Designing 3D Capabilities Into Handheld Displays
- Sunday, 01 November 2009
For 3D viewing to be convenient on small, portable devices, it must be autostereoscopic, thus eliminating the requirement for 3D “glasses.” The display devices must also provide 2D and 3D capability simultaneously, without sacrificing resolution or color vibrancy.
Several 2D/3D mobile autostereoscopic displays have been developed recently with spatially multiplexed solutions, using either parallax barriers or lenticular construction. But such devices are relatively thick, are difficult to assemble because of the need to align layers at the pixel resolution of the display, and reduce the perceived 3D spatial resolution by a factor of 2.
A new 3M solution presents a time-sequential approach that is resolution and color independent and requires no alignment during assembly. Its three major components are a directional backlight, a thin piece of 3D film, and a 120Hz LCD panel. The system easily integrates into today’s thin, high-resolution displays, and enables simultaneous 2D and 3D viewing on a frame-by-frame basis with no picture degradation in resolution or color.
The solution is a two-view, full-resolution display produced by optically forming distinct viewing regions for left and right eyes. Full resolution is achieved using time sequential left and right images displayed fast enough to avoid the perception of flicker. Further, the single 3D viewing zone eliminates view reversal, a common cause of eye fatigue.
The accompanying figure of the display stack shows the directional back-light, a layer of 3D film, and a 120Hz LCD panel. The autosteresopic 3D display is achieved by combining this display stack with page-flipped content and synchronizing the left light source with the left image on the LCD, and the right light source with the right image.
Note the schematic cross-section of the 3D film. Unlike other autostereoscopic approaches, this one does not require alignment of the single layer of 3D film to the LCD pixels or light guide features. The top lenticular and bottom prismatic features are carefully aligned during manufacture with a nanometer tolerance on individual feature registration.
The 3D film integrates into the backlight module of the LCD display. Only one LCD panel is required, operating at 120Hz refresh rate. Backlight module assembly is nearly identical to existing systems, allowing for simple integration. The usual optical film stack is replaced with a reflective film, custom light guide, and 3D film. Directional backlight technology with LEDs on the the left and right sides of the light guide is required to separately direct the left and right images sequentially into the viewer’s eyes, providing full resolution of the display panel. No precision registration during assembly is necessary.
A key feature of the 3D film is the nanometer scale difference in feature size between the lenticular and prism structures. While the exact size of the features can be adjusted to minimize Moiré effect with the LCD display, the individual prisms are between 1 and 30 nm larger than the corresponding lenticular features, depending on the desired viewing distance and the size of the panel.
As illustrated, the left and right image content must be synchronized on the display panel with the respective left and right light sources. The human visual system has a flicker fusion frequency somewhat below 60Hz so that a flicker-free display requires left/right image sequences to be displayed at 120 Hz. Although the display frequency is high, the content stream does not need to be updated at this speed. Buffering the display pictures or video stream will allow 24 fps or slower image streams to be displayed with seamless autostereoscopic effects.
In addition to a 120Hz refresh rate, response time of the panel is a critical factor. Turning on the left or right light source before the previous image has been removed from the display leads to timing crosstalk, in which one light source illuminates the correct image and the temporal “residue” of the incorrect image.