Hybrid Solid-State Lighting Design

The transition from incandescent lighting to more energyefficient options is projected as a major opportunity for solid-state lighting (SSL.) The current prevailing technology for white-light SSL uses a nitride semiconductor blue LED combined with a downconverting YAG:Ce yellow phosphor. The residential use of LED lights presents challenges beyond efficiency, because of very-high color-rendering requirements, and the practical aspects of the current costs of LED lamps. Finding the balance between energy efficiency, color quality, and price, such that the resulting light source is appealing to the majority of consumers, is the major obstacle to SSL acceptance in the residential sector. We show here a potential approach for balancing efficiency, color quality, and costs by using a hybrid LED source, where the light engine uses down-conversion combined with monochromatic LEDs. The results are obtained starting with simulations of calibrated sources, and then verified experimentally using the tunable Light Engine in our laboratories. The photometric efficiency of visible LEDs, both for pump-phosphor down-conversion devices as well as for monochromatic semiconductors, is a strong function of the emissive power in a given device. An ordinary warm-white high-power phosphor converted InGaN device operated at 50mW will show an efficacy of almost 200lm/W. However, the same device, operated at 2.25W will have only 45lm/W efficacy. In an ordinary residential application, such as R-30 downlight replacement, the source needs a 600 lm flux. The semiconductor costs alone constrain that the LEDs must be operated in their highest-power, yet lowest-efficacy mode. Even at that, this requires 6 LEDs in each lamp. To achieve this flux and keep the lamp affordable is accomplished by paying a price in color quality. Obtaining a CRI over 80 in this case is problematic, and if R9 (saturated red) is included, the metric is even lower. The color can be improved by adding an additional red phosphor; but at a further price of lost efficacy, since the blue LED is now downconverting two phosphors in parallel, and the red phosphor is typically less efficient, and less thermally stable, than the yellow phosphor. A proposed solution has been to use four monochromatic LEDs, without any down-conversion. This improves the efficacy substantially