vectogram(redirected from stereotest-housefly)
A polarized stereogram consisting of two polarized images at right angles to each other. When viewed through polarizing filters it presents one image to one eye and another image to the other eye. The Vectograph is a chart based on this principle in which almost one half of a chart is seen by one eye and almost the other half by the other eye while some lines, letters or numbers are seen binocularly to lock fusion. The Vectograph is useful for balancing refraction and to detect suppression and fixation disparity. The Titmus stereotest (Fig. V1) consists of various vectograms, including one with a stereoscopic pattern representing a housefly, to establish whether the patient has gross stereopsis (it produces approximately 3000 seconds of arc of retinal disparity at 40 cm). Children are often tested by asking them to hold one of the wings of the fly, which they will do above the plate if it is seen stereoscopically. The other vectograms of the test provide finer tests for stereoscopic acuity. See stereoscopic visual acuity; retinal disparity; random-dot stereogram; suppression; balancing test; two-dimensional test.