Quality Considerations for Aviation Head-up Displays (HUDs)

 

We have all seen the dramatic air combat sequence in movies—with a target sight, virtual guides help the pilot aim his weapon and lock on before firing. Those on-screen guides appear in the projections of a head-up display (HUD), so-called because the pilot’s head remains up with eyes on the outside environment, rather than down toward a screen or instrument panel.

A HUD is any transparent display that gives a pilot a seamless view of critical flight information, projected directly in the pilot’s line of sight (e.g., on a screen just inside the windshield). This allows the pilot’s eyes to remain focused outside the aircraft—the HUD’s virtual images may appear to be projected a distance in front of the aircraft, so that the pilot does not have to change focus to the HUD screen itself (this screen may be only centimeters away) or to look elsewhere for critical information (such as an instrument panel in the cockpit).

Aviation HUDs are designed so that flight information appears to be on the same visual plane as objects in the environment, so pilots don’t need to refocus their eyes when looking back and forth between projections on the screen and the exterior environment.

Rudimentary HUDs were first developed for World War II aircraft and became widely used in military applications during the 1960s. The first civil application of the technology was introduced in 1993.¹ Today, these systems are common in both military planes and large commercial jets.

The Boeing 787 is the first large commercial aircraft to offer a HUD as standard equipment, using a Rockwell Collins head-up guidance system.

Conventional HUDs display virtual shapes and symbols that provide weather, navigational, and other information, collectively referred to as “symbology”. The symbology can include aircraft position information like altitude, a horizon line, heading & flight path, turn/bank & slip/skid indicators, radar data, and airspeed, along with other data from the plane’s avionics and instrumentation (HUDs on military aircraft may also display information such as an attack target, weapons status, etc.)

HUDs are particularly useful if visibility conditions are poor. In fact, the Federal Aviation Administration (FAA) now allows pilots to make landings in “no natural vision” (zero-visibility) situations as long as there is an “enhanced flight vision system” (EFVS) installed, for example, an aircraft HUD system, or a helmet-mounted display (HMD) for the pilot.²

HUD System Components

To operate effectively, a HUD system typically includes the following components:

• A computer that receives data (including real-time metrics from the aircraft system sensors, avionics instrumentation, and satellite data).
• A transparent display screen, called a combiner. Typically made of glass or plastic, the combiner reflects information towards the pilot’s eyes without obstructing the exterior view through the windshield or blocking the passage of ambient light.
• A control panel that allows the pilot to select various display options and data to be displayed.
• A projector that projects the assembled images onto the combiner screen. Modern HUD systems have eliminated overhead projector units and instead are able to generate images directly on the display screen.

First-generation HUDs used a cathode-ray tube (CRT) display to generate images on a phosphor screen. Many HUDs still in use today are CRT displays, but the phosphor screen coating degrades over time. Next-generation HUDs introduced the use of solid-state light sources such as light-emitting diodes (LEDs), modulated by a liquid-crystal display (LCD) screen to display images. Many commercial aircraft today use this type of HUD.

Third-generation aviation HUDs use optical waveguides that produce images directly in the combiner, without the need for a projection system. Some of the latest HUD systems use a scanning laser, which can display images and video on a clear transparent medium, such as a windshield.

HUD makers are also beginning to work with imaging technologies like liquid crystal on silicon (LCoS), digital micro-mirrors (DMD), and Organic Light Emitting Diodes (OLED) to reduce the size, weight, and complexity of HUD systems. The next generation of HUD technology adds synthetic terrain or infrared video information to further enhance the display, as part of a broader category of EFVS that includes conventional HUDs.

Human Factors in Aviation HUDs

The study of human factors is about understanding human behavior and performance. In the aerospace industry, discussion of human factors often focuses on the element of human error in accidents and system failures. Here, “human factors” refers to specific aspects of human capabilities and performance such as visual perception. Consideration of innate human characteristics and responses helps with optimal design of systems that will be used by humans (the discipline of human-centered design). Well-designed equipment and the quality of systems and components help reduce human factors as a causal element in poor performance and accidents.

