LED Light Combiner Allows High-Intensity Headlight Design
Lauren I. Rugani
High luminosity, low power consumption and longevity are among the characteristics that make LEDs ideal lighting sources for applications such as automobile headlights. However, several hurdles remain to achieving an efficient optical design, including color and luminance variations among the LEDs that are visible in a headlight’s beam pattern. A team of researchers from Universidad Politécnica de Madrid, LPI Europe in Madrid and LPI LLC in Altadena, Calif., has addressed these and other challenges with the simultaneous multiple surface design method.
More than a dozen LEDs per headlight are required to attain a flux comparable to current high-intensity headlights. The team’s design incorporates an element that combines the flux of three 75-lm LEDs to produce a single light source with increased output. The LEDs consist of an InGaN blue chip with a phosphor coating, and each has a flat exit aperture of ~1.2 mm2. A tailored lightguide in contact with the LED surface collects light from each of the diodes and reduces the color and flux variations among them. The optical contact of this combiner to the LED surface restricts light loss and improves efficiency and output.
The simultaneous multiple surface design comprises an attached refractive free-form exit lens and a reflective free-form coated mirror that project the light from the virtual LED source to the far field, creating the vertical intensity gradient of the headlight pattern. The design requires two pairs of wavefronts for input parameters, two optical path lengths between the corresponding pairs and a seed curve on one of the surfaces, which defines the vertical extent of the projected images.
The input wavefronts are spherical surfaces emitted from the corners of the LED combiner exit aperture, while the output wavefronts contain the information of the vertical and horizontal spread and shape of the full beam pattern. Calculations then couple corresponding input and output wavefronts to ensure that the outgoing wavefronts control the exact path of the rays emitted from the edges of the combiner. All the points generated by these four wavefronts eventually define the full optical design.
The researchers created both low-and high-beam designs with 76 and 77.2 percent efficiency, respectively, and corresponding intensities of 52.9 and 115 lux — values that meet legal standards. Among the design advantages is the tolerance between the LEDs and the lightguide, which allows for ±0.2-mm placement error without affecting the emitted light pattern.
Optics Express, Dec. 25, 2006, pp. 113014-113020.
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