Head mounted display systems (HMDs) are devices, worn on a user's head, which display information to a user. Some HMDs project their display onto large, fixed surfaces in front of a user's eyes. The display surface can be a partially-reflective, partially-transmissive surface, known in the art as a beam-splitter, attached to the user's face and enabling the user to simultaneously view the display and the environment. Alternately, the display surface can be a retro-reflective screen as in U.S. Pat. No. 5,572,229 to Fisher, entitled “Head Mounted Projection Display System Featuring Beam-splitter and Method of Making Same.” Other HMDs use optics (like mirrors and beam-splitters) to project a display directly onto a user's eye.
HMDs are currently used for a wide range of applications, both military and civilian. Historically, head mounted displays have been used by pilots and soldiers to supplement the information they draw from their environment with real-time computer-processed information to better enhance their view and inform their decisions. Other users of head mounted display systems include: video gamer players seeking a virtual-reality playing experience; warehouse managers seeking an enhanced stock location and coordination system (see U.S. Pat. No. 6, 744,436 to Chirieleison, Jr. et al, which is incorporated herein by reference); television and movie viewers seeking privacy and/or an enhanced viewing experience; vehicle operators seeking greater feedback about vehicle and environmental conditions; and, computer users seeking a compact working environment. HMDs are used in many more environments than so-called “heads-up displays” or HUDs, as HMDs may be worn by a user in many environments, including those in which the use of a HUD might be difficult or impossible.
For all their uses, however, HMDs have historically had a number of problems for which solutions have seemed either impossibly costly, or impossibly difficult to execute. For example:                To produce even a small display area, HMDs have typically required equipment that is bulky, heavy, unwieldy, uncomfortable, and expensive.        Typical HMDs fail to cover all of the directions in which the eye can look, leading to a discontinuous image, an unenhanceable “blind spot,” or a lack of image altogether at the periphery. For such a head mounted display, see U.S. Pat. No. 6,452,572 to Fan et al.        Typical HMDs have elements that obscure a fixed portion of a user's field-of-view. Optics such as lenses, and mechanical elements such as servos, motors, and support structures occupy valuable property in the user's visual field. For an example of an early attempt at a high-resolution vision system which follows the line-of-sight with such elements, see U.S. Pat. No. 4,028,725 to Lewis.        Producing a large field-of-view with an HMD typically requires a reduction in display resolution, leading to less information, and less precision. Alternately, the use of a high display resolution requires a narrow and constricting field of view.        While the distribution of rods, cones, and other structures within the eye is not even or symmetrical, HMDs typically produce the same pixel density and image type for every part of the eye, leading to a discontinuous, uncomfortable, and suboptimal viewing experience, which wastes information density where the eye is ill equipped to handle it.        HMDs often produce images from a single source in space, leading to variations in the image perceived when the eye is at different angles from the source. Systems to compensate for this angular distortion can involve digital alteration of the image, which can squander processing power and require specialized hardware.        Delivering an image from a location on the HMD to the eye often requires a large number of optical components like mirrors, beam-splitters, and lenses. Each of these components not only increases the weight of the device, but can distort or dim the image; collectively, they require a source image of the highest precision and brightness to provide an image of acceptable quality to the eye. For an example of a HMD which uses multiple mirrors in this manner, see U.S. Patent Application Publication No. 2002/0167462 to Lewis et al.        HMDs which project directly onto the retina pose unique safety risks and alignment problems. For an example of a HMD of this type, see U.S. Pat. No. 5,467,104 to Furness, III et al.        The demand for a display that covers a large field of view but with sufficient resolution has led some to superimpose two or more images in a single HMD, adding a high resolution inset image to an image with a low resolution but a wide field of view. Such system designers often encounter an exponential increase in system complexity, as they must use twice the equipment and processing power to produce two images per eye, and then keep these images spatially and temporally synchronized at all times. Further, when the user turns his gaze outside of the inset area, the fovea of his eye is presented with low-density information, while his periphery receives high-density information, which is the opposite of ideal.        
The field of Head Mounted Displays would benefit greatly from a display which could continuously present very detailed imagery in the center of a user's eye, wherever the user looks, without requiring heavy equipment or demanding image processing for basic image projection. A novel invention providing these features will now be described.