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4.2. Polarization-multiplexed displays
The basic setup consists of two monitors arranged at a
right angle and the screens are covered by orthogonally
oriented filtersheets (linear or circular polarization). The
two views are combined by a beam combiner (half-silvered
mirror) and the observer wears polarized glasses. This
frequently used technique provides full color rendition at
full resolution and very little cross-talk in the stereo pair
(less than 0.1% with linear filters). However, over 60% of
the light emitted is lost through the filters, and the remaining
light flux is halved by the mirror.
Polarization techniques are very well suited for video-
projection. When using CRT-projectors with separate
optical systems for the primary colors, the left and right-
view color beams should be arranged in identical order to
avoid rivalry. The light flux in LC-projectors is polarized by
the light-valves. Commercial LC-projectors can be fitted for
stereo display by twisting the original polarization direction
via half-wave retardation sheets to achieve, e.g., the
prevalent V-formation.
Stereo projection screens must preserve polarization.
Optimum results have been reported for aluminized surfaces
and for translucent opalized acrylic screens. Typical TV
rear-projection screens (sandwiched Fresnel lens and lenti-
cular raster sheet) depolarize the passing light. LCD-based,
direct-view displays and overhead panels have recently
been marketed [2]. Their front sheet consists of pixel-
sized micro-polarizers, tuned in precise register with the
raster of the LCD. The left and right-eye views are elec-
tronically interlaced line-by-line and separated through a
line-by-line change of polarization.
4.3. Time-multiplexed displays
The human visual system is capable of merging the con-
stituents of a stereo pair across a time-lag of up to 50 ms.
This ‘memory effect’ is exploited by time-multiplexed
displays. The left and right-eye views are shown in rapid
alternation and synchronized with an LC-shutter, which
opens in turns for one eye, while occluding the other eye.
The shutter-system is usually integrated in a pair of specta-
cles and controlled via an infrared link. When the observer
turns away from the screen, both shutters are switched to be
transparent. Time-multiplexed displays are fully compatible
for 2-D presentations. Both constituent images are repro-
duced at full spatial resolution by a single monitor or
projector, thus avoiding geometrical and color differences.
The monitor-type systems have matured into a standard
technique for 3-D workstations.
Impairing cross-talk may result from the persistence of
CRT phosphors, particularly of the green phosphor. Ideally,
image extinction should be completed within the blanking
interval. Using ‘fast’ phosphors like P46 leads to noticeable
loss of brightness (50%) and life-span. P43 yields a some-
what optimum performance. With the standard phosphor
P22, cross-talk can amount to over 20% (the perception
threshold is at 0.3% [3]). Since the two views are displayed
with a time-delay, they should be generated with exactly
this delay. Otherwise, moving objects will appear at incor-
rect positions in depth.
4.4. Time-sequentially controlled polarization
Tektronix has developed a display which combines the
time and polarization-multiplex techniques. The monitor’s
faceplate is covered with a modulator, consisting of a linear
polarizer, a liquid-crystal r-cell and a quarter-wave retarda-
tion sheet (to turn linear polarization into circular polariza-
tion). The r-cell switches polarization in synchronism with
the change of the left and right-eye views. Circular polariz-
ing glasses serve for de-multiplexing.
This approach offers three advantages over systems with
active shutter-glasses: First, polarizing glasses are inexpen-
sive and light-weight. Second, the r-cell can be constructed
from several segments which operate independently on the
active portions of the screen, hence ensuring that each eye is
only reached by the intended image contents. This way,
cross-talk can be greatly reduced (down to 3.6% in a system
with five segments and the P22 standard phosphor [4]).
Third, multiple display arrays can be operated without any
extra synchronizing circuitry.
4.5. Location-multiplexed displays
Location multiplex means that the two views are created
at separate places and relayed to the appropriate eye through
separate channels (e.g. by means of lenses, mirrors and
optical fibres). In the following examples, the image plane
appears either in a fixed accommodation distance (HMD
and BOOM displays) or in a variable, gaze-controlled
position (3DDAC).
4.5.1. Helmet mounted display (HMD)
With HMDs, the perceived images subtend a large view-
ing angle, typically up to 120” (horizontal) by 80” (vertical).
As the natural surroundings are occluded from sight (some-
times optional, by a visor), HMDs are apt to convey a feel-
ing of total immersion in the scene displayed. Attached
headtracking devices are used to create eye-point dependent
changes in perspective when the user moves. Accommoda-
tion distance is usually below 2 m, or can be adjusted in see-
through displays to the distance of the fixated real world
objects [5]. Meanwhile, numerous manufacturers offer a
variety of systems with different performance [6].
HMDs allow free head movement without losing screen
contact, thus avoiding musculo-skeletal problems. There is
no need to dim ambient light (visor closed), and peripheral
vision must not necessarily be obstructed (visor open). On
the other hand, latencies and tracking-errors tend to provoke
odd, uneasy sensations through conflicting visual stimula-
tion and postural feedback, and adaptation can lead to