A Glossary of Stereoscopic Terms

Acknowledgments: This Glossary has taken as its starting point the original Stereo Glossary produced by William C. Dalgoutte in the 1960s (published in the Bulletin of The Stereoscopic Society and later, with amendments, in the Technical Supplements of Stereoscopy), and also draws on a revised and expanded version by Craig Daniels and Dale Hammerschmidt (published in the USA, 1990). The present version has been compiled, with substantial revision of both the earlier versions, by Don Wratten with further input from Mark Brailsford, Olivier Cahen, Colin Clay, Neville Jackson, Abram Klooswyk, Geoff Ogram and Charles Smith.

adv. = advanced concept
arch. = archaic; no longer in general use
coll. = colloquial term
LH/RH = left-hand/right-hand

accidental stereo effectsFound in fusing supposedly identical side-by-side pictures: for example, postage stamps or wall patterns, where irregularities in the printing produce a three-dimensional effect. See also false stereo effects.
accommodationThe refocusing of the eyes as their vision shifts from one distance plane to another.
accommodation-convergence link (adv.)The physiological link which causes the eyes to change focus as they change convergence, a link which has to be overcome in stereo viewing since the focus remains unchanged on the plane of the constituent flat images.
acuity (stereo)See stereo acuity.
alignment gauge (Also “mounting grid”, “mounting guide”)A positioning gauge to assist in achieving accurate placing of left and right film chips when mounting.
alternating perspective figures (adv.)Planar drawings which jump from inside-out to outside-in during viewing.
American stereoscope (arch.)See Holmes stereoscope.
anaglyphA type of stereogram (either printed, projected or viewed on a TV or computer screen) in which the two images are superimposed but are separated, so each eye sees only the desired image, by the use of coloured filters and viewing spectacles (commonly red and cyan, or red and green).
analysersDevices placed in front of the eyes to separate the left and right eye images, mainly when projected. Typically, these are polarising spectacles, anaglyph spectacles or liquid crystal “shutters”.
aperture separation (adv.)The distance between the centres or corresponding edges of the openings in the camera aperture plate; typically 71.25 mm in many traditional stereo cameras of 5-perforation format, or 63.5 mm in 7-perforation format.
autostereogramUsually describes a stereogram designed to be viewed without the aid of special apparatus in front of the eyes (e.g. a lenticular print or a single-image random-dot stereogram.
base, stereoThe distance between the left and right lenses when taking a stereo pair; i.e.
(1) the lens separation in a conventional stereo camera or a pair of mono cameras;
(2) the distance between the lens viewpoints when taking sequential exposures.
Bates stereoscopeSee Holmes stereoscope.
beamsplitterAn optical element that reflects and passes specified percentages of the light rays striking it.

(coll.) A device consisting of prisms and/or mirrors which can be attached to a mono camera to produce two side-by-side images (usually within a single frame). More accurately described as an image-splitter, as it does not split an individual beam into components. Because the groups of light rays forming the left and right images cross over as they pass through the camera lens, the recorded images end up in the correct configuration for stereo viewing without the need for the usual transposition
binocular viewerA two-lensed device for viewing planar pictures where the use of both eyes give more comfortable viewing and enhances the impression of depth.

A stereoscope.
Brewster-type stereoscopeNamed after Sir David Brewster, who invented the lensed, or ‘lenticular’, stereoscope in 1849. Now may be used to describe any type of box-form lensed stereoscope usually for viewing prints.
cardboarding (coll.)A condition where objects appear as if cut out of cardboard and lack individual solidity. Usually the result of inadequate depth resolution arising from, for example, a mismatch between the focal length of the taking lens, the stereo base and/or the focal length of the viewing system.
Cazes stereoscopeA development of the Wheatstone stereoscope by L. Cazes, dating from 1895. It incorporates an extra pair of angled mirrors to view large images of the type used in aerial surveying.
chromatic stereoscopy (or chromostereopsis) (adv.)An impression of depth which results from viewing a spectrum of coloured images through a light-bending device such as a prism, a pinhole or an embossed ‘holographic’ filter, caused by variations in the amount of bending according to the wavelength of the light from differing colours (chromatic dispersion). If such a device is placed in front of each eye, but arranged to shift planar images or displays of differing colours laterally in opposite directions, a 3-D effect will be seen. The effect may also be achieved by the lenses of the viewer’s eyes themselves when viewing a planar image with strong and differing colours.
chrono-stereoscopy (adv.)A stereoscopic effect obtained when in time-lapse photography there has been a change in the subject.

