Here is the mini-display optics article referenced in the immediately prior post.
June 23, 1997, TechWeb News
Two-stage magnification combined in low-profile device --
Mini-display integrates refined optics
By David Lieberman
Palo Alto, Calif. - The flat-panel display world has begun looking like a
display-of-the-month club lately, as startups with intriguing new technologies
seem to continually emerge from the woodwork. The latest to come to light is
Siliscape Inc. Based here, it is a three-year-old venture that hopes to be
producing miniature display engines by the middle of next year.
But Siliscape's expertise is in optics, not displays. It was formed in 1994
when two "old dyed-in-the-wool electro-optics guys"-Alfred Hildebrand and
Gregory Kintz-set out to solve two problems they saw in miniature displays:
illumination and magnification. "No one had developed a small display with
any sense of how they were going to magnify the image," said Hildebrand,
Siliscape's president and chief executive.
The duo came up with solutions (patents in progress) for both problems, with
the goal of creating a very compact display engine-i.e. an integrated package
containing the display, light source and lens. "If you do the display on silicon,"
said Hildebrand, "which is the economical way of doing things vs. TFTs
[thin-film transistors] on glass, that means you have to operate in reflective
mode and that means you have to use off-axis illumination."
Siliscape attacked the magnification issue by developing a compound
magnifier that performs two stages of magnification in one compact optical
device. "All the other [mini-display makers] use simple one-stage magnifiers,"
said Hildebrand, "which have a limitation of about 10x magnification. For
pixel sizes at 10 microns or below, you need greater than that."
Siliscape's optics essentially collapse the two-stage magnification of a
compound microscope into a single low-profile device. "A compound
microscope typically takes from 6 to 10 inches from the objective lens to the
eyepiece because it needs room to create an intermediary image," Hildebrand
said. "For portable products, you can't tolerate that. We've used a
combination of a reflective element and refractive element to put both stages
of magnification in one optic and get up to 30x without having to have two
different lens stages."
Siliscape's "special edge," Hildebrand said, is its high-magnification,
low-profile optics. The very compact optics of the company's display engine,
though, complicates the illumination problem. "Front to back, it's only 12 mm,
less than a half inch," said Hildebrand. The entire engine measures just 30 x
40 x 12 mm.
With its optics in place, Hildebrand and Kintz needed to select a reflective
display technology, and they initially considered a MEMS
(micro-electromechanical-system) display before settling on LCDs. These
LCDs, however, are not the twisted-nematic and supertwisted-nematic
LCDs that populate mainstream applications, but an LCD of a different type
that has hardly seen the light of day: the polymer-encapsulated LCD. It's one
variation of a class of LCDs generically known as PDLC for
polymer-dispersed LCD, whose claim to fame is high efficiency.
"As opposed to starting from scratch and building a microdisplay technology,
we set out to find one," Hildebrand said. "With the illumination problems in
reflective displays, especially when packaged tightly, the best way to go is to
use scatter-mode LCs [liquid crystals], not the [common]
polarization/retardation LCs. So we set out to find scatter-mode material and
settled on polymer-dispersed [LCD], which we can use well with off-axis
illumination."
Siliscape found the LCD technology it needed at Raychem Corp. (Menlo
Park, Calif.), which for the past many years has been developing its NCAP
(Nematic Curvilinear Aligned Phase) PDLC technology-initially with a variety
of applications in mind and then honing in on projection-display applications.
Siliscape also found ready-made the other critical component it would need
to build a complete display engine: the silicon to control the LCD.
After a relatively short development relationship with Hitachi fizzled a few
years ago, Raychem found a new silicon partner in National Semiconductor,
whose ASIC work for the NCAP display provided Siliscape with a
ready-made solution. "We joined the cooperative [National-Raychem] effort
and National supplies prototypes to us," Hildebrand said.
