Greater Lighting Efficiency
LED Optics Made from Liquid Silicone Rubber Reduce Light Output Losses
In conventional construction of an LED high-beam light module, the LED semiconductor crystal is first encapsulated with a potting compound. Because of the limited design freedom, the light emitted by the LED is not projected directly onto the secondary optics. The losses at the transition between the primary optics and preliminary optics reduce the light output by up to 15 %. With this in mind, LED optics were injection molded using liquid silicone rubber (LSR) as part of a research project. The LSR optics encapsulate the LED and also function as preliminary optics. This reduces the losses in light output.
The LSR optics are injection molded directly onto the LED circuit board. This requires placing the LED circuit board into the open mold and holding it in the correct position with locating pins. The liquid silicone rubber is then injected through an opening in the back of the circuit board using a cold runner system. Costly and time-consuming assembly steps on the LED chip can thus be eliminated.
The mold temperature has a significant effect on the cross-linking behavior of the LSR. In addition, it determines the heat load on the chip during the injection molding process. To rule out unnecessarily long application of heat to the chip and make the process as economical as possible, the cross-linking time was adjusted on the basis of the mold temperature in a series of tests. As soon as the optical components are stable enough for demolding, the cross-linking time is ended.
Currently, alternating temperature tests are being conducted between -40 °C and 125 °C over a period of 1000 hours, along with a continuous operation test of the LED at the maximum permissible current over a period of 4000 hours. The results from these tests will decide whether industrialization of the process at this stage of the research is possible. (hs)