Scanne
d Laser
Pico-Pro
j
e
c
t
ors:
Seeing th
eB
i
g Picture
(with a S
mall Dev
ice)
Mark Fre
eman, M
ark Cham
and Sid
pion
Madhav
an
28 | OPN May 2009
1047-6938/09/0??/0028/6-$15.00 ©OSA
www.osa-opn.org
Pico-projectors are the latest technology to prove that big
things often do come in small packages. These tiny projectors
are embedded in mobile devices to provide large-screen
displays that can be viewed from anywhere. The authors
provide an overview of these technologies—which are poised
to hit the market by next year—with a special focus on
2D scanned laser projection.
Miguel Khoury/Microvision
I
t’s amazing what we carry in our pockets these days. From cell phones to iPods to
PDAs, we have at our fingertips connectivity with friends and colleagues around
the world, libraries of text, music, photos, videos and more.
Unfortunately, the displays that we use to view all this information are also small;
they are flat-panel screens with just a few square inches of display area. No wonder
that projectors that display large images from within hand-held electronic devices—
the so-called pico-projectors—are drawing so much attention in the tech world.
With pico-projectors, you can project a full-size image onto whatever is near at hand,
whether it be the wall, your shirt, or a piece of paper. Pico-projectors represent a core
enabling technology for the future growth of portable devices.
Moreover, scanned laser pico-projectors have an infinite focus that can be projected onto surfaces with any 3D depth profile and remain in focus. The possible applications are various and many. Aside from having a larger format for typical mobile
applications, such as reading e-mail and sharing pictures, pico-projectors could be
used for impromptu business presentations or perhaps scientific visualizations.
From a business point of view, the size of the market for pico projectors is extremely
large. In 2007, there were worldwide sales of more than 1 billion mobile phones, 200
million personal media players, 125 million digital cameras, and 25 million Nintendo
DS handheld game devices. And it’s not just device manufacturers who stand to benefit by offering a larger-screen viewing experience. Service providers would also have
the opportunity to sell bandwidth, and content providers could sell more compelling
and rich multi-media content. A handful of manufacturers, including Samsung and
3M, are already beginning to offer pico projectors this year as stand-alone accessories
that can be attached to other handheld devices. By 2010, pico-projectors that are
embedded into devices will start to become available.
OPN May 2009 | 29
Red LD
Green SHG
laser
Blue LD
PicoP Integrated Photonics
Module (IPM) scan engine.
MEMS
scanner
Miguel Khoury/Microvision
Scanned laser: A simple
projector design
The figure above shows the basic layout
of the pico-projector being developed
by Microvision Inc., called PicoP. The
architecture is quite simple, consisting
of one red, one green, and one blue laser,
each with a lens near the laser output
that collects the light from the laser and
provides a very low NA (numerical aperture) beam at the output. The light from
the three lasers is then combined with
dichroic elements into a single white
beam. Using a beamsplitter or basic foldmirror optics, the beam is relayed onto
a biaxial MEMS scanning mirror that
scans the beam in a raster pattern. The
projected image is created by modulating
the three lasers synchronously with the
position of the scanned beam. The complete projector engine, also known as the
Integrated Photonics Module, or IPM, is
just 7 mm in height and less than 5 cc in
total volume.
The essence of the design is that,
with the exception of the scanner, the
rest of the optics engine deals with
making a single pixel. All three lasers
are driven simultaneously at the levels
needed to create the proper color mix
30 | OPN May 2009
With pico-projectors,
you can project a
full-size image onto
whatever is near at
hand, whether it be the
wall, your shirt, or a
piece of paper.
for each pixel. This produces brilliant
images with the wide color gamut available from RGB lasers.
Direct-driving of the lasers pixel-bypixel at just the levels required brings
good power efficiency and inherently
high contrast. The efficiency is maximized, since the lasers are only on at
the level needed for each pixel. The contrast is high because the lasers are completely off for black pixels rather than
using an SLM (spatial light modulator)
to deflect or absorb any excess intensity.
The single-pixel collection optics are
optimized to take the particular beam
properties of the red, green, or blue
laser and relay it through the scanner
and onto the screen with high efficiency
and image quality. The pixel profile is
designed to provide high resolution
and infinite focus with a smooth nonpixellated image.
However, with the simple optomechanical design, the electronics
design becomes more challenging. This
is because some of the display complexity is moved to the electronics. This
approach requires that engineers be able
to accurately place pixels and to modulate the laser at pixel rates.