Clemetson attachment

Device (named after the inventor) like a rectangular lens hood mounted on a twin-lens stereo attachment used with a mono camera to prevent light spilling into the ‘wrong’ areas, thus performing a similar function to the internal septum.
Colardeau progressionThe advancement of film in a (4- or 5-perforation) stereo camera by a constant two frames, resulting in stereo pairs separated by two unrelated frames. This progression sequence was designed by L.J.E. Colardeau for use with the first 35mm stereo camera, the Homeos, in 1914, and adopted for use with the first mass-produced stereo camera in the 1950s, the Stereo Realist.
The advantage of this progression is that the film travels the same distance each time it is advanced, allowing a simpler and probably more reliable film advance mechanism; it also achieves greater film economy.. The disadvantage is that the individual frame width is limited to one-third of the distance between the centres of the left and right images, which in practice (with a 71.25 mm aperture separation) restricts it to the 5-perforation (23mm) image size.
confocal stereoscopy (adv.)Focussing the two camera lenses on different planes of the subject matter in order to increase the depth of field perceived in viewing the resulting stereogram.
convergenceThe meeting of lines of sight through the eyes, or light rays through the optical system, at a common point closer than infinity.
corresponding points (on a stereogram)See homologues.
cross-eyed viewingSee free-viewing.
Cyclopean imageThe single (three-dimensional) mental image obtained by the brain’s fusion of the individual view from each eye (from Cyclops, the legendary giant with a single eye centred in his forehead). See also stereopsis.
diplopia‘Double vision’: in stereo viewing, a condition where the left and right homologues in a stereogram remain separate instead of being fused into a single image.
disparate imagesA pair of images which fail as a stereogram (eg, due to distortion, poor trimming, masking, mismatched camera lenses, or the like).
displacementAccidental (or, sometimes, deliberate) misalignment of homologues in a stereogram, often due to movement of the two frames on the mask or mount from their true positions.
distortionIn general usage, any change in the shape of an image that causes it to differ in appearance from the ideal or perfect form.

In stereo, usually applied to an exaggeration or reduction of the front-to-back dimension (coll. stretch and squeeze respectively).
divergenceThe opposite of convergence: a condition where the eyes’ lines of sight ‘toe-out’ from each other.
dwarfism (coll.)See Lilliputism.
European format – (coll.)Term often applied (especially in USA) to denote a 7-perforation 35mm stereo slide image (i.e., one which is 28mm to 30mm wide). Derived from the fact that this format was first used in the Verascope F40 and some other post-war stereo cameras made in Europe.
false stereo effectsCaused, e.g. in water or snow scenes, where one lens picks up a reflection which the other misses and this image is fused with a different reflection picked up by the other lens. Also caused by subject movement between two separate, sequential exposures; eg, drifting of clouds or branches blowing in the wind. See also accidental stereo effects.
far pointThe feature in a stereo image which appears to be farthest from the viewer.
fifty-by-fifty (50 x 50)Transparency mount size, referring in millimetres to images in normal 35mm slide mounts. A stereo pair in 50×50 mounts is usually referred to as 2x50x50 or 2/50×50.
five-perforation formatSee format.
floating edgesUnnatural strips down the window frame of the scene where monocular images ‘float’, caused by the stereo window being located behind some or all of the subject matter. Masking (for transparencies) or trimming (for prints) the outside edge of the picture area gets rid of them by bringing the window sufficiently forward.
foil mountA thin sheet of aluminium in a standard mount size, with window frames cut to the appropriate size for a given format, usually with some means of allowing each film chip to be held firmly yet also to be capable of re-positioning.
fore-window imageAn image that appears in front of the stereo window frame; ie, “coming through the window”. Where an image cuts the edge of the window-frame, the effect is usually referred to as floating edges.
formatIn stereography, usually refers to the dimensions of the window-frame of an image pair, or the method of mounting the pair of images. For 35 mm transparencies, the format is often identified by reference to the number of film-edge perforations (P) in the width dimension of each frame, or by the name of the camera most closely associated with it; e.g., 4P (also ‘Nimslo’ or ‘half-frame’); 5P (also ‘Realist’ or ‘American’); 7P (also ‘Verascope’ or ‘European); 8P (also ‘full-frame’). Print formats are usually identified by the type of mounting; e.g., ‘traditional’ (for side-by-side pairs) or ‘ViewMagic’ (for over-and-under pairs).
frame identification notchA notch on the top or side edge of, usually, the right-hand frame of some traditional stereo cameras, to provide a means of distinguishing it from the left-hand frame to facilitate mounting.
framingThe adjustment of film chips in their mount(s) to include or exclude parts of the scene and to set up the stereo window.

