A HUD or Head Up Display is a device that overlays information on the user’s normal field of view. This includes head and helmet mounted displays, but the main thread of this page is the HUD mounted above the glare shield in military fighter and attack aircraft.

These HUDs display key flight and weapons information, helping to keep the pilot’s attention outside the cockpit. If you’re into fighter-style cockpit simulations, having a functioning HUD simulation is a huge step up in realism. Since the HUD is in the forward field of view, it’s a key element in the overall experience.

The sim game software you’re running paints the HUD data on top of the forward view. As you build your pit, you’ll want to separate the exterior view from the instruments. You’ll want a HUD.

HUD Basics

A HUD uses an optical combiner to add computer generated symbols to the pilot’s forward field of view. The system is designed to place this added imagery at optical infinity so that the pilot does not need to refocus eyes when switching between threat scanning and reading the HUD data. This means the optical system is more complex than simply a partially reflective mirror. A HUD is a class of collimated display.

Collimating Lens

A spherical, plano-convex lens will collimate an image, however such a simple approach comes with a few shortcomings. It will introduce geometric distortion in the image, not all colors will be in focus for the same lens position and when the image center is in focus, the image edges will not be.

We use spherical lenses in spite of these limitations because spherical shapes can be made with great accuracy for only moderate costs. Aspherical lenses made with similar accuracy can have substantially better optical properties, but manufacturing complexities generally make the cost prohibitive.

In any case, aspherical lenses are not the final answer because they can only correct for geometric distortions. Lenses, spherical or otherwise, work by refracting light, and refraction angles change with the wavelength of the light. Aspherical lenses are subject to chromatic aberration the same as are spherical lenses.

A typical HUD collimating lens is an assembly of several simple lenses. At least one of the lenses will be made of a material with a different index of refraction. The individual lenses and the lens materials are chosen such that the aberrations of the individual lenses tend to cancel, resulting in a composite lens having performance superior to that of its components.

Combiners

Partially reflective

Combiners come in several varieties. A simple one is a partially silvered, flat mirror. It’s workable but looses light. If it has a 50% transmission ratio, half goes through and half is reflected. This is not a problem for the HUD data, as the HUD source can be made brighter. It is a problem, however, with respect to the pilot’s forward vision. Half of that gets reflected away too. You can play with the transmission ratio to reduce the forward vision loss, but that only works so far.

Another annoying feature of these combiners is the presence of internal reflections. Light will reflect from both the front and back surfaces. The reflections appear as ghost images.

Internal reflections can be minimized by using an anti-reflection coating on the non-reflecting surface of the mirror substrate. The coating generates reflections off both its front and back surfaces. The thickness of the coating is controlled so that back surface reflection is out of phase with the front surface reflection. Being out of phase, they cancel each other. The index of refraction is chosen so that the two reflections are of the same magnitude to ensure the best cancellation. Anti-reflection coatings are tuned to a single wavelength of light, but give adequate results over a range of colors.

Dichroic

An improved combiner makes use of a dichroic mirror. This is similar to an anti-reflection coating, but many layers are used and coating thickness is chosen to enhance reflection of a particular wavelength. Other wavelengths pass through with minimal attenuation. When a dichroic mirror is used, a narrow band of colors is blocked out of the pilot’s forward view and replaced by the HUD data. Because the color of the HUD display matches the dichroic characteristic, virtually all of the HUD generated light is directed to the pilot, and only a small amount of outside light is blocked.

A dichroic combiner is also subject to internal reflections. An anti-reflection coating is used on the backside of the combiner to control this.

Catadioptric

The combiner does not have to be flat. A curved combiner can be used to implement part of all of the collimation. This is generally referred to as a catadioptric HUD. Catadioptric refers to an optical system that incorporates both reflective and refractive elements. A big advantage of reflective optics is that they do not suffer from chromatic aberration. Similar to lenses, they are most easily made in spherical shapes, and so also suffer from spherical aberration. This can be corrected through optical system design just as a lens-based system is corrected.

Holographic

The term “holographic HUD” brings to mind something out of Star Trek or Star Wars, perhaps a little 3D image of a gremlin standing on the aircraft’s nose alerting the pilot to potential threats or targets by making obscene gestures. Sadly, such is not the case. The reality is much more mundane.  A holographic HUD simply uses a holographic optical element or HOE as the combiner. This is a specialized diffraction grating that can  both combine and collimate. HOEs can be made very wavelength specific to allow the maximum amount of light from the forward field of view to pass through to the pilot. A holographic HUD can deliver a larger field of view for a given weight than can a HUD based solely on lenses and/or mirrors.

Options for simulating a HUD

Caveat: I haven’t quite gotten around to building a HUD as yet. What follows are my thoughts on developing a HUD sim, before allowing reality to correct my mistakes. (i.e. you are now entering the vaporware zone.)

