This article needs to be updated.(June 2016)
A virtual reality headset is a head-mounted device that provides virtual reality for the wearer. VR headsets are widely used with computer games but they are also used in other applications, including simulators and trainers. They comprise a stereoscopic head-mounted display (providing separate images for each eye), stereo sound, and head motion tracking sensors (which may include gyroscopes, accelerometers, structured light systems, etc.). Some VR headsets also have eye tracking sensors and gaming controllers.
The Sega VR, announced in 1991 and seen in early 1993 at the Winter CES, was never released. An early VR headset, the Forte VFX1, was announced at CES in 1994. The VFX-1 has stereoscopic displays, 3-axis head-tracking, and stereo headphones. Sony, another pioneer, released the Glasstron in 1997, which has an optional positional sensor, allowing the wearer to view the surroundings, with the perspective moving as his head moves, giving a deep sense of immersion. These VR headsets gave MechWarrior 2 players a new visual perspective of seeing the battlefield from inside the cockpit of their craft. However, these early headsets failed commercially due to their limited technology and were described by John Carmack as like "looking through toilet paper tubes".
In 2012, a crowdfunding campaign began for a VR headset known as Oculus Rift; the project was led by several prominent video game developers, including John Carmack who later became the company's CTO. In March 2014, the project's parent company Oculus VR was acquired by Facebook for US$2 billion. The final consumer-oriented release of Oculus Rift began shipping on 28 March 2016.
In March 2014, Sony demonstrated a prototype headset for PlayStation 4, which was later named PlayStation VR. In 2014, Valve Corporation demonstrated some headset prototypes, which lead to a partnership with HTC to produce the Vive, which focuses on "room scale" VR environments that users can naturally navigate within and interact with. The Vive was released in April 2016 and PlayStation VR in October 2016.
Virtual reality headsets and viewers have also been designed for smartphones. Unlike headsets with integrated displays, these units are essentially enclosures which a smartphone can be inserted into. VR content is viewed from the screen of the device itself through lenses acting as a stereoscope, rather than using dedicated internal displays. Google released a series of specifications and associated DIY kits for virtual reality viewers known as Google Cardboard; these viewers are capable of being constructed using low-cost materials, such as cardboard (hence the naming).
Samsung Electronics partnered with Oculus VR to co-develop the Samsung Gear VR (which is only compatible with recent Samsung Galaxy devices), while LG Electronics developed a headset with dedicated displays for its LG G5 smartphone known as LG 360 VR.
Asian hardware manufacturers like Xion and Kolke have developed inexpensive virtual reality headsets. In late April 2017, the Chinese company Tencent announced it was preparing to launch its virtual reality headset that year.
Virtual reality headsets have significantly higher requirements for latency—the time it takes from a change in input to have a visual effect—than ordinary video games. If the system is too sluggish to react to head movement, then it can cause the user to experience virtual reality sickness, a kind of motion sickness. According to a Valve engineer, the ideal latency would be 7-15 milliseconds. A major component of this latency is the refresh rate of the display, which has driven the adoption of displays with a refresh rate from 90 Hz (Oculus Rift and HTC Vive) to 120 Hz (PlayStation VR).
The graphics processing unit (GPU) also needs to be more powerful to render frames more frequently. Oculus cited the limited processing power of Xbox One and PlayStation 4 as the reason why they are targeting the PC gaming market with their first devices.
Because virtual reality headsets stretch a single display across a wide field of view (up to 110° for some devices according to manufacturers), the magnification factor makes flaws in display technology much more apparent. One issue is the so-called screen-door effect, where the gaps between rows and columns of pixels become visible, kind of like looking through a screen door. This was especially noticeable in earlier prototypes and development kits, which had lower resolutions than the retail versions.
The lenses of the headset are responsible for mapping the up-close display to a wide field of view, while also providing a more comfortable distant point of focus. One challenge with this is providing consistency of focus: because eyes are free to turn within the headset, it's important to avoid having to refocus to prevent eye strain.
The lenses can also be adjusted dynamically to account for a user's eyeglass prescription, so that the user can use the headset without corrective eyeglasses.
Virtual reality headsets are being currently used as a means to train medical students for surgery. It allows them to perform essential procedures in a virtual, controlled environment. Students perform surgeries on virtual patients, which allows them to acquire the skills needed to perform surgeries on real patients. It also allows the students to revisit the surgeries from the perspective of the lead surgeon.
Traditionally, students had to participate in surgeries and often they would miss essential parts. Now, with the use of VR headsets, students can watch surgical procedures from the perspective of the lead surgeon without missing essential parts. Students can also pause, rewind, and fast forward surgeries. They also can perfect their techniques in a real-time simulation in a risk free environment. 
Virtual reality headsets have been used for training United States soldiers of the army, air-force, and navy. It is a particularly useful tool for training soldiers without putting them in harm's way.
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