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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 ...
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.
Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters. In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres.
The Jaeger chart is an eye chart used in testing near visual acuity. It is a card on which paragraphs of text are printed, with the text sizes increasing from 0.37 mm to 2.5 mm. [ 1 ] This card is to be held by a patient at a fixed distance from the eye dependent on the J size being read.
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.
Count fingers vision – state distance c/o or c.o. Complains of CT: Cover test c/u: ... Visual acuity with eye chart at Near 15.7 inches (400 mm) and without (sc ...
A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether corrective lenses (i.e. glasses or contact lenses) were used: Distance from the chart D (distant) for the evaluation done at 20 feet (6 m). N (near) for the evaluation done at 15.7 inches (400 mm).
The test is based on the principle of diplopic projection. [1] Dissociation of the deviation is brought about by presenting a red line image to one eye and a white light to the other, while prisms are used to superimpose these and effectively measure the angle of deviation (horizontal and vertical).