Light guides are used in a wide variety of applications to transmit light from one location to another. Light guides having a variety of different geometries have been developed for different applications. For example, a light guide configuration that is particularly useful in display applications is a planar light guide. In a typical display configuration, a light source is positioned along an edge of a planar light guide that is configured to receive light generated by the light source. The light propagates within the light guide by total internal reflection, and is extracted from one of the planar surfaces of the light guide. The extracted light, which is usable to illuminate a display field, is optionally passed through subsequent optical components, such as diffusers, light recycling films, and/or spatial modulators.
Light is extracted from a planar surface of the light guide using one or more of a wide variety of extraction elements. Generally, the extraction elements comprise a feature that causes light to propagate out of the light guide instead of being totally internally reflected within the light guide. Typical extraction elements comprise, for example, raised or recessed surface features such as protrusions or dimples, as well as localized material differences or other surface or volume perturbations in the light guide. Often the extraction elements are arranged to extract a substantially uniform light field from the light guide, which is particularly advantageous in the context of providing a uniform illumination field for a display surface.
When light is extracted from a planar light guide, nonuniformities in the illumination of the light guide are manifested in a nonuniform illumination field extracted from the light guide. In the context of a planar light guide used to illuminate a display field, such nonuniformities are manifested as brightly illuminated regions and poorly illuminated regions of the display field. A more uniform illumination can be produced by modifying the spatial distribution of extraction elements on the light guide, such that more light is extracted from the light guide in poorly-illuminated regions, and less light is extracted from the light guide in well-illuminated regions. While this approach is usable to provide a more uniform field of light extracted from the light guide, it fails to provide a generalized approach for producing a desired illumination field, such as a spatially nonuniform illumination field.