Evolving Technologies for LCD-Based 3-D Entertainment
Two technologies using polarized glasses with retarders to create home-based 3-D display imagery are presented. The leading 3-D technology on the market today is glasses-based, and the author proposes that polarizer-glasses-based technology, from a viewer-friendly point of view, will lead the premium 3-D market in the near future.
by Jeong Hyun Kim
THE PAST 2 YEARS have seen rapid growth in the 3-D industry, in both content and display technology. In terms of content, 3-D movies have made viewers comfortable with 3-D imagery. And the industry for 3-D broadcast content is also growing, with users enjoying events such as the World Cup in 3-D in their own homes. Gaming, too, has become a strong content area for 3-D.
In terms of display technology, 3-D is divided into two major categories: autostereoscopic (non-glasses-based 3-D) and stereoscopic (glasses-based 3-D). The autostereoscopic displays use lenses or barrier arrays in front of the displays, and the lenses or barriers control the paths of light from the display, projecting them to where the viewer is positioned. Viewers do not have to wear glasses, but they must be situated in the correct position with regard to the display in order to view the 3-D imagery correctly. Moreover, at this point in time, most autostereoscopic displays produce sub-optimal 3-D image quality compared to stereoscopic displays.
In glasses-type 3-D displays, the light infor-mation is separated by both the display and the lenses of the glasses. Although stereoscopic displays have the disadvantage of requiring the use of glasses, they offer more freedom in terms of viewing angle and distance. Additionally, because the different left and right images are clearly separated by the 3-D glasses, they produce very clear 3-D imagery compared to that of autostereoscopic displays.
Until autostereoscopic displays improve, stereoscopic 3-D displays will continue to drive the initial 3-D market despite the inconvenience of glasses.
Polarizer and Shutter Glasses
The process of realizing stereoscopic images involves the following: Based on binocular disparity, different left and right images are transmitted to the viewer's left and right eye through the cooperation of the 3-D display and the glasses, respectively. As we combine the two different streams of information in our brains, this technology allows us to recognize the depth of given objects in or out of the display window. This is the basic concept of stereoscopic technology with glasses.
There are three major technologies that have been used to realize stereoscopic views with glasses: anaglyph, polarization, and active shutter. Anaglyph 3-D technology, which involves two-color glasses, with typically red on one side and cyan or green on the other, has obvious color problems, so polarizer and shutter glasses types have become the major candidates for high-image-quality 3-D.
There are two methods for displaying the left and right images to the display panel. One, generally used with polarized glasses, is the spatial-multiplexing method and the other, generally used with shutter glasses, is the time-multiplexing method. Both are depicted in Fig. 1.
Fig. 1: The principles of (a) spatial-multiplexing and (b) time-multiplexing are shown.
For the spatial-multiplexing method, the left and right images are displayed in the same frame with different pixels or lines, while in the time-multiplexing method, the left and right images are displayed alternately in different frames. The time-multiplexing method requires a display with a high response speed. If the display is not fast enough, the two images overlap, leading to deterioration in the 3-D picture quality. This overlap is called ghosting or ghost effects. So, in order to reduce the ghosting in the time-multiplexing method, a display with a fast response and high frame rate is required. The spatial-multiplexing method, however, offers good 3-D picture quality without ghosting even with low-frame-rate displays because it does not depend on the response speed of the display. With spatial multiplexing, the left and right eyes each receive only half the resolution of the total frame. However, in the author's opinion, if the images are truly 3-D and contain significant depth information, then when the images are combined, a portion of the resolution is effectively restored through the sum of the right and left resolution images.
The leading 3-D technology used along with the time-multiplexing display method incor-porates shutter glasses. The glasses use left and right shutter lenses (switching liquid-crystal cell). The left and right shutters are alternately opened in each left and right frame. The shutters blink in front of each eye very quickly, but users are often aware of flicker phenomena. Because the retina corresponding to the outer part of the viewing field is very sensitive to flicker, the shutter-glasses technology used can cause eye fatigue. Moreover, because the shutter glasses contain electronics and power supplies, they are heavier and bulkier than polarizer glasses.
In this article, the current technology used for 3-D polarizer glasses and, also, a future technology that will be used for 3-D active-retarder polarizer glasses are described.
Patterned-Polarizer 3-D Displays
Patterned-polarizer 3-D, the most common technology used with polarizer-glasses-based 3-D, incorporates left and right images in the same frame. The technical concept of patterned-polarizer 3-D is shown in Fig. 2.
Fig. 2: The basic concept of patterned-polarizer 3-D display technology involves a patterned plate in front of an LCD that converts light to either left- or right-circular polarization in conjunction with polarized glasses.
As shown in the figure, there is a patterned plate in front of a conventional LCD panel, which corresponds to the odd and even lines of the LCD, respectively. This patterned polarizer converts light from the LCD to either left- or right-circular polarization. The LCD interlaces left and right images; for example, a left image is displayed in each odd line and a right image is displayed in each even line. As the two different images pass through the patterned polarizer, the left image would be left-circular polarization and the right image would be right-circular polarization. The polarizer glasses are designed to transmit left-circular polarization to the left eye and right-circular polarization to the right eye. Consequently, if a viewer sees a patterned-polarizer 3-D display through the polarizer glasses, different images will be shown to the left and right eyes of the viewer, providing stereoscopic imagery.
