HMD- (Head Mounted Display)

Introduction:

HMD- it stands for Head Mounted Display. As the name implies, its a computer display you wear on your head in the form of Helmet or a set of Goggles. These are personal information-viewing devices that can provide information in a way that no other display can. While they can be used as hands-off information sources, the displayed video can also be made reactive to head and body movements, replicating the way we view, navigate through, and explore the world.

On the basis of image representation, it is categorized into 3 modes. The Monocular mode has one display, which can (usually) be placed at a suitable viewing angle in front of either the right or the left eye. In the Binocular mode two disparate images are presented on two separate displays and the image can be viewed as a Stereoscopic 3D image, while in the Bi-ocular mode, the same image is presented on 2 displays and therefore the image can be viewed as a 2D image.

Its built-in technology and capabilities enables it to various exciting and useful applications such as Virtual reality for creating artificial environment, to Medical Visualization as an aid in surgical procedures, to Military Vehicles for viewing sensor imagery, to Airborne workstation applications reducing size, weight, and power over conventional displays, to Aircraft simulation and training for ﬁxed and rotary wing avionics display applications.

What is HMD:

A typical HMD has either one or two small displays with lenses and semi-transparent mirrors embedded in a helmet, eye-glasses (also known as data glasses) or visor. The monitors in an HMD are most often Liquid Crystal 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.

In addition to displays and image generators, a typical HMD system includes loudspeakers, battery chargers, and different input/output cables. Because of the small size and light weight, commercial HMDs as accessories of wearable technology can be used in various contexts.

Different Types of Image Sources for HMD:

There are two main image sources, which are used in HMD, such as emissive and non-emissive.

Emissive Image Source: emissive devices represent a large category of image sources in which the image plane of the device emits light without the need for supplemental illumination. Such devices include:

Active Matrix Electroluminescent (AMEL): a thin-ﬁlm layer of luminescent phosphor is sandwiched between two electrodes, one transparent, in a pixilated array. The pixels are digitally addressed using high-frequency pulses to achieve grayscale. Recent improvements use a quasi-analog addressing to achieve greater grayscale range and improved luminance. These are compact and very rugged devices.

Cathode Ray Tube (CRT): this is a vacuum tube with an electron gun at one end and a phosphor screen at the other. A beam from the electron gun is modulated by deﬂection grids and directed onto the screen. The incident electrons excite the phosphor, emitting visible light. CRTs can be very bright and very rugged for the aviation environment, though they are larger than ﬂat-panel displays and require high voltage.

Vacuum Fluorescent Display (VFD): most commonly seen in alphanumeric displays, the VFD uses a vacuum package containing phosphors that are excited by a series of ﬁlaments. These capabilities are being expanded as imaging devices. Though currently available only in low-resolution devices, VFDs may in time become more prevalent.

Organic Light Emitting Diodes (OLED): a low-voltage drive across a thin layer of organic material causes it to emit visible light when the charge carriers recombine within the material. A very promising technology, though as of this writing it is still in the developmental stages.

Non-emissive Image Source: the non-emissive image sources modulate a separate illumination on a pixel-by-pixel basis to create the desired imagery. Examples are:

Transmissive Liquid Crystal Displays (LCD): the pixel matrix is illuminated from the rear. A modulated electric ﬁeld controls the transmission of the backlight through the individual liquid crystal-ﬁlled cells. Quality transmissive LCDs are manufactured in large quantity in Japan, though in limited quantity domestically.

Reﬂective Liquid Crystal on Silicon Displays (LCOS): this is the same as the transmissive device except that the image source is illuminated from the front. The light transmits through the cell and reﬂects off a mirror-like surface when the pixel is transmitting and is scattered when the pixel is turned off. This is a fast-growing area of development in the U.S. because the manufacturing technology is similar to silicon wafer fabrication.

Scanning Display: a point source (such as a laser) or line of point sources (such as LEDs) is modulated in one or more directions using resonance scanners or opto-acoustic modulators to produce imagery. One example is the Retinal Scanning Display.

Applications of HMD: because of it extra-ordinary technology, it has following applications,

Aviation and Tactical Ground: Ruggedized HMDs are increasingly being integrated into the cockpits of modern helicopters and fighter aircraft. These are usually fully integrated with the pilot's flying helmet and may include protective visors, night vision devices and displays of other symbology. Military, police and firefighters use HMDs to display tactical information such as maps or thermal imaging data while viewing the real scene.

Gaming and Video: Low cost HMD devices are available for use with 3D games and entertainment applications. One of the first commercially available HMDs was the Forte VFX-1 which was announced at CES in 1994. The VFX-1 had stereoscopic displays, 3-axis head-tracking, and stereo headphones.

Sports: system has been developed for Formula One drivers by Kopin Corp. and the BMW Group. According to BMW, “The HMD is part of an advanced telemetry system approved for installation by the Formula One racing committee to communicate to the driver wirelessly from the heart of the race pit.” The HMD will display critical race data while allowing the driver to continue focusing on the track. Pit crews control the data and messages sent to their drivers through two-way radio.

Engineering and Medical Science: Engineers and scientists use HMDs to provide stereoscopic views of CAD schematics. These systems are also used in the maintenance of complex systems, as they can give a technician what is effectively "x-ray vision" by combining computer graphics such as system diagrams and imagery with the technician's natural vision. There are also applications in surgery, wherein a combination of radiographic data (CAT scans and MRI imaging) is combined with the surgeon's natural view of the operation, and anesthesia, where the patient vital signs are within the anesthesiologist's field of view at all times. Research universities often use HMDs to conduct studies related to vision, balance, cognition and neuroscience.