Page 422 - Mechatronics for Safety, Security and Dependability in a New Era

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passive methods are the completely flickerfree image and lighter eyewear. All of the passive methods
suffer from less than perfect channel separation.
The most common active stereo approach is to use one CRT-projector per one display surface. Images
for user's both eyes are shown sequentially. Shutter glasses are used to select the correct image for
each eye. The speed of the phosphor material on the CRT is a compromise between a flickerfree image
and an adequate response time. Problems arise especially in stereographic presentations where the
CRT is forced to display two (or more) different images sequentially at a high rate. The previous
image leaks to the next one so that the channel separation cannot be perfect. A flickerfree visual
experience requires at least 75Hz refresh rate per eye at ordinaiy range of image brightnesses. As the
brightness increases, the required image refresh rate rises up to and possibly beyond 500 Hz [2]!

LCD-projectors and displays use a completely different image reproduction method. A constant light
source is modulated by a liquid crystal panel. The typical response time of an lcd-panel is 20 - 40 ms.
This is just enough to show animated graphics at 30 - 50 frames per second but it is way too slow for
time division multiplexing required for shutter glasses. One major advantage of the LCD-projectors is
the light output, which is significantly higher than in their CRT counterparts. Also the prices of LCD-
projectors are very competitive compared to CRT-projectors with adequate light output. As mentioned
above, the LCD-panel can not switch the image fast enough so the switching must be performed
externally. This paper introduces a method where the light output of each individual projector is
controlled by an external shutter disc. Shutter glasses, worn by the user, select which eye is allowed to
see the image of the currently active projector.

THEORY OF OPERATION

The operation principle is simple and elegant. Both eyes have their own projector. The projector
modulates its ligh source with the appropriate image and the external shutter switches the picture on
and off to the screen. The inherent slowness of the LCD-panel poses no obstacle to a stereographic
projection. In our construction the shutter is common for both projectors. Nevertheless, it is possible to
use two separate shutters which give more flexibility to projector mounting.

The LCD-projector
The nature of the LCD-projector makes it very attractive to be used with an external shutter. Individual
pixels maintain their states over the whole frame period. The external shutter could - but it does not
have to - be synchronized with the projector or with the graphics generating computer. In a fast
moving scene the unsynchronized image might bring out a 'tearing effect' where the image splits to
horizontally unaligned upper and lower portions. If the projector updates its LCD-panels directly with
the incoming RGB-signal it would be useful to use the vertical synchronization pulse to control the
shutter motor. It would remove the tearing problem and give a constant response time from a graphics
drawing to the screen. Both of these are desired aspects in a simulation environment.

For comparison, the DLP projectors offer a slightly better contrast and light output as their LCD
counterparts. The DLP technology also enables sequential stereo by itself it still have its limitations.
Currently commercial DLP-stereo enabled projectors have frame rates limited to 60Hz per eye which
is quite low for high bright images. [3] [4]
The electro-mechanical shutter

In our system, a mechanical shutter does the switching between the two images forming the stereo
image pair. The shutter is disc shaped and positioned in very front of the projectors. The disc rotation