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
humans 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.
((c)) ((a)) ((b))
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)
(m)
Transmittance
PMMA
1.49
57.44
1.16
0.3651.06
>90
glass
1.441.95
20--90
2.36.2
0.3701.5
8595
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. 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.