Enhancing brightness of Lambertian light sources with luminescent concentrators: the light extraction issue.

T Gallinelli,S Chenais,T Billeton,S Forget,A Barbet,F Balembois,F Druon,P Georges

doi:10.1364/oe.27.011830

Abstract

Luminescent concentrators (LC) enable breaking the limit of geometrical concentration imposed by the brightness theorem. They enable increasing the brightness of Lambertian light sources such as (organic) light-emitting diodes. However, for illumination applications, light emitted in the high-index material needs to be outcoupled to free space, raising important light extraction issues. Supported by an intuitive graphical representation, we propose a simple design for light extraction: a wedged output side facet, breaking the symmetry of the traditional rectangular slab design. Angular emission patterns as well as ray-tracing simulations are reported on Ce:YAG single crystal concentrators cut with different wedge angles, and are compared with devices having flat or roughened exit facets. The wedge output provides a simple and versatile way to simultaneously enhance the extracted power (up to a factor of 2) and the light directivity (radiant intensity increased by up to 2.2.).

Highlights

Light-emitting diodes (LEDs) are the new reference for general lighting, they have proved to be reliable and economically-viable alternatives to halogen or arc lamps for more specific, bright illumination sources useful for e.g. automotive [1] or medical applications [2]
Using LED + Luminescent Concentrators (LC) combinations to build bright directional sources requires that the radiation emitted in a high-index material is eventually outcoupled to air, which raises the important issue of light extraction, a long-studied issue in other contexts like device design of LEDs [13], OLEDs [14] or scintillators [15]
Light extraction in wedged luminescent concentrators We investigated the influence of a wedge on the exit facet, a way to eliminate closed ray paths that are stable upon successive Total Internal Reflection (TIR) processes and are responsible for the significant percentage of trapped rays propagating in a high index medium surrounded by air

Summary

Introduction

Light-emitting diodes (LEDs) are the new reference for general lighting, they have proved to be reliable and economically-viable alternatives to halogen or arc lamps for more specific, bright illumination sources useful for e.g. automotive [1] or medical applications [2].

Results

Conclusion