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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, [18] and a record Secchi depth of 79 m has been reported from a coastal polynya of the Eastern Weddell Sea ...
The human eye is optimised for air vision, and when it is immersed in direct contact with water, visual acuity is adversely affected by the difference in refractive index between water and air. Provision of an airspace between the cornea and the water can compensate, but causes scale and distance distortion.
Secchi depth is the depth at which a disk is no longer visible to the human eye. This measurement was created in 1865 and represents one of the oldest oceanographic methods. [ 23 ] [ 24 ] To measure Secchi depth, a white or black-and-white disk is mounted on a pole or line and lowered slowly down in the water.
This similarity in refractive index is the reason a diver cannot see clearly underwater without a diving mask with an internal airspace. [3] Absorption of light depends on wavelength, this causes loss of colour underwater. [20] [21] The red end of the spectrum of light is absorbed over a short distance, and is lost even in shallow water. [20]
The PADI recreational dive planner, in "Wheel" format. The Recreational Dive Planner (or RDP) is a decompression table in which no-stop time underwater is calculated. [1] The RDP was developed by DSAT and was the first dive table developed exclusively for no-stop recreational diving. [2]
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
While ocean color is a key theme of ocean optics, optics is a broader term that also includes the development of underwater sensors using optical methods to study much more than just color, including ocean chemistry, particle size, imaging of microscopic plants and animals, and more.
The science of underwater diving includes those concepts which are useful for understanding the underwater environment in which diving takes place, and its influence on the diver. It includes aspects of physics , physiology and oceanography .