Optical system and image display device

An optical system comprising:

• a light guide for guiding an image light ray which is output from a display element and forms an image, to a field of view region of a user as a virtual image,
• wherein:
• the light guide includes a body having a plate shape and a periodic structure defined in the body;
• the periodic structure has periodicity in three predetermined directions intersecting each other within a predetermined plane perpendicular to a thickness direction of the body;
• the periodic structure includes an in-coupling region and an exit region;
• the in-coupling region is configured to:
• (i) divide the image light ray incident from the display element into a plurality of image light rays;
• and (ii) allow the plurality of image light rays to propagate within the body in a plurality of branch directions including first, second, and third branch directions parallel to the three predetermined directions, respectively;
• the exit region is configured to allow the plurality of image light rays propagating in the plurality of branch directions within the body to emerge from the body toward the field of view region;
• the exit region includes a plurality of exit parts adjacent to the in-coupling region in the plurality of branch directions individually;
• at least two of the plurality of exit parts have different diffraction efficiencies for incident light under a same condition;
• at least one of the plurality of exit parts includes recessed or protruded parts arranged to have periodicity in the three predetermined directions within the predetermined plane;
• and
• a ratio of a size of the recessed or protruded parts relative to a period of arrangement of the recessed or protruded parts is larger in a direction perpendicular to a branch direction different from a branch direction of the plurality of branch directions, in which a corresponding exit part is adjacent to the in-coupling region, than in a direction perpendicular to a branch direction of the plurality of the branch directions, in which the corresponding exit part is adjacent to the in-coupling region.