For humans, the eyes (and the associated optic system and visual processing centers of our brain) are the most important source of information we use to assess and understand the world around us. Human vision has driven much of the evolution in cockpit technology. “In contrast to the complicated, gauge-based systems of the past, the electronic flight displays of today’s modern airliners are testament to advances in human factors engineering.”³ Some of the most important human factors considerations include:

• Focus & Accommodation. For the eye to “register a sharply focused image, certain structural alterations are required depending on the focal length or distance to the object of interest. The process of adapting focal length from a distant object to a near point is known as visual accommodation and involves three separate, but coordinated functions—lens accommodation, pupil accommodation, and convergence. The speed at which accommodation occurs varies between individuals and with age but it is generally a split-second affair.”³ Accordingly, a display configuration that requires the pilot to switch focal point from near (display screen) to far (exterior landscape) could potentially diminish the pilot’s performance, not enhance it.

 

• Visual Attention.Our brains are only able to process a limited amount of visual information simultaneously. We have visual working memory that helps process and buffer the information we take in, effectively “metering” competing stimuli. However, focusing on specific items also blocks out others, potentially causing an “attentional blindness.” This selectivity is essential for a human’s ability to operate in complex environments but also potentially dangerous when flying an aircraft. “To efficiently attend to various information sources, and appropriately balance their time between focused and divided attention, pilots are taught the process of ‘scanning’, or attending briefly to each information source sequentially in a systematic fashion.”³ HUD displays reduce complexity by overlaying visual information on the exterior environment, making it easier to take in both types of visual input at once.

• Color and Contrast. Correct color and contrast values in a HUD display are essential for usability and safety in all operating conditions. The human eye is very sensitive to color and luminance (brightness). We are more sensitive to contrast than absolute luminance, allowing us to see accurately over a wide range of lighting conditions. High contrast (for example, black text on a white page) is easier to perceive than shades of gray. Successive contrast is the effect on our perception in a dynamic situation when shifting our eyes between one or more objects or views in succession. For example, looking at bright cockpit lights then transferring attention to a dark sky causes reduced perception because our eyes take longer to adjust to the darker view. HUD systems typically use green light for their display symbology because the human eye is most sensitive to these wavelengths.

Design Factors

Constructing an effective HUD system relies heavily on the design of the display itself. Considerations about the size, form factor, lighting, and more must be carefully evaluated. Factors include:

• Field of View (FOV) – FOV is the scope of the angle (vertical, horizontal, and diagonal) that a display captures and transmits back to the pilot. For example, a combiner with a narrow FOV might show only a runway; a wider FOV could include more information around the perimeter of the runway, allowing the pilot to see peripheral objects like another plane approaching from the side.
• Parallax – Because human eyes are separated by a slight distance, each eye receives a slightly different image, which is combined in our brains to create our binocular vision. Parallax errors occur when the image presented on a HUD does not align eye-to-eye. A HUD image needs to be clearly viewable by one or both eyes. This issue is typically addressed by collimation.
• Collimation – The human eye can focus on only one point at a time, thus HUD images need to be collimated: the projected light rays need to appear parallel out to infinity, rather than appear to converge at a point on the physical display screen. With collimation, a pilot does not need to refocus to view both projected symbols and the outside environment since both appear to be on the same “infinite plane.” In time-sensitive and safety-critical maneuvers such as landings, eliminating even the brief time it takes a pilot to refocus from the digital projection to the outside view can be vital. A collimator is a key component of high-quality HUD systems.
• Eyebox – To enable collimation and clarity of the display, the user’s eyes cannot be too far outside of an optimal viewing position, defined as the head motion box or “eyebox” area of the HUD system. Move to far left/right, up/down, and the image may not display clearly or fully, or may be distorted. Modern HUDs allow some freedom of movement within an eyebox of roughly 5 inches lateral by 3 inches vertical by 6 inches longitudinally (front to back). For a quality HUD, the pilot needs to be able to view the entire display as long as one eye is inside the eyebox.
• Luminance/contrast – A HUD must adjust luminance and contrast depending on ambient lighting (sunlight, night conditions, weather, etc.) to ensure readability under all conditions.
• Boresight – Aircraft HUD components need to be precisely aligned with three axes of an aircraft, so that data on the display conforms to the plane’s real position in space—that is, relative to the artificial horizon. This alignment process is called boresighting. This is typically done to an accuracy of ±7.0 milliradians (±24 minutes of arc) and may vary across the HUD’s FOV.
• Scaling – The images displayed on the HUD must be scaled to overlay the outside view with a 1:1 relationship with respect to the flight path, (pitch and yaw scaling, landscape details, etc.). “For example, objects (such as a runway threshold) that are 3 degrees below the horizon as viewed from the cockpit must appear at the −3° index on the HUD display.”⁴

