Bi-level focusing plate of phase grating type

摘要

There is disclosed a bi-level focusing plate of phase grating type in which micro unit patterns, each of which is comprised of one or more projections and one or more recessions, are arranged regularly and repeatedly at a pitch of 10 to 30 mu m. The phase difference between each of projections and each of recessions is set at pi radian against a light having a wavelength of 400 to 700 nm and the ratio in area of total area of projections or recessions in one unit pattern to the area of said one unit pattern is set so as to fall into a range of 0.3 to 0.5. The focusing plate is characterized in that the boundaries in each unit pattern each defined between the projection and the recession adjacent to each other therein are comprised of straight-lined portions and curved portions.