Light Guides for Illumination Applications

In optics, a “light guide” usually refers to a flexible and transparent tube for information transmission purposes. The guided light is confined to the tube’s core via total internal reflection by a cladding with a lower refractive index. For illumination purposes, a light guide frequently consists of a rigid polymer body, immersed in air without separate cladding. This generally larger index difference, hence smaller critical angle, reduces the efficiency of light coupling – equally well into as out of the light guide. Illumination applications often strive for a homogeneously perceived light distribution with maximum luminance over the entire solid body or illuminated surface. For this, the optical designer has different engineering options available, to name a few: light-source adapted input surfaces; mode mixing surfaces; non-periodic reflecting prism arrays; and arrays of surface scattering structures. Any option frequently exhibits advantages and disadvantages at the same time w.r.t. light distribution and luminance. We present and discuss optical design details for polymer light guides produced by injection moulding with emphasis on examples currently under production.