Mounting a display on the human head may be one of the most revolutionary developments in this century. Separating the display from its host, be it a PC, TV, or a DVD, offers a number of benefits. These include the potential for far lower display cost, much lower weight—from pounds to grams—as well as privacy, convenience, and user mobility. A recipe for a head-mounted display (HMD) begins with a pair of fashionable sunglass frames. Then add a pair of XGA (1024- by 768-pixel) microdisplays, plus ancillary electronics and a cable, or perhaps later on a transceiver to communicate with its host, and there you have it.
It may not be very long before such HMDs are playing an indispensible role in our daily lives. In fact, ten years from now it might be hard to remember that we once got along without them (see "An HMD Odyssey," p. 110). Expected applications include use with laptop computers and games. In the area of medicine, surgeons will be able to monitor vital signs continuously. Industrial applications will be improved because technicians will be able to observe oscilloscope waveforms without bothering to turn and look at a scope display. Or, they could control a process while watching instrument data superimposed upon their HMD's primary viewing area.
If the HMD is to achieve the popularity and ac-ceptance that the Sony Walkman now enjoys, being only convenient and comfortable to use isn't enough. It also must be fashionable, styled in accordance with the trendiest sunglasses.
There are HMDs in the consumer market now, but prices far exceed consumer targets. Sony, Canon, and Olympus have each introduced HMDs for use with portable DVD players. Prices begin at $1000 and spiral upwards from there. And that's for very low resolution—less than 200,000 pixels. At those prices, there isn't any rush by consumers to snatch them up.
Chuck McLaughlin, president of the McLaughlin Consulting Group, Menlo Park, Calif., has been involved in display technology for some 20 years. He's adamant that HMD innovators must drive the price down from approximately $1000 to the $25 to $40 range for the market to take off. Looking at what lies be-tween now and the hoped-for ac-ceptance of the HMD in the fu-ture, he adds, "Breakthrough ar-chitectural de-signs and components will need continuous im-provement if we are to get there."
A formidable list of obstacles stand in the way. For starters, consider resolution. McLaughlin believes that although many companies currently offer SVGA (800- by 600-pixel) images, XGA will become essential for competitiveness.
Undaunted, a number of other companies have committed to the HMD and have invested sizeable amounts of research. They bet that HMDs are most likely to first gain acceptance when used as a monitor for desktops and laptops. In these situations, privacy, low power consumption, and user mobility are important.
InViso, Sunnyvale, Calif., has completed a significant amount of research in an attempt to quantify the crucial parameters that are appropriate for a successful HMD. Shown in the illustration is the result of their efforts to date (Fig. 1). Note the small height of this pair of binocular HMDs called eShades. It epitomizes a form factor that will fulfill ergonomic requirements, which are limiting our degree of immersion in a display environment. For most of us, prolonged immersion in an artificial environment is disturbing because we lose a sense of connection with our surroundings. But if the height of the display glasses is quite small, then the user will be able to quickly reconnect with the environment beyond the display glasses by simply glancing up or down.
InViso's eShades offers the visual equivalent of a 19-in. desktop monitor at 30 in. The product consumes less than one-fourth the energy of the desktop display, thereby extending battery life. Resolution is SVGA. In addition, the design of the display glasses was achieved with a microdisplay that's capable of providing an enhanced image quality when compared to TFT-based displays.
In particular, use of liquid crystal on silicon (LCoS) in combination with field sequential color, high-pixel fill factors, RGB LED illumination, and a frame rate of greater than 60 frames/s creates a vivid and sharp image. Even with the large magnification involved, there's no perceived screen door effect caused by pixel border. Also, the large field of view provides an extraordinary viewing experience.
If a binocular HMD is to be mounted on the bridge of the nose, with stems over the ears, it has to be lightweight and centered beyond the bridge of the nose. Through user testing, InViso has figured out that the weight should be no more than 100 g.
Power consumption limits depend on battery size and weight required for a binocular viewer. If 40 g is budgeted for battery weight and use is made of Li-ion technology, this would supply about 4 W-hrs of energy. A four-hour continuous operation time then requires that the total power consumption not exceed 1 W. These requirements are summarized in the table.
Currently, microdisplay power requirements far exceed power budgets of cell phones and PDAs. Mounting a battery in the HMD while holding the weight below 100 g, therefore, simply doesn't seem feasible. If the weight is going to approach a target value of 40 g, then it seems that tethered operation is inescapable, at least for now.