Advanced Helmet Mount Display (AHMD)
When the U.S. Department of Defense needed a new battlefield landscape simulator to train pilots, they specified a helmet-mounted simulation instead of traditional rear-projection screens, so as to eliminate the need for large display surfaces and the associated large enclosure space.
The imaging surfaces had to be transparent so the pilot would still be able see the physical instrument panel. Other helmet-based simulator systems had involved a specific integrated helmet, which meant trainees could not use their own personal fitted helmet, and would often feel cut off from the cabin environment. The imaging system had to have a wide field of view and large eyebox, and be lightweight and have a balanced center of gravity so that pilots could be trained over an extended period of time without significantly adding to their fatigue.
Zygo embraced the challenge, designing and manufacturing an advanced high-resolution off-axis optical system that provides an immersive virtual experience for training. It serves as stellar example of Zygo’s expertise in electro-optical design and assembly.
The AHMD overlays computer-generated battlefield scenery on the real-world environment in flight simulator applications. It provides a 100 degree (Horizontal) by 50 degree (Vertical) field of view, with a 30 degree binocular overlap region. With an eye relief exceeding 50 mm, the AHMD significantly reduces the claustrophobic feeling caused by previous-generation helmet mount displays. The AHMD also tracks the pilot’s head movement such that the overlaid information is automatically aligned with the pilot’s instantaneous view of the world.
The compact, lightweight, unit fits on top of the pilot’s personal flight helmet, and is connected to the Image Generator via a high-speed digital fiber optical cable, which provides excellent performance while being flexible enough to allow virtually unrestricted head movement. The AHMD is designed so the user can quickly self-adjust the unit to optimize the image, which eliminates time-consuming interpupillary distance measurements and specialized fitting procedures.
The brain uses visual binocular disparity to create a 3D perception of the world using information from 2D retinal images. The electro-optical assembly needed to convert the optical fiber signal into discrete projected images with the correct amount of binocular disparity so as to mimic what the pilot would see in the real world. However, the brain is not easily deceived, so innovative engineering was required to make the scene appear as natural as possible. A wide field of view, low-latency of the projected image, and custom mapping to minimize image distortion were all key elements of the design. The net effect is a natural and believable sense of depth and presence for the pilot viewing the computer-generated images.
The AHMD, designed and manufactured by Zygo’s Electro-Optics group, met all of the Military’s requirements, and received very positive reviews from personnel who trained with the unit. Zygo received multiple orders over a period of several years.