2 glass HUD question

Alan Dicey wrote in
:

B2431 wrote:
I notice some HUDs have 2 glasses. How does this work without
reflections from the second one and interferance from the first?

Dan, U. S. Air Force, retired

Put simply, a dual combiner increases the vertical field of view of
the HUD symbology.

The sheets of glass in front of the pilot are called combiners: they
combine the HUD display symbology (projected from beneath by a very
bright CRT and some big lenses) with the pilots forward view by
reflecting the symbols projected from a very bright CRT under the
glareshield.

http://www.cmcelectronics.ca/En/Prod...hud_nthawk_faq
_en.html

http://www.mikesflightdeck.com/head_up_displays.htm

The combiners are cleverly-engineered partial reflectors. The
simplest design is just a semi-silvered mirror. The pilots view of
the outside world is dimmed a little, but the HUD symbology is
overlaid onto the pilots Field of View (FoV), reflected from below.

More advanced combiners use dichroic coatings to partially reflect
only the narrow set of wavelengths emitted by the (usually green) crt
tube. All other light is passed straight through, resulting in a
brighter real-world view (important in bad weather, dusk or night
operations).

The surfaces that are not meant to reflect are given anti-reflection
coatings to minimise the ghost images.

From the pilots point of view, the two combiners are one on top of
the
other, hence the increase in the vertical FoV. A single combiner
would need too much space above the glareshield and behind the
windscreen, and need a truly enormous final condensing lens in the CRT
projection optics.



I wonder if LCD projection optics will replace the vacuum tube CRT?
And maybe LEDs for a light source?


--
Jim Yanik
jyanik-at-kua.net