ECEN 4616/5616 Project Report Student “#4” 12/09/2010
Introduction
While the quality of human life has been greatly improved since the invention of the first incandescent bulbs in thelate 19th century, light-emitting diode (LED) starts to take part in lighting human’s life until the late 90s.Before that, due to the lack of blue LED, LEDs were only used as indicators in various electronic products. Until 1993, Shuji Nakamura of Nichia Corporation demonstrated the first high-brightness blue LED. Thisenabled the production of white-light LEDs, and thus created a new direction for lighting techniques.
The most important advantages of LED over traditionalincandescent lamps are its relative high energy-converting efficiency and the stability due to its solid nature. About 90--95% of the electrical energy supplied to incandescent lamps emits as the form of heat; only a very small portion of the energy is converted into light. On the contrast, LEDs typically have about 30 % of energy conversion efficiency, and this efficiency is still being enhanced by advancing epitaxial, processing, and packaging technologies. As the brightness of LEDsgoes up and up, and the prices inversely goes lower and lower, LED becomes a promising candidate for various lighting applications. This project is mainly focus on applying white-light LED on the design of automotive headlamps.
As headlamps for cars, LEDs suffer from the heat problems. The heat generated after long-period operation would damage the contacts of the LEDs, and thus the causing the failure after long-term continuous operation. In this design, a compact “LED bulb” module containing a reflective mirror and a projection lens is developed. The module facilitates the exchange of failed LED bulbs, and can project the light to a distance of 150 meters. Finally, due to its compact size, a bunch of LED bulbs can be put together and produce enough amount of light for automotive headlamp application.
Components
White-light LED from Philips Lumileds, LUXEON®Rebel series.
The dimensions, spectrum, and spatial radiation pattern are shown in Fig. 1.
Figure 1. (a) Neutral-white color spectrum. (b) Typical representative spatial radiationPattern for neutral white Lambertian. (c) Package outline drawing.
Projection lens: PMMA
The bi-convex and positive meniscus lenses are tested using ZEMAX.
The optical properties are shown in Table 1. (Optical glass is also listed for comparison)
Index (nd) / Abbe # (vd) / Density (g/cm3) / λ (μm) / TransmittancePMMA / 1.49 / 57.44 / 1.16 / 0.365—1.06 / >90
glass / 1.44—1.95 / 20--90 / 2.3—6.2 / 0.370—1.5 / 85—95
Table 1.The comparison of PMMA and optical glass.
Method
In order to make use of all the light generated from LED, letting LED face toward the image plane is not a good choice. It is impractical to use a lens with huge diameter to focus the light due to the wide emission angle (170°for 90 % of the intensity). Therefore I decided to use a mirror to collect all the LED light. Parabolic lens seemed at first a good choice, because it produces light in a collimated way. However, a negative lens will be needed to expand the light into a wide angle for headlamp application, and it is not possible to create real image in front of a negative lens with collimated source. So I decided to use an elliptical mirror, which will perfectly image an object at one focus to its other focus. The Zemax layout of the elliptical mirror is shown in Fig. 2.
Figure 2.Elliptical mirror after optimization.
The purpose is to generate a real image of the light spot produced by elliptical mirror as shown above. This elliptical mirror generated image, which is the “object” for the following projection lens, is to be magnified to a diameter at least 5 m at a distance of 150 m.ThusI picked initial parameters ( l = 10 mm, l’ = 150,000 mm, M = l’/l = 15,000, u = 30°), and setting the edge thickness of the projection lens to be -2 mm. The layout is shown in Figure 3. Then I tried to optimize and set the PMAG as the operand to define merit function. However the result is really bad. The rays converge very fast after going through the lens, and resulting a very wide illumination in the image plane.
Figure 3.The Layout of the designed system. As can be seen the spherical aberration
is very large, resulting a non-uniform illumination in the boundary. The diameter of the illumination is about 16 meters.
To minimize the spherical aberration, I tried to use a negative surface. I also set the conic values of both surfaces of the lens to be variable, so that ZEMAX can create aspherical surfaces to eliminate spherical aberration. In this step, the same operand for merit function, PMAG = 1333 was used again to optimize, and the results are shown in Figure 4.
Figure 3.Layout of the system with positive meniscus lens.The spherical aberration is largely eliminated, resulting a uniform, almost collimated rays. However, in the illumination diagram, the diameter of the illuminated region is only about300mm, meaning a very concentrated spot in the center. This is an undesirable property for illumination application.
Conclusion
The objective of this project is to design a compact optical system for LED projector headlamp. The system consists of an elliptical mirror, which reflects all the rays generated by LED to its other focus, and a projection lens. When the projection lens is biconvex, the output illumination has serious spherical aberrationin a distance of 150 m from the lens. However, this configuration produces a quasi-uniform illumination, except that the intensity in center and boundary is stronger. While using meniscus projection lens with aspherical surfaces can eliminate spherical aberration, the illumination becomes extreme non-uniform. Thus spherical lens is a desirable property for this lighting design.