Choosing an HMD

So, you decide to purchase a new head-mounted display (HMD) for your office's immersive videoconferencing system. You go down to Circuit City, and there in the VR Department is a bewildering array of HMD selections. You try one on. "Too heavy", you say. You try another. "Not enough resolution". And another. "The colors don't look right". Finally, you find the perfect HMD, which another customer has just rejected as "not rugged enough". She buys the heavy one to throw in the back of her pick-up truck for emergency response visualization in the Rocky Mountains, while you go for lighter weight and maximum visual performance.

Sound familiar? It should, because variety, application specificity, feature/benefit trade-offs, hands-on demonstration, and retail distribution are key aspects to most mass-market electronics purchasing decisions. But for HMDs, this is a story from early in the next century.

Today, HMD selection is much more challenging for a number of reasons. First, the only opportunities to "wear and compare" HMDs occur at tradeshows, where you might wait in line an hour for each HMD, and where each will display different imagery generated by a different graphics engine. Some of these HMDs might be field-proven production products, while others have been "shipping soon" for many months. And every supplier shouts "ours is best" in a market where the pros and cons of each model for your particular application are not yet clear. In the face of so much hype, what can you do to be a smart HMD buyer?

The answer is to go beyond the "field-of-view, pixel count, and price" questions, and instead use an analytical approach, in which you understand the trade-offs involved in HMD design, and base your analysis on the specific application(s) for which the HMD will be used. By starting with analysis, you can narrow your options down to two or three alternative models. At that point, there is no alternative but negotiating with potential suppliers to ensure that you try before you buy.

To support your analysis, General Reality Company has developed a chart called the "Wear and Compare HMD Selection Tool". This tool is organized into five sections designed to ensure that your shopping trip into Cyberspace is as organized and productive as possible. For each decision or selection criterion, the first blank column allows you to flag your most important needs, while the other three columns let you rank or rate 3 HMDs using any scale you prefer.

Selecting The Right Category of HMD For Your Application

The first step in the analysis is to decide what kind of HMD you need. This is analogous to deciding between a pick-up truck and a sports car before visiting auto row. Here are the key trade-offs:

1) Extended Wear vs. Many Users:The first decision to make is whether your application demands an HMD to be used for long periods of time by a small number of users, or for short periods of time by many users. The extreme examples would be a doctor's personal display for endoscopic surgery control compared to an arcade game.

If extended wear is anticipated, minimizing eye-strain and maximizing comfort are critical, which would suggest an HMD designed to maximize visual quality and minimize weight. According to the latest HMD safety research, a complete set of adjustments is also required, including focus and interpupillary distance (which aligns the HMD optics to the different distances between people's eyes). An example is General Reality's original CyberEye(tm) Model CE-100M, designed specifically for extended wear applications.

In contrast, no amount of bulletproofing is ever enough for an arcade, while every moment spent adjusting an HMD is dead time for the arcade's cash flow. But, since time of use will be short, much higher levels of eye strain, discomfort, and weight can be tolerated. Liquid Image (Winnipeg, Canada) has built a solid business by offering arcade operators the MRG-2, which is itself solid enough to stand on.

2) Stereoscopic vs. Monoscopic:The human brain perceives depth using a number of subconscious rules such as "known items are certain sizes at certain distances", or "close objects block views of more distant objects". Many of these cues can be provided in software, without a stereoscopic HMD. But, if your application needs every 3D cue possible, then a stereoscopic system should be used. In such a system, your two eyes are presented with slightly different images, which drives your eyes to "converge", in which they roll further towards each other as objects get closer. While stereoscopy adds a powerful depth cue, it will typically entail greater software complexity, two image rendering boards instead of one, and a slower graphics frame rate.

3) Field-Of-View vs. Optical Performance:In applications where maximum immersion is desired, wide fields-of-view can be used to increase the user's sense of "being there". For example, in games, objects can approach from your peripheral vision, causing you to turn your head to shoot them. Alternative ways to achieve greater immersion include using 3D audio to attract attention, and minimizing tracking lag to improve the user's ability to "look around" in an intuitive way. One penalty for maximizing immersion through a wider field-of-view (FOV) is larger, heavier optics, since the lens closest to your eye must have a diameter at least as wide as the FOV angle seen from your eye. As FOV grows, it also becomes more and more difficult to maintain sharp, undistorted images across the entire display, and aligning the display to your eyes generally becomes more difficult.

