Two highly-transparent liquid silicone rubber (LSR) compounds with a high refractive index for LEDs and optical components are newly available from Germany’s Wacker (U.S. office in Adrian, Mich.). Lumisil 590 and 591 boast a refractive index of 1.53, which means they rank among the high-refractive-index (HRI) encapsulants. Such grades are particularly suitable for manufacturing highly-efficient LEDs. The HRI silicone protects the sensitive LED chip against mechanical influences and corrosive gases. At the same time, the silicone’s high-refractive index enables optimum light efficiency.
Semiconductor chips used to generate light in LEDs have a high-refractive index. To maximize the amount of light emitted by the LED chop that can pass through the encapsulation, the refractive index of the chip and encapsulant must be roughly the same value, as is the case with the new Lumisil compounds. Moreover, both materials are said to be easy to process—heat-resistant and totally tack-free after curing, are almost completely transparent for light in the visible spectral range (approx. 400 to 700 nm) and, reportedly do not yellow even when radiation is extremely intense. Transmission tests with Lumisil 590 show that one-millimeter-thick (0.039-inch-thick) layer lets over 91% of visible light through.
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