**4.2.1 Head mounted displays**

Head mounted displays place a screen in front of each of the viewer's eyes at all times. The view, the segment of the virtual environment generated and displayed, is controlled by orientation sensors mounted on the "helmet". Head movement is recognized by the computer, and a new perspective of the scene is generated. In most cases, a set of optical lens and mirrors are used to enlarge the view to fill the field of view and to direct the scene to the eyes (Lane, 1993). Four types of Head Mounted Displays (HMDs) are: (a) Liquid Crystal Display (LCD) head mounted display; (b) Projected head mounted display; (c) Small CRT head mounted display; and (d) Single Column LED head mounted display.

LCD head mounted display uses LCD technology to display a scene. When a liquid crystal pixel is activated, it blocks the passage of light through it. Thousands of these pixels are arranged in a two dimensional matrix for each display. Since liquid crystals block the passage of light, to display a scene a light must be shone from behind the LCD matrix toward the eye to provide brightness for the scene (Aukstakalnis & Blatner, 1992). LCD head mounted display is lighter than most HMDs. As with most HMDs, it does provide an immersive effect, but the resolution and the contrast are low. The problem associated with low resolution is inability to identify objects and inability to locate the exact position of objects. Since the crystals are polarized to control the color of a pixel, the actual polarizing of the crystal creates a small delay while forming the image on the screen. Such a delay may cause the viewer to misjudge the position of objects (Bolas, 1994).

Projected head mounted display uses fiber optic cables to transmit a scene to the screen. The screen is similar to a cathode ray tube (CRT) except the phosphor is illuminated by the light transmitted through fiber optic cables. Ideally, each fiber would control one pixel. But due to the limitation in cost and manufacturing, each fiber controls a honeycomb section of pixels (Lane, 1993). Projected head mounted display provides better resolution and contrast than LCD displays. This HMD is also light weight. Higher resolution and contrast means that the viewer is able to see an image with greater detail. The downside of this type of HMD is that it is expensive and difficult to manufacture (Bolas, 1994).

mainframe computer terminal or a personal computer (PC) connected to a network. Workstations had offered higher performance than personal computers, especially with respect to CPU and graphics, memory capacity and multitasking capability. They are optimized for the visualization and manipulation of different types of complex data such as 3D mechanical design, engineering simulation animation and rendering of images, and mathematical plots. Workstations are the first segment of the computer market to present advanced accessories and collaboration tools. Presently, the workstation market is highly commoditized and is dominated by large PC vendors, such as Dell and HP, selling Microsoft Windows/Linux running on Intel Xeon/AMD Opteron. Alternative UNIX based platforms are provided by Apple Inc., Sun Microsystems, and Silicon Graphics International

Sensory displays are used to display the simulated virtual worlds to the user. The most common sensory displays are the computer visual display unit, the head-mounted display

Head mounted displays place a screen in front of each of the viewer's eyes at all times. The view, the segment of the virtual environment generated and displayed, is controlled by orientation sensors mounted on the "helmet". Head movement is recognized by the computer, and a new perspective of the scene is generated. In most cases, a set of optical lens and mirrors are used to enlarge the view to fill the field of view and to direct the scene to the eyes (Lane, 1993). Four types of Head Mounted Displays (HMDs) are: (a) Liquid Crystal Display (LCD) head mounted display; (b) Projected head mounted display; (c) Small

LCD head mounted display uses LCD technology to display a scene. When a liquid crystal pixel is activated, it blocks the passage of light through it. Thousands of these pixels are arranged in a two dimensional matrix for each display. Since liquid crystals block the passage of light, to display a scene a light must be shone from behind the LCD matrix toward the eye to provide brightness for the scene (Aukstakalnis & Blatner, 1992). LCD head mounted display is lighter than most HMDs. As with most HMDs, it does provide an immersive effect, but the resolution and the contrast are low. The problem associated with low resolution is inability to identify objects and inability to locate the exact position of objects. Since the crystals are polarized to control the color of a pixel, the actual polarizing of the crystal creates a small delay while forming the image on the screen. Such a delay may

Projected head mounted display uses fiber optic cables to transmit a scene to the screen. The screen is similar to a cathode ray tube (CRT) except the phosphor is illuminated by the light transmitted through fiber optic cables. Ideally, each fiber would control one pixel. But due to the limitation in cost and manufacturing, each fiber controls a honeycomb section of pixels (Lane, 1993). Projected head mounted display provides better resolution and contrast than LCD displays. This HMD is also light weight. Higher resolution and contrast means that the viewer is able to see an image with greater detail. The downside of this type of

CRT head mounted display; and (d) Single Column LED head mounted display.

cause the viewer to misjudge the position of objects (Bolas, 1994).

