Challenges and improvements in LED-pumped luminescent concentrators

As the brightness of high-power LEDs is generally limited to less than ca 200 Mnit (200 x 108 lm/m2sr), and expectations are that this will stay limited to a few hundred Mnit for optimized devices, high luminance light modules have been developed during the past few years based on luminescent concentrators. With these light sources the requirements can be met for most high luminous flux applications with limited etendue, like in stage and entertainment lighting or in digital projection, where LEDs don’t meet the specifications. In this paper we report on the challenges of High Lumen Density (HLD) light engine concepts based on transparent luminescent concentrators pumped by blue LEDs and on the large improvements that were recently made with respect to luminance and module efficacy while significantly simplifying the architecture. For mainstream LCD-based front projection systems, typically a yellow-green light source with an etendue of less than 14 mm2sr and a luminous flux of more than 14 klm (DC) is requested to enable > 4k ANSI-lm while meeting a high-quality color gamut. By optimizing the pump LEDs and the light coupling configuration and by decoupling the thermal channels for converter and pump LEDs in a simplified module architecture, we have improved the efficacy from 55 lm/W to more than 70 lm/W for 15 klm yellow-green output with a luminance well over 1 Gnit while reducing the module complexity considerably. With the same concept a DC luminous flux of 19 klm was achieved within an etendue of 13.6 mm2sr (i.e., 1.4 Gnit). By design, the preferred trade-offs can be made between efficiency, luminance, luminous flux, module size, and cost. Thanks to this new architecture, further optimization for the specific applications is possible, enabling also more temperature-sensitive converter materials to be applied successfully.