How Virtual Reality Gear Works

A Head-mounted Display (HMD) is just what it sounds like -- a computer display you wear on your head. Most HMDs are mounted in a helmet or a set of goggles. Engineers designed head-mounted displays to ensure that no matter in what direction a user might look, a monitor would stay in front of his eyes. Most HMDs have a screen for each eye, which gives the user the sense that the images he's looking at have depth.

The monitors in an HMD are most often Liquid Cystal Displays (LCD), though you might come across older models that use Cathode Ray Tube (CRT) displays. LCD monitors are more compact, lightweight, efficient and inexpensive than CRT displays. The two major advantages CRT displays have over LCDs are screen resolution and brightness. Unfortunately, CRT displays are usually bulky and heavy. Almost every HMD using them is either uncomfortable to wear or requires a suspension mechanism to help offset the weight. Suspension mechanisms limit a user's movement, which in turn can impact his sense of immersion.

A few HMD models use other display technologies, though they are very rare. Other display technologies include:

There are many reasons engineers rarely use these display technologies in HMDs. Most of these technologies have limited resolution and brightness. Several are unable to produce anything other than a monochromatic image. Some, like the VRD and plasma display technologies, might work very well in an HMD but are prohibitively expensive.

Many head-mounted displays include speakers or headphones so that it can provide both video and audio output. Almost all sophisticated HMDs are tethered to the VR system's CPU by one or more cables -- wireless systems lack the response time necessary to avoid lag or latency issues. HMDs almost always include a tracking device so that the point of view displayed in the monitors changes as the user moves his head. (We'll examine tracking devices in a later section.)

Some systems use a special set of glasses or goggles in conjunction with other display hardware. In the next section, we'll look at such a system -- the CAVE display.

Students and researchers at the University of Illinois - Chicago developed what many VR specialists feel is the most immersive display system for VR environments. It's called the CAVE system, which stands for Cave Automatic Virtual Environment.

A CAVE is a small room or cubicle where at least three walls (and sometimes the floor and ceiling) act as giant monitors. The display gives the user a very wide field of view -- something that most head-mounted displays can't do. Users can also move around in a CAVE system without being tethered to a computer, though they still must wear a pair of funky goggles that are similar to 3-D glasses.

The active walls are actually rear-projection screens. A computer provides the images projected on each screen, creating a cohesive virtual environment. The projected images are in a stereoscopic format and are projected in a fast alternating pattern. The lenses in the user's goggles have shutters that open and shut in synchronization with the alternating images, providing the user with the illusion of depth.

Tracking devices attached to the glasses tell the computer how to adjust the projected images as you walk around the environment. Users normally carry a controller wand in order to interact with virtual objects or navigate through parts of the environment. More than one user can be in a CAVE at the same time, though only the user wearing the tracking device will be able to adjust the point of view -- all other users will be passive observers.

In the next section, we'll look at another kind of virtual reality display called the workbench.

One display system that some VR researchers feel is only tangentially related to virtual environments is the workbench display. In the early '90s, the Office of Naval Research's Computer Science Liaison Scientist, Larry Rosenbaum, led a team of Navy VR engineers in the creation of a large display monitor that allowed multiple users to view it at the same time. Users can view the display vertically or tilted horizontally like a table or bench.

Users wear special goggles while looking at the workbench, just as they would in a CAVE system. Each user sees the same image projected from the workbench display. Because of the stereoscopic projection and the goggles' lens shutters, the objects displayed on the workbench appear to be three-dimensional.

The reason some VR researchers feel the workbench isn't a true representation of creating a virtual environment is due to immersion. Because the user is looking at a display that doesn't fill his field of vision, he remains aware that he is in the real world, although that world now includes virtual objects that he can manipulate. There is no virtual environment to explore -- if the user looks away from the display, he'll see an ordinary, physical room. However, this doesn't change the fact that workbench displays can be very useful.

One use for the workbench display is medical training. A surgeon can practice a procedure on a three-dimensional virtual patient while surrounded by a real medical staff. If the surgeon were to perform the same procedure while wearing an HMD, the people around him would either be characters under computer control or computer avatars representing other humans. With the workbench display, interaction with other people is natural and completely real.

Workbench displays are also helpful to military tacticians. Programmers can create realistic, three-dimensional representations of battlefields, giving military personnel an accurate view of battle situations. A good model can also reveal potential bottlenecks or hidden enemy encampments.

Other applications using workbench displays include visualization of scientific research or product research and development.

In the next section, we'll look at some of the devices that allow users to interact with virtual environments.

While creating impressive graphics and display systems remains an important part of the VR experience, many researchers feel that developing intuitive devices for user interaction is more crucial. Basic interactive devices like keyboards or joysticks are easy to use, but they also tend to hamper the sense of immersion. Ideally a user would become unaware of the interaction device completely.

Though progress has been slow, there are still some exciting developments in human-machine interfaces (HMI). While many industries help drive developments in graphics technology, not many have a vested interest in exploring new kinds of HMI. Typically, the industries involved in advancing HMI include the entertainment field, academic institutions and small VR firms. Still, there are several interesting HMI devices used in some VR systems. Particularly interesting are the devices designed to be worn by users. These include gloves and bodysuits.