HMD is that it is expensive and difficult to manufacture (Bolas, 1994).

(SGI) (http://en.wikipedia.org/wiki/Workstation).

(HMD) for 3D visual and headphones for 3D audio.

**4.2 Sensory displays** 

**4.2.1 Head mounted displays** 

Fig. 1. Visette 45 SXGA head mounted display (HMD)

Small CRT head mounted display uses two CRTs that are positioned on the side of the HMD. Mirrors are used to direct the scene to the viewers' eye. Unlike the projected HMD where the phosphor is illuminated by fiber optic cables, here the phosphor is illuminated by an electron gun as usual (Lane, 1993). CRT head mounted display is in many ways similar to the projected HMD. This type of HMD is heavier than most other types because of added electronic components (which also generate large amounts of heat). The user wearing this type of HMD may feel discomfort due to the heat and the weight of the HMD (Bolas, 1994).

Single Column LED head mounted display uses one column of 280 LEDs. A mirror rapidly oscillates opposite from the LEDs, reflecting the image to the user's eye. The LEDs are updated 720 times per oscillation of the mirror. As the LED column updates for each column of the virtual screen, the mirror redirects the light to the viewers' eye, one column at a time, to form the image of the entire virtual screen (Aukstakalnis & Blatner, 1992). Single Column LED HMDs allow the user to interact with a virtual world and the real world simultaneously. This type of display can be used to create a virtual screen that seems to float in the real world.

One of the common problems of HMDs is that the cable connecting the HMD and a computer restricts the mobility of the user. The user can only move as far as the cable allows. If the cable is not properly managed, the user could trip over it or become entangled in it. In addition, switching frequently between a virtual world and the real world is tedious and tiresome.

#### **4.2.2 Binocular Omni-Orientation Monitor (BOOM)**

The BOOM is mounted on a jointed mechanical arm with tracking sensors located at the joints. A counterbalance is used to stabilize the monitor, so that when the user releases the monitor, it remains in place. To view the virtual environment, the user must take hold of the monitor and put her face up to it. The computer will generate an appropriate scene based on the position and orientation of the joints on the mechanical arm (Aukstakalnis & Blatner, 1992).

Some of the problems associated with HMDs can be solved by using a BOOM display. The user does not have to wear a BOOM display as in the case of an HMD. This means that crossing the boundary between a virtual world and the real world is simply a matter of moving your eyes away from the BOOM.

A Survey of Some Virtual Reality Tools and Resources 29

A mechanical tracker is similar to a robot arm and consists of a jointed structure with rigid links, a supporting base, and an "active end" which is attached to the body part being tracked (Sowizral, 1995) often the hand. This type of tracker is fast, accurate, and is not susceptible to jitter. However, it also tends to encumber the movement of the user, has a restricted area of operation, and the technical problem of tracking the head and two hands

An electromagnetic tracker (EMT) allows several body parts to be tracked simultaneously and will function correctly if objects come between the source and the detector. In this type of tracker, the source produces three electromagnetic fields each of which is perpendicular to the others. The detector on the user's body then measures field attenuation (the strength and direction of the electromagnetic field) and sends this information back to a computer. The computer triangulates the distance and orientation of the three perpendicular axes in the detector relative to the three electromagnetic fields produced by the source (Baratoff & Blanksteen, 1993). Electromagnetic trackers are popular, but they are inaccurate. They suffer from latency problems, distortion of data, and they can be thrown off by large amounts of metal in the surrounding work area or by other electromagnetic fields, such as those from

Fig. 3. Patriot wireless electromagnetic tracker

Fig. 4. A.R.T. optical tracking system

at the same time is still difficult.

Fig. 2. A binocular omni-orientation monitor (BOOM)
