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.
A HUD uses an optical combiner to add computer generated symbols to the pilot’s forward field of view. A HUD is a class of collimated display, the added imagery appears to be at optical infinity. This provides two major advantages. The imagery stays aligned with the outside view regardless of how the pilot viewpoint changes, (for example, the aiming symbol always indicates weapon system boresight), and 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 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 can 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.
The HUD collimating optics assembly consists 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 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.
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.
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.
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.
A Simulator HUD
Unless your sim has a collimated display, a HUD for your sim does not need to collimate. You want the HUD aiming symbol to stay aligned with the weapon system boresight and the virtual horizon to remain on top of the "real" horizon. What a simulator HUD should do is make the HUD imagery be coincident with the external scenery display. If the scenery is projected on a screen 60 inches away, the HUD imagery should be created an apparent 60 inches away.
The HUD imagery is virtual. It isn't projected on the screen that displays the imagery. The HUD imagery is only visible when viewed through the combiner. Look around the combiner, and the HUD imagery is gone.
The optical paths in a simulator HUD are the same as in a real HUD. The imagery is formed on a display device. Light from the devices passes through a lens, is redirected by a mirror, and is aligned with the light from the external scenery with a partially reflective combiner. However, the details are different.
The HUD image source doesn't need to be nearly so bright as the source in a real HUD. In a simulator HUD the HUD imagery is competing only with an LCD monitor or a projection system, not noon time sun reflecting off white cloud tops. A small LCD monitor will work well in a simulator HUD.
A fold mirror isn't strictly necessary if you're got the room to orient the optical path vertically. Probably though your sim does not have the room and you will need to fold the path. Use a front surface mirror. A standard glass mirror has a rear reflective surface. Light also reflects off the front of the glass, forming a ghost image. When you look straight into the mirror, the ghost image lines up with the main reflection from the back of the mirror. You don't normally see the ghost image. But when used as a fold mirror, the ghost is shifted relative to the main reflection. The ghost reflection is very apparent and very annoying.
Admittedly, it would be nice to use high precision optics corrected for geometric distortion, but it really isn't necessary. If you're willing to accept a bit of distortion, a simple convex lens will create a virtual image at a useful distance for considerably less money. You need a fairly large diameter lens, something around 5 inches or so. A focal length of 10 to 15 inches will work. I'd lean more to the longer focal length as the geometric distortion is likely to be smaller. If the image source is placed somewhat closer to the lens than its focal length, a virtual image will be formed some distance beyond the lens focal point. Use the lens maker's formula to determine these distances.
[1/(distance to image source)] = [1/(lens focal length)] + [1/(distance to virtual image)]
A teleprompter mirror makes a great combiner, because that's what a teleprompter mirror really is. It has a partially reflective coating on one side and an anti-reflection coating on the the other to suppress ghost images.
If you were to prototype a simulator HUD using easily worked, low cost materials, you might choose foam core board, and the prototype might look like this. (The yellow ruler is 6 inches long.)
Rather than initially using a small LCD monitor for an image source, you might choose to build a static test pattern which you could backlight with a flashlight, and slide within the prototype enclosure to adjust the focal distance. Such a test pattern would help determine if any geometric distortion was objectionable.
Large diameter lenses are expensive. You can buy a 5 inch diameter lens from optics houses like Edmund Optics for around USD50. This diameter is used in some of the smaller theater lighting fixtures, and lenses for these fixtures show up from time to time on Ebay. The optical quality isn't likely to be as good, but the price should be more attractive.
For some REAL information on HUDs...
The Avionics Handbook ed. by Cary R. Spitzer. (2001, CRC Press LLC, ISBN 0-8493-8348-X) The Head Up Display chapter in this handbook provides the best overview of the topic I've come across. You probably don't want to buy your own copy, but if you're really interested in HUDs, I recommend having your local library check into getting one through inter-library loan.
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. It's a good resource for HUD symbols.