Hildebrand describes Raychem's LC formulation, licensed from the Kent
State Liquid Crystal Institute, as "LCs encapsulated/dispersed in a polymer
emulsion, elliptical droplets in 1- to 2-micron domains." The top surface of
the ASIC on which the LCD sits is highly reflective and the LCD serves as a
light shutter-the control valve for manipulating light. By electrically addressing
the different LCD domains, the physical orientation of the droplets is switched
between an ordered and a random state, which changes their scattering
properties.
How efficient is NCAP compared with mainstream LCDs, which are only
about 5 percent efficient? "Very efficient," Hildebrand said, "probably 98
percent in terms of light that's scattered. There are no cross polarizers, so you
don't throw away half of the light to begin with. How much light you actually
collect, though, depends on the optical system."
Raychem/National and Siliscape have different target applications for their
LCDs, with the twosome squarely focused on high-efficiency (that is, cool
and quiet) projectors and Siliscape on handheld cell phones and fax viewers.
The companies thus use the display differently.
With a projector and its on-axis illumination, Hildebrand explained, "if there's
voltage across the LC, it goes clear and reflects light into the collection
aperture of the lens system ['on']. If there's no voltage, it scatters light out of
the f-stop of the optical system ['off']."
For the off-axis illumination in Siliscape's application, however, "we use the
chip another way. When there's no voltage, light is scattered into our exit
pupil. When it's clear because there's a voltage across it, the mirror reflects
light obliquely out in another direction."
Siliscape has been seeding cell-phone makers with its display engine since
the end of last year, with hopes of ramping up into production in mid-1998.
The display itself is an SVGA (800- x 600-pixel) device measuring 6 x 8 mm,
or just under 0.5-inch in diagonal. In operation, it draws about 60 mW when
it's switched on and 2 mW in standby. First products will be monochrome,
with color expected to be demonstrated this summer. Like several other mini
makers, Siliscape is using a monochrome LED for illumination, with plans to
move to a tricolor scheme using red, green and blue LEDs.
Crying need
Hildebrand and others point to the crying need in mobile computing and
communications equipment for small, lightweight, low-power, low-cost
displays. The only way to meet all those needs and generate a readable
image, he said, is the "virtual" display.
"The only way to satisfy demands for nearly computer-sized images from a
display the size of a postage stamp is to use optical techniques to enlarge the
image from these displays," Hildebrand said. "Put them near your eye, and
you can see a magnified image of the pixels about a meter in front of you, thus
making them 'virtual image' displays."
Siliscape's target applications, primarily cell phones with facsimile capabilities,
dictated one important criterion for the design of its first product: enough
pixels horizontally to display the entire width of a conventional facsimile page.
That meant at least Super VGA resolution.
"If you don't have a full 800 dots, you have to scroll the page sideways," said
Hildebrand. "And from an ergonomics standpoint, that's intolerable."
Being able to display a full fax line also has certain system implications. "The
thing about fax is that you're stuck-you must print the full width," said
Hildebrand. "A fax is compressed when it's sent, it's stored compressed, and
you don't decompress it until it's displayed. It's stored as a compressed
stream of bits, not something like ASCII text, so you can't easily create
character returns when you want to. You don't know where the line breaks
are. So unless you have 800 dots, you can't do a full line without sacrificing
resolution, and you have to go through some very complicated mathematical
algorithms, which becomes impractical."
Full-page world
New capabilities that will migrate to tomorrow's multifunction cell phones will
also dictate at least 800-pixel horizontal resolution. "Web-page people aren't
going to rewrite Web pages for less than VGA displays," said Hildebrand,
"and the standard Web page is designed around SVGA. We've talked to
software people trying to make gateways to wireless servers that serve
portable devices, and they say it's a full-page world.
"How to deal with, say, a quarter Web page at a time is very problematic.
There's no software going to be written to pick a Web page apart and make
it fit on somebody's random display."
Copyright (c) 1997 CMP Media Inc.
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