The purpose of the invention is to provide a
light guide of a variety of forms having a uniform distribution of brightness in the plane and a planer
light source device for a
liquid crystal display device which uses such
light guide. The
light guide comprises a first surface which is a surface to which a natural polarization light is incident and a second surface other than the first surface which is an
exit surface of a specific polarization light into which, said natural polarization light is modulated, wherein;said light guide has an interface of two materials of different indices of
refraction oriented at an angle of thetaB+ / -alpha degrees relative to the primary propagation direction of said incident light, said thetaB being an angle satisfying Brewster's condition, more than two orientations of said interface exist in a single light guide, and the difference between the indices of
refraction of the two materials of different indices of
refraction is between 0.001 and 1.0. thetaB is typically about 45 degrees. The light guide comprises a first transparent member having a plurality of upwardly convex right angle isosceles triangles on a first surface thereof and a first index of refraction and a second transparent member having a plurality of downwardly convex right angle isosceles triangles on a second surface thereof and a second index of refraction, and said first surface and said second surface contact each other.