Scaling Up: Kilolumen Solid-State Lighting Exceeding 100 LPW via Remote Phosphor (2008)

This thirty-month project was successful in attaining its ambitious objectives of demonstrating a radically novel 'remote-phosphor' LED light source that can out-perform conventional conformal coated phosphor LED sources. Numerous technical challenges were met with innovative techniques and optical configurations. This product development program for a new generation of solid-state light sources has attained unprecedented luminosity (over 1 kilo-lumen) and efficacy (based on the criterion lumens per 100mw radiant blue). LPI has successfully demonstrated its proprietary technology for optical synthesis of large uniform sources out of the light output of an array of separated LEDs. Numerous multiple blue LEDs illuminate single a phosphor patch. By separating the LEDs from the phosphor, the phosphor and LEDs operate cooler and with higher efficiency over a wide range of operating conditions (from startup to steady state). Other benefits of the system include: better source uniformity, more types of phosphor can be used (chemical interaction and high temperatures are no longer an issue), and the phosphor can be made up from a pre-manufactured sheet (thereby lowering cost and complexity of phosphor deposition). Several laboratory prototypes were built and operated at the expected high performance level. The project fully explored two types of remote phosphor system: transmissive and reflective. The first was found to be well suited for a replacement for A19 type incandescent bulbs, as it was able to replicate the beam pattern of a traditional filament bulb. The second type has the advantages that it is pre-collimate source that has an adjustable color temperature. The project was divided in two phases: Phase I explored a transmissive design and Phase II of the project developed reflective architectures. Additionally, in Phase II the design of a spherical emitting transmissive remote phosphor bulb was developed that is suitable for replacement of A19 and similar light bulbs. In Phase II several new reflective remote phosphor systems were developed and patents applied for. This research included the development of reflective systems in which the short-pass filter operated at a nominal incidence angle of 15{sup o}, a major advancement of this technology. Another goal of the project was to show that it is possible to align multiple optics to multiple LEDs (spaced apart for better thermal management) to within an accuracy in the z-direction of 10 microns or less. This goal was achieved. A further goal was to show it is possible to combine and homogenize the output from multiple LEDs without any flux loss or significant increase in etendue. This goal also was achieved. The following color-coded computer drawing of the Phase 2 reflective remote phosphor prototype gives an idea of the accuracy challenges encountered in such an assembly. The actual setup has less functional clarity due to the numerous items of auxiliary equipment involved. Not only did 10 degrees of freedoms alignment have to be supplied to the LEDs and component prisms as well, but there were also micro-titrating glue dispensers and vacuum hoses. The project also utilized a recently introduced high-index glass, available in small customized prisms. This prototype also embodies a significant advance in thin-film design, by which an unprecedented 98% single-pass efficiency was attained over a 30 degree range of incidence angle (Patents Pending). Such high efficiency is especially important since it applies to the blue light going to the phosphor and then again to the phosphor's light, so that the 'system' efficiency associated with short-pass filter was 95.5%. Other losses have to be kept equally small, towards which a new type of ultra-clear injection-moldable acrylic was discovered and used to make ultra-transparent CPC optics. Several transmissive remote phosphor prototypes were manufactured that could replace screw-in type incandescent bulbs. The CRI of the white light from these prototypes varied from 55 to 93. The system efficiency achieved was between 27 to 29.5 lumens per 100 mw radiant blue. For blue LEDs providing 200mw blue for 1 watt electrical input, these prototypes produced 54 to 59 lumens per watt. The latest blue LEDs (from several manufacturers) are more than twice as efficient as this. So in this case the efficiency of the systems (exclusive of driver losses) would be minimally 108 to 118 lumens per watt. So the systems efficiency of the transmissive remote phosphor was proven to be equal to or greater than if the same LEDs use conformal coated phosphor; with the stated advantages of the remote phosphor system.