4) Acuity vs. Field-Of-View:Acuity refers to how well the HMD displays fine detail in images. In the absence of degradation caused by loosely toleranced lens systems, acuity is set by the number of picture elements (pixels) available in the display, and by the HMD's FOV.

With a limited number of display pixels available in today's displays, you as the application developer or user have a critical trade-off to assess. You can use an HMD that spreads the available number of pixels over a wide FOV and thus lose fine detail in the image, or you can maintain image detail but give up wide FOV. For shoot-em-up games, wide FOV is usually more important than image detail, while for industrial applications detail is typically more important.

A useful way to quantify this trade-off is to express an HMD's visual acuity in the same manner used by standard eye charts. In this system, a subject's vision is deemed 20/D, where D is an "effective distance" at which a perfect eye can see objects that the subject barely resolves at 20 feet. For example, the limit for legal blindness is 20/200, which means that a perfect eye can see an object at 200 feet that the subject can only see at 20 feet.

To calculate visual acuity for an HMD, find the number of pixels spread across the display in one dimension (ie; 640 for a 640x480 display). Using the angular FOV in that same direction, solve the equation:

To illustrate, take a typical low-cost game HMD with a 70^ohorizontal FOV and 420 pixels in the horizontal direction. This HMD's effective distance D is 70*1200/420 = 200, which results in visual acuity of 20/200, right at the legal limit of blindness. Clearly, this HMD would not be useful in a driver training simulator.

As a general rule, high-priced HMDs at $5,000 to $150,000 currently use cathode ray tube display sources capable of 640x480 to 1280x1024 resolutions, while lower-priced HMDs at $499 to $5,000 use active matrix liquid crystal displays (AMLCDs) ranging from 320x200 to 640x480. Over the next few years, the pixel count for commercial AMLCDs can be expected to double every two years, which should result in significant ongoing HMD price/performance advances.

At present, there are two methods commercially available for expanding the performance envelope created by the acuity vs FOV trade-off. The more common method works only for stereoscopic systems, and reduces overlap between the images presented to each eye. For example, an HMD specified at 70% overlap means that the outer 30% of each eye's FOV is not projected to the other eye. You can demonstrate this effect to yourself by closing one eye and then the other while looking straight ahead. Unfortunately, your optical cortex has had years of experience at ignoring the overlap effects caused by your nose, while the unfamiliar optical geometry presented by an HMD's partial overlap can cause disturbing visual artifacts.

A second method for attacking the performance envelope has been pioneered by Kaiser Electro-Optics, and is commercially available in its VIM-1000. This technique uses multiple AMLCDs and lenses for each eye, and is known as tiling. While the potential performance improvement is significant, visual artifacts between the tiles are an issue. In addition, the bulk of an HMD's cost-of-goods is currently driven by the AMLCD, which makes tiling a relatively expensive approach.

5) External Light Block vs. Safety Risks:Today's HMDs display video to only a portion of the user's FOV, so you need to decide what to display to the rest. To maximize suspension of disbelief, you may want immersive eyecups, which block all outside light and force you to see only the HMD's display. However, many HMD designers are now moving away from fully-immersive designs for two reasons. First, if the user is allowed to move about, the risk of falling rises astronomically when you can't see your feet. More curiously, all VR systems suffer from a time lag between turning your head and seeing updated graphics for the new line-of-sight in the HMD. The resulting mismatch between the user's visual perception and sense of physical motion can cause simulator sickness symptoms such as nausea, disorientation, and fatigue.

6) Heads-Up Viewing vs. Field-Of-View:In many HMD applications, suspension of disbelief is not important, but simultaneous visual access to the real world and the HMD display is important. For example, several companies are now introducing wearable computers, which display maintenance instructions from a belt-mounted hard-drive. In these cases a monochromatic, single-eye display might be adequate, while in medical imaging uses, full-color stereoscopic imagery might be called for. Either way, your eyes only have so much FOV to go around, so if you use most of it to view the real world, the FOV of the HMD will be limited.

7) See-Through Display vs. Light Loss:One technology area with great promise is augmented reality, in which computer graphics are visually-overlaid on a direct view of the physical world. Applications range from real-time maintenance instructions to the surgical equivalent of "paint by numbers". Achieving augmented reality requires a see-through HMD, such as the "i-glasses" from Virtual I/O (Seattle, WA). Because see-through HMDs use beamsplitters to combine the virtual and real views, half of the total light is thrown away. Since today's display sources are brightness-limited, the image will appear dimmer to the extent the discarded light is from the virtual image.

