Stereoscopy (also called stereo-view, stereoscopic, 3-D) is a technique capable of recording three-dimensional visual information or creating the illusion of depth in an image.

Human vision uses several cues to determine relative depths in a perceived scene.

Some of these cues include:

• Stereopsis
• Accommodation of the eyeball (focus)
• Occlusion of one object by another
• Subtended visual angle of an object of known size
• Linear perspective (convergence of parallel edges)
• Vertical position (objects higher in the scene generally tend to be perceived as further away)
• Haze, desaturation, and a shift to bluishness
• Change in size of textured pattern detail

All the above cues, with the exception of the first two, are present in traditional two-dimensional images such as paintings, photographs, and television. Stereoscopy is the enhancement of the illusion of depth in a photograph, movie, or other two-dimensional image by presenting a slightly different image to each eye, and thereby adding the first of these cues (stereopsis) as well. It is important to note that the second cue is still not satisfied and therefore the illusion of depth is incomplete.

Many 3D displays use this method to convey images. It was first invented by Sir Charles Wheatstone in 1838. Stereoscopy is used in photography and also for entertainment through the production of stereo-grams aka stereo-view cards. Stereoscopy is useful in viewing images rendered from large multi-dimensional data sets such as are produced by experimental data. Modern industrial three dimensional photography may use 3D scanners to detect and record 3 dimensional information. The three-dimensional depth information can be reconstructed from two images using a computer by corresponding the pixels in the left and right images. Solving the Correspondence problem in the field of Computer Vision aims to create meaningful depth information from two images. Traditional stereoscopic photography consists of creating a 3-D illusion starting from a pair of 2-D images. The easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation exactly equal to the perspectives that both eyes naturally receive in binocular vision. If eyestrain and distortion are to be avoided, each of the two 2-D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented straight ahead, the viewer’s eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together. During the 1800s and early 1900s many people enjoyed viewing these stereoview3-D Images typically with a wooden viewer comprised of 2 glass lenses enclosed in a viewing “shroud” attached to a length of wood with an intersecting wood support that has a wire holder on each side to hold the card. Several companies manufactured the stereo-viewers and the stereo-view cards to view. Many themes and types of images were printed on the cards and several collectible sets (although rare) still exist. One set is a self promotional series of cards published by Sears Roebuck and Company, other notable sets include exotic destinations, historical sites, personal events.

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