Infinite focus
In a raster-scanned laser projector, there
is no projection lens. The projected beam
directly leaves the MEMS scanner and
creates an image on whatever surface it
is shone upon. Because of the scanned
single pixel design, light-collection
efficiency is kept high by placing the
collection lenses near the output of the
lasers, while the output beam NA is very
low. By design, the rate of expansion of
the single-pixel beam is matched to the
rate that the scanned image size grows.
As a result, the projected image is always
in focus.
www.osa-opn.org
PICO-PROJECTOR TECHNOLOGIES
A number of companies around the world are actively pursuing pico-projector technology. One way to view the big
picture is to categorize the projector technologies based on the approaches being used to generate the projected
images, including those that result from (1) scanning a pixel in two dimensions, (2) imaging a 2D array of pixels, and
(3) mixing imaging and scanning.
c
Imaging pico projectors
c
These projectors follow the basic design used in smallbusiness projectors. A small spatial light modulator
(SLM), which has an individually addressable modulator
for each pixel, is used to create the picture. The SLM is
imaged by a projection lens onto the projection surface.
Unlike business projectors, which sometimes use three
SLM panels (one for each color), these projectors use
just a single SLM to meet the pico form factor.
Illumination in these systems comes from either
lasers or LEDs. The light is collected and optically
conditioned into a uniform beam that illuminates all the
pixels in the SLM at once. The pixel modulators absorb
or deflect light from the illumination beam to create the
modulated pixel gray levels. These projectors require
focusing and have a potential power disadvantage, since
the SLM illumination beam must always be on.
Color sequential: Color sequential is the most common
approach to produce full color and requires a fast modulator technology capable of cycling through all colors
during a single video frame interval. SLM technologies
that support color sequential include the DLP (Digital
Light Processing from Texas Instruments) and FLCOS
(ferro-electric liquid crystal on silicon; made by Displaytech and others). Color sequential has the advantage of
good color gamut and one-third the pixel count of color
filter system with the same display resolution. Color
breakup is an artifact associated with the color sequential approach that may be disturbing to some.
Color filter array: The color filter approach uses an array
of color filters built into the SLM. It is used with liquidcrystal-on-silicon modulators based on the more commonly used nematic liquid crystals. It sacrifices some
of the color gamut available with RGB light sources
(assuming white LED illumination), and requires a pixel
count that is 3X the display resolution; however, it avoids
color breakup. HiMax is one company that offers a
color-filter-based LCOS panel.
This special property comes from
dividing the task of projecting an image
into using a low NA single-pixel beam
to establish the focus and a 2D scanner
to paint the image. In fact, the MEMS
scanner plays the role of fast projection optics by producing an image that
expands with a 43-degree horizontal
projection angle.
Scanning pico projectors
Scanning pico-projectors directly utilize the narrow
divergence of laser beams, and some form of 2D scanning to “paint” an image, pixel by pixel. Usually the scan
pattern is similar to the raster pattern used in traditional
television, albeit in this case it is photons, rather than
electrons, being scanned. Some designs use separate
scanners for the horizontal and vertical scanning directions, while others use a single biaxial scanner. The specific beam trajectory also varies depending on the type
of scanner used. Since the scanner replaces an array of
pixels and projection optics, these projectors can be very
small while remaining in focus at any projection distance.
Microvision’s PicoP falls into this category.
c
Mixed imaging and scanning pico projectors
Mixed imaging and scanning pico projectors use a onedimensional spatial light modulator to create a single
column of pixels and a 1D scanner to sweep the column
horizontally, thereby creating a 2D image. Like the imaging-type projectors, a projection lens is used to image the
SLM onto the projection surface.
The 1D SLM is based on the diffraction of laser light.
Each pixel consists of a set of reflective ribbons located
side-by-side that can be individually shifted in a direction perpendicular to their flat surface to create either a
plane mirror or a diffraction grating. Modulation occurs by
illuminating the SLM with a uniform beam and adjusting
the diffraction grating for each pixel to diffract the desired
amount of light.
A gray level is achieved by allowing only the diffracted
(or the remaining undiffracted) light to pass through the
projector and blocking the rest. One-dimensional SLMs
of this type include Sony’s GLV (grating light valve),
Samsung’s SOM (spatial optical modulator), and Kodak’s
GEMS (grating electro mechanical system). To the best of
our knowledge, of these three, only Samsung is actively
pursuing this approach for a pico-projector.
