Just as an aside, several years ago I tinkered with a concept for a low-cost
HUD.  I built a cross-shaped raster of LED's, just like in some CDI's.  I
then hooked them (through appropriate electronics) to the CDI input for the
localizer and glide slope needles.  I laid the raster on the dash of my 182
so that it shined up onto the windshield.  The reflection was then a CDI
indication that could be used to fly an ILS.  Of course, it wasn't much if
any good in direct sunlight, but the theory was you don't need a HUD on a
sunny day.  In clouds or at night it worked okay, but there was a double
image due to the reflection from both sides of the windshield, a problem I
thought that could be cured by adding a selective reflective surface to the
inside of the glass.  It never went any further than that first successful
experiment, but I thought at the time it would be a good aid to an ILS
approach.

Gary Casey