LED-based projection source based on luminescent concentration

Abstract

The concept of an LED-based source with high lumen density is described. It contains a luminescent rod in which LED light is converted to light with a longer wavelength that is extracted from a small face of the rod. The fundamental limitations and possibilities are discussed, as well as the constituents needed. Results are shown for two realized high lumen density sources. A source with YAG:Ce as phosphor is extensively characterized and the results are compared to modeling results. A source with an optimized green emitting phosphor is used for projection. With 64 pump LEDs at 490 W peak electrical input and 50% duty cycle, a peak luminous flux of 18000 lm and a peak luminance of over 1000 cd/mm 2 is obtained, with an efficacy of 37 peak lm/W.

Extraction optics for high lumen density sources

“…The mentioned conversion efficiencies have been determined in an integrating sphere (Instrument Systems) by separate measurements of the blue flux from the LED module and the green flux from the complete HLD module (at the same input electrical power). More details on the set-ups for the experimental results, as well as on the module construction, can be found in literature [1][2][3][4][5].…”

“…The main reasons for this relatively low conversion efficiency were discussed before [1,4] and will be repeated briefly here. Firstly, the injection of blue pump light is not fully efficient since part is reflected and part is transmitted.…”

“…Moreover, both discharge lamps and laser-based sources degrade relatively fast. Recently [1][2][3][4][5], we developed a new type of LED-based light source, the high lumen density (HLD) source, which is very stable and enables the mentioned applications (luminance > 2·10 9 cd/m 2 ). Similar concepts were investigated by others [6,7].…”

“…The underlying microscopic cause is that the luminescent sites behave as independent sources after conversion. The conversion efficiency, defined as the ratio of the green flux emitted by the module and the blue flux emitted by the LEDs, is 0.28 for a typical HLD module [4], consisting of a block-shaped rod made of ceriumdoped lutetium aluminum garnet (LuAG:Ce, dimensions 64 × 1.9 × 1.2 mm 3 ) with a glass CPC attached to it. Figure 2 shows the measured output spectrum from the module (red solid line), together with its absorption spectrum (blue) and its intrinsic emission spectrum (measured using a thin layer of powder, orange).…”

“…The red dashed line shows the simulated output spectrum, obtained from ray-tracing simulations [1,4] using the measured absorption (blue) and emission (orange) spectra as input. The simulated value for the conversion efficiency is 0.29, slightly more than the experimental value.…”