Head-Mounted Displays Provide User Mobility, Privacy, and Convenience

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.

A good start point for seamlessly making the transition to a wearable eye display is information that has been learned about resolution and im-age size from the evolution of desktop and laptop monitors and flat-panel displays. Their resolution is typically up to 100 dots/in. (0.25-mm pixels). If resolution were any lower, the legibility of office documents and other fixed-format information would be compromised. That's why SVGA has become the de facto screen resolution for 15- to 17-in. diagonal CRT monitors as well as for 10.4- to 12.1-in. liquid-crystal-display (LCD) flat panels.

Therefore, to use an HMD for viewing pages or web images in the format in which they were prepared, the HMD must deliver an 800- by 600-pixel minimum resolution. Otherwise, the user will have to deal with the annoyance of scrolling up or down in order to locate the required information.

Also known as the field of view, the "viewing angle" is measured from the apex at the retina of the eye to the diagonal corners of the display. It must be adequate for the eye to resolve a pixel, given the eye's resolution of one to two minutes of arc (Fig. 2). Based upon subjective evaluation by users, InViso has determined that 32° (1.92 min./SVGA pixel) provides a useful near-eye viewing angle for a binocular design.

Though luminance and image quality usually depend heavily upon ambient light levels, this just isn't true in an HMD. The reason is that an HMD user actually views a virtual image. Therefore, ambient light has almost no effect on image contrast. A luminance of 20 ft-lamberts and a contrast ratio of 20:1 are sufficient under almost all viewing conditions, as is a refresh rate somewhere between 60 and 120 frames/s.

Aperture ratio is the percentage of the total picture area from which light actually emanates. InViso has determined that 80% to 85% is required in order to produce a smooth, "paper-like" image.

Furthermore, there are optometric considerations. In a binocular HMD, the location of each image relative to each eye is critical for comfortable viewing over long time intervals. To avoid eyestrain, a distance of more than 1 m is essential, especially among older viewers. Also, it's vital that the images for the right and left eyes converge at the same distance at which the images are focused. This means the images must precisely overlap so that the pair of binocular images merge into one. The occurrence of the brain perceiving the two images as one is known as "neural fusion."

In addition, the display must provide for a large range of motion relative to the eye. This will make sure that the image perceived by the viewer remains immobile and undistorted regardless of eye movement. Called "eye relief," the distance from the pupil to the display must be at least 25 mm so that it won't interfere with prescription glasses.

Moreover, the optical system must tolerate side-to-side motion of the eye, thereby accommodating rotation of the eyeball and viewer movement. If the aperture of the magnifying optics is too small for the desired field of view, the image will be clipped. This viewing window itself might define the aperture. Or, in many cases, a pupil is formed within the optical system itself. Clipping (or vignetting) can be easily observed by moving the display relative to the eye. The amount of lateral movement that can be tolerated, without diminishing the display's fullness of view, is the "eye box."

To quantify this, a human's pupil size is normally 4 mm in diameter and moves approximately 6 mm laterally as the eye scans over a 34° field of view. As a result, the box must have a 6-mm diagonal, laterally, as the eye scans over the 34°. So, an eye box of 12 mm is a good choice. This also is the dimension of the magnifier.

Having worked extensively on military HMDs, eMagin Corp., Hopewell Junction, N.Y., is now turning to the consumer market. The company sees a self-emitting organic light-emitting diode (OLED) structure as compatible with silicon CMOS active matrix substrates and, therefore, well suited for HMDs. The company has developed an industrial SXGA headset with 12-mm pixels and a 15.3- by 12.3-mm active area (Fig. 3). Its OLED-based microdisplay reproduces 256 gray levels at a 60-Hz video response rate. Power consumption is 360 mW at 200 candelas/m². A digital interface employing low-voltage differential signaling (LVDS) interconnects it to the host.

MicroOptical Corp., Westwood, Mass., is field testing a monitor clip-on information display. It's the company's model C-1, which provides quarter-VGA (320- by 240-pixel) color at a 60-Hz refresh rate (Fig. 4). It fits most adult frames and safety glasses. The input is a standard VGA, female DB-15 connector, and a standard National Television Standards Committee (NTSC) RCA plug. Power requirements are 100 mW for the display and backlight, 2.9 W for the VGA interface, 1.9 W for the NTSC interface, and 300 mW for an RS-170 interface. The RS-170 is the Electronic Industries Association standard for the combination of signals that are required to form NTSC monochrome video. Head-supported weight is less than 50 g.

Likewise, the company has field-tested its model EG-6C, which has VGA color or monochrome resolution, with a 60-Hz refresh rate. The field of view is approximately 10° on the diagonal corresponding to a 6.5-in. diagonal display, three feet away. The focus range is from 20 cm to infinity. The power required is 100 mW for the display and backlight, 2.9 W for the VGA interface, 1.8 W for the NTSC interface, and 300 mW for the RS-170 interface. The head-supported weight is 100 g. (See "Candidates For Head-Mounted Displays," p. 112, for a discussion of the three microdisplay technologies that are likely to end up in HMDs.)

Summing up, McLaughlin remarks, "The issue isn't the microdisplays. It's the optics, the power, and all the other stuff." The innovators haven't yet turned the corner from a price/performance point of view. He further says, "I'm optimistic about what they're trying to do, but rather skeptical as to how fast they're going to get it done. An audio headset weighs in at 14 g, so a 100-g HMD isn't going to do it. Do you want to wear a 100-g headset all day?"

Probably not. "I just hope that the developers are looking for new and exciting architectures and designs to try to meet the performance and price points for the consumer market. There's no doubt that over the next year substantial market opportunities are going to develop for HMDs," concludes McLaughlin.

Bergstrom, N.; Chuang, C.L.; Curley, M.; Hildebrand, A.; and Li, Z.W., "Ergonomic Wearable Personal Display," Digest of Technical Papers, Society for Information Display International Symposium, vol. XXXI, May 16-18, 2000.