1. Field of the Invention
The present invention generally relates to the field of helmet mountable displays (HMDs).
2. Description of the Related Art
U.S. Pat. No. 5,091,719 describes a helmet mountable display (HMD) in which left and right channels share a common optical path that includes first and second relay lenses having respective optical axes, and horizontal and vertical deflecting mirrors. The lenses are arranged so that their axes form a V, with the deflecting mirrors disposed at the apex. For the left (right) eye, an 8.times.1 array of fibers is modulated to project the intensity pattern of a 1k.times.1k image through the first (second) relay lens. A pair of motors oscillate the horizontal and vertical mirrors to deflect light through the second (first) lens, and raster scan the image onto a back projection screen which projects the image onto the helmet's visor.
One of the primary concerns in HMD design is the optical system's overall weight, which is dominated by the weight of the motors used to oscillate the mirrors. Although the described HMD uses a common optical path to reduce the number of lenses, it requires two motors to drive the respective mirrors. Another design consideration is that people who wear the helmet have different interpupillary distances (IPDs). The IPD of male pilots in the 5-95 percentile varies from about 55 mm to about 75 mm. In the `V` design, variations in IPD are compensated for by rotating the first and second lenses with respect to the apex point to achieve the correct spacing. However, this induces `parallelogram` distortion in the output image, i.e., a square input appears as a parallelogram, and requires electronic precompensation of the digital imagery.
U.S. Pat. No. 5,166,778 discloses a single lens HMD that employs a single 8.times.1 array of fibers to modulate both left and right images through a single lens. Horizontal and vertical mirrors are oscillated by respective motors to reflect the intensity patterns back through the lens and onto respective back projection screens. The single lens design reduces the number of lenses, but the single lens must be very large to simultaneously provide images for the left and right eyes, and the system still requires two motors to scan the image. Variations in IPD are compensated by adjusting optical relays between the respective back projection screens and the visor.
A single-eye head mounted projection display called "The Private Eye" is produced by Reflection Technology Corporation of Waltham, Mass. The system includes a column of LEDs which project successive columns of an image through a magnifying lens onto a mirror. The mirror is oscillated to horizontally sweep the projected patterns onto the pupil of the person wearing The Private Eye. The mirror must be relatively large and located near the eye to provide a reasonably large field of view. Additionally, for a two-eyed display, the system would require two drive motors. Since this system is one-eyed, IPD adjustment is not an issue.