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Abstract
Lens arrays are introduced to diminish the total internal reflection (TIR) that happens at chip-encapsulant and encapsulant-air interfaces of chip-on-board light-emitting diodes (COB-LEDs), so as to improve the light extraction efficiency (LEE) of the COB-LEDs. However, the LEE of COB-LEDs with lens array depends on the refractive index of the encapsulant layer ${n_{{\rm encap}}}$ and lens array ${n_{{\rm lens}}}$, which was rarely concerned so far. Optical simulations based on a Monte Carlo ray tracing method, and experiments were conducted to investigate the effect of ${n_{{\rm encap}}}$ and ${n_{{\rm lens}}}$ on the LEE of COB-LEDs with millilens array. The simulated results show that the TIR at chip-encapsulant, encapsulant-lens, and lens-air interfaces can be significantly diminished by regulating the ${n_{{\rm encap}}}$ and ${n_{{\rm lens}}}$, and the LEE of COB-LEDs decreases as the refractive difference of encapsulant layer and lens array $|{n_{{\rm lens} -}}{n_{{\rm encap}}}|$ increases. Compared to the COB-LEDs with only a flat encapsulant layer, the LEEs of blue and white COB-LEDs with ${n_{{\rm lens}}} = {n_{{\rm encap}}} = {n_{{\rm ITO}}} = {2}$ are enhanced by 246.2% and 50.6%, where ${n_{{\rm ITO}}}$ is the refractive index of the top layer of the conventional LED chip. The experimental results agree well with the simulated results with normalized LEE deviation within 7.3%.
© 2021 Optical Society of America
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