How do HUDs work?

[VNugget]

I find this a very interesting topic, and would be curious to find out how they work. I have searched the internet high and low, but came up with nothing. I imagine that they build upon the principles of red-dot gunsights, but when searching for information on those, all I have come up with are online sales... no educational information on the optical principles behind them.

So... any help, references to websites, books, etc. would be greatly appreciated.

Thanks,

 

[CCDiscoB]

Its actually quite simple, it falls under the PFM principle.

You don't realize how much you use it until it fails.

 

[Rush Limbaugh]

I thought it used the Thermos principle.

 

[Pistlpetet]

I assume that HUDs in other aircraft work on the same principle. In the Apache you wear the HUD on an eyepiece attached to your helmet. This HDU (Helmet Display Unit) contains a small one inch TV screen that is reflected onto a piece of glass that you look through. Nothing really gee-whiz about it. You can adjust the brightness of the symbology during daytime ops, and during nite ops you use the FLIR picture displayed on this HDU to fly the aircraft. Apache pilots do not use NVGs to fly the aircraft, however some front seat Apache pilots are allowed to use NVGs for navigation purposes, but the back seat PIC guy still has to use the FLIR.

Anyways, HUDs have been around for a long time as gun/missle sighting systems, they then evolved into displaying Flight data information.

Pete

 

[VNugget]

Hmm, thanks for the interesting info, but I was actually wondering more along the lines of how it actually projects the symbology in a manner that it's always over the same piece of backgorund, no matter where your head is, i.e., how it eliminates parallax error.

 

[Pistlpetet]

The glass has a special coating to it so that is absorbs and displays the symbology. I would imagine that the display properties of video projection also needs to be very strong.

Does that answer things?

 

[VNugget]

Well, that didn't, but after giving it some more thought and drawing it out, I answered it myself... and it's so stupid, that I feel really dumb!

The projector and the glass are fixed with respect to each other, and that's the common fixed base of reference! (In reality, those are a bunch of words I threw together, but it's the best way I could describe it ). Any given location on the projector is projected up at the glass, and then reflected towards you in only one possible way. Your eye can "like it or leave it."

Sure you can move your head, and the "piece of background" opposite a certain location on the glass will shift tens of miles, but so will the point on the projector that you're "looking at." Hence, when your eye "intercepts" a symbol, it can only be looking at one possible point across the glass.

And the whole time I started off with the concept that it was projected on there like a movie (i.e. you could get the same effect by drawing on it with a marker) and thought it was some esoteric concept that you'd need a degree in optics to understand. DUMB DUMB DUMB!! Of course I had to embarass myself here before I figured it out. Sorry for wasting anyone's time.