frontline technology Toward the Ultimate 3-D Display Holographic stereography and a new photorefractive material may help create a 3-D display that reproduces all human visual cues. by Pierre-Alexandre Blanche LET US START WITH a statement artistic touches can be misleading in cases display refresh rate only needs to be doubled about displays: “It is all about the viewer.” where the display is used for critical analysis. to 60 or 120 Hz – reasonable frequencies for Understanding human physiology and how we You do not want a surgeon to think the artery the current technology in both cinema and perceive depth should lead display technology is farther away than it is in reality; you do not television. This convenience is the main toward a flawless and thus successful 3-D want the intelligence analyst to think the reason for stereoscopy’s recent commercial system. What is true for the consumer market structure or building is smaller than it is on success, in theaters at least. Nonetheless, is even more relevant for special applications the field. Discrimination in these cases can be there are a few issues with this approach, one such as medical imaging, remote training, or improved when more cues are provided, thus of which is that almost every stereoscopic intelligence analysis. In these fields, the 3-D the focus on integrating stereopsis, vergence, technique today requires some sort of eye- image can incorporate elements or data from and accommodation into the display environment. wear. The public has been fairly accepting of different instruments or sensors, making the this eyewear tradeoff when it comes to experi- fused information complex to analyze. But a Stereopsis, Vergence, and Accommodation encing immersive 3-D, as theater attendance complex image does not, and should not, be Stereopsis and vergence are phenomena has shown over the last couple of years. Like- difficult to interpret. In areas where informa- resulting from binocular vision. Different wise, millions of people wear prescription tion interpretation is critical, the display ought images are perceived by the left and right eye glasses without complaint – because the bene- to be perfectly adapted to the decision maker. due to their lateral separation, and these fits far outweigh the discomfort. Several cues are used by the brain to images are interpreted by the brain to deduce Another limitation with stereoscopy is that determine absolute and relative distances1: distance. Stereopsis is a consequence of the it does not reproduce motion parallax: When occlusion, relative size of objects, atmospheric binocular disparity (mostly the parallax shift), the viewer moves in front of the display, the scattering, texture, and shading are already while vergence is the rotation of the eyes in viewpoint does not change. You cannot look exploited in the case of 2-D displays. Indeed, opposite directions around their vertical axis around an object as in real life. For a station- we did not wait for the introduction of stereo- to fixate on the same point of an image [see ary audience such as one in a theater, this is scopic theater or television to understand Figs. 1(a) and 1(b)]. not an issue at all. Motion parallax can also relative positions of elements in a scene. Stereoscopic displays use those cues to be addressed by a head-tracking system that When an explosion happens in the back- reproduce 3-D by employing a technique that determines the position of the viewer and ground in a 2-D movie, we understand why was introduced in 1838 by Sir Charles Wheat- calculates the viewpoint in real time.3 the hero remains unharmed. A film director stone. In a variety of implementations – such However, head tracking can be implemented can exploit these cues (and the absence of as color filters (anaglyph), polarizers, or shut- for one and only one viewer at any given others) to make objects appear closer or ters – stereoscopic displays present a different moment. For a multi-viewer audience, there farther away than they are in reality. The use image to the left and the right eye and prevent would be one master directing the display, of a telephoto lens is such an example because crosstalk by using some sort of eyewear.2 resulting in awkward sensations for the other it normalizes the size and distance difference Stereoscopy has many advantages: it is viewers. between near and far objects. However, these robust, quite easy to implement, and effective While the mandatory eyewear and the lack to a certain extent. It only requires a pair of of motion parallax are mild concerns with Pierre-Alexandre Blanche is with the images to be recorded and does not dramati- stereoscopic approaches, the most serious College of Optical Sciences, The University cally change the image-capture techniques problem is the accommodation-vergence of Arizona, Tucson, AZ. He can be reached at (side-by-side cameras). In the case of polar- conflict. Indeed, even if the vergence cue is [email protected]. ization and active-shutter technologies, the correctly reproduced by a stereoscopic 32 Information Display 2&3/12 0362-0972/03/2012-032$1.00 + .00 © SID 2012 vergence changes and is correctly reproduced, Vergence but the accommodation is fixed and thus Accommodation incorrect. This conflict can lead to eye fatigue and related physiological effects such as a) c) headache, dizziness, and nausea. However, there is a certain degree of mismatch that the brain can tolerate, especially when the screen is far away, as in a theater. At large distances, accommodation is not used, and the eye lens Far stays at rest. This is why the vast majority of people can appreciate a 3-hour movie in a stereoscopic theater. But for displays that are located much closer to the viewer, such as a television or a workstation, the conflict becomes more pronounced and the effects can be intolerable.5 This is especially the case in professional environments where workers spend most of their time looking at the display, or for young children, whose visual b) d) systems are still in development.6 This is the fundamental reason why better techniques are needed to reproduce 3-D. That brings us to holography. Near Holography Holography is the reproduction of both the amplitude and the phase of a scene by a diffractive pattern. We are familiar with systems that display amplitude, or intensity. From a photograph to an LCD TV, every display reproduces the light intensity. The Fig. 1: Accommodation and vergence cues differ for a far or near object. phase, or wavefront of a light field, is less common and describes how the light wave is system, the accommodation is not. This image must form on the retina. For such a particularly curved at each given point of a infor-mation conflict inside the brain leads to system, a sharp image is obtained for only one scene. For a 2-D image, the wavefront is flat asthenopia, i.e., visual fatigue, including position of the object. When the object posi- because each emission point is at the same headache, nausea, and motion sickness.4,5 tion changes, the image position changes distance from the viewer (the Huygens– Vergence is the rotation of both eyes along accordingly unless the focal length of the lens Fresnel principle) but for a real scene, objects their vertical axis to fix on a common point in is adjusted, exactly describing what happens at the forefront have a more convex wave a scene. It is shown in Figs. 1(a) and 1(b) in the eye. The eye lens is not rigid and can pattern than elements in the background. This how the eyes rotate so images are formed near be deformed by the ciliary muscles to change is precisely why the eye needs to accommo- the fova spot, which is responsible for sharp its optical power. This is the basis of the date. It is now obvious why holography is the central vision. The brain interprets the tension accommodation cue: the adjustment of the eye ultimate technique to display 3-D: it recon- on the eye muscles (media and lateral rectus) lens to focus on objects at various distances structs the correct light field, and in doing so to aid in determining the object’s distance. If [see Figs. 1(c) and 1(d)]. Accommodation is all of the optical cues are reproduced. that object is in the forefront of the scene, the not related to stereoscopic vision, since you There is a catch. The reason why we still eyes rotate inward, causing the lines of sight still need to accommodate when viewing with do not have holographic television or theater, to cross near the viewer. If the object is a single eye. It should be clear by now why no even though holography was discovered in farther away, the eyes rotate outward to make stereoscopic technique correctly reproduces the 1947, is due to the last two terms of the the lines of sights more parallel. One can see accommodation required by a true 3-D scene: above-mentioned holographic definition: the why this cue is correctly reproduced by stere- when looking at the display, your eyes accom- “diffractive pattern.” To diffract light, the oscopy, since the basis of this technique is modate to the position of the emitter plane pattern (you can read pixel) needs to be of the laterally shearing the left and right image (the screen), and this position, or distance scale of the wavelength. For visible light, this elements according to depth. from observer to image, remains constant. is around 500 nm. Now, if you want a reason- But the eye is also a single-lens optical As shown in Fig.