In this TI DLP
Labs video, we will
discuss a convenient
method and tool
for estimating the
volume of a HUD system.
As head-up displays grow in
popularity and market share,
size of the displayed
image also grows.
Moving to augmented reality
head-up displays, or AR HUDs,
display sizes can exceed 15
degrees field of view, or FOV.
While this increased FOV
allows displays covering
more of the road in
front of the driver,
it also presents
several challenges.
One main challenge with AR
HUDs is physical volume,
with the largest systems
approaching 20 liters of space.
This can be difficult to
fit in many small cars
due to interference with
the steering column, HVAC
systems, and brake pedal.
This is a typical
layout of an AR HUD.
The HUD is located below
the car's dashboard
and consists primarily of
the projector, diffuser,
and large freeform mirror.
The projector and
diffuser screen
make up the picture
generation unit, or PGU,
and form a real image
on the diffuser screen.
The light from
this image is then
reflected off the freeform
mirror and again off
the windshield to be redirected
into the driver's eye box.
The virtual image is
projected at the virtual image
distance, or VID.
And the size of the
image is determined
by the horizontal and vertical
field of view, or FOV.
There are three main
steps in the process
for estimating the volume
requirements of a AR HUD.
First, from the FOV, VID,
and eyebox requirements,
the size of the large
mirror must be estimated.
Once this is calculated, the
size of the diffuser and mirror
can be used to find the
total volume occupied
by the main optical path.
Finally, the size of the
mechanical components,
thermal solution
and layout must be
estimated to determine the final
volume of the AR HUD system.
Taking a closer look at
the layout of an AR HUD,
we see the projector
and diffuser screen
create an image which is
reflected off the freeform
mirror, here labeled M1, to
the windshield and eyebox.
There was a large cavity created
between the diffuser screen
and mirror which must be left
open for the optical path.
Overall volume in a
HUD system is primarily
affected by the size of
the large freeform mirror.
The larger this mirror is,
the larger the optical cavity
becomes.
The size of this
mirror is mainly
determined by three HUD system
parameters, FOV, eyebox,
and to a lesser extent, VID.
Increasing FOV or eyebox
size will cause a nearly
linear increase
in overall volume,
whereas increasing VID has a
more logarithmic relationship.
This graph shows
the relationship
between eyebox size, mirror
size, and virtual image
distance in the
horizontal dimension.
We can see that by changing
the FOV or eyebox size
specifications, the mirror
size is significantly changed.
But when the VID is reduced
from 10 meters to 7.5 meters,
the mirror size remains
almost unchanged.
Increasing or decreasing the FOV
shows an almost linear change
in overall volume.
Smaller FOV and
eyebox combinations
can have volumes
below eight liters,
while AR HUDs with the largest
FOV and eyebox specifications
can approach 18 liters
with traditional optics.
Changing VID will have
a significant effect
on mirror size and overall
volume at short VIDs.
There is a significant
volume increase
changing the VID from
two meters to six meters,
but very little difference for
VIDs greater than six meters.
To help understand the impact
of different parameters
on overall volume, TI
has created an Excel tool
to estimate the necessary
required volume of an AR HUD
system.
This tool can be
downloaded by going
to the link in the
video description.
The AR HUD Volume
Estimation Calculator
uses standard HUD parameters
such as FOV, eyebox size,
and VID to calculate an
estimated overall volume
occupied by the HUD system.
The included graphs
show approximate sizes
of vertical and
horizontal dimensions
of the virtual image
formed allowing the user
to estimate what
portion of the road
may be displayed upon for
a given set of parameters.
Near the top of the calculator
are the input parameters.
Included are high-level
parameters of the overall HUD
design as well as some
system-dependent parameters.
Updating these parameters will
update the volume estimates
in real time.
Field of view, virtual image
distance, and eyebox size
can be updated to understand
the relative volume requirements
for different HUD designs.
Mirror 1 obliquity
angle will typically
be fixed by the design
of the car windshield.
HUD diffuser screen image width
is calculated automatically
based on the FOV and
VID requirements above.
It is scaled along with
a mirror magnification
to maintain reasonable
design constraints.
Mechanical volume increase
is an estimated parameter
to factor in mechanical
housings, mounting features,
thermal solutions, and other
non-optical features of a HUD
system.
30% is a typical value
for some designs,
but may be adjusted
for individual designs.
This calculator estimates
volume for a one-mirror system.
If designing with multiple
mirrors, increasing the M1-M2
overlap fraction can account for
the portion of the optical path
occupying the same volume.
The calculator output section
shows estimated values
for mirror dimensions, virtual
image size, magnification,
and volume estimates.
All calculator outputs are
calculated in real time
when inputs are updated.
There are two volume
estimates included.
The first is the
estimated volume
of only the optical components.
The total mechanical
volume estimation
includes the increased volume
from the mechanical components
included in the input section.
In putting moderate
HUD specifications
to create a medium-sized AR HUD
image and using FOV of 10 by 3,
a VID of 7.5 meters, an eyebox
size of 140 millimeters by 60
millimeters, and the
other default parameters,
we see a total estimated
required volume of 6.9 liters.
In putting HUD specifications
to create a much larger AR HUD
image and using FOV of 15
by 5, a VID of 15 meters,
and eyebox size
of 140 millimeters
by 120 millimeters, and the
other default parameters,
we see a total estimated
required volume of 19 liters.
Total volume is not
the only consideration
when fitting in AR HUD
system into a vehicle.
There are often other
systems using the same space
under the dashboard that the
HUD needs to fit next to.
Some of these systems
include the steering column,
brake pedal, HVAC ducts,
and the car frame.
Because most of these components
limit the vertical space
available, height is often the
limiting constraint, not just
total volume.
Vertical FOV and eyebox height
have the greatest impact
on overall HUD height and
can often be traded off
for a smaller volume system.
Total volume of an AR HUD
should be considered early
in the automotive
design process.
Space should be allocated
when designing the car frame
and dashboard to minimize
design challenges later
in the process.
For more information
on this topic,
please see the other
resources on TI.com/DLPAuto.
Thanks for watching.
