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A similar method better preserving colours apply narrow-band colour filters,
separating the left and right images with wavelength triplets biased in a few 10 nm
range, less visible to human perception [10].
2.3 Polarization Based Separation
Polarization based separation exploits the possibility of polarizing light and filtering
them with polar filters. The two images are projected through different polarization
filters onto a surface that reflects light toward viewers, keeping the polarization of
the incoming light (mostly) unmodified. Viewers wearing glasses with the respec-
tive filters in front of the eyes can then perceive a stereoscopic view. A popular
example of this technology can be experienced in most 3D cinemas [11,12].
Light can be polarized either linearly or circularly. In the first case, the left and
right images pass through two perpendicular linear polarizers and then projected
onto a surface. The reflected images then pass through the respective polarizing
filters that are embedded into glasses, separating the left and right images. The
downside of linear polarization is that the image degrades when a user tilts her
head, as separation does not work as intended with this orientation. Circular po-
larization overcomes this problem being invariant to head tilt. In this case one im-
age is polarized with clockwise, the other with counter-clockwise direction.
The advantage of the polarization based stereoscopic technique is that it keeps
image colours intact (unlike anaglyph), with glasses that are relatively cheap, how-
ever the overall brightness is challenged and some cross-talk is always present.
One way of generating a pair of polarized images is by using two projectors,
one projecting the left eye image with a polarizing filter in front of it, the other
projecting the right eye image with orthogonal polarization [13,14]. There is also a
single-projector technique, in which a rotating polarizator wheel or an LCD po-
larization modulator is used in the projector to change the direction of polarization
of every second frame [15]. One needs a special projection screen to reflect polar-
ized images, as surfaces used for 2D projection do not maintain the polarization of
the reflected light. Previously silver screens have been used, now specialized ma-
terials are available for this purpose [16]. Polarized stereo images can also be cre-
ated using two LCD monitors with perpendicular polarization arranged with a pas-
sive beamsplitter (half-mirror) at a bisecting angle between the displays. The
resulting stereo image pair can be seen directly with polarizing glasses [17,18], as
shown in. Fig. 3.
Another approach to create polarized images is using a patterned micro-
polarizer sheet (also called x-pol or micro-pol), which is placed on the surface of a
2D LCD panel. The sheet is aligned with the rows on the LCD panel so that pixels
in the even row will be polarized clockwise, pixels in the odd row will polarized in
reverse, as shown in Fig. 4. Providing corresponding line interleaved stereoscopic
images for the display will result in a 3D effect when using circularly polarized
glasses (although with resolution reduced by half). Some manufacturers providing
such displays are LG [4] and Zalman [19], but 3D laptops using this technology
also appeared from Acer [20].
