stereogram, random-dot (RDS)
A stereogram in which the eye sees an array of little characters or dots of a roughly uniform texture and containing no recognizable shape or contours. The only difference is that a certain region in one target has been laterally displaced with respect to the other, to produce some retinal disparity. When they are viewed in a stereoscope, that region is seen in stereoscopic relief. The shape in that region can be any pattern. The effect is remarkable as the shape usually appears to float out from the surround. Syn. Julesz random-dot stereogram.The random-dot E test uses a polarized random test pattern and requires the use of Polaroid spectacles to detect whether a subject has stereopsis. The subject will see a raised letter E in the random-dot pattern of one of the test plates. At 50cm, the retinal disparity induced by the E is 500 seconds of arc. The TNO test for stereoscopic vision also uses random-dot stereograms in which the half-images have been superimposed and printed in complementary colours, like anaglyphs. The test plates, when viewed with red and green spectacles, elicit stereopsis. There is a series of plates inducing retinal disparities ranging from 15 to 480 seconds of arc. See stereoscopic visual acuity; anaglyph; retinal disparity; Frisby stereotest; Lang stereotest; two-dimensional test; vectogram.