Abstract: The luminaire for a prism light guide system consists of a longitudinal hollow structure made of transparent dielectric material. The structure has inner and outer surfaces which are in octature, and at least a portion of one outer surface that has a light release mechanism. The light release mechanisms include a non-planar surface, a rough outer surface, or round corners in a corrugated outer surface. The luminaire can further include a device positioned within the hollow structure for increasing the divergence angle .theta. of the light. It can consist of a diffusion screen or a mirror. Some of the walls may also be covered by a reflective material to redirect any escaping light back into them.