Accurate Reector Design for Light Emitting Diodes
By Lee Boon Kheng, Kwong Yin Leong and Gurbir Singh
Design Guide
Abstract
The design of optical elements using the speed and
accuracy of computer-aided programs has helped in
bringing out the right product in the shortest time..
This paper will describe a method of utilizing the optical
simulation software to design a reector for LEDs with
accuracy and speed.
Introduction
This case study is a parabolic reector design with LEDs
for torch light applications. The end goal is to produce
a light beam that is bright and focused, with a narrow
viewing angle and a high intensity value.
The optical design process consists of source modeling,
correlation, optimization, tolerance and verication. Each
of the design steps are vital and interlink to each other,
hence none should be overlooked.
Reector design steps
A typical reector design involves the following steps:
1. Generate the source model (package and LED die)
2. Intensity and radiation pattern correlation between
simulated and actual measurements
3. Optimize the reector design (compound parabolic
concentrator, CPC angle) to achieve the desired
intensity with an acceptable viewing angle
4. Tolerance study of the fabrication part
5. Verication with optical measurement equipments
Generate the source model (Step 1)
The LED light source used here is a 1W Power LED (Avago
part ASMT-MW00). The source model is generated by
taking the following steps:
1. The radiation pattern of the LED light source is
measured or obtained from the datasheet.
2. The LED radiation pattern data is fed into the ray-
tracing program (Figure 1) using the model for
multiple intensity points of radial source in the
simulation software.
3. The geometry of the LED light source is built and the
ray-tracing program is executed.
4. The radiation pattern data from the simulation result
is matched with the measured radiation pattern of the
actual light source (Figure 2).
Figure 1. 1W Power LED package and ray-tracing layout
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-90 -70 -50 -30 -10 10 30 50 70 90
Angle (degree)
Relative Intensity
1W Power
LED Actual
Simulation
Figure 2. Radiation pattern (RPT) overlay
2
Intensity and radiation pattern correlation (Step 2)
This correlation is done to ensure the accuracy of
the primary model (LED source). A known cylindrical
reector, of dimension shown in Figure 3b, is coupled
to the source model generated in Step 1. The simulation
result obtained is correlated to the measured data from
an actual setup shown in Figure 3a.
Figure 3a. Cylindrical reector coupled with ASMT-MW00 LED
8.05 mm
32 mm
29 mm
Figure 3b. Cylindrical reector dimension
The reector shape and material reectivity, including
the surface roughness and scattering parameter, is
chosen so that the radiation pattern and intensity is
closely matched between the actual measured and simu-
lation results. Figure 4 show the simulation layout of the
ASMT-MW00 light source coupled with the known cylin-
drical reector.
Figure 4. Simulation layout
Figure 5. Reector surface properties
Figure 5 shows the reector surface properties assigned
for the LED light source model in the ray-tracing
program. The built-in surface properties and coating
in the software has greatly increased the speed of the
designing process. The simulation result matched with
the actual measurement.
3
Figure 6. Overlap RPT of the simulation and measurement result
Figure 6 shows the overlap RPT of the simulation
and the measurement result of the known reector.
Intensity and RPT correlation is achieved successfully
when the reector surface is set to metal (mirror nish)
with specular reection. With the surface properties
and source model selected, the accuracy and speed of
producing the right design is increased.
Optimize reector design (Step 3)
Next, optimize the reector. The base surface used is a
parabolic shape of the compound parabolic concentrator
(CPC). This is a built-in surface in the simulation software,
and optimization is done by tweaking the CPC collima-
tion angle.
The nal design target for the torchlight is to achieve an
intensity value of about 170cd with a viewing angle of
less than 20°.
Figure 7. Simulation result of the radiation pattern
Due to the dimension constraint and smooth beam
pattern required, the intensity value achieved in the sim-
ulation is 184 cd with a viewing angle of 16°.
Tolerance study (Step 4)
The fabrication part will have a certain level of accuracy.
This has to be consider in the simulation. The critical
parameter is the surface accuracy with a tolerance of +/-
0.1 mm.
0.00
100.00
200.00
-90.00 -70.00 -50.00 -30.00 -10.00 10.00 30.00 50.00 70.00 90.00
degree
Intensity
Surface (nominal)
Surface accuracy (- 0.1mm)
Surface accuracy (+0.1mm)
Figure 8. Radiation pattern overlay for nominal surface and tolerances
The tolerance study shows that with the variation of +/-
0.1mm, the
a) viewing angle is still matched
b) intensity is still within acceptable range (+/- 5%) of the
nominal intensity (i.e. 184cd)
The results also showed that dimensional variation of the
fabricated part due to manufacturing tolerances should
not be an issue.
Intensity (cd) 184
Viewing Angle (degree) 16
Verication (Step 5)
The measuring equipment, Imaging Sphere was used
to gauge the intensity value and viewing angle of the
reector with the LED. A closely matched result was
achieved in Figure 9.
Figure 9. Measurement result of the radiation pattern
Conclusion
The design of reectors using software simulation must
be done correctly at each design steps. Using this process
will help to achieve the design completion in a single
iteration, thereby helping to reduce the time to market.
Intensity (cd) 180
Viewing Angle (degree) 16
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Data subject to change. Copyright © 2007 Avago Technologies Limited. All rights reserved.
AV02-0633EN - September 3, 2007
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