Know-How

3D Display Technologies Overview(2)--No Device required

Autostereoscopic Displays
No viewing devices required

Free Viewing
With practice, most readers can view stereo pairs without the aid of blocking devices using a technique called free viewing. There are two types of free viewing, distinguished by how the left and right eye images are arranged. In parallel, or uncrossed viewing the left eye image is to the left of the right eye image. In transverse or cross viewing, they are reversed and crossing the eyes to form an image in the center is required. Some people can do both types of viewing, some only one, some neither. In Fig. 15, the eye views have been arranged in left/right/left order. To parallel view, look at the left two images. To cross view, look at the right two images.
Figure 15 Free viewing examples
Figure 16 is a random dot autostereogram in which the scene is encoded in a single image as opposed to a stereo pair [9]. There are no depth cues other than binocular disparity. Using cross viewing, merge the two two dots beneath the image to view the functional surface. Crossing your eyes even further will produce other images. (See [10] for a description of how to generate these interesting images).
Figure 16 A random dot autostereogram
Cos[(x2 + y2)(1/2)] for -10 x,y 10

Holographic Stereograms
Most readers are familiar with holographic displays, which reconstruct solid images.
Normally a holographic image of a three dimensional scene will have the “look around” property.A popular combination of holography and stereo pair technology, called a holographic stereogram, involves recording a set of 2D images, often perspective views of a scene, on a piece of holographic film. The film can be bent to form a cylinder so the user can walk around the cylinder to view the scene from any aspect. At any point the left eye will see one view of the scene and the right eye another or the user is viewing a stereo pair.
Conventional display holography has long been hampered by many constraints such as
limitations with regard to color, view angle, subject matter limitations, and final image size. Even with the proliferation of holographic stereogram techniques in the 1980s, the majority of the constraints remained. Zebra Imaging, Inc. expanded on the developments in one-step holographic stereogram printing techniques and has developed the technology to print digital full-color reflection holographic stereograms with a very wide view angle (up to 110°), unlimited in size, and with full parallax.
Zebra Imaging’s holographic stereogram technique is based on creating an array of small (1mm or 2mm) square elemental holographic elements (hogels). Much like the pixels of two-dimensional digital images, hogel arrays can be used to form complete images of any size and resolution. Each hogel is a reflection holographic recording on pan-chromatic photopolymer film. The image recorded in each hogel is of a two-dimensional digital image on a spatial light modulator (SLM) illuminated with laser light in the three primary colors: red, green, and blue (Figure 17)
Figure 17 Zebra Imaging holographic stereogram recording

Parallax Barrier Displays
A parallax barrier [2] consists of a series of fine vertical slits in an otherwise opaque medium. The barrier is positioned close to an image that has been recorded in vertical slits and back lit. If the vertical slits in the image have been sampled with the correct frequency relative to the slits in the parallax barrier, and the viewer is the required distance from the barrier, then the barrier will occlude the appropriate image slits to the right and left eyes respectively and the viewer will perceive an autostereoscopic image (Fig. 18). The images can be made panoramic to some extent by recording multiple views of a scene. As the viewer changes position, different views of the scene will be directed by the barrier to the visual system. The number of views is limited by the optics and, hence, moving horizontally beyond a certain point will produce “image flipping” or a cycling of the different views of the scene.
High resolution laser printing has made it possible to produce very high quality images: the barrier is printed on one side of a transparent medium and the image on the other. This technique was pioneered by Art^n in the early 1990’s to produce hard copy displays and is now being used by Sanyo for CRT displays.
Figure 18 Parallax Barrier Display

Lenticular Sheets
A lenticular sheet ([1], [2]) consists of a series of semi-cylindrical vertical lenses called
“lenticles,” typically made of plastic. The sheet is designed so the parallel light entering the front of the sheet will be focused onto strips on the flat rear surface (Fig. 19). By recording an image in strips consistent with the optics of the lenticles as in the case of the parallax barrier display, an autostereoscopic panoramic image can be produced. Because the displays depend on refraction vs. occlusion the brightness of a lenticular sheet display is usually superior to the parallax barrier and requires no back-lighting. Such displays have been mass produced for many years for such hardcopy media as postcards.
Figure 19 Lenticular Sheet Display
In these two techniques the image is recorded in strips behind the parallax barrier or the
lenticular sheet. Although the techniques are old, recent advances in printing and optics have increased their popularity for both hardcopy and autostereoscopic CRT devices.
In both the lenticular and parallax barrier cases, multiple views of a scene can be included
providing motion parallax as the viewer moves his/her head from side to side creating what is called a panoramagram. Recently, parallax barrier liquid-crystal imaging devices have been developed that can be driven by a microprocessor and used to view stereo pairs in real time without glasses.
Some of these techniques are discussed later.

Alternating Pairs
The output from two vertically mounted video cameras are combined. A integrating circuit was designed to merge the two video streams by recording a fixed number of frames from one camera followed by the same number of frames from the other camera. The technique imparts a vertical rocking motion to the image. If the scene has sufficient detail and the speed of the rocking motion and the angle of rotation is appropriate for the individual viewing the system, most viewers will fuse a 3D image. The system was commercialized under the name VISIDEP. The technique can be improved using graphical and image processing methods. More details can be found in [2].

