Patent ID: 8281510

Claim:
A plastic sheet having optical transparency, comprising: an exit surface; and an incident surface on which a plurality of prism lines, having a smooth surface and a shape of a prism in a cross-section perpendicular to a longitudinal direction of a prism line, are aligned, wherein: the shape of the prism is formed by a side A on a lower side at an angle of α degrees to a normal line of the exit surface and a side B on an upper side at an angle of β degrees to the normal line of the exit surface; α<β; 40 degrees≦β≦80 degrees; and an integral value of a downwardly emitted light flux is 55% or more of the total integral value of an upwardly emitted light flux and the downwardly emitting light flux when predetermined diffusion light enters one surface of the plastic sheet in a direction perpendicular to the plastic sheet.