A small black and white television with a video input appears to be a good choice for the image source. (A small point-of-sale VGA monitor would likely be a better choice, but these items rarely turn up on the surplus market.) Small format color LCDs are becoming widely available, but don’t have the resolution and are more expensive. Five inch BW televisions occasionally turn up as loss-leaders with after rebate prices as low as $15. The normal price is on the order of $30~40.

The television can be driven with a TV video output capable video card. HUD graphics are not particularly complex (assuming no FLIR). There are no smoothly shaded 3-D images, just simple line drawings. The image can be built up using Windows® GDI calls. (At this point, I’m thinking in terms of MSFS and FSUIPC. G2Interactive has some plans for getting more data out of Falcon 5, but has not released details.)

Ideas for the combiner revolve around reflective window film or aluminized Mylar®. Rather than applying the film to a glass substrate, the thought is to use only the film. This should avoid ghosting from internal reflections. There reflections will still be there, but because the film is so thin, the ghost image will be almost on top of the main image.

The collimator does not need to be particularly strong in this application. The HUD image does not need to be placed at optical infinity, but only at the depth of the external view imagery. As I’m looking at a monitor only a few feet away, that shouldn’t be a big problem. A simple lens should do the job. I plan on playing with plastic page magnifiers from an office supply store.

For some REAL information on HUDs...

Aircraft Display Systems by Malcolm Jukes. (2004, American Institute of Aeronautics and Astronautics, Inc. ISBN 1-56347-657-6) This provides broad coverage of all displays used in aircraft. Of particular interest are chapters 6, "The Head-Up Display", and 7, "Civilian Head-Up Displays". 

Head Up Displays: Designing the Way Ahead by Richard L Newman. (1995, Ashgate Publishing ISBN 0291398111) is a comprehensive survey work. It targets engineering teams developing new technology. Its main goal is to improve the design process. There is less about the internal workings of existing HUDs than about what functions a HUD should perform. It's up to the design team to figure out how.

Here's an overview of the book:

Chapter 1 - Introduction. Newman lays out the purpose and scope of his book: to provide an historical review of the HUD usage, successes and shortcomings, and to recommend design criteria to avoid repeating past mistakes.

Chapter 2 - Historical Review. Overviews early work, HUD applications, evolution of the technology, and shortcomings.

Chapter 3 - A review of HUD Technology. Lists the different approaches that have been used in HUDs. Provides some block diagrams and sample HUD displays to demonstrate the symbology. Defines a number of terms.

Chapter 4 - Symbology lessons learned. This is the GUI (graphical user interface) of the HUD. If Microsoft, Apple and the various Unix camps can't agree today on what's best, imagine what HUD designers decades ago went through. This chapter takes a detailed look at the HUD symbols, what they are supposed to communicate to the pilots and how successful they are.

Chapter 5 - Primary flight reference criteria. Takes a specific look at subsets of HUD data which provide "sufficient information for the pilot to fly the airplane during a particular mission segment". This is information like airspeed, altitude and attitude. Systems and navigation information would not normally be included unless critical to the particular segment.

Chapter 6 - Equipment considerations. Lists operation requirements: field of view, image quality, reliability, and so on.

Chapter 7 - Display criteria. Addresses what information should be displayed by the HUD at any given portion of the flight.

Chapter 8 - Recommended standard symbology. The title just about says it all.

Chapter 9 - HUD evaluations. Makes the case for objective-based test methodologies rather than individual opinions.

Chapter 10 - HUD training. Answers, "when should HUDs be covered during flight training?"

Chapter 11- Summary. Lists a number of unresolved (as of 1995) HUD related issues.

Chapter 12 - Glossary. Presents standardized definitions and abbreviations.

Appendix: HUD Symbologies. Presents a number of sample HUD displays which are "significant from a historical perspective or which are current production HUDs".

Bibliography. An extensive list of publications about HUD technology arranged by year of publication.

Subject index to bibliography. 

Index.

This book is a bit on the pricey side (US$180), though if you order online from Ashgate (www.ashgate.com) they may offer a 15% discount. I wouldn't recommend buying a copy unless you have a professional interest in HUD design. However, if you're very interested in HUD technology and can find a library copy, it's definitely worth looking at.

It's possible that I'm not as smart as I think I am. (Occasionally, I have moments when I know this to be true. Fortunately the feeling passes quickly.) Although I have tried to make this information as accurate as I can, it is not only possible, but also quite likely, that errors lurk within. I cannot and do not warrant these pages to be error free and correct. Further I accept no liability for the use of this information (or misinformation). If, after reading this, you are still interested, please be aware that the contents of this site are protected by copyright (copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 by John M. Powell). Nonetheless, you may copy this material subject to these three conditions: (1) the copyright notice is copied and presented along with the material, (2) the copy is used for non-commercial purposes, and (3) the source of the material is properly credited. And of course, you may link to this page.