Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph, movie, or other two-dimensional image is created by presenting a slightly different image to each eye. Many 3D displays use this method to convey images. It was first invented by Sir Charles Wheatstone in 1838. Stereoscopy is used in photogrammetry and also for entertainment through the production of stereograms. 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 3 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 create depth perception in the brain is to provide to the eyes of the viewer two different images, representing two perspectives of the same object, with a minor deviation similar 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

Little or no additional image processing is required. Under some circumstances, such as when a pair of images is presented for crossed or diverged eye viewing, no device or additional optical equipment is needed.

The principal advantages of side-by-side viewers is that there is no diminution of brightness so images may be presented at very high resolution and in full spectrum color. The ghosting associated
with polarized projection or when color filtering is used is totally eliminated. The images are
discretely presented to the eyes and visual center of the brain, with no co-mingling of the views.
The recent advent of wider HD and computer flat screens has made wider 3D digital images practical
in this side by side mode, which hitherto has been used mainly with paired photos or in print form.

By exchanging the right and left views, so that the image to be viewed with the left eye is on the right side, it is possible to obtain a 3-D effect with some effort but without any equipment. To view the crossed-eye view shown here, the viewer should move slightly back from his or her normal viewing distance and place his viewpoint on a line perpendicular to the center of the image. A finger should be placed halfway between the eyes and the image, then the finger should be viewed. The three bright spots between the pictures should become four spots, and the two images become three. If the focus of the eyes is now allowed to drift to the surface of the screen without uncrossing the eyes, a three dimensional depth illusion will appear in the central image. The finger may now be removed from the view. A viewer may find that the extra side images disappear once in-depth view of the central image is stable. This is a popular way of presenting images on computers but it is difficult to learn and for many viewers the method produces substantial eye-strain, and is not comfortable enough for extended viewing. Another disadvantage is that after prolonged viewing, the eyes may become accustomed to "over-crossing", which may lead to temporary double-vision. It also offers none of the advantages enumerated above that are provided by the stereoscope. When images are presented as for the stereoscope, with the image to be viewed by the left eye on the left, they can be viewed by diverging the eyes. This gives much less eye-strain than crossing the eyes, and requires a smaller adjustment of focus, but can be even harder to learn. Without the use of viewing equipment, the size of a stereoscopic image viewable is significantly limited by one's eye-spacing and the inability of one's eyes to diverge to a large extent. The major advantage of cross-eye viewing is that the images can be substantially larger, and no glasses are needed by those who have the viewing knack. Prismatic glasses, with built-in masking, make the trick easy for most people, but they tend to be a bit expensive.

In the 1940s, a modified and miniaturized variation of this technology was introduced as the View-Master. Pairs of stereo views are printed on translucent film which is then mounted around the edge of a cardboard disk, images of each pair being diametrically opposite. A lever is used to move the disk so as to present the next image pair. A series of seven views can thus be seen on each card when it was inserted into the View-Master viewer. These viewers were available in many forms both non-lighted and self-lighted and may still be found today. One type of material presented is children's fairy tale story scenes or brief stories using popular cartoon characters. These use photographs of three dimensional model sets and characters. Another type of material is a series of scenic views associated with some tourist destination, typically sold at gift shops located at the attraction.

Another important development in the late 1940s was the introduction of the Stereo Realist camera and viewer system. Using color slide film, this equipment made stereo photography available to the masses and caused a surge in its popularity. The Stereo Realist and competing products can still be found (in estate sales and elsewhere) and utilized today.

Low-cost folding cardboard viewers with plastic lenses have been used to view images from a sliding card and have been used by computer technical groups as part of annual convention proceedings. These have been supplanted by the DVD recording and display on a television set. By exhibiting moving images of rotating objects a three dimensional effect is obtained through other than stereoscopic means.

An advantage offered by transparency viewing is that a wider field of view may be presented since images, being illuminated from the rear, may be placed much closer to the lenses. Note that with simple viewers the images are limited in size as they must be adjacent and so the field of view is determined by the distance between each lens and its corresponding image.

Good quality wide angle lenses are quite expensive and they are not found in most stereo viewers.

The user typically wears a helmet or glasses with two small LCD or OLED displays with magnifying lenses, one for each eye. The technology can be used to show stereo films, images or games, but it can also be used to create a virtual display. Head-mounted displays may also be coupled with head-tracking devices, allowing the user to "look around" the virtual world by moving their head, eliminating the need for a separate controller. Performing this update quickly enough to avoid inducing nausea in the user requires a great amount of computer image processing. If six axis position sensing (direction and position) is used then wearer may move about within the limitations of the equiment used. Owing to rapid advancements in computer graphics and the continuing miniaturization of video and other equipment these devices are beginning to become available at more reasonable cost.

Head-mounted or wearable glasses may be used to view a see-through image imposed upon the real world view, creating what is called augmented reality. This is done by reflecting the video images through partially reflective mirrors. The real world view is seen through the mirrors' reflective surface. Experimental systems have been used for gaming, where virtual opponents may peek from real windows as a player moves about. This type of system is expected to have wide application in the maintenance of complex systems, as it can give a technician what is effectively "x-ray vision" by combining computer graphics rendering of hidden elements with the technician's natural vision. Additionally, technical data and schematic diagrams may be delivered to this same equiment, eliminating the need to obtain and carry bulky paper documents.

Augmented stereoscopic vision is also expected to have applications in surgery, as it allows the combination of radiographic data (CAT scans and MRI imaging) with the surgeon's vision.

A recent variation on the anaglyph technique is called "Anachrome method" . This approach is an attempt to provide images that look fairly normal without glasses as 2D images to be "compatible" for posting in conventional websites or magazines. The 3D effect is generally more subtle, as the images are shot with a narrower stereo base, (the distance between the camera lenses). Pains are taken to adjust for a better overlay fit of the two images, which are layered one on top of another. Only a few pixels of non-registration give the depth cues. The range of color is perhaps three times wider in Anachrome due to the deliberate passage of a small amount of the red information through the cyan filter. Warmer tones can be boosted, and this provides warmer skin tones and vividness.


As of January 2006, more than 3,000 educational, or scientific images were offered on-line in this and similar "compatible" formats.

For making stereo images of a distant object (e.g., a mountain with foothills), one can separate the camera positions by a larger distance than usual. This will enhance the depth perception of these distant objects, but is not suitable for use when foreground objects are present. In the red-cyan anaglyphed example at right, a ten-meter baseline atop the roof ridge of a house was used to image the mountain. The two foothill ridges are about 6.5km (4mi.) distant and are separated in depth from each other and the background. The baseline is still too short to resolve the depth of the two more distant major peaks from each other. Owing to various trees that appeared in only one of the images the final image had to be severely cropped at each side and the bottom.

In the wider image, taken from a different location, a single camera was walked about 100 ft (30m) between pictures. The images were converted to monochrome before combination.