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A virtual reality headset is a head-mounted device that uses 3D near-eye displays and positional tracking to provide a virtual reality environment for the user. VR headsets are widely used with VR video games, but they are also used in other applications, including simulators and trainers. VR headsets typically include a stereoscopic display, stereo sound, and sensors like accelerometers and gyroscopes for tracking the pose of the user's head to match the orientation of the virtual camera with the user's eye positions in the real world. VR headsets typically use at least one MEMS IMU for 3DOF motion tracking, and optionally more tracking technology for 6DOF motion tracking. 6DOF devices typically use a sensor fusion algorithm to merge the data from the IMU and any other tracking sources. The sensor fusion algorithms that are used are often variants of a Kalman filter. Some VR headsets also have eye-tracking sensors and gaming controllers. The VR glasses use a technology called head-tracking, which changes the field of vision as a person turns their head. The technology may not be perfect, as there is latency if the head moves too fast. Still, it does offer an immersive experience. 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.