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 the late 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. This enabled the production of

white-light LEDs, and thus created a new direction for lighting techniques.

The most important advantages of LED over traditional incandescent 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 LEDs goes 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 radiation

Pattern 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 >90glass 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

(a) (b)

(c)

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. Thus I 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 about 300 mm, 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 aberration in 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.