ILLUMINATION DEVICE, METHOD OF DESIGNING ILLUMINATION DEVICE, AND DESIGNING DEVICE

An illumination device, which projects a projection pattern including information onto a projection plane, includes a light source that emits a coherent light beam, and a diffractive optical element that diffracts the coherent light beam from the light source and projects the projection pattern. The diffractive optical element has a diffraction characteristic that allow an observer who sees the projection pattern from a viewpoint position in a first line-of-sight direction based on a position and a direction of the diffractive optical element, to visually recognize the information with a predetermined size, a predetermined shape and a predetermined inclination angle.