Frame-Sequential Stereoscopic Computer Graphics Displays
A frame-sequential stereo display presents alternating left-eye and right-eye images on a CRT monitor. Shutter glasses are synchronized with the monitor to allow only the appropriate eye to view the current image. To avoid flicker, the monitor must be operated at twice the usual frame-rate.
The advantages of a frame-sequential display are principally that the use of a single monitor saves space and the cost of an extra monitor and graphics controller, which are the most expensive components of a computer graphics system. In addition, there are never alignment problems such as result from the use of multiple CRTs on optical channels. However, there are some disadvantages. The liquid-crystal shutters have imperfect performance. Most importantly, most CRT phosphors have sufficiently long image persistence that the image for each eye persists into the following frame, which is viewed by the opposite eye. These conditions produce crosstalk. Our monochrome monitor and ferro-electric shutter glasses (described below) largely eliminate these problems.
Our economy glasses are suitable for development and testing. Our medium-priced shutter glasses provide performance similar to other high quality shutter glasses on the market. Switching times are about 2 msec., which is longer than the vertical blanking period and causes cross talk at the top of the frame. Contrast ratios are typically 150:1 measured with tungsten. This ratio does not describe performance with a CRT.
The persistence of standard CRT phosphors--either RGB or monochrome--are the major factor limiting performance. Phosphors usually have 2 or 3 components, each having a different luminance decay characteristic. One of the components, present in low quantity, has very long persistence and limits performance over the entire screen. This afterglow is called “tailing.” The major phosphor components have more rapid decay. A standard RGB monitor, used with the Pi-Cell glasses, will produce extinction ratios of roughly 40:1 for red, 20:1 for blue, and 8:1 for green, in the center of the screen. Standard P-4 monochrome phosphors are not significantly better. In both cases, performance is worst at the bottom of the screen, because the pixels there have little time to decay before the next frame. Performance is worse at the top because of the switching time of the Pi-Cell glasses.
To improve performance, Vision Research Graphics (VRG) has introduced two products:
1) a monochrome monitor with a custom phosphor compounded to eliminate tailing,
2) Ferro-Electric shutter glasses with extremely fast (50 msec.) switching times. Used together, these products result in an excellent monochrome display that virtually eliminates cross talk. The cross talk is so low that we have been unable to measure it, although it is visible in a darkened room. The monochrome phosphor is yellowish in color. The Ferro-Electric glasses are essentially optically perfect but have relatively small apertures (1”) and are designed to be held close to the eyes. The shutter positions are fully adjustable, and can be used by children.