Reflective diffractive gratings with variable reflectivity and display systems having the same

An optical device comprising:

• an optical waveguide structure comprising:
• an incoupling optical element configured to redirect incident light at angles such that the incident light propagates through the optical waveguide structure by total internal reflection, wherein the incoupling optical element comprises:
• a diffractive optical grating structure comprising:
• a plurality of protrusions;
• and
• a reflective layer on surfaces of the protrusions, and
• wherein at least one parameter of the reflective layer varies across an area occupied by the plurality of protrusions, and
• wherein a reflectivity of the diffractive optical grating structure varies with the at least one parameter of the reflective layer across the area occupied by the plurality of protrusions.