Selecting The Best HMD Model In A Category

Once you've decided on your key requirements, you can ignore all of the inapplicable models and get down to a detailed comparison of individual features and parameters. These are often what separate professional HMDs (over $1,000) from consumer toys (under $1,000). Unfortunately, many of these parameters are not specified by most HMD manufacturers, and you would need an extensive laboratory to quantify most of them yourself. Where a subjective evaluation is required, erring on the side of choosing the most pleasing image you can afford is probably a safe bet.

Optical Requirements

Eye Relief:Eye relief is the distance from your eye to the first lens your eye sees. Eye relief of at least 25mm is required to allow users to wear prescription glasses, which is a critical requirement in public attractions and for many individual users. Even in cases where eyeglass compatibility can be given up, short eye relief distances can cause severe discomfort, and even eye injury if contact occurs between the lens and eye. For a given FOV, the larger the eye relief, the larger (and heavier) the lens needs to be. Thus eye relief often limits FOV. To test eye relief, just wear the thickest glasses you can find.

Exit Pupil Diameter:An HMD's exit pupils can be thought of as portholes through which you view the scene in the HMD. Like portholes, if you move too far to one side, you lose sight of the center of the scene. With small exit pupils, the left and right optical systems must be aligned very precisely to the wearer's eyes at all times, both vertically and by adjusting for the interpupillary distance between the user's eyes. Any adjustment inaccuracy or shifting during active movement will cut off the edges of the images or cause them to blur severely. To minimize this, the optics can be designed with wide exit pupils, which in extreme cases like General Reality's "public" HMDs, can even eliminate the need for any interpupillary adjustments. However, wide exit pupils tend to conflict with wide FOV, so you have another trade-off to assess. To evaluate exit pupils, swing your head around a few times and see if the images still look centered.

Visual Comfort:The most perfect optical design in the world will look terrible if accurate optomechanical alignment tolerances are not maintained during assembly. In particular, accurate alignment between the left and right optical systems is crucial if eyestrain is to be avoided. For example, if the difference in vertical alignment, rotation, or magnification between the left and right images is only fractions of a percentage point, you will have difficulty fusing the left and right eye images into a single image, and you will likely feel uncomfortable after even brief use. To assess this most critical of HMD parameters, wear any HMD you are considering for 15 minutes, then describe how you feel in detail.

Brightness:An HMD's apparent brightness is a complex function of the display source's brightness, the HMD's FOV, and numerous individual factors such as how many lenses are used, the quality of the anti-reflection coating on each lens, and losses from beam-splitters for see-through capability. To see whether brightness is adequate, wear the HMD in bright light in its most non-immersive mode, and decide subjectively.

Contrast:The difference in brightness between the darkest and brightest pixels can be expressed as a ratio, with 100:1 a good minimum to require. Any less, and the display will appear to be "washed out"

Color:Are colors saturated or subdued? Assuming you are connected to an image generator that outputs a large color palette, you should also check to ensure that an HMD's colors are correct. Does skin look like skin and sky like sky?

Distortion:One key imperfection in every optical system is distortion, in which portions of the image are shifted with respect to their ideal positions as a function of distance from the image center. In general, larger fields of view and lower-cost optical systems result in greater distortion, with a fish-eye lens representing the extreme example. In low-performance HMDs, distortion makes a rectangular image boundary resemble the profile of a barrel or a pincushion. To evaluate distortion, view large straight lines near the edges of the visual field.

Aberrations:Every optical system generates aberrations, which are a series of unavoidable image imperfections created when light propagates through a series of air/glass interfaces. Severe aberrations can result in problems such as inability to achieve crisp focus, difficulty in focusing the image center and edges simultaneously, colored shadows at object edges, and points being imaged as comet shapes. In general, higher-priced HMDs successfully attack these problems by incorporating extra lens surfaces to correct the more severe aberrations. To evaluate aberrations, look for the problems just described.

Defective Pixels:Most AMLCDs include one or more defective pixels, which either remain stuck on, stuck off, randomly fluctuating, or tied to a neighboring pixel. Specifying "zero defective pixels" eliminates the resulting artifacts, at a correspondingly higher cost. To evaluate this issue, look for errors in "all white", "all black", and small checkerboard scenes.