In setting up for stereo projection, the sideways adjustment of the images so that the vertical edges of the frames are coincident.
free-viewingUsually understood to mean the fusion of adjacent left and right image pairs into a stereo image without a viewing aid, by the ability to overcome the accommodation-convergence link, in order to diverge or converge the eyes (while they remain focussed at the viewing distance).
In the case of parallel viewing, the eyesight is made to depart from normal convergence so that the left eye looks only at the left image, and the right eye looks only at the right image.
With cross-eyed viewing, the left eye is made to look at the right image and the right eye at the left image (the images themselves being, of course, transposed from the normal arrangement). An advantage of the latter is that, once the technique has been mastered, larger and more widely separated images can be fused although it has the disadvantage of producing the effect of Lilliputism.
frozen water effectThe appearance (in a stereograph) of running water, particularly in rapids or a waterfall, when shot with too fast a shutter speed; noticeable because the image does not convey the expected sense of movement.
frustum effect (adv.)Front-to-back keystone distortion in the space-image so that a cube parallel to the lens-base is portrayed as the frustum of a regular four-sided truncated pyramid with the smaller face towards the observer. In reverse frustum distortion, the larger face is forward.
fusionThe merging (by the action of the brain) of the two separate views of a stereo pair into a single three-dimensional (or Cyclopean) image.
fusion, irregularFusion of points that are not homologous, as with accidental and false stereo effects and multiple diplopia.
ghost imageThe effect produced by inadequate extinction of a pair of projected overlapping stereo images, so that one eye sees a proportion of the other-eye view as well as the intended view. Hence, ghosting – to describe thecondition.
giantism (coll.)Jargon term for the impression of enlarged size of objects in a stereo image due to the use of a stereo base separation less than normal for the focal length of the taking lens(es). See also hypostereo.
height error (or vertical error)A fault present in a stereogram when the two film chips or prints are not aligned vertically in mounting, so that homologous points are at different heights.
Holmes stereoscopeUsual name for the common type of hand-held stereoscope with an open skeletal frame. Named after its inventor in 1859, the American physician and author, Oliver Wendell Holmes. Where, as is normally the case, the stereoscope includes a hood to shade the eyes and an adjustable card holder, it is more correctly termed a Holmes-Bates (or just Bates) stereoscope (after Joseph Bates who introduced these refinements).
holography“whole drawing”. A technique for producing an image (hologram) which conveys a sense of depth, but is not a stereogram in the usual sense of providing fixed binocular parallax information. In outline, this is accomplished by the recreation on a photographic plate of (spectrally restricted) wave fronts of light that appear as if they emanated from the subjects that were holographed, due to the recording of interference patterns between beams of coherent light scattered by the subject matter. Under certain viewing conditions, an image is obtained which is fully three-dimensional in all directions so that a movement of the head presents another aspect of the object (when in stereo viewing this would present a distortion of the same object). The production of such an image, using lasers and mirrors or prisms, is laborious and difficult and requires laboratory conditions of working.
homologues (or homologous points)Identical features in the left and right image points of a stereo pair. The spacing between any two homologous points in a view is referred to as the separation of the two images (which varies according to the apparent distance of the points) and this can be used in determining the correct positioning of the images when mounting as a stereo pair.
hyperfocal distanceThe distance setting on the focussing scale of a lens mount which will produce a sharply focussed image from infinity to half the distance of the focus setting at any specific lens aperture. Of particular value in stereo photography to ensure maximum ‘depth of field’, so that viewing is not confused by out-of-focus subject matter.
hyperstereoUse of a longer than normal stereo base in order to achieve the effect of enhanced stereo depth and reduced scale of a scene; it produces an effect known as Lilliputism because of the miniaturisation of the subject matter which appears as a result. Often used in order to reveal depth discrimination in architectural and geological features. The converse of hypostereo.
hypostereoUse of a shorter than normal stereo base in order to reduce depth distortion in portraiture and close-up stereography; it produces an effect known as Giantism (qv). The converse of hyperstereo.
image-splitterA device mounted on the front of a single lens which, through the use of mirrors or prisms, divides the image captured on film into two halves, which are the two images of a stereoscopic pair. Sometimes called a frame-splitter, and often imprecisely called a beamsplitter.
infinity, stereoSee stereo infinity.
inter-lens separation (ILS)The distance between the optical centres of the two lenses of a stereo camera or stereoscope, or (in wide-base stereography) between two photographic or viewing positions. Similar to base, stereo.
interocular distanceThe separation between the optical centres of a twin-lens stereo viewer (which may be adjustable). Not necessarily the same as the interpupillary distance of the eyes.
interpupillary distance (IPD)The distance between the centres of the pupils of the eyes when vision is at infinity. IPDs can range from 55 to 75 millimetres in adults, but the average is usually taken to be around 65 mm, the distance used for most resolving calculations and viewer designs.
inversionThe visual effect achieved when the planes of depth in a stereograph are seen in reverse order; eg, when the left-hand image is seen by the right eye, and vice-versa. Often referred to as pseudostereo.
keystoningTerm used to describe the result arising when the film plane in a camera or projector is not parallel to the view or screen. The perspective distortion which follows from this produces an outline of, or border to, the picture which is trapezoidal in shape, resembling the keystone of a masonry arch.
In stereo, the term is applied to the taking or projecting of two images where the cameras or projectors are ‘toed-in’ so that the principal objects coincide when viewed. The proportions of the scene will then have slight differences which produce some mismatching of the outlines or borders of the two images. Gross departures from orthostereoscopic practice (eg, if using telephoto lenses) can produce keystoning in depth; more properly called a frustum effect.
lenticular(arch.) Pertaining to a lens. As used by Brewster to describe his lensed stereoscope.