Gloves have played a role in the VR craze from the very beginning, even though the original designers didn't necessarily intend for them to be used in VR systems. Using a wired glove, you can interact with virtual objects by making various hand gestures. Many people call the gloves DataGloves or Power Gloves, though both those terms specifically refer to particular models of gloves and are not generic terms. Not all gloves work the same way, though all share the same purpose: allowing the user to manipulate computer data in an intuitive way.

Some gloves measure finger extension through a series of fiber-optic cables. Light passes through the cables from an emitter to a sensor. The amount of light that makes it to the sensor changes depending on how the user holds his fingers -- if he curls his fingers into a fist, less light will make it to the sensor, which in turn sends this data to the VR system's CPU. In general, these sort of gloves need to be calibrated for each user in order to work properly. The official DataGlove is a fiber-optic glove.

Other gloves use strips of flexible material coated in an electrically conductive ink to measure a user's finger position. As the user bends or straightens his fingers, the electrical resistance along the strips changes. The CPU interprets the changes in resistance and responds accordingly. These gloves are less accurate than fiber-optic gloves, but they also tend to be much less expensive.

Of course, if you want a really accurate and responsive glove, you should use a dexterous hand master (DHM). The DHM uses sensors attached to each finger joint. You attach the sensors to your joints with mechanical links, which means the glove is like an exoskeleton. These gloves are more accurate than either fiber-optic gloves or those using electrically conductive material, but they are also cumbersome and clunky.

In the next section, we'll look at VR input devices.

If you don't have a CAVE system or the cash to drop on a DataSuit, there are still a few options available to you to provide users a way of getting around a virtual environment without using a wand or joystick. Researchers believe devices that allow for more natural navigation within a VR environment also increase the user's sense of immersion. With this in mind, engineers and scientists developed a few different systems for user navigation.

One system is the treadmill. A treadmill is useful because the user remains stationary with respect to the real world, but feels as if he is actually walking through the virtual environment. Researchers have found it relatively simple to link a treadmill to a computer system so that a user's steps result in an appropriate adjustment in the system's graphics. An obvious limitation of normal treadmills is that you can only walk in two directions: backward or forward.

Some companies have developed omni-directional treadmills. These devices allow a user to step in any direction. Normal treadmills use a single motor, which exerts force either forward or backward relative to the user. Omni-directional treadmills use two motors -- from the user's perspective the treadmill can exert force forward, backward, left or right. With both motors working together, the treadmill can allow a user to walk in any direction he chooses on a walking surface wrapped around a complex system of belts and cables.

An alternative to a treadmill is a pressure mat. You may have seen pressure mats used with video games like "Dance Dance Revolution." There are many kinds of pressure sensors, though the most common are electromechanical pressure sensors. An electromechanical pressure sensor is a relay that activates when pressure is applied to the sensor. When the circuit closes, an electric current runs through it, signaling the CPU to make changes to the graphic output sent to the user.

The company VirtuSphere, Inc. offers a unique way for users to move around inside a virtual environment. It looks like a human-size hamster ball -- the user gets inside the sphere and walks around in it. The sphere rests on a stable platform that has several wheels resting against the sphere, allowing it to roll in any direction while staying in the same fixed position. Sensors in the wheels tell the CPU which way the user is walking, and the view within the user's HMD changes accordingly.

Within CAVE systems, some VR researchers are experimenting with a technique called passive haptics. "Haptics" refers to the sense of touch, so a haptic system is one that provides the user with physical feedback. A joystick with force-feedback technology is one example of a haptic interface device. Passive haptics are a little different in that they don't actively exert force against a user. Instead, passive haptics are objects that physically represent virtual elements in a VR environment. For instance, a real folding table might double as a virtual kitchen counter. Having something real to touch in a virtual environment enhances the user's sense of immersion and helps him navigate through the simulation.

In the next section, we'll look at the kinds of tracking systems you find in HMDs, DataGloves and other VR gear.

Tracking devices are intrinsic components in any VR system. These devices communicate with the system's processing unit, telling it the orientation of a user's point of view. In systems that allow a user to move around within a physical space, trackers detect where the user is, the direction he is moving and his speed.

There are several different kinds of tracking systems used in VR systems, but all of them have a few things in common. They can detect six degrees of freedom (6-DOF) -- these are the object's position within the x, y and z coordinates of a space and the object's orientation. Orientation includes an object's yaw, pitch and roll.

From a user's perspective, this means that when you wear an HMD, the view shifts as you look up, down, left and right. It also changes if you tilt your head at an angle or move your head forward or backward without changing the angle of your gaze. The trackers on the HMD tell the CPU where you are looking, and the CPU sends the right images to your HMD's screens.

Every tracking system has a device that generates a signal, a sensor that detects the signal and a control unit that processes the signal and sends information to the CPU. Some systems require you to attach the sensor component to the user (or the user's equipment). In that kind of system, you place the signal emitters at fixed points in the environment. Some systems are the other way around, with the user wearing the emitters while surrounded by sensors attached to the environment.

The signals sent from emitters to sensors can take many forms, including electromagnetic signals, acoustic signals, optical signals and mechanical signals. Each technology has its own set of advantages and disadvantages.

To learn more about gear used in virtual reality systems, check out the links on the next page.