Non-telecentric holographic optical element in an eye tracking system

A head mounted device comprising:

• a frame;
• and
• a lens assembly coupled to the frame, wherein the lens assembly includes:
• a waveguide;
• an out-coupling diffraction grating at least partially disposed within the waveguide and configured to out-couple light from the waveguide;
• and
• a holographic optical element (HOE) at least partially disposed within the waveguide, wherein:
• an input surface of the HOE shares an input surface of the waveguide;
• the HOE is configured to gradually change diffraction angles of a plurality of incident light rays based on a plurality of diffraction grating planes comprising slanted grating planes with differing grating periods across the HOE from a first end to a second end of the HOE;
• and
• the HOE is configured to in-couple oblique light rays from a field-of-view (FOV) of an eyebox region that extends laterally beyond a peripheral border of the input surface of the HOE.

A method comprising:

• illuminating a holographic optical element (HOE) with a first light ray that is oblique to an input surface of the HOE, the HOE partially disposed within a waveguide and sharing an input surface of the HOE with an input surface of the waveguide and the HOE configured to gradually change diffraction angles of a plurality of incident light rays based on a plurality of diffraction grating planes comprising slanted grating planes with differing grating periods across the HOE from a first end to a second end of the HOE, wherein the first light ray extends laterally beyond a peripheral border of the input surface of the HOE;
• adjusting characteristics of a first grating plane in the HOE until a first diffraction angle of the first light ray reaches a first predetermined diffraction angle value;
• determining a first angular bandwidth of the first grating plane;
• illuminating the HOE with one or more additional light rays, wherein at least some of the one or more additional light rays originate from outside the peripheral border of the input surface of the HOE, wherein incident angles of the one or more additional light rays are at least partially based on the first angular bandwidth;
• and
• adjusting characteristics of one or more additional grating planes in the HOE until one or more additional diffraction angles of the one or more additional light rays become one or more additional predetermined diffraction angle values.