Shaped like a lens. In stereo, used to describe:
(1) A method of producing a depth effect without the use of viewing equipment, using an overlay of semi-cylindrical (or part-cylindrical) lens-type material which exactly matches alternating left and right images on a specially-produced print, thereby enabling each eye to see only one image from any viewing position, as in an autostereogram.
(2) A projection screen with a surface made up of tiny silvered convex surfaces which spread the reflected polarised light to increase the viewing angle.
Lilliputism (coll.)Jargon term for the miniature model appearance resulting from using a wider-than-normal stereo base in hyperstereography.
macro stereoUltra close-up images, photographed with a much-reduced stereo base in order to maintain correct stereo recession.
mirror stereoscopeA stereo viewer incorporating angled mirrors, as in the Wheatstone and Cazes stereoscopes.
misalignmentIn stereo usage, a condition where one homologue or view is higher or lower than the other. Where the misalignment is rotational in both views, there is tilt; in one view only, twist. Viewing a misaligned stereogram can result in diplopia or produce eyestrain.
monocular areasParts of the scene in a stereo image which appear in one view and not in the other. These can be natural (if behind the stereo window) or unnatural, as in the case of floating edges (if in front of the stereo window).
mountIn stereo usage, a special holder or card used to secure, locate and protect the two images of a stereo pair. Usually, the term includes any framing device or mask which may be incorporated.
mountingThe process of fixing the left and right views to a mask or mount (single or double) so that they are in correct register, both vertically (to avoid misalignment) and horizontally (so that the stereo view is held in correct relationship to the stereo window).
mounting jigA device used to assist in the process of mounting stereo pairs in correct register, usually incorporating an alignment grid placed below the mount holder and a pair of viewing lenses above the film chips to enable each eye to focus on the appropriate image and fuse the pairs.
near pointThe feature in a stereo image which appears to be nearest to the viewer.
NimsloThe brand name, taken from the surnames of inventors Jerry Nims and Allen Lo, for a camera system intended primarily to produce lenticular autostereo prints, incorporating four lenses to record the same number of images (each of 4-perforations width) on 35mm film. The name is often used to identify the size of mask or mount developed to hold 4-perforation-wide pairs of transparencies made with this camera and its derivatives.
one-in-thirty ruleA rule-of-thumb calculation for determining the stereo base when using a non-standard camera lens separation, eg in hyper- or macro- stereography. To achieve optimum stereo depth, the separation of the centres of the camera lenses should be around one-thirtieth of the distance from the lenses to the closest subject matter in a scene. This ‘rule’ only holds good under certain optical conditions (eg where ‘standard’ focal-length lenses are used), and usually needs to be varied when, for example, lenses of longer or shorter than normal focal length are used.
orthostereoscopic image (adv.)An image which appears to be correctly spaced as in the original view. See also, tautomorphic image.
orthoscopic imageA stereoscopic image viewed with its planes of depth in proper sequence, as opposed to an inverse (or pseudo) stereoscopic image.
over-and-under (coll.)A form of stereo recording (on cine film) or viewing (of prints) in which the left and right images are positioned one above the other rather than side-by-side, and viewed with the aid of prisms or mirrors which deflect the light path to each eye accordingly, as in the proprietary Elmo (cine), KMQ and ViewMagic (prints) systems.
Panum phenomenon (adv.)A trick of stereo viewing whereby, if a single vertical line is presented to one eye and two vertical lines to the other, and one of the double lines is fused with the single line in binocular viewing, the unmatched line is perceived to be nearer or further away than the fused line. A concept used in the design of stereo mounting grids. A phenomenon first described by the scientist Panum in 1858.
parallaxGenerally, the differences in a scene when viewed from different points (as, photographically, between the viewfinder and the taking lens of a camera).

