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.
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Imaging pico projectors
These projectors follow the basic design used in small-business 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: This is the most common approach to
producing full color. It 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 a 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 liquid-crystal-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.
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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.
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Mixed imaging and scanning pico projectors
Mixed imaging and scanning pico projectors use a one-dimensional 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 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.
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.
