High brightness light emitting diodes have been used to develop high illuminance headlight for medical applications. It
provides various advantages such as high illuminance, long life time, reduced infra red light, extended operation time
with battery and light weight. A 3 W LED was employed to achieve the high performance medical headlight. The optical
design includes two lenses for high energy transmission and high illuminance. The LED headlight shows 42,000 lux with
spot diameter of 80 mm at the distance of 300 mm. For comparison purpose, 5 W LED was also used to obtain the high
illuminance headlight. However, the large divergence angle and large spot size of the 5 W LED limits the illuminance to
31,000 lux with increased burden on heat dissipation. The thermal images of the heat sinks indicate that the temperature
of the headlight using a 3 W LED is below 50 degree C, which is suitable for medical applications.
The two dimensional Finite-Difference-Time-Domain (FDTD) algorithm is used to study the optical behavior of nano-composite encapsulants. As the size of the nano-particles in an encapsulant decreases, the scattering from particles also decreases and the nano-mixture eventually becomes an optically uniform medium. Calculations of FDTD reveal the size limit of nano-particles when the transition from scatterers to an optically uniform medium occurs. As the size of the nano-particles is reduced to 0.02 λ, scattering substantially disappears and the transmission efficiency improves two-fold compared to that without nano-particles. The numerical results show that the use of a nano-composite encapsulant can improve the extraction efficiency of high-brightness light-emitting-diodes (LEDs). In addition, we simulated the roughened surface of a high-index resin layer using FDTD. The transmission efficiency of roughened surface increases 37% compared to that of the flat surface. Therefore, the combination of high-index nano-composites and a roughened surface can increase the extraction efficiency of the LEDs.
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