Patent US 6,285,505

Light emitted from a virtual retinal display light source passes through a beamsplitter to a scanning subsystem and on to an eyepiece and the viewer's eye. Some of the light is reflected from the viewer's eye passing back along the same path. Such light however is deflected at the beamsplitter toward a photodetector. The reflected light is detected and correlated to the display scanner's position. The content of the reflected light and the scanner position for such sample is used to generate a map of the viewer's retina. Such map includes `landmarks` such as the viewer's optic nerve, fovea, and blood vessels. The map of the viewer's retina is stored and used for purposes of viewer identification. The viewer's fovea position is monitored to track where the viewer is looking