HUD Technical Question

[Micro]

I'll cut to the chase... Why is it that some (mostly early model) HUDs have 2 pieces of glass? 

[KursadA]

Woohoo - some obscure technical question that I actually can answer. A couple years ago, I helped a friend rebuild a HUD for a flight simulator; and had to go through lots of literature to understand the technology.

 

The correct term for these are "dual combiners". Dual combiner technology was an early technique to extend the vertical field of view. Advances in combiner and projector technology and materials rendered this unnecessary in later HUD designs, which can provide a pretty good field of view using a single combiner glass.

 

If you really need a detailed technical description, the Rochester Avionic Archives site has a good, short description. Just click on one of the examples using dual combiners on their HUD archive page; e.g. the Tornado HUD or Mirage III HUD.

http://rochesteravionicarchives.co.uk/manufacturer/pilkington-pe

 

Quote

The vertical field of view can be extended by the addition of a second Combiner Glass forward of the first glass and extending higher up. The optical system then provides an IFoV with a second but smaller double circular field above the first. Now, the image has to transition from the aft glass to the forward glass in a smooth manner and this done by using graded optical coatings which are traditionally a neutral density coating. Thus the coating on the aft glass reduces in reflectance at the top as the forward glass coating increases in reflectance and the transmittance, of course, does the opposite. The whole effect must be achieved without any obvious display artefacts and even more important there must be no false horizon lines created. This last issue is aided by making the top of the Combiner Glass angle down in the forward direction on the sight line from the design eye position. To achieve this gradation of coatings requires a sophisticated deposition technique. 

 

Hope this helps

 

K

 

 

 

[Micro]

Ah.... Very interesting stuff. Thanks for the info!

[dnl42]

Wow, cool info! Didn't know about this early form.

 

I've got to say, a HUD in a car is a Wonderful Thing(TM). My '92 Bonneville projected directly on the windshield. My '15 Mini S has a separate display glass that raises into position when the car is started and retracts when shut own.