In stereo, often used to describe the small relative displacements between homologues, more correctly termed deviation.
parallax stereogramA form of autostereogram which currently describes a technique in which alternate thin vertical strips of the left and right hand views are printed in a composite form and then overlaid with a grating (originally), or (nowadays) a lenticular sheet of cylindrical lenses which presents each view to the correct eye for viewing stereoscopically.
photogrammetryA professional discipline which uses stereography as a basis for scientific measurement and map-making.
planar (image)Flat, or in a single plane; as opposed to stereoscopic or three-dimensional.
polarisation (of light)The division of beams of light into separate planes or vectors by means of polarising filters (first practically applied by Edwin Land of the Polaroid company in the 1930s). When two vectors are crossed at right angles, vision or light rays are obscured. By polarising the light from two projectors in opposite vectors, and by providing viewers with spectacles having similarly polarising filters, each image from a stereo pair – when projected onto a type of screen surface which does not then depolarise the light rays – is seen only by the left and right eye respectively.
progression (in film transport)The amount or method by which film is advanced between exposures in a purpose-built stereo camera. The Colardeau progression moves by an even two frames; the Verascope progression moves by one and three frames alternately.
pseudoscopic, or pseudo (coll.)The presentation of three-dimensional images in inverse order, so that the farthest object is seen as closest and vice-versa: more correctly referred to as inversion. Achieved (either accidentally or deliberately, for effect) when the left and right images are transposed for viewing.
Pulfrich effectTerm now used to describe an illusory stereoscopic effect which is produced when two-dimensional images moving laterally on a single plane (as on a film or television screen) are viewed at slightly different time intervals by each eye, the perceived delay between the eyes being achieved by means of reduced vision in one of them; eg, through the use of a neutral-density filter. The apparent positional displacement which results from this is interpreted by the brain as a change in the distance of the fused image. A scene is produced giving a depth effect, the depth being proportionate to the rate of movement of the object, not to the object distance. The phenomenon was first adequately described in 1922 by Carl Pulfrich, a physicist employed by Carl Zeiss, Jena, in relation to a moving object (a laterally-swinging pendulum).
random-dot stereogramA type of stereogram in which a three-dimensional image is formed by the fusing of apparently randomly-placed dots in a stereo pair: an effect first created manually by Herbert Mobbs of The Stereoscopic Society in the 1920s but scientifically developed, using computer-generated images, by Bela Julesz in the 1960s.
RBTShort for Raumbildtechnik GmbH of Stuttgart/Germany, a company manufacturing stereo equipment mainly for the traditional R-mount size (41 x 101.6 mm), comprising cameras (derived from the siamesing of two individual cameras), projectors, mounts and viewers.
Realist formatThe 5-perforation 35 mm slide format of 23 x 24 mm, originally created by the specification of the Stereo Realist (USA) camera, and subsequently adopted by many other manufacturers.
retinal rivalry (adv.)Retinal rivalry is the simultaneous transmission of incompatible images from each eye.
R-mount, RochwiteName of the designer of the pioneering Stereo Realist camera from the 1950s, and its image-mounting system. A Rochwite mount (or R-mount) is one with overall dimensions of 41 x 101.6 mm (1 x 4 ins.), not more than 3 mm deep; adopted as the standard for traditionally-mounted stereo pairs of 35mm transparencies.
rotationTilting of the images through not holding the camera horizontally, causing one lens to be higher than the other at the picture-taking stage. If the tilting is not too severe, it may be possible to straighten both images when mounting but there will be a height error, however small, in part of the image.

