LED-based Train Headlight Devised With Two Parabolic Reflectors

Researchers from the National Taiwan University have devised a new LED-based train headlight that consumes about one-tenth of the energy needed for headlights with conventional light sources. In addition to using less energy, LEDs last longer and are more compact and more rugged than traditional light sources.

Trains are difficult to stop. Therefore, train headlights must be visible from far enough away to give people or vehicles on the tracks sufficient time to move out of the way. While conventional train headlights, which employ incandescent or halogen bulbs, are bright enough, they are not very energy efficient. In fact, with traditional train headlights, most of the energy powering the light converts into heat rather than visible light.

National Taiwan University researchers develop train headlight prototype with two parabolic reflectors.

National Taiwan University researchers develop train headlight prototype with two parabolic reflectors.

Engineering and design company Lab H2 Inc. approached a team of researchers led by Guo-Dung J. Su from the Micro Optics Device Laboratory of the Graduate Institute of Photonics and Optoelectronics at National Taiwan University, Taiwan, to design locomotive headlights that featuring LEDs as a light source.

“Some LED headlight products sold on the market are designed with many LEDs that have outputs that overlap in large sections. These designs waste a lot of energy,” said Wei-Lun Liang of the Micro Optics Device Laboratory, who was helpful in designing the new train headlight. “Our research showed that electricity use can be reduced by focusing on the best way to distribute the LED energy equally.”

The design combines multiple, high-efficiency LEDs to produce the needed intensity. Two half-circular parabolic reflectors combine to form a cup-shaped aluminized reflector that directs the light from the high-efficiency LEDs positioned in the plane where the two reflectors are joined.

The design consumes a total of 20.18 Watts to accomplish the same light intensity as an incandescent or halogen lamp powered with several hundred watts. Some of the LEDs can be turned off to dim the headlight to avoid blinding waiting passengers as the train passes a platform, for example.

“Combining several LEDs is more expensive and consumes more electricity than using a few single LEDs,” said Liang. “Thus, we needed to determine how to best position the lowest possible number of high-efficiency LEDs needed to meet the requirements by analyzing how the parabolic surface reflected the LED lights.”

The goal was to create a headlight that would deliver light with 1.25 times the brightness required by U.S. federal regulations that require a peak intensity of at least 200,000 candelas to illuminate a person at least 800 feet away.

Emission Overlap had to be Minimized for Train Headlight

To maximize the efficiency of the output, the designers needed to minimize the overlap of the light emission from the various LEDs. They first used estimation, then tests and simulations to fine-tune the positions of the LEDs based on the corresponding illumination pattern. In addition to looking at the light output, the LEDs needed to be placed far enough from each other for heat dissipation and the prevention of circuit damage.

“Other scientists can use the linear equation we derived for deciding the approximate positions of LEDs for other applications,” said Liang. “This can substantially shorten the time required to determine LED positioning before fine-tuning the positions.”

“We believe this is the first design to use a combination of two semi-parabolic reflector surfaces,” said Liang.

Despite the design’s low power consumption, it still generates some waste heat. To commercialize the new design, the researchers will have to develop and test a heat dissipation system. The researchers published a paper detailing the design and the process that they used to create it in the journal Applied Optics.

Reference

W.-L. Liang, G.-D. J. Su, “Design of a high-efficiency train headlamp with low power consumption using dual half-parabolic aluminized reflectors,” Applied Optics, Volume 57, Issue 6, 1305-1314 (2018). DOI: 10.1364/AO.57.001305

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