Digital dash

[mark koenig]

Just wanted to add my 2 cents worth to the discussion.  My experience
comes from owning and driving a vehicle with a factory HUD, and 
researching and using display technology in various industrial projects,
although not in a HUD application.
The car is a 1990 Pontiac Grand Prix Turbo.  It came with the HUD as
standard equipment.  I understand it has been available as an option
on various GM models since 1990, with an option price in the $200 to
$300US range.  As an aside, since this was a limited production vehicle,
I have kept my eyes open for 'parts sources'.  I found a GPTurbo at a
local body shop about 8 years ago.  It had recently been brought in, and
the insurance company had just determined that the car was totaled.
Although the damage wasn't all that bad, The accident had cracked the
sealed optical path of the HUD, and the body shop found out that the
HUD was only available as a complete unit from GM, at a cost of 
$2,500US, which was the last straw for the insurance company.
My knowledge of the theory behind this unit's design is based on my
own research and conversations with various body shop and GM sources.
The optical path to the HUD must be long enough to allow the display
to appear to be out over the hood of the car.  On my GPTurbo, it
appears to be about a foot or two behind the front bumper.  The reason
for the long focus length is to allow you to switch between reading
the display and watching what's in front of you (like other cars,
cows, blondes, etc.) without having to drastically re-focus your eyes.
Without a long focus length, you would end up with a monster headache
and eye fatigue after driving the car for any length of time. The
only way to get this long focal length in a car is to use a 'folded'
(with mirrors) optical path tucked between the dash and the firewall.
Actually, using mirrors to fold the path allows you to solve a couple 
of other problems.  One is that it allows you to use a non-reversed
display.  Another is that the actual display screen ends up being
buried deep in the dash, away from temperature extremes on the top
of the dash.  My unit has two adjustments - one for brightness, and
one for changing the apparent vertical position of the display to
allow for different driver heights and preferences.  The vertical
position adjustment is done by allowing the final mirror to pivot
along one edge.
The brightness adjustment is absolutely necessary. Mine has a sliding
adjustment for driver preference, and is also tied into the headlight
switch.  With the headlights on, the brightness dims to about half
normal.  Without this, you will either not be able to see the display
in sunlight (display too dim), or your eyes will not adjust properly
to nighttime driving (display too bright).
On the subject of brightness - my display is a custom fluorescent
display.  My windshield is supposedly specially coated on the inside,
although I can detect no coating on it.  And it doesn't seem to be
affected by how dirty the windshield is on the inside.  Others have
mentioned military/space applications using a CRT as the display
source, but these use very efficient (expensive) mirrors, and a
detectable coating on the inside of the windshield.  IMHO, a CRT
will not work for everyday driving in HUD display because: you
probably can't afford the CRT that has been ruggedized for 
temperature and vibration as needed for use in a car, and your
probably not going to like replacing the CRT every so often due to
'CRT burn' and loss of brightness through time.  And I don't think
an Electroluminescent-backlit LCD screen will come anywhere close
to having enough brightness for daylight operation, unless you
overdrive the EL backlight and plan on replacing it every year or 
less.
My guess for a useable display source would be either a dot-matrix
LCD with either an incandescent or LED-array backlight source,
or a dot-matrix LED array.  Finding LEDs and/or incandescent
light sources that can withstand the vibration/temperature
environment should not be difficult.  Finding an LCD display for
this environment, without an EL backlight, and with enough pixels,
is another matter.  If someone finds one, PLEASE LET US KNOW.
The LED array seems like the best choice to me, but I don't know
of anyone who makes one for this application or anything close to 
it.
I would recommend to anyone considering building a home-brew HUD,
to scour the junkyards for one of these GM HUD displays, even if it
isn't operable, just to get a feel for how it works and how they
made it fit in the dash.  I've often hoped someone would dissect
one of these things and create a web site with lots of pictures
and measurements and stuff (hint hint).

You are free to disregard anything I say.

Good Luck.

Mark K.
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