(3) binoculars. All three systems are Stereoscopic visual displays: Principles, improved by providing a viewing hood to position the O's head in front of the device to reduce extraneous viewing devices, alignment procedures* stimulation. The viewing hood and prisms (or mirrors) usually are GEORGE SPERLINGt attached to one end of a sturdy, Bell Telephone Laboratories, Murray Hill, New Jersey 07974 lightweight frame. The other end of and the frame should be quickly and New York University, New York, New York 10003 conveniently attachable to the display device itself in order to provide rapid The principle of binocular (stereoscopic) depth perception is that the visual changeovers between normal and system interprets the slight differences between the views seen by the two eyes stereoscopic viewing of the visual as depth cues. In computer-generated displays, two slightly different images are display. The binoculars system also produced on the left and right halves of the display surface and viewed by a prism, mirror, or binoculars system that delivers the appropriate image to each eye. The prism system is the simplest, the mirror system gives the best optical quality, and the binoculars system is useful for producing large apparent images from small display surfaces. All three systems can be adapted for group viewing and all require careful alignment (null adjustment of accommodative distance and vergence distance). Objective and subjective methods of alignment are described. (0) THE PRINCIPLE OF STEREOSCOPIC limitations of image quality and of DEPTH PERCEPTION information content.1 In normal vision, the two eyes receive slightly different views of the VIEWING DEVICES world, and the differences between the I shall be concerned here not with views are interpreted by the visual the psychological problem of how to system as visual depth. The name given compose a stereogram in order to to the depth perception that arises provide the desired illusions of depth,2 from differences between the images but rather with the simple technical viewed by the eyes is stereoscopic problem of how to use a cathode ray (b) depth perception; the two oscilloscope (CRO) display system corresponding images are called a economically and efficiently to stereo pair (of images) or a stereogram, provide two different views. Basically, L R when they are produced artificially. there are only two methods of The principle of stereoscopic depth producing the two images of the perception is illustrated in Fig. 1. stereogram: (1) to use two different In normal vision there are many CROs, one for each image, and (2) to other important cues to depth, such as use two different subareas of a single those provided by the change in view CRO display surface (e.g., the left and produced by a head movement, the right halves), and to view each subarea interposition of objects, perspective separately with each eye. transformations, sizes of known The method of two CROs is more objects, distance haze, vergence of the expensive and has inherent in it the (c) eyes, etc. Of these, only the depth cue problem of adjusting the two CROs to produced by head movements is not be electronically equivalent. The readily simulated by a visual display two-CRO solution is useful mainly system which presents an appropriate when it is necessary to separate the stereogram to the two eyes. In fact, stereogram members (not readily Fig. 1. The principle of stereoscopic most stereoscopic viewing devices possible on a single display surface depth perception. (a) Top view of an require the head to remain still; even because the two display areas are o viewing two surfaces (8" 8, ). The when they do not, head movements constrained to be adjacent to each two eyes see the nearer surface against are relatively easy to measure. As this other) and when independent different parts of the background, as head-movement information readily hardware control of such parameters indicated. The displacement (between can be incorporated into a as x- and y-axis magnification is the two views) of the foreground stereoscopic display system, the desired. For example, to control relative to the background is the simulation of normal binocular vision independently the stimulus area in a stereoscopic depth cue. (b) The by a computer display system large annular neighborhood of each stereogram, i.e., the views seen by the introduces virtually no limitations that computer-generated display, it is left 'and right eye. The stereo pair of are not already inherent in convenient to separate the stereogram images may, of course, be produced monocularly viewed displays, i.e., the members. In principle, however, the either on a visual display surface or by problems of viewing two CROs are the viewing a natural object. *Originally presented at the North same as those in viewing two subareas (c) Perspective drawing of the illusion Atlantic Treaty