Head-mounted Displays (HMDs)

Augmented reality (AR), virtual reality (VR), mixed reality (MR) and extended reality (XR) are new technologies that find applications in the industrial, educational, healthcare and entertainment field. AR enhances the view of the real world by overlaying computer-generated information. VR replaces the real world and emphasizes full immersion. MR sits somewhere between AR and VR, as it merges the real and virtual worlds. XR is a kind of catch-all phrase that encompasses AR, VR, and MR. While all three ‘realities’ share common overlapping features and requirements, each has different purposes and underlying technologies. For example, if you are using googleMaps / Live View to navigate roads on your smart phone, you are using AR. If you are playing a 3D video game or are using a full-fledged flight simulator, you are using VR technology. And if you have watched in amazement the scene where Tony Stark is designing his famous armouredsuit in the Iron Man movie, you have seen MR in action. It is pertinent to note here that the first Iron Man movie was released in 2008, and the MR technology, which looked quite futuristic in the movie scene, is now becoming a reality. MR devices can today overlay digital images on a patient’s body and help the medical professional to perform real-time scan. And if the need arises, the patients can take a look at these scans real-time as well.

These are only a few real life applications of AR, VR, and MR; plenty others exist. All AR, VR, and MR technologies offer different levels of immersive experience and are therefore also called ‘immersive technologies’.

HMDs – Way to Access AR, VR, MR
While everyone is aware of the benefits of using AR, VR and MR, not everyone is aware how exactly it is experienced. In this article, we will talk a little bit about that.

AR / VR / MR is experienced through the use of special devices called head mounted displays, or HMD in short.Most of the HMDs are based on the stereoscopic 3D display technique. It takes advantage of a depth cue referred to as stereopsis, which is nothing but the depth human eyes perceive because each of our two eyes sees a scene from a slightly different perspective. This slight difference in perception is used to advantage in a HMD, and then both 2D offset images are combined in the brain to give the perception of 3D depth.HMDs are display devices that are adjusted directly in front of one or both of the wearer’s eyes using a head mount. In this way, digital content is superimposed on the wearer’s field of vision. Usually, a camera is integrated in the device so that the wearer can film his or her field of view and share it via digital channels.

The HMDs made for AR applications are different than those used for VR apps. The simple reason is that AR is partially immersive experience in which virtual objects are superimposed on real world surroundings. VR on the other hand is a totally immersive experience in which the user is completely detached from the surroundings.In AR, the display is usually transparent and digital information is superimposed onto real life objects. These HMDs are called Optical head-mounted displays or OHMDs. In VR, the display is not transparent and only virtual information and images are displayed in front of wearer's eyes.

HMDs for AR
Like mentioned above, HMDs used for AR can be as simple as glasses. They use optical combiners and display devices to project virtual objects onto real-world ones. The most common method employed for this is the use of transparent optical elements, which ensure that both virtual and real images can reach the human eyes simultaneously. These types of HMDs augment the physical world by superimposing media over the device in real time. AR-HMDs are currently widely employed in the fields of medicine, aviation, manufacturing, information acquisition, and entertainment. The emergence of smartphones has accelerated the miniaturization of integrated circuits and high-brightness micro displays. This miniaturization coupled with emergence of high-resolution displays has resulted in the increased popularity ofARapps and HMDs.

A few popular AR devices today include Oculus Quest, Microsoft HoloLens, Google Glass Enterprise Edition, and others.

HMDs for VR
The prime objective of a VR HMD is to provide complete isolation to the viewer in order to provide them a totally immersive experience. As such when users put on aVR HMD, they can no longer see the world around them, but instead only see VR content projected on the display screen such as 360-degree videos. A VR HMD contains components like display screen, stereo sound, sensors, and compatible controllers to deliver an immersive and interactive audiovisual experience. There are cameras inside and maybe outside of headsets to track the position of the headset and any hand-held controllers. Like a smartphone, the headset’s internal sensors sense the tilt and rotation of the user’s head. Computers generate graphics to match the position of the user’s head and hands. The quality of VR HMDs is primarily affected by:

Tracking: The more precise the tracking is, the better the VR immersive experience
Field of View (FOV): Of the 360 degrees, human eyes can see about 220 degrees. The greater the FOV, better the immersive experience    

Other key factors that affect quality of HMDs include pixel density, color fidelity, dynamic range, and brightness.

A few popular VR HMDs include HTC Vive, Oculus Rift and Samsung Gear VR.

HMDs can also be categorized as per their type:

• Slide-on HMD: these consist of a smartphone holder, lenses and some form of basic input. A smartphone is inserted into the HMD and the entire device is played upon the eyes of the user to create the VR experience. They are inexpensive as compared to other types of HMDs
• Discrete HMD: These HMDs make use of a VR source transmitting video to the user by means of a cabled connection to the source. While less flexible, they provide one of the best VR experience. Since they are connected to a device, they are also called as tethered HMDs.
• Integrated HMD: As the name says, these HMDs integrate the functions of PCs / smart phones, and mobile computing within a very hand pair of eyeglasses. There is no need for anything separate; all the source and display technology is consolidated into a single unit. Currently, most of Integrated HMDs are AR OHMDs, and are relatively expensive.