This can’t be achieved in more traditional projector designs, where projection optics are used to image a spatial
light modulator onto the projection
screen due to conflicting constraints on
the projection lens. On the one hand,
a short focal length lens is needed to
create an image that grows quickly with
projection distance, while, on the other,
the lens aperture must be kept large to
maximize the projector’s brightness.
This dictates the need for a fast projection lens, with F/2 lenses being typical.
Depth of focus is proportional to F-stop.
The trade-off for traditional projector designs balances the rate the image
grows with distance, light efficiency and
depth of focus.
OPN May 2009 | 31
[ Scanned vs. imaging projectors ]
Horizontal trajectory: drive vs. time
Scanned laser focus advantage
3.00
Typical imaging projector spot size
focused for 0.5 m projection distance
2.00
1.50
1.00
Dis
pla
yp
ixe
e
l s iz
=
im
4 3°
Pi c
age
oP
s i ze
e
pix
4
l di
num
am
ber
e te
of p
ixe
ls
r
0.50
0.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Distance from projector [m]
“Infinite focus” property of the scanned laser projector compared with an
imaging-type projector.
The figure above shows the spot size
as a function of projection distance
for the PicoP. Notice that the spot size
grows at a rate matched to the growth
of a single pixel. For comparison, the
figure also includes the estimated spot
growth for an imaging-type projector
that has been focused for a 0.5-m projection distance. We assume a moderately
fast F/4 projection lens and the focal
length chosen to give the same 43° rate
of growth with projection distance for
the projected image. The depth of focus
for an imaging-type projector is much
reduced compared to the scanned laser.
To the user, this means that the
imaging-type projector must be
refocused as the projection distance
is changed, and that portions of the
image will be out of focus when one
projects onto surfaces that present a
range of projection distances within the
image—for example, projecting onto a
flat surface at an angle or onto surfaces
with a significant 3D profile.
Shifting projector functions
from optics to electronics
With the simplification of the optomechanical projector engine design, a
greater portion of the display emphasis
is shifted to the electronics. This allows
the physical size of the projector engine
to be minimized to fit with hand-held
32 | OPN May 2009
Vertical trajectory: drive vs. time
2.50
Spot size [mm]
[ Bidirectional raster scan ]
Blanking
Generation of raster pattern
[ PicoP projection display system ]
Frame buffer
memory
Safety
subsystem
Video ASIC
Laser drive
ASIC
System controller
and SW
Beam shaping
optics, combiner
MEMS drive
ASIC
Display
MEMS device
consumer products. The electronics,
which can be integrated more straightforwardly into consumer products, take
over tasks that are done optically with
other projector designs. Some of the
tasks that are shifted include pixel positioning, color alignment and brightness
uniformity. With the PicoP, the video
processor and MEMS controller have
been implemented as custom ASICs that
drive the IPM scan engine.
MEMS Drive ASIC
The MEMS Drive ASIC drives the
MEMS scanner under closed loop
control. The horizontal scan motion is
created by running the horizontal axis
at its resonant frequency—which is
typically about 18 KHz for the WVGA
(Wide Video Graphics Array) scanner.
The horizontal scan velocity varies sinusoidally with position. The MEMS controller uses feedback from sensors on the
MEMS scanner to keep the system on
resonance and at fixed scan amplitude.
The image is drawn in both directions as the scanner sweeps the beam
back and forth. This helps the system
efficiency in two ways. First, by running on resonance, the power required
to drive the scan mirror is minimized.
Second, bi-directional video maximizes
the laser use efficiency by minimizing
the video blanking interval. This results
in a brighter projector for any given
laser output power.
The vertical scan direction is driven
with a standard sawtooth waveform to
provide constant velocity from the top
to the bottom of the image and a rapid
retrace back to the top to begin a new
frame. This is also managed in closedloop fashion by the MEMS controller
based on position feedback from the
MEMS scanner to maintain a smooth
and linear trajectory. The frame rate
is typically 60 Hz for an 848 3 480
WVGA resolution; it can be increased
when the projector is used in lower
resolution applications.
www.osa-opn.org
Video ASIC
The video processor accepts either RGB
or YUV (NTSC/PAL) input. A video
buffer is provided to allow artifact-free
scan conversion of input video. Gamma
correction and colorspace conversion
are applied to enable accurate mapping
of input colors to the wide laser color
gamut. A scaling engine is available
for upconverting lower resolution
video content.
