In tracing the winding lines between the two X's, both the normal and those with colour vision deficiencies are able to trace the line. Instead of reading a number, subjects are asked to trace a visible line across the plate. To pass each test you must correctly trace the wiggly lines. Each tracing should be completed within ten seconds. There are also Ishihara tests consisting of 10, 14 or 24 test plates, and plates in some versions ask the viewer to trace a line rather than read a number. The full test consists of 38 plates, but the existence of a severe deficiency is usually apparent after only a few plates. Other plates are intentionally designed to reveal numbers only to those with a red-green color vision deficiency, and be invisible to those with normal red-green color vision. Within the pattern are dots which form a number or shape clearly visible to those with normal color vision, and invisible, or difficult to see, to those with a red-green color vision defect. The test consists of a number of Ishihara plates, each of which depicts a solid circle of colored dots appearing randomized in color and size. Since then this is the most widely used and well known color vision deficiency test and still used by most optometrists and ophthalmologists all around the world. It was named after its designer, Shinobu Ishihara, a professor at the University of Tokyo, who first published his tests in 1917. The first in a class of successful color vision tests called pseudo-isochromatic plates ("PIP"). The Ishihara test is a color blind test for red-green color deficiencies.
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