Pixelization:Most HMDs under $5,000 use AMLCDs as display sources for light weight and low cost. Unfortunately, a significant percentage of any LCD panel is comprised of black spaces between the individual pixels. Especially at larger FOVs, this makes the HMD image appear as if you were looking through a screen door. Some HMD manufacturers leave this pixelation effect alone, while others blur the entire image to eliminate it, with severe and obvious impacts on visual quality. Some of the more advanced HMDs now use holographic filtering techniques to depixelize HMD images without introducing visual degradation, which can have a major positive impact on perceived image acuity.

Mechanical Requirements

Weight:Beyond visual comfort, a wide variety of issues affect physical comfort. One is weight, which is easily measurable and thus often used as a proxy for comfort by unsophisticated HMD shoppers. Commercial HMDs range in weight from 8 ounces to 80 ounces, with comfort and muscle fatigue determining the acceptable limit. For safety reasons, HMDs used on young children should be as light as possible.

Nose Weight:You'll get a sinus headache much faster from a one-pound pair of eyeglasses than from a three pound hat. So, until three-ounce VR glasses really exist, it might be best to avoid any nasal contact whatsoever.

Balance:Ideally, an HMD's center of gravity should be the same as your head's, ie; somewhere around the middle of your brain. Unfortunately, most of the HMD's functionality is usually in front of your eyes. Some designs attack this problem with a counterweight at the back of your head, which raises total weight and increases inertial resistance to turning your head. Other designs attack the problem using lighter weight optical components, such as injection molded plastic lenses.

Fit:Early HMDs fit so poorly that users typically held the HMD in place with one hand throughout VR experiences, thus giving rise to the legendary dance "VR Grasp". Today a wide range of fit-enhancing solutions are available, ranging from ratchet knobs to air bladders. To test, adjust HMD and start jumping around. Does the HMD stay on and do the images stay clear?

Air Flow:Many still call head-mounted displays "helmets" because early designs were exactly that. Now that HMDs are not just for soldiers anymore, most users prefer an open-frame design with minimum head contact, an open top, and no nose immersion. These features maximize hygiene and minimize sweating or lens fogging.

Ruggedness:Even if not for arcade use, your new HMD should be rugged enough to withstand a few bumps and bruises. Look out for small parts, weak hinges, protruding screws, or wires without strain relief. Are the lenses protected by a window from dust and fingerprints? Also, if the HMD needs to travel, can you get a case?

Adjustments:Are brightness/contrast or audio volume adjustable? Does the unit have the focus and interpupillary adjustments if you need them? Are they easy to adjust properly? Do they stay adjusted?

Support Electronics Size/Weight:Many early HMDs required an electronics support cabinet the size of a shoe box, making portable use impossible. Newer designs have compressed this to the size of a pack of cigarettes, or mounted all electronics on the HMD. If it matters, ask.

Maintenance & Storage:Can the HMD be swabbed with alcohol to clean it and kill germs? Are wear items like headbands easily replaceable? Can you hang the HMD on a hook for storage or does it need a shelf?

Electronic Requirements

Audio:If your application calls for audio, make sure volume is adequate but not deafening, and listen for solid bass response. Is the HMD compatible with 3D spatial audio generators? If you do not need audio, consider an HMD that lets you remove the audio speakers to reduce weight.

Standards Compatibility:Connecting an HMD can be easy or nightmarish depending upon its signal interface. The most common and easiest interfaces (in the US) are NTSC (the HMD plugs in like a TV to a VCR), and VGA (it plugs into a computer VGA port). VGA can be converted to NTSC easily at low cost, so for most VR platforms, either will do. For extreme-performance systems running on high-end workstations, separate RGB connections are more common, but using such an HMD on non-RGB systems can be problematic.

Power Consumption:If you need to run off a battery pack or computer bus, select an HMD using less than a watt or two. Otherwise, ignore the issue.

Connectors:For most applications, RCA jacks are acceptable and quite common. For higher-performance applications, a BNC video connector is more desirable.

Approvals:Is the HMD approved by Underwriters Laboratories or the FCC if needed?

Vendor Quality

By now, you are an expert HMD shopper, so all you need to worry about are the things you always worry about when buying a new technology from a new company. A full discussion would require a Purchasing Monthly article, so please refer to the few suggestions included on the Selection Tool. What it all comes down to is "will the supplier make sure you end up a happy customer"? Some may not, but many of them will, and you can help by making sure the HMD you select is the right one for your needs.