Light pipe, plate light source unit and reflection type liquid-crystal display device

A light pipe comprising:

• a plate-like member including light output means formed in an upper surface of said plate-like member so that light incident on an incidence side surface of said plate-like member is exited from a lower surface of said plate-like member through said light output means;
• an adhesive layer having a refractive index lower than that of said plate-like member;
• and
• a light-diffusing layer including fine prismatic structures formed in a surface thereof and bonded to said lower surface of said plate-like member through said adhesive layer,
• wherein said light output means formed in said upper surface of said plate-like member is formed by a repetitive structure of prismatic structures arranged at intervals of a pitch of from 50 μm to 1.5 mm and each having an optical path changing face and a long side face;
• each of said optical path changing faces is constituted by a slope inclined downward from said incidence side surface side to a counter end side at an inclination angle in a range of from 35 to 48 degrees with respect to a reference plane of said lower surface so that a projected width of each of said slopes on said reference plane is not larger than 40 μm;
• and each of said long side faces is constituted by a slope inclined at an inclination angle in a range of from 0 to 10 degrees with respect to said reference plane so that an angle difference between any two long side faces over a surface of said plate-like member is not larger than 5 degrees, so that a difference between inclination angles of adjacent ones of said long side faces is not larger than 1 degree and so that a projected area of said long side faces on said reference plane is not smaller than 5 times as large as a projected area of said optical path changing faces on said reference plane.