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Vernier acuity (from the term "vernier scale", named after astronomer Pierre Vernier) is a type of visual acuity – more precisely of hyperacuity – that measures the ability to discern a disalignment among two line segments or gratings.
Ewald Hering's model, published 1899, of how a Vernier acuity stimulus is coded by a receptor array. Receptors marked c signal a different position code along the horizontal direction from either the position a code or the position b code. [1] The sharpness of our senses is defined by the finest detail we can discriminate.
PHP takes advantage of visual hyperacuity, also known as vernier acuity—the ability to identify the misalignment of visual objects. Visual hyperacuity is at least 10 times more sensitive than visual acuity , the ability to separate between distinct objects. [ 4 ]
Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes (6/4) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone.
The use of the vernier scale is shown on a vernier caliper which measures the internal and the external diameters of an object. The vernier scale is constructed so that it is spaced at a constant fraction of the fixed main scale. So for a vernier with a constant of 0.1, each mark on the vernier is spaced 9/10 of those on the main scale.
Projectional radiography, X-ray computed tomography and nuclear medicine result in some degree of ionizing radiation exposure but have with a few exceptions much lower radiation doses than what is associated with fetal harm. [1] They are indicated when ultrasonography or MRI is not readily available or not feasible for the diagnostic question ...
A pregnancy test is used to determine whether a female is pregnant or not. The two primary methods are testing for the female pregnancy hormone (human chorionic gonadotropin (hCG)) in blood or urine using a pregnancy test kit, and scanning with ultrasonography. [1] Testing blood for hCG results in the earliest detection of pregnancy. [2]
Snellen chart. The Snellen chart, which dates back to 1862, is also commonly used to estimate visual acuity.A Snellen score of 6/6 (20/20), indicating that an observer can resolve details as small as 1 minute of visual angle, corresponds to a LogMAR of 0 (since the base-10 logarithm of 1 is 0); a Snellen score of 6/12 (20/40), indicating an observer can resolve details as small as 2 minutes of ...