Challenges and performance of LED-pumped polycrystalline luminescent concentrators

High luminance light modules have been developed during the past few years based on luminescent concentrators. By using wavelength conversion, the etendue limitation that prohibits brightness increase of light that has been generated is removed, which is the principle of e.g. solar luminescent concentrators and the High Lumen Density (HLD) light engines developed for projection systems. While high-power LEDs are generally limited to a brightness of ca 200 Mnit (2 x 108 lm/m2sr), LED-pumped HLD modules have demonstrated brightness values of more than 1 Gnit. With these light sources requirements can be met for high flux applications with limited source size that are out of reach of LEDs. Such cases are found in e.g. stage and entertainment lighting or in front projection. So far, the luminescent concentrator light sources were based on single-crystalline (SC) converters. In this paper we report on the development of HLD light engines based on LED-pumped polycrystalline (PC) luminescent concentrators and simplified light source architectures. We demonstrate that with LuAG:Ce and LuYAG:Ce PC luminescent concentrators, emitting in the green-yellow spectral range, identical emission spectra and module performance characteristics can be achieved as with SC luminescent concentrators. This is successfully combined with improvements that were made with respect to HLD luminance and module efficacy. Key challenges for the PC luminescent converters are found in the minimization of light scattering. We conclude that with HLD modules based on PC concentrators, over 15 klm @ <70 lm/W can be delivered with a source brightness well over 1 Gnit. By design, the preferred trade-offs can be made between efficiency, luminance, luminous flux, module size, and cost, by which flux values over 20 klm are feasible as well. Thanks to this simplified concept, further optimization for specific applications is enabled, including the application of more temperature-sensitive converter materials for e.g. longer wavelengths.