ABOUT THIS TEST
A random-dot stereogram contains no shape that either eye can see by itself. The dots are
random; a region of them is shifted sideways between the left and right images. Only when the
brain fuses the two views does the shape appear, floating in front of the noise. Béla Julesz
introduced the method in 1960 precisely to remove monocular cues — outline, shading, size —
so that what remains is stereopsis alone.
Can a computer screen measure stereo vision?
Only partly. A flat screen shows both eyes the same image, so depth has to be created
artificially — with red-cyan anaglyph glasses, or by free-fusing two side-by-side panels.
Without one of those, no browser test can measure stereopsis at all. This test screens for
whether gross stereopsis is present; it cannot produce a clinical stereoacuity value in arc
seconds the way a Titmus, TNO or Randot test can.
What is a random-dot stereogram?
A field of random dots that contains no visible shape to either eye alone. A region of the
dots is shifted horizontally between the left and right images, so the shape only appears when
the brain fuses the two views. It was introduced by Béla Julesz in 1960 and is the standard way
to test stereopsis without monocular cues — you cannot pass it by guessing from one eye.
Why did I fail if my eyes are fine?
Failing this screen does not necessarily mean absent stereopsis. In free-fusion mode, many
people simply cannot free-fuse, which is a learned skill, not a vision defect. In anaglyph mode,
poor-quality or wrongly-oriented red-cyan glasses, screen color calibration, and low display
brightness all interfere. A real result requires a clinician with calibrated targets.
Do I need red-cyan glasses?
For the anaglyph mode, yes — standard red-left, cyan-right 3D glasses. If you do not have
them, the free-fusion mode shows two panels side by side that you can fuse by crossing or
diverging your eyes, though that mode is harder and its failures are less meaningful.
What does the number mean?
The result is the smallest horizontal disparity, in screen pixels, at which you could still
locate the shape. Converting pixels to arc seconds requires knowing your exact pixel pitch and
viewing distance, neither of which a web page can measure — so this test reports pixels and
stops there rather than inventing a precision it does not have.
Not a diagnosis. If you have double vision, a turned
eye, or a sudden change in depth perception, see an optometrist or ophthalmologist. Children in
particular should be checked in person, since untreated amblyopia is time-sensitive.