A proprietary Virtual Pixel Synthesis
(VPS) engine uses a high-resolution
interpolation to map the input pixels to
the sinusoidal horizontal trajectory. The
VPS engine demonstrates how projector
functions have been shifted from the
optics to the electronics in the scanned
laser paradigm. It effectively maps the
input pixels onto a high-resolution
virtual coordinate grid. Besides enabling
the repositioning of video information
with subpixel accuracy onto the sinusoidal scan, the VPS engine optimizes the
image quality. Brightness uniformity is
These super-small,
low-power projector
systems will open up
the display bottleneck
for mobile devices,
allowing information
to be accessed and
shared more easily from
portable devices.
managed in the VPS engine by adjusting coefficients that control the overall
brightness map for the display.
Optical distortions—including keystone, parallelogram, and some types of
pincushion distortion—can be compensated by using the VPS engine to adjust
the pixel positions. The VPS engine
also allows the pixel positions for each
color to be adjusted independently. This
simplifies the manufacturing alignment
of the IPM by relaxing the requirement
that the three laser beams be perfectly
aligned. The positions of the red, green
and blue pixels can be adjusted electronically to bring the video into perfect
alignment, even if the laser beams are
not. This capability can also be used to
compensate for some types of chromatic
aberration if the PicoP is used as an
engine in a larger optical system.
Mapping from digital video coding to
laser drive is performed by the Adaptive
Laser Drive (ALD) system. The ALD is
a closed-loop system that uses optical
feedback from each laser to actively
compensate for changes in the laser
characteristics over temperature and
aging. This ensures optimum brightness,
color and grayscale performance. Unlike
other display systems, optical feedback
is incorporated to ensure optimum color
balance and grayscale.
REQUIREMENTS FOR PICO-PROJECTORS
In order to operate in a mobile format, pico-projectors will need to meet a
number of requirements related to the following dimensions.
Size: The height or thickness of the projector is its most
important characteristic; the technology must be able to
be embedded in thin handheld devices. Both height and
volume should be minimized. The initial products will have
projector engine sizes ranging in height from 7 to 14 mm
and in overall volume from 5 to 10 cc.
Brightness: The brighter the better. Brightness is limited
by the available brightness of the light sources, either
lasers or LEDs, the optical efficiency of the projector
design, and by the need for low-power operation in order
to maximize battery life. Initial product offerings will be in
the range of 5-10 lumens.
Image size: In general, the shorter the distance it takes
for the projector to produce a large image, the better—at
least up to a point. A one-to-one distance/image-size ratio
(projection angle of 53 degrees) is probably a good target.
The first round of products on the market will have projection angles in the range of 30-45 degrees.
Focus-free operation: Focus-free operation is a very
desirable attribute in mobile applications. It is like “point
and shoot” in a camera. Unlike typical projectors used in
business settings, pico-projectors are designed for mobile
use (in other words, the distance from the projector to the
image will likely change often).
Resolution: Resolution can be expected to continue to
grow as product technology matures. The wide screen format is generally desirable for viewing video content. Initial
product offerings will typically offer resolutions from QVGA
(320 x 240) to WVGA (848 x 480).
Color: Pico-projectors typically use either lasers or red,
green and blue LEDs for light sources. In both cases, the
result is large color gamuts that far exceed the usual color
range experience of TVs, monitors, and conference-roomtype projectors. White LEDs used with color filters yield a
reduced color gamut.
Contrast: The higher the better. Just as brightness is
a measure of the absolute whiteness of pico- projectors, contrast is the measure of their absolute darkness.
Contrast is the dynamic range of a display, making the
difference between washed out images and crisp dramaticlooking images.
Battery life: A starting goal for mobile devices is that they
should last for the length of a complete movie. This puts
the lower limit at around 1.5 hours.
OPN May 2009 | 33
Components
Biaxial MEMS scanner
The biaxial MEMS scanner is made
using standard bulk silicon MEMS
fabrication processes. The WVGA
pico-projector scanner has a scan mirror
diameter of approximately 1 mm, and
it produces an active video scan cone of
43.2° by 24.3°.
The scanner uses moving-coil actuation with a single drive coil, which can
be seen on the vertical scan frame in the
photo with just two drive lines as shown.
The single coil design simplifies the
fabrication of the MEMS scanner and
reduces the number of required interconnects. The MEMS die is housed in a
package with small magnets that provide
a magnetic field oriented at approximately
45° to the scan axes. A single composite
drive signal is applied that contains the
superposition of the fast-scan horizontal
drive at the resonant frequency of the
horizontal mirror motion and the 60 Hz
vertical drive sawtooth waveform.
