Efficient LED for Train Headlights

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 have designed a new LED-based train headlight that uses a tenth of the energy required for head­lights using conven­tional light sources. If operated 8 hours every day, the elec­tricity savings of the new design would reduce emissions of the green­house gas carbon dioxide by about 152 kilograms per year.

The new train headlight design uses two half-circular parabolic, or cup-shaped, aluminized reflectors with high-efficiency LEDs placed in the plane where the two reflectors come together. Combining the strong beams from each reflector generates the light intensity necessary to meet safety guidelines. (Source: W.L. Liang, Nat. Taiwan U.)

Train headlights not only illu­minate the tracks ahead, they also play an important role in rail transpor­tation. Because trains are difficult to stop, the head­lights must be visible from a distance far enough away to give people or vehicles on the tracks ample time to move out of the way. Tradi­tional train head­lights, which use incan­descent or halogen bulbs, are bright enough to meet safety regu­lations but are not very energy efficient because most of the energy powering the light is converted into heat rather than visible light.

Su and his colleagues were approached by the engi­neering and design company Lab H2 Inc., to design loco­motive head­lights that use LEDs as a light source. In addition to requiring less energy, LEDs also last longer and are smaller and more rugged than tradi­tional light sources. “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, who was instru­mental 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 new train headlight design is based on ten precisely posi­tioned high effi­ciency LEDs. The design uses a total of 20.18 Watts to accomplish the same light inten­sity as an incan­descent or halogen lamp that uses several hundred watts. The new head­light can also be dimmed by turning off some of the LEDs to avoid blinding waiting passengers when the train passes a platform, for example. Much like those used for cars, train head­lights typi­cally combine a light source with a para­bolic, or cup-shaped, reflec­tive surface that focuses the emitted light into a beam. Although LEDs are a great option for saving energy, the most energy-effi­cient LEDs emit smaller spots of light. For this reason, the researchers had to combine the small outputs of multiple high-effi­ciency LEDs into a larger circular output to create a beam large enough to use as a train headlight.

“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 researchers’ goal was a headlight that would provide light 1.25 times the bright­ness required by U.S. federal regu­lations. These regu­lations require train head­lights to have a peak inten­sity of at least 200,000 candelas and illu­minate a person at least 800 feet in front of the head­light.

Posi­tioning the LEDs to create a high-effi­ciency train head­light, the researchers used two half-circular para­bolic alu­minized reflectors. When used together, the strong beams from each reflector combine to generate the light inten­­sity necessary to meet federal guide­lines. This design also simpli­­fied placement of the circuits needed to power the LEDs because they could be housed in the hori­­zontal divider separa­ting the reflectors.

To determine where to place the LEDs in the reflectors, the researchers first estimated the best location of each LED and then used a series of tests and simu­lations to fine-tune the final position for each LED based on its corres­ponding illu­mination pattern. “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.”

They point out that headlights typi­cally use a complete para­bolic reflector surface. “We believe this is the first design to use a combi­nation of two semi-para­bolic reflector surfaces,” said Liang. “By systema­tically analyzing the design to determine the best placement of the LEDs in the reflector, we were able to minimize elec­tricity consumption while satisfying require­ments asso­ciated with traffic safety.” The researchers are now working to turn their design into a commercial product. Even though the new design exhibits low power consump­tion, it still generates some waste heat. Before the design can be commer­cialized the researchers will need to develop and test a heat dissi­pation system for the new headlight. (Source: OSA)

Reference: W.-L. Liang and G.-D. J. Su: Design of a high-efficiency train headlamp with low power consumption using dual half-parabolic aluminized reflectors, Appl. Opt. 57, 1305 (2018): DOI: 10.1364/AO.57.001305

Link: Graduate Inst. of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan

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