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A more sophisticated method involves using eyeglasses with LCD shutters instead of the lenses. The shutters are being closed and opened alternately. This technology doesn’t work for static pictures but works for movies: all the odd-numbered frames must be prepared for one eye and all the even-numbered frames, for the other eye. Syncing the moments of the opening and closing of the shutters with the alteration of the frames, we can make each eye see only those frames that it is meant to see.

This technology has no color reproduction limitations but has others instead. First, each eye sees only half the frames with rather long intervals in between, so the frame rate must be high enough for the eye not to notice the flickering. Second, the glasses must be strictly in sync with the alteration of the frames. Otherwise the picture will lose the 3D effect and divide in two. If the eyeglasses are used together with an LCD monitor, there is a third problem: the matrix’s response time must be low enough for adjacent frames not to merge into each other.

As the consequence of these limitations, such eyeglasses were produced for a while but didn’t really take off.

However, an LCD monitor has one special property due to the very technology of producing an image by means of liquid crystals. It is an inherent but rarely mentioned property. An LCD monitor yields polarized light. Take any polarizer (e.g. an appropriate filter for your camera) and look through it at any LCD monitor: you will be changing its transparency from zero to maximum by simply turning the filter around.

Can this be of any use, though? A filter will block all light from the screen, just like the above-mentioned eyeglasses with shutters. Yes, it will – with an ordinary monitor. But the monitor’s screen can be divided into zones with different polarization of light. This can be achieved by means of two polarizer films placed on both sides of the liquid crystals. Now if we take a look through a polarizing filter, some zones will look darker and others lighter depending on the position of the filter. If we take two filters, one for the left eye and the other, rotated by 90 degrees, for the right eye, one eye will see some zones on the screen and the other eye will see different zones!

If the different-polarity zones are small enough and distributed uniformly on the screen, each eye will see the whole picture, these pictures will be different!

So, here they are: the Zalman Trimon ZM-M220W monitor and the polarizing eyeglasses.

I guess the point of the technology is obvious from this photograph: the odd- and even-numbered lines of pixels of the monitor’s screen have different directions of light polarization. But the human eye itself doesn’t perceive polarization. Without special eyeglasses, you just see an ordinary image.

The lenses of the eyeglasses are polarizers whose polarization planes are rotated relative to each other. So, you can see odd-numbered lines through one lens and even-numbered lines through the other lens. If the monitor shows a stereo-picture in which the image for one eye is shown in the odd-numbered lines of pixels and for the other eye in the even-numbered lines, each eye will only see the picture meant for it.

Take note that the eyeglasses are a passive device here (they do not contain any electronics and do not need to be in sync with the monitor) and do not distort color reproduction.

This can be illustrated by the picture from the Zalman website.

There is only one thing that I haven’t mentioned: there is an additional film on the LCD matrix that transforms linear polarization of light into circular. This prevents the picture from getting distorted when the position of the eyeglasses relative to the screen changes.

The obvious drawback of this technology is that the effective vertical resolution of the monitor is reduced twofold. If the screen has a native resolution of 1680x1050 pixels, each eye will only see 1680x525. Anyway, this is a far higher resolution than what you can get with a VR-helmet. It is quite appropriate for games and movies.

 
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