A difference in the alignment of the two images in a stereogram caused by faulty mounting (see also twist).
Savoy format (adv.)A stereo format produced by prisms or other forms of image-splitter on a planar camera, side-by-side for still images and over-and-under for cine images.
separation (interaxial)The distance between two taking positions in a stereo photograph.

Sometimes used to denote the distance between two homologues.
septumThe partition used in a stereo camera to separate the two image paths.

Any partition or design element that effectively separates the lines of sight of the eyes such that only their respective left and right images are seen by each one.
sequential stereographA stereo pair of images made with one camera which is moved by an appropriate separation between the making of the LH and the RH exposures.
siamese (coll.)Used as a verb, to assemble a stereo camera from the relevant parts of two similar planar cameras. Therefore, siamesed (adjective) to describe the finished assembly.
single-image random dot stereogram (SIRDS)A form of random dot stereogram in which the stereo pair is encoded into a single composite image which each eye has to decipher separately. Popularised in the ‘Magic Eye’ type books of the 1990s.
slide barA device for taking sequential stereo pairs of non-moving subjects, enabling a planar camera to move by an appropriate separation (qv) whilst holding the camera in correct horizontal register with the optical axes either parallel or ‘toed-in’ to create a convenient stereo window.
squeeze (coll.)Diminution of depth in a stereogram in relation to the other two dimensions, usually resulting from a viewing distance closer than the optimum (especially in projection). The opposite effect to stretch.
stereo- [from Greek stereos, = ‘solid’]Having depth, or three-dimensional: used as a prefix to describe, or (coll.) as a contraction to refer to, various stereographic or stereoscopic artefacts or phenomena.
stereo acuityThe ability to distinguish different planes of depth, measured by the smallest angular differences of parallax that can be resolved binocularly.
stereo infinityThe farthest distance at which spatial depth effects are normally discernible, usually regarded as 200 metres for practical purposes.
stereo windowThe viewing frame or border of a stereo pair, defining a spatial plane through which the three-dimensional image can be seen beyond (or, for a special effect, ‘coming through’).

A design feature in some stereo cameras whereby the axes of the lenses are offset slightly inwards from the axes of the film apertures, so as to create a self-determining window in the resulting images which is usually set at around an apparent 2 metres distance from the viewer. (See toeing-in.)
stereogramA general term for any arrangement of LH and RH views which produces a three-dimensional result, which may consist of
(1) a side-by-side or over-and-under pair of images,
(2) superimposed images projected onto a screen,
(3) a colour-coded composite (anaglyph),
(4) lenticular images,
(5) a vectograph or
(6) in film or video, alternate projected LH and RH images which fuse by means of the persistence of vision.
stereographThe original term, coined by Wheatstone, for a three-dimensional image produced by drawing; now denoting any image viewed from a stereogram.