As explained above, due to the characteristics of displaying both left and right images in one frame, patterned-polarizer 3-D technology minimizes light losses and guarantees higher brightness. Also, because the two images are clearly separated by the patterned polarizer, we experience remarkably fewer ghost phenomena; i.e., 3-D cross-talk, a numerical index of this ghost effect, is very low. Therefore, the patterned polarizer can provide an outstanding stereoscopic view to the user.
Table 1 shows a comparison of 3-D TV specifications between patterned-polarizer 3-D and shutter-glasses 3-D. Patterned-polarizer technology has superiority over shutter-glasses technology in picture-quality parameters such as 3-D cross-talk and 3-D picture brightness. 3-D cross-talk is about 0.5% and 3-D luminance is three times as bright as that of shutter-glasses 3-D. Shutter-glasses 3-D greatly depends on the response time of displays in order to reduce 3-D cross-talk. But patterned-polarizer 3-D is unrelated to response time and frame rate. Moreover, since shutter glasses are electronic units with circuits and batteries, many users find them less comfortable – and less environmentally friendly. Polarizer glasses also are very light in weight – of about 10 grams.
*Note: Glass-based retarder displays used in TVs have to date led to a TV product still noticeably more
expensive than 2-D TVs, but LG Display is preparing a less-expensive film-based patterned retarder that may
help close the gap on the 3-D price premium.
Active-Retarder 3-D Displays
Unlike patterned-polarizer 3-D technology, which spatially separates left and right images, active-retarder 3-D technology is based on the time-multiplexing method mentioned above that separates left and right images. As shown in Fig. 3, an active-retarder 3-D display is composed of two panels.
Fig. 3: The active-retarder concept for 3-D display technology uses two panels – a conventional LCD and a panel with one LC layer and two glass substrates.
One is a conventional LCD for 3-D images and the other is an active-retarder panel consisting of one liquid-crystal layer and two glass substrates to control polarization. As shown in Fig. 3, each even and odd frame corresponding to the left and right images is written alternately on the image panel. When two images are alternated on the image panel, the active retarder in front of the image panel converts the polarization state of the input polarization from the image panel. The LC panel switches the polarization of alternate frames between left and right circular.
This technology can provide viewers with high-resolution displays because the full resolution of one frame is projected to each eye. Also, as the scanning active retarder helps with the writing of the display panel, the result is a brightness higher than that of shutter-glasses 3-D technology. It should be noted that compared with patterned-polarizer 3-D technology, active-retarder technology requires an additional LC panel, which leads to additional cost. Thus, cost is an issue that needs to be overcome.
Table 2 shows a comparisons of 3-D performance between conventional shutter-glasses 3-D displays and active-retarder 3-D displays currently under development in sizes suitable for monitor use.
Since two of the 3-D technologies are based on the time-multiplexing method, their resolutions are full HD without resolution loss in both 2-D and 3-D modes. In the comparison of luminance under the same 2-D brightness, the 3-D luminance of the active-retarder 3-D is measured to be about 100 nits, more than twice that of shutter-glasses 3-D displays.
The white–black 3-D cross-talk of active-retarder 3-D is similar to that of the shutter-glasses 3-D, but the average gray-to-gray 3-D cross-talk is optimized to be about 2%, which is half that of shutter-glasses 3-D displays. And with the help of scanning technology, the cross-talk deviation over the entire display area is measured at 0.5% for active-retarder technology. But the cross-talk deviation for shutter-glasses 3-D is 10%, due to the difference in the data writing time of the first and last line. In this sense, the image produced in the active-retarder 3-D technology is much better than that of shutter-glasses 3-D technology due to the high-brightness. gray-to-gray 3-D cross-talk, and cross-talk deviations. Therefore, active-retarder 3-D technology is introduced as a novel technology that satisfies brightness compared to that of shutter-glasses 3-D technology in monitors. Besides, it provides wearing convenience to users because it employs polarizer glasses just like that of patterned polarizers. With such high brightness, high resolution, and user convenience, this technology should be suitable for the premium monitor or TV market.
The Future for 3-D Polarizer-Glasses Technology
In this author's experience, in certain cases the shutter-glasses-type 3-D display can cause dizziness due to flicker, cross-talk, and low overall luminance. The relatively expensive shutter glasses appear to be an inconvenient burden to consumers that could limit adoption. The polarizer-glasses-type 3-D display with simple and inexpensive glasses is more user-friendly. In terms of 3-D display quality, the patterned-retarder-type 3-D display offers high brightness and is flicker and cross-talk-free, reducing visual fatigue. Thus, at some point in the future, the patterned-retarder type may emerge as a good candidate for mainstream 3-D technology in the home. •