Quality Regulations

Because of their use in real-time flight situations, the visual performance of HUD systems is critical. The FAA has issued several Advisory Circulars on topics related to HUD displays and electronic flight displays. Among many operational considerations, the agency specifies parameters related to a display’s size, resolution, symbology line width, luminance (in all light conditions), contrast ratio, chromaticity, grayscale, response, refresh rate and update rate, defects (such as element defects and stroke tails), reflectivity/glare, and the size of the flight deck viewing envelope.

For more detailed specifications, refer to the FAA Advisory Circulars:

• AC-25-11B– Electronic Flight Displays
• AC 90-106A– Enhanced Flight Vision Systems
• AC-25_1329-1C– Approval of Flight Guidance Systems
• AC-20-167A– Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment

Testing Head-Up Display Quality

How can aerospace manufacturers ensure that HUD equipment and systems are designed effectively to mitigate human factors, address the design and functional considerations, and adhere to FAA guidelines? A rigorous display testing regimen must be put in place. Thorough design and quality control inspection ensures that HUD projections are properly aligned and clear for in-focus binocular viewing, and that light and colors are vivid enough to be clearly discernible from surroundings in any lighting condition.

Low-quality projections put aircraft at risk if operators are unable to interpret poorly projected objects in the viewing area of the display. This can lead to misinterpretation, loss of critical environmental data (such as navigation, object proximity, and other alerts), and pilot distraction.

To accurately assess these elements, an optical measurement device and complementary test and measurement software is used to inspect HUD projections at several points within the eyebox area (to account for the scope of potential viewing angles). Radiant Vision Systems has provided the leading solutions for conventional display, near-eye-display (NED), and HUD testing in consumer electronics, automotive, and aerospace industries, with equipment advantages that optimize testing speed and simplicity.

In contrast to test methods that use spot meters (for instance, spectroradiometers) or traditional human inspection, Radiant’s HUD test platform is an all-in-one, automated system that relies on imaging to evaluate an entire display for all photometric (light, color, contrast) and dimensional requirements (defects, distortion, ghosting) in sequence.

Radiant’s ProMetric® Imaging Photometers and Colorimeters have been applied in testing environments to measure see-through display technologies from OLED to waveguide, using a range of projection methods.

Want to know more? Let us show you how Radiant imaging photometers and colorimeters solve several test and measurement challenges in the aerospace industry. See a demo of Radiant’s automated HUD test and measurement solution.

For more information, visit www.RadiantVisionSystems.com.

Citations:

1. “Head-Up Display” on SkyBrary, https://www.skybrary.aero/index.php/Head_Up_Display
2. Refer to FAA Advisory Circular 90-106A, issued 3/2/17
3. Nichol, Ryan J., “Airline Head-up Display Systems: Human Factors Considerations”. International Journal of Economics and Management Sciences, 4:248, May 3, 2015. https://www.omicsonline.org/open-access/airline-headup-display-systems-human-factors-considerations-2162-6359-1000248.php?aid=54170
4. “Head-Up Display” on Wikipedia, https://en.wikipedia.org/wiki/Head-up_display

Additional References:

Wood, R.B. and Howells, P.J., “Head-Up Displays”. Chapter 4 in The Avionics Handbook, CRC Press LLC: 2001. http://www.davi.ws/avionics/TheAvionicsHandbook_ Cap_4.pdf

Howells, P.J., “Head-up dispaly: not as easy as it seems!” SPIE Newsroom on www.SPIE.org, September 30, 2007. http://www.spie.org/newsroom/0859-head-up-display- not-as-easy-as-it-seems?SSO=1