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.
Although the maximum brightness of LEDs has been increasing continuously during the past decade, their luminance is still far from what is required for multiple applications that still rely on the high brightness of discharge lamps. In particular for high brightness applications with limited étendue, e.g. front projection, only very modest luminance values in the beam can be achieved with LEDs compared to systems based on discharge lamps or lasers. With dedicated architectures, phosphor-converted green LEDs for projection may achieve luminance values up to 200-300 Mnit. In this paper we report on the progress made in the development of light engines based on an elongated luminescent concentrator pumped by blue LEDs. This concept has recently been introduced to the market as ColorSpark High Lumen Density LED technology. These sources outperform the maximum brightness of LEDs by multiple factors. In LED front projection, green LEDs are the main limiting factor. With our green modules, we now have achieved peak luminance values of 2 Gnit, enabling LED-based projection systems with over 4000 ANSI lm. Extension of this concept to yellow and red light sources is presented. The light source efficiency has been increased considerably, reaching 45-60 lm/W for green under practical application conditions. The module architecture, beam shaping, and performance characteristics are reviewed, as well as system aspects. The performance increase, spectral range extensions, beam-shaping flexibility, and cost reductions realized with the new module architecture enable a breakthrough in LED-based projection systems and in a wide variety of other high brightness applications.
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