Helmet-Mounted Displays: Cathode-Ray Tube (CRT) vs. Flat-Panel

The helmet-mounted display (HMD) system designed for the RAH-66 Comanche helicopter must meet numerous design and system specifications regarding optical performance, weight, center of gravity (c.g.), military equipment compatibility, and human factors integration constraints. The HMD technology currently used in the AH-64 Apache consists of a monocular miniature cathode-ray tube (CRT) on which the pilot can view both imagery and symbology. The Comanche HMD design specifies a binocular display having a minimum field-of-view of 30° vertical x 52° horizontal with at least 30° binocular overlap. The alignment and registration of the oculars must be precisely aligned to reduce pilot visual fatigue and negate other perception problems. The Comanche specification also requires a minimum eye relief of 25mm to accommodate a chemical/biological protective mask. The interpupillary distance (IPD) is required to be adjustable from 58-74mm with a 0 to -1 diopter adjustment. The binocular displays must have a vertical adjustment range of ±5mm and a fore-aft adjustment range of +8mm forward and -4mm aft. This complex optical system must be mounted on an Army flight-qualified helmet that meets the prescribed weight, c.g., crash survivability, and noise attenuation requirements, in accordance with current military standards. In addition, the Comanche helmet system must accommodate the 5th percentile female through the 95th percentile male aviator. Current design guidance is to separate the helmet and the optical system. The helmet system is referred to as the Pilot Retained Unit (PRU) while the optics and displays are known as the Aircraft Retained Unit (ARU). These requirements are only a partial set of the system and performance specifications that must be met by what is termed the Comanche Helmet Integrated Display Sighting System (HIDSS). Technology and market forces now have entered the HIDSS picture and forced the developer to consider the integration of advanced helmet-mounted, flat-panel displays in lieu of miniaturized CRTs. Advancements in the field of small flat-panel displays make them viable candidates to replace the heavier, bulkier, higher voltage, monochromatic miniature CRTs that are near the end of product maturity with regard to electro-optical performance. Flat-panel displays are in their infancy with respect to brightness, gray scale range, switching speed, resolution, durability, power, cost, and safety. The decision point is fast approaching to select the HMD technology that will be integrated into the RAH-66 Comanche. Whatever display technology is chosen will serve as the major information interface between crewmembers and their aircraft and sensors. Decision-makers must decide between the currently available CRT displays or emerging active-matrix flat-panel displays. A wrong decision could be costly in terms of dollars and pilot performance.