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.
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 étendue, 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 étendue 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 étendue 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.
KEYWORDS: Solar concentrators, Light emitting diodes, Light sources, Semiconductor lasers, Projection systems, Light sources and illumination, Collimation, Laser systems engineering, Digital Light Processing, Light, Ceramics, High power diode lasers
Although LEDs have penetrated successfully in many lighting domains, high brightness light source applications are still suffering from their limited luminance. High power LEDs are generally limited to less than 100 Mnit (108 lm/m2sr), while dedicated devices for projection may achieve pulsed peak luminance values up to 200 Mnit for phosphorconverted green. For high luminous flux applications with limited etendue, like in stage or architecture spot lighting or in front projection, in the beam only very modest luminance values can be achieved with LEDs compared to systems based on discharge lamps. In this paper we evaluate light engine concepts based on static luminescent converters pumped by blue laser diodes, and concepts based on luminescent concentrators pumped by blue LEDs. Both concepts break through the flux and brightness requirements for these applications by enabling luminance values that are a factor five to ten higher than what can be achieved with LEDs. With continuous wave irradiation of a 10 mm2 static converter by multiple laser diodes, 47 klm yellow-green emission was achieved at 1.5 Gnit source luminance, or 40 klm @1.2 Gnit in a collimated beam. With yellow-green light concentrator modules, 16 klm yellow-green emission was achieved at 1.2 Gnit collimated beam luminance. Thermal conditions are much more relaxed in luminescent concentrator modules than for static laser diode (LD) pumped converter systems. The High Lumen Density (HLD) LED-based luminescent concentrator, with its advantage of scalability in both flux and luminance, enables breakthrough performance in projection systems and in a wide variety of other applications. Laser-pumped converters, on the other hand, easily scale in flux proportional to their source size at constant luminance. They show very high flux capability and comparable brightness, enabling scope extension with extremely high flux solid state light sources.
Tens of blue diode lasers are focused onto a ceramic phosphor to create a high-brightness high-lumen output light engine. Illumination uniformity and scattering properties of the phosphor impact the efficiency of the system.
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