3D displays using light from an undiffused point-source array
To form parallax-based 3D representations, one must produce a set of images that are seen from different viewing directions. These viewing zones are typically formed by a combination of optics and a display.1 There are many different kinds of viewing-zone-forming optics, but lenticular and parallax-barrier plates, in particular, are frequently used to realize ‘contact-type’ 3D displays. In this arrangement, the optical element is laminated on the front surface of the display panel. However, this results in reduced image quality due to brightness reduction, submersion of the image, creation of moiré patterns, and projection of the pattern of the optics. These undesirable visual effects can be removed completely by replacing the contact optic plate in front of the liquid-crystal (LC) panel with a 2D array of point light sources behind it.
Most current LCDs consist of an LC panel illuminated from behind by a back-light unit (BLU). A typical BLU is designed to produce diffuse light with a specific diffusing angle. By replacing the BLU with an array of point light sources, the LC panel is now illuminated by undiffused light. This change in the structure of the LCD enables 3D image display. The optical geometry of a typical contact-type 3D-display system is shown in Figure 1. The LC panel is divided into pixel cells of equal shape and size. (The pixel cell is a basic unit of the multiview images on the panel.) The image on each pixel cell can be viewed from a direction defined by the relative position of the pixel cell in the display panel and a corresponding point light source in each image.
The number of elements in the viewing-zone-forming optics is the same as the number of pixel cells. We designed a geometry such that each pixel cell is imaged by its corresponding lens and its image overlaps with other pixel-cell images at the viewing-zone cross-section. In this geometry, the relative positions of the viewing-zone-forming optics and the point-light-source array are the front- and back-node planes of the display panel, respectively, which are defined when the four corners of the viewing-zone cross-section are aligned with those of the pixel cells.
The point light sources in the array should have the following properties: the emitting surface area should be much smaller than the pixel size and the beam-divergence angle should be as large as possible. This angle should at least cover a pixel cell uniformly and the light should satisfy the brightness requirement of the display system. Figure 2 shows our 3D image display using a full-color LED array as the point-light-source array. Since each full-color LED has an emitting area larger than 400μm on a side, four pixels are combined to represent each image pixel. This dramatically reduces the resolution, so only characters and objects with simple shapes can be displayed.
Other groups have developed different methods to form a point-source array. The Holovizio system,2 commercialized by Holografica, uses the light from many projector chips, which are all focused in a plane. In this case, the plane works as a point-light-source array. That system places a diffusing screen a certain distance from the image plane to form a pixel cell from each of the focused points. Another technique, the focused-light-array method,3 is based on a similar principle.
Although our demonstration used large pixels,4 a development path is apparent. From the geometry in Figure 1 one can imagine that if the distance between the point light source and its corresponding pixel cell becomes increasingly smaller, the pixel cell also becomes smaller and smaller. Eventually, the cell will have the same size as the emitting surface area of the point light source. This should allow higher-resolution 3D displays based on existing technologies.
Jung-Young Son is a chair professor at the School of Computer and Communication Engineering of Daegue University. His areas of interest include 3D imaging, electroholography, and laser-based optical instrumentation.