The mechanical design of the MEMS
scanner allows motion along only the two
orthogonal scan directions. Mechanical
filtering, resulting from the different mass
and flexure stiffness governing horizontal
and vertical motion, sorts the drive signals by frequency content, inducing the
18 KHz resonant motion of the horizontal axis and the 60 Hz sawtooth motion
of the vertical axis. Piezo-resistive sensors
provide scan mirror position feedback to
the MEMS controller ASIC to maintain
closed loop accuracy of the desired scan
mirror motion.
Lasers
The technology for the red and blue
lasers in the PicoP leverages the technology of similar lasers that are used for
the optical disk storage industry. The
wavelength requirements are shifted
somewhat, but the basic technology is
the same—GaAlInP red laser diodes
and GaN blue laser diodes.
Pico-projectors incorporate green
lasers as well. Prior to the push for
laser projectors, green lasers had not
been used for any similar high-volume
applications. The technology for the
34 | OPN May 2009
[ Bi-axial MEMS scanner ]
Only two
drive
lines
Vertical scan
frame with
drive coils
Piezo
resistive
sensor
Scan
mirror
green laser in the PicoP is based on
infrared lasers developed for the telecom
industry, the other massive market for
laser technology. Robust near-infra-red
laser diodes with very high modulation bandwidths are combined with
a frequency-doubling crystal, usually
periodically poled lithium niobate, to
produce a green laser that can be directly
modulated. With an eye toward the
burgeoning pico-projector industry, several companies, including Corning and
OSRAM, are ramping up production of
suitable green lasers.
The choice of which wavelength to
use for the lasers is based on two considerations. First is the response of the
human eye (the photopic response) to
different wavelengths. This response is a
somewhat Gaussian-looking curve that
peaks in the green-wavelength region
and falls off significantly in red and
blue. The amount of red and blue power
needed to get a white-balanced display
varies rapidly with wavelength.
For example, eye response increases
by a factor of two when the wavelength
is changed from 650 nm (the wavelength used for DVD drives) to 635 nm.
This allows the required laser power
to drop by the same factor, making a
projector that is lower power. Similarly,
the blue laser should be chosen to have
as long a wavelength as possible. Currently, blue lasers in the range of 440 to
445 nm are the best practical choice. As
the industry grows, longer wavelengths
in the range of 460 to 470 nm may
become a better option.
The second consideration is color
gamut. Since the photopic response is
peaked all through the green wavelength
range, the green wavelength should
be chosen where it will be most useful
for enhancing the color of the display.
Green lasers at 530 nm are a good choice
for maximizing the color gamut.
The ability to directly modulate the
lasers is at the heart of the scanned laser
pico-projector technology. Pixel times
at the center of the WVGA scanned
display are on the order of 20 ns. Lasers
therefore need modulation bandwidths
on the order of 100 MHz.
A path forward
The first generation of pico-projectors
are being launched this year. These
super-small, low-power projector systems
will open up the display bottleneck for
mobile devices, allowing information to
be accessed and shared more easily from
portable devices. With the high volumes
expected, there is great market opportunity for key components such as red,
green and blue lasers.
The scanned laser projector paradigm
provides a path forward to higher-resolution projectors without growth in size.
Unlike SLM-based projector technologies—in which increased resolution
means growth in the number of pixels
in the SLM—the single-pixel, singlescan-mirror nature of the engine remains
the same, even as the resolution of the
projected display increases. t
Visit OPN online (www.osa-opn.org) to
view videos that demonstrate the technical
concepts behind scanned laser projection and
how the MEMS scanner works.
Mark Freeman (mark_freeman@microvision.
com) is a principal engineer in optics at
Member Microvision Inc., in Redmond, Wash.,
U.S.A. Mark Champion is a principal engineer
in electronics and Sid Madhavan is a vice president of engineering at Microvision.
[ References and Resources ]
>> R. Sprague et al. “Mobile Projectors
Using Scanned beam Displays”, from
Mobile Displays, Technology and Applications, Bhowmik, Li, and Bos editors,
John Wiley and Sons, Chapter 21, 2008.
>> W. Davis et. al. “MEMS-Based Pico
Projector Display”, Proceedings of IEEE/
LEOS Optical MEMS & Nanophotonics,
Freiburg, Germany, 2008.
www.osa-opn.org