A single-image random-dot stereogram (SIRDS) is a set of printed or displayed dots that appear to form a three-dimensional ( 3D ) scene when viewed close-up with the eyes focused at infinity. Sometimes text characters are used in place of dots; then the composite is called a single-image random-text stereogram (SIRTS).
Viewing a SIRDS or SIRTS involves deceiving the eye-and-brain mechanism. The scheme does not work for everyone. Deliberate defocusing can cause eye strain in some people. The trick is to position the SIRDS or SIRTS 12 to 18 inches (30 to 45 centimeters) away from your eyes, look momentarily at something far away, and then look back at the image again. The image can be moved alternately closer to, and farther from, your eyes, until adjacent sets of dots or characters overlap in the lines of sight. Alternatively, you can try looking at the image cross-eyed.
An optical instrument consisting of prisms or mirrors can force your left and right eyes to look at adjacent sets of dots in a SIRDS or SIRTS. This works for almost everyone and reduces the risk of eye strain.