Moving Slit Parallax Barrier
A variation of the parallax barrier is a mechanical moving slit display popularized by Homer Tilton he called the Parallactiscope [2]. A single vertical slit is vibrated horizontally in front of a point plotting output display such as a CRT or oscilloscope. The image on the display is syncronized with the vibrating to produce an autostereoscopic image. Many variants have been proposed but to date the author knows of no commercially viable products using the technique.
The DTI System
The Dimension Technologies, Inc. (DTI) illuminator is used to produce what is known as a multiperspective autostereoscopic display. Such a display produces multiple images of a scene, each of which is visible from a well defined region of space called a viewing zone. The images are all 2D perspective views of the scene as it would appear form the center of the zones. The viewing zones are of such a size and position that an observer sitting in front of the display always has one eye in one zone and the other eye in another. Since the two eyes see different images with different perspective, a 3D image is perceived. The DTI system is designed for use with an LCD or other transmissive display. The LCD is illuminated from behind, and the amount of light passing through individual elements is controlled in order to form a full color image.
The DTI system uses an LCD back-light technology which they call parallax illumination [11]. Figs. 20 and 21 illustrate the basic concept. As shown in Figure 20, a special illuminator is located behind the LCD.

Figure 20 DTI Illuminator
The illuminator generates a set of very thin, very bright, uniformly spaced vertical lines. The lines are spaced with respect to pixel columns such that (because of parallax) the left eye sees all the lines through the odd columns of the LCD while the right eye sees them through even columns. There is a fixed relation between the distance of the LCD to the illumination plate, and the distance of the viewer from the display. This in part determines the extent of the “viewing zones.” As shown in Fig. 21,

Figure 21 Viewing Zones
viewing zones are diamond shaped areas in front of the display where all the light lines are seen behind the odd or even pixel columns of the LCD.
To display 3D images, left and right eye images of a stereoscopic pair are placed on alternate columns of elements. The left image appears on the odd columns, while the right image is displayed on even columns. Both left and right images are displayed simultaneously and hence the display is time parallel. Since the left eye sees the light lines behind the odd columns, it only sees the left eye image displayed on the odd columns. Similarly, the right eye, sees only the right eye image displayed on the even columns.

The 2D/3D Back-light System
There are many ways to create the precise light lines described above. One method that is used in DTI products is illustrated in Figure 22 ([12], [13]). The first component is a standard off the shelf back-light of the type used for conventional 2D LCD monitors. This type of back-light uses one or two miniature fluorescent lamps as light sources in combination with a flat, rectangular light guide. For large displays, two straight lamps along the top and bottom of the guide are typically used. For smaller displays a single U shaped lamp is typically used. An aluminized reflector is placed around the lamp(s) to reflect light into the light guide

Figure 22 Back-light System
The flat, rectangular light guide is typically made of acrylic or some other clear plastic. Light from the lamp enters the light guide from the sides and travels through it due to total internal reflection from the front and back surfaces of the guide.
The side of the light guide facing away from the LCD possesses a pattern of reflective
structures designed to reflect light into the guide and out the front surface. Several possible choices for such structures exist, but current manufacturers usually us a simple pattern of white ink dots applied to the rear surface of the light guide in combination with a white reflective sheet placed behind the light guide.
The second component is a simple, secondary LCD which, when in the “on” state, displays a pattern of dozens of thin, transparent lines with thicker opaque black stripes between them. These lines are used for 3D imaging as described in the previous section
The third major component is a lenticular lens, again shown in Figure 5.8. This lens consists of a flat substrate upon the front surface of which are molded hundreds of vertical, parallel cylindrical lenslets. Light coming through the dozens of thin transparent lines on the secondary LCD is reimaged into thousands of very thin, evenly spaced vertical lines by a lenticular lens array spaced apart from and in front of the secondary LCD. The lines can be imaged onto an optional front diffuser located in a plane at one focal length from the lenticular lenslets. The pitch (center to center
distance) of the lines on the light guide and the lenticular lenses must be chosen so that the pitch of the light lines re imaged by the lenticular lenslets bears a certain relationship to the pitch of the LCD pixels.
Since the displays will likely all be used for conventional 2D applications (such a word
processing and spreadsheets) as well a 3D graphics, the system must be capable of generating illumination in such a way that each eye sees all the pixels of the LCD so that a conventional full resolution 2D image can be displayed with conventional software.
Note that when the secondary LCD is off, in other words in the clear state where the lines are not generated, the even diffuse light from the back-light passes through it freely, and remains even and diffuse after being focused by the lenticular lens. Therefore, when the secondary LCD is off, no light lines are imaged and the observer sees even, diffuse illumination behind all the pixels of the LCD. Therefore, each of the observer’s eyes can see all the pixels on the LCD and full resolution 2D images can be viewed.
DTI sells two displays: a 15” at $1699, with optional video input at $300 extra, and an 18.1” at $6999 video included. Both have 2D and 3D modes and accept the standard stereo formats (field sequential, frame sequential, side by side, top/bottom).

Tags:3D,stereo, hologram,LCD,DTI