The Basic Components
Let’s have a closer look at our HMD. The basic parts are comparable to other HMDs
We have a body case which is 3d-printed with head-mount straps, an 5.6″ HD display. There is an orientation tracker running on Arduino-basis and we use the aspheric lenses from the Oculus DK1 to enable a wide field-of-view.
For binocular eye tracking we now added two infrared cameras for binocular eye tracking on both sides of the body outside the user‘s field of view.
We then added two dichroic mirrors (also called „hot mirrors“) to virtually move the camera view in front of the eyes. This way, we achieve a more uniform tracking precision compared to systems working with a lower camera position.
And we created 3d-printed lens holders that are fully adjustable in three dimensions for precise and flexible user calibration. The lens holders contain a ring of infrared LED lights to illuminate the user‘s eyes for the tracking cameras, while being invisible to the user.
Finally we added a front camera for positional tracking and potentially blending in the physical environment if sensible.
Disjoint Light Paths
I will now explain the light transport within our HMD. In the next sketch you see the basic parts of the system from above.
The infrared LEDs illuminate the eyes of the user. The reflected light then passes through the lenses. The rays are then reflected at the mirrors and received at the eye tracking cameras.
Now the visible light visualized by the green lines: The visible light emitted from the display passes the mirros unhindered, is refracted at the lenses and finally reaches the eyes. This results in two disjoint light paths that enable eye tracking functionality without affecting the user‘s viewing comfort