Tech Briefs Magazine - June 2022 - Sensor-5

RasterTrajectory
Lissajous Trajectory
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Figure 4. Raster scan left; Lissajous scan, right. (Image: TriLite Technologies GmbH)
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Figure 5. The electronics PCB feeds the scanner module, which is directly coupled to the optical waveguide. (Image: TriLite Technologies GmbH)
and opens it up. This increases the " eye
box " - the area within which an effectively
viewable image is formed by a visual
display. This is very important because
if your glasses shift a little, you still
need to see the image. With a large eye
box, you see the same image wherever
your eyes are aimed, thus making the
glasses fit more universally.
There are two important functions of
the wave guide. It conveys the image to
the lens and also combines the incoming
light with the " digital light " so that
you can simultaneously see both the
digital image and the real-world scene
coming through the eyeglass lens.
Aligning the Lasers
It is necessary to align the outputs of
the three - RGB - lasers in order to
get a perfect image, or the image would
Sensor Technology, June 2022
be blurred (see Figure 3). You can spend
lots of effort, energy, and money to do
the alignment mechanically. However, in
order to minimize size, weight, and
power consumption, TriLite uses software
instead - proprietary multiparameter
algorithms for laser and mirror
control. This enables them to tolerate a
certain degree of misalignment.
Scanning Patterns
The images in Figure 4 illustrate two
alternate scan patterns for a single image
frame. On the left is a typical raster scan
of the type used on a CRT - the pattern
is painted in lines from left to right proceeding
from top to bottom. On the
right is a Lissajous scan, which through a
complex series of motions, gradually
paints the entire image instead of doing
it line by line. This is much like a jpg
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image gradually coming into focus and
produces a more comfortable image for
the eye.
In order to produce a Lissajous trajectory,
TriLite puts the MEMS mirror in
resonance in both axes. The pattern is
generated by a difference in frequency
and phase between the two axes. The
mirror is controlled to make sure it's
moving at the selected frequency and
the laser is controlled to define exactly
where both are aimed. They are then
able to write the pixel information. If it's
a red pixel, for example, only the red
laser is pulsed. The color information
comes by controlling the laser diodes;
the spatial information is constant and
comes from the mirror.
Controlling the Lissajous is complex,
for both timing and pulsing - the mirrors
are moving very fast, and the laser
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Tech Briefs Magazine - June 2022

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