Broadcom Accurate Reflector Design for Light Emitting Diodes, Design User guide

Brand: Broadcom

Model: Accurate Reflector Design for Light Emitting Diodes, Design

Type: User guide

Accurate Reector 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 reector for LEDs with

accuracy and speed.

Introduction

This case study is a parabolic reector 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 verication. Each

of the design steps are vital and interlink to each other,

hence none should be overlooked.

Reector design steps

A typical reector 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 reector design (compound parabolic

concentrator, CPC angle) to achieve the desired

intensity with an acceptable viewing angle

4. Tolerance study of the fabrication part

5. Verication 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

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-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

Intensity and radiation pattern correlation (Step 2)

This correlation is done to ensure the accuracy of

the primary model (LED source). A known cylindrical

reector, 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 reector coupled with ASMT-MW00 LED

8.05 mm

32 mm

29 mm

Figure 3b. Cylindrical reector dimension

The reector shape and material reectivity, 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 reector.

Figure 4. Simulation layout

Figure 5. Reector surface properties

Figure 5 shows the reector 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.

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 reector.

Intensity and RPT correlation is achieved successfully

when the reector surface is set to metal (mirror nish)

with specular reection. With the surface properties

and source model selected, the accuracy and speed of

producing the right design is increased.

Optimize reector design (Step 3)

Next, optimize the reector. 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

Verication (Step 5)

The measuring equipment, Imaging Sphere was used

to gauge the intensity value and viewing angle of the

reector with the LED. A closely matched result was

achieved in Figure 9.

Figure 9. Measurement result of the radiation pattern

Conclusion

The design of reectors 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

For product information and a complete list of distributors, please go to our web site: www.avagotech.com

Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies, Limited in the United States and other countries.

AV02-0633EN - September 3, 2007

Broadcom Accurate Reflector Design for Light Emitting Diodes, Design User guide

Broadcom Accurate Reflector Design for Light Emitting Diodes, Design User guide

Broadcom Accurate Reflector Design for Light Emitting Diodes, Design User guide

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