In more general but erroneous usage as the equivalent of stereogram.
stereographoscopeAn early type of stereoscope which also carries a large monocular lens (above the two regular stereoscopic lenses) for the viewing of planar photographs.
stereographerA person who makes stereo pictures.
stereographyThe art and practice of three-dimensional image making.
stereopsisThe physiological and mental process of converting the individual LH and RH images seen by the eyes into the sensation and awareness of depth in a single three-dimensional concept (or Cyclopean image).
stereopticon (arch.)Term sometimes (erroneously) used to describe a stereoscope. First used (1875) to identify a dissolving twin-image magic lantern which could be used to convey information about depth by the blended sequential presentation of a series of planar views of a subject; later applied to some other kinds of non-stereo projectors.
stereoscopeA device for viewing stereograms, employing prisms, lenses or mirrors to facilitate vision and the fusion of images.
stereoscopic‘Solid looking’: having visible depth as well as height and width. May refer to any experience or device that is associated with binocular depth perception.
stereoscopyThe reproduction of the effects of binocular vision by photographic or other graphic means.

stretch (coll.)The elongation of depth in a stereogram in relation to the other two dimensions, usually caused by viewing from more than the optimum distance, especially in projection. The opposite effect to squeeze.
tautomorphic image (adv.)A stereoscopic image which presents the original scene to the viewer exactly as it would have been perceived in life; ie, with the same apparent scale, positions of scenic elements, and a stereo magnification of x1 for all subject matter in the view.
Taxiphote viewerA form of cabinet viewer devised by the Jules Richard company for viewing a collection of stereograms in sequence, and continuously.
three-dimensional (or 3-D)A modern-usage alternative to the term stereoscopic, derived from Descartes’ system of coordinates which assigns two dimensions to a plane (usually denoted x and y) and three dimensions (x, y and z) to a solid space. [Note: The term ‘3-D’ has been misappropriated by the computer industry to denote the process of on-screen perspective modelling with a depth coordinate but in a single plane – pseudo 3-D.]
tissueIn stereo usage, an early type of stereogram on translucent paper in a card frame, often tinted and sometimes with pin-pricked highlights designed for viewing with backlighting.
toeing-inThe technique of causing the optical axes of twin planar cameras to converge at a distance point equivalent to that of a desired stereo window, so that the borders of the images are coincident at that distance (apart from any keystoning which results).
transpositionThe changing over of the inverted images produced by a stereo camera to the upright and left/right presentation necessary for normal viewing. May be achieved optically by means of a transposing camera or viewer, or mechanically by means of a special printing frame, as well as manually during the mounting of images.
Tru-VueProprietary name of a commercial stereo transparency viewing system which presents a series of views in a film-strip sequence on a single card mount.
twist (coll.)Rotational displacement of one view in a stereo pair in relation to the other.
vectograph (adv.)A form of polarisation-coded stereogram (originally devised by the Polaroid company) in which the images are mounted on the front and rear surfaces of a transparent base, and are viewed by polarised light or through polarised filters. The polarised equivalent of an anaglyph stereogram.
Verascope format, progressionsee format, progression and European format.
ViewMagicProprietary name of a commercial stereo print viewing system (utilising angled periscope-type mirrors) for over-and-under mounted prints; the name now also being used to identify this mounting format.
View-MasterProprietary name of a commercial stereo transparency image display and viewing system utilising stereo pairs (7 in total) mounted in a circular rotating holder, and viewed with a purpose-made stereo viewer.
virtual realityA system of computer-generated 3-D images (still or moving) viewed by means of a headset linked to the computer which incorporates left-eye and right-eye electronic displays. The controlling software programs often enables the viewer to move interactively within the environment or ‘see’ 360° around a scene by turning the head, and also to ‘grasp’ virtual objects in the scene by means of an electronically-linked glove.
Wheatstone stereoscopeA ‘reflecting’ or mirror stereoscope in which a pair of images (which need to be reversed) are placed facing each other at either end of a horizontal bar and viewed through a pair of angled mirrors fixed midway between them. Named after Sir Charles Wheatstone who devised this earliest form of stereoscope in 1832, prior to the advent of photography.