Airborne pilots depend not only on information concerning the aircraft's airspeed, altitude, and attitude but also visual data that provides a view of the outside world and the ground. For example, in landing a plane in low-visibility conditions, a pilot will rely on a visual image provided by a weather-penetrating sensor, in this case, a 94GHz scanning radar. As an image-generating source, the cathode ray tubes (CRTs) used in head-up displays produce very high quality images. But CRTs have drawbacks that limit their effectiveness, particularly in bright ambient conditions. A CRT works by ‘writing’ on a phosphor screen with an electron beam. The beam is steered by magnetic deflection coils that in turn are driven by high-performance and agile deflection amplifiers. The phosphor luminesces visible light at a brightness determined by the energy of the incident electron beam. Because there is a vacuum behind the screen and glass in front of it—and neither medium is noted for its thermal conductivity—the phosphor screen produces heat during this process that is difficult to dissipate. Moreover, the mode of operation of the display in this system is raster graphic (a series of lines drawn in the same manner as a television scanned image), which presents particular technical challenges to CRT-based imaging sources like head-up displays. Moreover, the phosphor ‘blooms’ when the drive energy becomes too high, and it can also burn over time.
As a result, the raster graphic brightness that can be displayed to the pilot is strictly limited in the hundreds of foot-lamberts (FL) region. Unfortunately, theoretical work performed by BAE systems engineers, confirmed by flight testing, has indicated that we need to be an order of magnitude brighter in performance for our system to be effective in all conditions, a requirement that CRTs cannot satisfy.
Figure 1. The CRT (left) and its digital replacement (right).
The quest to replace the CRT with a solid-state, higher-performance arrangement is not new. It has been a research objective ever since designers of head-down displays abandoned the use of CRTs in the last century.1 The various technologies explored have generally involved the use of liquid crystal devices (LCDs). But LCDs are not particularly efficient and thus require a very intense illumination device, or backlight. Until very recently, in fact, the lack of a suitable backlight prevented the adoption of LCDs. Liquid crystals are also easily damaged by temperature extremes when they stored without being powered. For this reason, they have limited, if any, military application.
Figure 2. Relative luminance levels. CRT: Cathode ray tube. HUD: Head-up display. DLE: Digital light engine.
We have developed a low-powered green laser, with power output of approximately 1W, that we use to flood the face of a digital micromirror device on which our image is drawn. Our approach has enabled us to achieve the order of magnitude increase in luminance demanded by the system within a reasonable power budget.
Figure 3. The HUD image.
We started by modifying an existing CRT design by replacing the CRT with a ‘digital light engine’ that is the same size and shape as the tube it replaces (see Figure 1). The laser is a frequency-doubled Nd:YAG (neodymium-doped yttrium aluminum garnet) device that produces green light at 532nm with powers up to 1W. The device is compact and has undergone full environmental testing with vibration levels up to 26G. Figure 2 graphs the image intensity levels that we have reached with these devices in our displays, roughly 4000FL. By comparison, previous CRT systems have at best produced images that achieve raster-graphic brightness values of less than 1000FL. This is insufficient to produce a usable image against the bright conditions often experienced in practice of up to 10,000FL. The system was constructed and has undergone flight testing with the radar sensor at Edwards Air Force Base. The luminance provided by the system was adequate to supply sufficient ‘shades of gray’ for the radar image to be effective in very bright conditions, the only exception being flying directly into the sun, a scenario that under simulated category 3 (i.e., low-visibility) weather conditions is unrealistic. Figure 3 shows the image produced by the system.
In summary, we have demonstrated an entirely feasible head-up display design, in real conditions, that fully meets the requirements for a device that is part of an airborne autonomous landing concept system.2,3 A laser with 20% more power output has been constructed that will achieve the luminance requirements that our theoretical work indicates as ideal for such a system. Further work will focus on improvements in image generation and optical efficiency to reduce the laser energy required and to perform further flight testing to verify the system's performance in actual conditions.
Paul Wisely is the head-up display product manager at BAE Systems. He has many years' experience in the aerospace industry, including design and development of airborne electronic systems, and technical, project, and program management on a variety of military and civil display programs. He is currently involved in developing advanced display products incorporating advanced and emerging technologies. He received the Chairman's Gold Award for technical innovation in 2005