Stereo cards intended to be viewed in a stereoscope mode. 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.
Stereographic cards and the stereoscope
Two separate images are printed side-by-side. When viewed without a stereoscopic viewer the user is required to force his eyes either to cross, or to diverge, so that the two images appear to be three. Then as each eye sees a different image, the effect of depth is achieved in the central image of the three.
The stereoscope offers several advantages:
Using positive curvature (magnifying) lenses, the focus point of the image is changed from its short distance (about 30 to 40 cm) to a virtual distance at infinity. This allows the focus of the eyes to be consistent with the parallel lines of sight, greatly reducing eye strain.
The card image is magnified, offering a wider field of view and the ability to examine the detail of the photograph.
The viewer provides a partition between the images, avoiding a potential distraction to the user.
Stereograms cards are frequently used by orthoptists and vision therapists in the treatment of many binocular vision and accommodative disorders.
Source:
Wikipedia
