IEEE Spectrum - North American - March 2014 - 37

Blue
SlM chip
Dichroic
mirror

Blue
laser module

Dichroic
mirror

Green laser
module

Green
SlM chip

projection
lens
Red laser
module

Red SlM
chip

inside
a laser
projecTor

red, green, and blue laser beams each fall
upon a separate spatial light-modulator chip.
For each of these colors, the corresponding spatial
light modulator determines, for every pixel of every
frame, how much light of that color gets reflected
through the projector optics to the screen. The path
length for each colored beam from the modulator
chip to the projection optics must be the same,
so that the three images focus on the same plane.

In a 3-D projector setup, the polarization is actually circular,
but the phenomenon is no less useful for discriminating between
two beams of light. In the projector, the alternating polarizations
are accomplished with an electro-optical polarizing filter that
switches from one polarization to the other with each image
flash. The viewer, meanwhile, wears eyeglasses that have corresponding polarization filters, so only the appropriate image
reaches each eye. The projector displays images at 24 times 6
(3 for the right eye and 3 for the left)-or 144-flashes per second.
In Sony's approach, both views are projected at the same
time and on the same screen. The Sony spatial light modulators
can't flash at a rate of 144 per second. So Sony subdivides the
4K chip into two different 2K subpanels, one for the left-eye images and one for the right. Thus in this 3-D system, the left- and
right-eye images are 2K wide but only 0.8K tall (some pixels are
lost to a guard band). The Sony 3-D projector has two lenses and
a polarizing filter in front of each. The images projected to the
illustration by

James Provost

left eye go through one lens and the right-eye images go through
the other. Then the two images move onto the screen, where
they are superimposed on each other. As with the TI system,
the viewer wears glasses with corresponding polarized lenses.
Both approaches waste a lot of light-not just at the polarizing
filters but also at the spatial light modulators. Consider a viewer
watching a 3-D movie projected with TI chips: Each eye sees light
slightly less than half the time. With the Sony system, the lamp
inside the projector illuminates the entire spatial light-modulator
chip, but the projector uses less than half the chip's area to illuminate the screen-again, a roughly 50 percent light reduction.
All told, the losses at the projector, including the polarizing
filters, are about 75 percent. Then, 20 percent of what's left is
lost at the viewer's 3-D glasses. So the overall loss is approximately 80 percent. Even highly reflective screens can't make up
for that much loss. And remember, the brightness of the bulb
declines with time.
So perhaps it's no surprise that the popularity of 3-D has been
declining, with the exception of the recent motion picture Gravity
(which often kept the screen filled with the blackness of space,
making dimness less of an issue).

why not just Make brighter xenon arc laMps? The short
answer is: Because it wouldn't help. Like any arc lamp, projector
bulbs throw out light in all directions. And then this white light
must be split into red, green, and blue bands, which are focused
onto the light-modulator chips inside the projector. These chips
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Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2014