various standard corrective lenses are available that can be mounted. The plastic self-adhesive lens that can be applied to the inside of the mask may fall out if the mask is flooded for significant periods. Contact lenses may be worn under a mask or helmet, but there is some risk of losing them if the mask flooded.

Maps Underwater vision

Color vision

Water attenuates light because of its varying absorption as a function of frequency. In other words, when the light passes through the color distance the larger water is selectively absorbed by water. The absorption of color is also affected by the turbidity of water and the solute.

The special water absorbs red light, and to a lesser extent, yellow, green and purple, so the color most absorbed by water is the blue light. Particulate and dissolved materials can absorb different frequencies, and this will affect the color at depth, with results such as the green color usually in many coastal waters, and the dark red-brown color of many freshwater rivers and lakes due to dissolved organic matter.

Fluorescent paints absorb higher frequency lights in which the human eye is relatively insensitive and emits lower frequencies, which are easier to detect. The emitted light and reflected light join and may be much more visible than the original light. The most visible frequencies are also the fastest attenuated in water, so the effect is to increase the color contrast in the short range, until the longer wavelength is attenuated by water.

The best colors to use for visibility in water are shown by Luria et al. and quoted from Adolfson and Berghage below:

A. For cloudy and turbid water with low visibility (river, harbor, etc.)

1. With natural lighting:

a. Yellow fluorescent, orange, and red.

b. Common yellow, orange and white.

2. With the incandescent illumination:

a. Fluorescent and yellow, orange, red and white.

3. With a light source of mercury:

a. Fluorescent yellow-green and yellow-orange.

b. Usually yellow and white.

B. For slightly turbid water (sound, bay, coastal water).

1. With natural lighting or incandescent light sources:

a. Every neon in yellow, orange, and red.

b. Common yellow, orange and white.

2. With a light source of mercury:

a. Fluorescent yellow-green and yellow-orange.

b. Usually yellow and white.

C. For clear water (southern water, deep waters offshore, etc.).

1. With all kinds of fluorescent fluorescent paint is superior.

a. With long visibility, green fluorescent and yellow-green.

b. With short visibility, fluorescent orange color is very good.

2. With natural lighting:

a. Fluorescent paints.

b. Common yellow, orange and white.

3. With an incandescent light source:

a. Fluorescent paints.

b. Common yellow, orange and white.

4. With a light source of mercury:

a. Fluorescent paints.

b. Ordinary yellow, white.

The most difficult color on the visibility limit with the background water is a dark color like gray or black.

Physiological variations

A very close-vision person (a vision disorder resulting from a refractive index of the wrong eye due to distant objects being blurred) can see more or less normally underwater. Scuba divers who are interested in underwater photography can pay attention to presbyopic changes while diving before they recognize symptoms in their normal routine because of the nearest focus in low light conditions.

Moken people in Southeast Asia can focus under water to pick up small shells and other foods. GislÃƒÆ'Ã… Â© n et al. have compared Moken and untrained European children and found that Moken's underwater visual acuity is twice as much as their untrained counterparts in Europe. European children after 1 month of training also show the same level of underwater vision acuity. This is due to the contraction of the pupil, not the usual dilation (midriasis) experienced when the eye is normal, untrained, accustomed to seeing in the air, submerged.

src: i.ytimg.com

Visibility

Visibility is a measure of the distance at which an object or light can be seen. Theoretical black body visibility of pure water based on values â€‹â€‹for the optical properties of water to light 550 m is estimated to have reached 74 m.

The standard measurement for underwater visibility is the distance at which Secchi discs can be seen. The range of underwater vision is usually limited by turbidity. In very clear visibility the water can extend as far as about 80m, and the Secchi depth record of 79 m has been reported from the poly beach of the East Weddell Sea, Antarctica. In other sea waters, the Secchi depth in the 50 to 70 m range is sometimes recorded, including records 1985 53 m in the East and up to 62 m in the tropical Pacific Ocean. This level of visibility is rarely found in freshwater surfaces. Crater Lake, Oregon, is often cited for clarity, but the maximum Secchi depth recorded using a 2 m disk is 44 m. The Dry Antarctic and Silfra Valley Lakes in Iceland are also reported to be very clear.

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

Low visibility

Low visibility is defined by NOAA for operational purposes as: "When visual contact with a dive buddy can not be maintained anymore."

Africa AND-South indicates that limited visibility is when a "friend can not be seen at a distance of more than 3 meters."

src: static1.squarespace.com

References

src: static1.squarespace.com

Underwater vision

Senin, 04 Juni 2018