Crosstalk (Ghost Images)

The biggest drawback of LC-Shutterglasses besides the compatibility and ergonomy issue is Crosstalk. The right eye sees some residue of the image dedicated to the left eye and vice versa.  Images with high contrasts are very critical here. This is due to the persistance of the phosphors in the monitor tube and to a lesser degree the inability of the LC-panels to block the light entirely. Sync-errors can also play a role, especially on the upper or lower border of the screen.

• Persistence time of the monitor tube - the shorter the less ghosting
• Contrasts within the image -  hard contrasts produce strong ghosting
• LCD-quality - light blocking ratio, the manufacturers call it 'dynamic-range'
• Voltage/battery status of the glasses
• Refresh rate - the higher the more ghosting !?
• Brightness & Contrast setting of monitor
• Parallax size
• Accuracy of synchronization between glasses and monitor
• Resolution
• Display technique - interlace vs. page-flipping

• get refresh rate down !?
• get brightness up
• get contrast down
• make parallax smaller
• use fresh batteries
• use fast-phosphor projector (from Barco)
• use fast-phosphor monitor (very very hard to find)
• use low contrast applications and games, e.g. prefer day-scenes over night-scenarios (example: racing game, red desert, daylight, rainy weather -> almost no ghosting)

NEW:  For an in-depth analysis of the causes of ghosting in CRT-setups check:
Woods, A., and Tan, S.S.L. (2002) "Characterising Sources of Ghosting in Time-Sequential Stereoscopic Video Displays", presented at Stereoscopic Displays and Applications XIII, published in Stereoscopic Displays and Virtual Reality Systems IX, Proceedings of SPIE Vol. 4660, San Jose, California, 21-23 January 2003. (677kB pdf)

The IBM research laboratory adresses crosstalk in this paper. They even offer some hints and algorithms for programmers to reduce crosstalk. Companies like VRG experiment with faster non-LC-Shutterglasses and short persistance monitors.
Ghost images can also be produced in the mind of the viewer if the parallax is just too big.  Dave Bixer told me about his brute force method to avoid ghosting. I'm sceptical if ghosts can really completely avoided by this, but read for yourself and share your thoughts with us. BTW, there were Shutterglasses before the age of Liquid Crystal. They showed up shortly after the invention of TV. They worked mechanically, like a movie camera shutter. They produced noise and vibrations and the frequency was limited. They had an advantage though, they blocked the light entirely!

The cure for ghosting - choosing the right phosphor (July 17, 1999)

The main cause of ghosting is the slow phosphor decay time. Monitor tubes with P22 phosphors (zinc sulfide for blue/green) are very bad in this respect. Much better would be P43 tubes (rare earth for green). The problem is: P22 dominates the market. I couldn't find a mainstream non-P22 monitor yet. If you can find any please tell me or post it on the discussion board.
 

Ghosting and  projection systems:  BARCO a leader in the large screen business is aware of the problem and offers special 'fast green phosphor' versions of some of it's 3-tube-CRT-projector models.

BARCO has an excellent page about stereoscopic 3D projection covering shutterglasses, shutterscreens, polarization glasses, single and dual projector setups with tubes and LCD. Also get the Barco color brochure on stereo projection in Acrobat Format  - a must have!

My favorite idea - using a single DLP (micro mirror) projector with shutterglasses isn't addressed there. I'm still looking for a DLP which accepts high refresh rates >100 Hz and doesn't screw the stereo-information by line-doubling or other digital buffer functions.