Design of light-emitting-diode array for solving problems of irregular radiation pattern and signal attenuation for infrared electronic-toll-collection systems

Infrared light emitting diodes (LEDs) with a half-intensity angle Φ_1/2≃ 10° are suitable for electronic-toll-collection (ETC) applications. However, the radiation patterns of commonly available low-cost commercial infrared LEDs are irregular to some extent, which result in difficulties to construct an infrared ETC system with a proper communication area. Most low-cost infrared LEDs with a half-intensity angle Φ_1/2≃ 10° exhibit a common irregularity in their radiation pattern - a notch in the central radiant direction. The adverse influence bccause of such irregularities can be reduced by a proper design of an LED array in the emitter. High attenuation for the infrared signal of some windshields is also a serious problem for infrared ETC systems, which leads to severe shrinkage of the communication area if the emitter has not been properly designed. In this study, the authors present some successful designs with the aid of an optimisation algorithm to adjust the mounting angle of each constituent LED. In spite of the aforementioned irregularity, the proposed design still exhibits good performance in terms of the ability to withstand high signal attenuation and extended communication area in the vehicle-travelling direction.