We have used the following types of head-mounted displays (HMDs). They offer different levels of resolution, weight and field-of-view (FOV):

3x Oculus Rift CV1  5x HTC Vive
- FOV: up to 110 (circular view) 
- Resolution: <1080x1200 per eye, 90Hz
- Lens IPD adjustment 

- FOV: up to 110 (circular view)
- Resolution: <1080x1200 per eye, 90Hz 
- Lens IPD adjustment 

4x GearVR FOV: up to 100 (circular view)  1x Oculus Rift DK2 with integrated SMI eyetracking 
- Resolution: <1280x1440 per eye, 60Hz 

- 60Hz binocular eyetracking 

6x Oculus Rift DK2   2x HoloLens Mixed Reality Display  
- FOV: 100 (nominal, circular view) 
- Resolution: <960x1080 per eye 

- FOV:30 (horizontal) 
- Resolution: 1280x720 per eye, 60Hz 

1x HTC Vive with integrated SMI eyetracking
- 250Hz binocular eyetracking 

Oculus Rift CV1 VR HMD units were used for walking experiments in the TrackingLab. We have developed a mobile experiment solution which uses MSIVR backpacks (MSI) to generate the graphics for the headset with a fast desktop GPU (NVIDIA GTX 1070). Head pose is calculated by sensor fusing the onboard IMU of the CV1 with external reference data from the Vicon system (position and reference orientation) to provide smooth drift-free low-latency head orientation. The experimenter is able to see the same view as the subject without impacting rendering performance on the backpack PC. This is achieved by using hardware video encoding on the VR backpack and decoding it using SteamLink hardware (Valve).

HTC Vive VR HMD allows room scale tracking of areas of up to 4.5m x 4.5m using its own tracking system. It consists of two laser emitter base stations which can be used to turn an unused office or lab space into a small VR lab. It comes with tracked hand controllers which make it a good choice for experiments where user body and position movement is needed. Additional body parts can be tracked with the separate Vive Tracker puck units.

Samsung GearVR HMDs are providing a completely tetherless VR solution where a Samsung smartphone provides the high resolution display as well as the graphics rendering capability. We use the same sensor fusion approach as our CV1 setup to use these devices for lightweight large-room scale VR in the TrackingLab.

For HMD experiments where eyetracking information is needed we can use the SMI HTC Vive or SMI DK2 units which offer integrated eyetracking and a plugin for integration into existing Unity experiments.

Microsoft HoloLens is a mobile tetherless Mixed Reality HMD which offers inside-out 6DOF head tracking and the ability to overlay computer generated 3D content using a see-through display. The inside-out tracking system allows it to be used in many locations without an external tracking system. Due to the limited FOV it is however not suitable for all visual stimuli. 

Alternative Reality Headset
The Alternative Reality Headset was designed with the aim to present participants with visual roll-tilt stimuli that have the highest degree of ecological validity possible. To realize this, we mounted a stereo camera via a servo motor to a Head-Mounted-Display (HMD). The axis of rotation is aligned with the naso-occipital axis, and the image captured by the cameras is fed through to the HMD screens. The Alternative Reality Headset consists of a HTC Vive VR headset (HTC, New Taipei City, Taiwan), with a resolution of 1080x1200 per eye and a refresh rate of 90 Hertz. An OVRVision Pro stereo camera (Wizapply, Osaka, Japan) is attached to the front of the headset via a Dynamixel AX12A servo (Robotis, Lake Forest, California, United States) that allows rotating the stereo camera by up to ±150°. The camera resolution and framerate can be adjusted. Positional information provided by the HTC Vive Lighthouse positional tracking system can be used to correct the camera angle for head rotations.

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