Using micro - structures to couple light into thin light - guides

Abstract: The task of this project is to investigate the possibilities of using micro-structuring on the surface of thin light-guides to efficiently couple light from top-emitting LEDs into such light-guides. Areas of application are backlighting for LCD TV and flexible light emitting layers for lighting purposes. The micro-structures considered are prisms and pyramids. The micro-structures can be on the same side of light-guide as the LEDs or on the opposite side. When located on the opposite side, the micro-structures are coated with a specular reflecting layer or with a diffuse reflecting layer. The LEDs can either be in optical contact with the light-guide or not in optical contact. Optical ray-tracing software package Light-Tools is used for all ray-tracing simulations of these geometries. In simulations, a two-step approach is taken: Firstly, we build in Light-Tools a simple model of an LED in proximity to a micro-structured light-guide in order to ascertain which geometry is most likely to show a high in-coupling efficiency. In the next step, we made a more elaborate and more accurate model of the most promising geometries. The results of these simulations show that, for micro-structures that are located on the light-guide on the side opposite to the LEDs, a diffuse reflecting layer (i.e. a Lambertian scatterer) is more useful than a specular reflecting layer (i.e. a mirror). Also, in general, the prisms structures perform better than the pyramid structures. The highest efficiency reached in the simulations is 60% for Model I1  in which the light source is not in optical contact with the light guide and the mirror is on the opposite side of the light guide. Compared to the result reported in a previous paper, the incoupling efficiency improvement is 8%. We also looked into using Retro-reflection Films for improving the in-coupling efficiency. Several prototypes with a Retro-reflection Film were made and tested in the laboratory. Measurements were performed for three thicknesses of the light guide: 0.25 mm, 0.5 mm and 1 mm. The best in-coupling efficiencies measured are 37% for a 0.25 mm thick light guide, 53.1% for a 0.5 mm thick light guide and 57.1% for a 1 mm thick light guide. Compared to the samples without Retro-reflection Film, the best Retro-reflection Film results in a 7% increase for the 0.25 mm thick light guide, 6.2% increase for the 0.5 mm case and 2.8% increase for the 1 mm case, respectively.