Organization, Advanced of one CRO, and the examples will of depth that results from viewing the Studies Institute on: On-Line Computing stereogram in (b). Note that the for Behavioral Science, Department of refer only to the latter case. Psychology. Sheffield. England, July 19. The three main methods of viewing projection of 8, against 8, in (b) 1970. subareas of a display surface separately exaggerates its size; when S, is seen in tRequests for reprints should be sent to: with each eye are illustrated in Fig. 2; stereoscopic depth (c), the apparent G. Sperling, Room 20518. Bell Telephone Laboratories, Murray Hill, New Jersey they are (1) a prism viewing system, size of 8, approximates its actual size 07974. (2) a mirror viewing system, and (a) rather than its projected size (b). 154 Behav, Res. Meth. & Instru., 1971, Vol. 3 (3) "smaller" stereogram than a near viewer, but he perceives differently shaped objects because perceived eRO depth relations do not vary linearly with viewing distance. Whereas the shape of a two-dimensional object remains invariant with changes in viewing distance, the three-dimensional shape of an object defined by a stereogram changes as the viewing distance changes. In producing M stereograms, the absolute retinal size at which they will be viewed must be taken into account, and the viewer must also consider this. Mirror Method (Wheatstone, 1838) The advantages of the mirror method are: it is virtually free from optical distortion when good quality front-surface mirrors are used; it is extremely easy to adjust by rotating (0) (b) (e) the mirrors slightly; and it allows Fig. 2. Three stereoscopic viewing systems: (a) prisms, (b) mirrors convenient placement of masks to (c) bin~culars. Land R indicate the nominal centers of the left and right images: limit the field of view, thereby giving respectively, of the stereogram: M = mask, eRO =cathode ray oscilloscope (the good control of the overlap area (the visual display surface). Dashed lines indicate lines of sight extended to their shared display area between the two intersection. In (a) and (b) their intersection occurs at the accommodative subareas which serves as a buffer zone distance of the display indicating correct alignment of the systems. In (c), the for each subarea). The disadvantage of accommodative distance is assumed to be infinity, and the lines of sight are the mirror system relative to the prism parallel, again indicating correct alignment. system is that it is more complicated to construct. Like prisms, mirrors can can be made detachable, but as the kinds of glass, combined so as to be worn as spectacles to facilitate binoculars can be located far from the minimize the chromatic aberration group viewing. display, they do not interfere with inherent in the use of an elementary normal viewing and can be left prism.) In a useful variation of the permanently in place. prism method, the single prism (in either or in both eyes) is replaced by a Prism System (Brewster, 1856) pair of equal prisms. The prisms in The advantage of the prism viewing such a pair can be rotated against each system is that it is the easiest to other to produce-in effect-a single H---+--················V implement. A prism is needed only in prism of variable power. This is a one eye, and this may even be held by convenience, but it adds to optical hand. In fact, with practice, Os can distortion. learn to deviate their eyes (either The prism method for group diverged or crossed) so that the prism viewing. With some slight is superfluous. However, unnatural modifications, the prism method can deviation of the eyes is undesirable for be adapted for group viewing. The reasons to be discussed later. prism (or prisms), preferably I The disadvantages of the prism adjustable, is fitted on spectacle , I method are: the prism produces a frames that are worn either alone or I distortion of straight lines ("prism immediately in front of the viewer's \ distortion"); the power of a prism usual spectacles (when these are used). \ varies depending on the eye's direction Because there is now no \ of gaze and on their vergence, and this centerline-separation shield, each eye effect interacts with any lenses the 0 will see both images of the stereogram, H-.J-~LI....u may be using (Fry, 1937; Ogle, 1951); i.e., the entire display. This extraneous the prism reduces the optical quality stimulation may create serious viewing of the image being viewed (particularly problems. The simplest way to restrict by introducing chromatic aberration, the viewing of each eye to its intended which is disturbing against dark member of the stereo pair is by means backgrounds); and the of a corresponding pair of crossed cen terline-separation shield, which Polaroids, one pair of which is fixed to F extends forward from the nose to the the display surface and the other to center of the display, is an awkward the spectacle lenses (Fig.