3.00
Typical imaging projector spot size
focused for 0.5 m projection distance
Display pixel size = 43° image size number of pixels
PicoP pixel diameter
4
2. 50
Spot size [mm]
2.00
1. 50
1.00
0.50
0.00
0.0
0.5
1.0 1. 5 2.0
Distance from projector [m]
2. 5
3.0
“Infinite focus” property of the scanned laser projector compared with an
imaging-type projector.
Horizontal trajectory: drive vs. time
Vertical trajectory: drive vs. time
Display Blanking
Generation of raster pattern
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 opto-
mechanical 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 handheld
[ PicoP projection display system ]
Frame buffer
memory
Safety
subsystem
Video ASIC
Laser drive
ASIC
System controller
and SW
MEMS drive
ASIC
Beam shaping
optics, combiner
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
(application-specific integrated circuits)
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, bidirectional 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 closed-loop 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.
