ARL-CR-0781 ● SEP 2015

US Army Research Laboratory

A Process for the Development of Rapid 

Prototype Light Pipes

prepared by Barry J Kline

TKC Global Solutions LLC, Suite 400 North

13873 Park Center Road, Herndon, VA 

under contract W911QX-14-C-0016

Approved for public release; distribution is unlimited.
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Army position unless so designated by other authorized documents.

Citation of manufacturer’s or trade names does not constitute an official 

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Destroy this report when it is no longer needed. Do not return it to the originator.
ARL-CR-0781 ● SEP 2015

US Army Research Laboratory

A Process for the Development of Rapid 

Prototype Light Pipes

prepared by Barry J Kline

TKC Global Solutions LLC, Suite 400 North

13873 Park Center Road, Herndon, VA 

under contract W911QX-14-C-0016

Approved for public release; distribution is unlimited.

FOR OFFICIAL USE ONLY (delete if not FOUO)
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September 2015

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Contractor

3. DATES COVERED (From - To)

1–30 June 2015

4. TITLE AND SUBTITLE

A Process for the Development of Rapid Prototype Light Pipes

5a. CONTRACT NUMBER

W911QX-14-C-0016

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Barry J Kline

5d. PROJECT NUMBER

350.14100.000.001

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

TKC Global

Suite 400 North

13873 Park Center Rd

Herndon, VA 20171

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US Army Research Laboratory

ATTN: RDRL-WML-F

Aberdeen Proving Ground, MD 5066

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ARL

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ARL-CR-0781

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Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Researchers at the US Army Research Laboratory’s Guidance Technology Branch have demonstrated the ability to rapidly 

fabricate uniquely configured optical light pipes using stereolithography and postprocessing with an airbrush. This technique 

represents an improved option for transmitting optical status indicators from electronics inside a smart projectile to a location 

on the exterior of the projectile body.

15. SUBJECT TERMS

light pipe, rapid prototype, 3-D systems viper, objet connex 500

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OF ABSTRACT

UU

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OF PAGES

20

19a. NAME OF RESPONSIBLE PERSON

Barry Kline

a. REPORT

Unclassified

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Unclassified

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410-306-1203

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iii

Contents

List of Figures iv

Preface v

1. Background 1

2. Technical Approach 4

3. Results 7

4. Future Work 8

5. References 9

Appendix. Light Pipe Schematic 11

List of Symbols, Abbreviations, and Acronyms 13

Distribution List 14
iv

List of Figures

Fig. 1 Laser TIR in poly(methyl methacrylate)................................................2

Fig. 2 Snell’s law = ( nnnn ∗ ss ss = nnnn ∗ ss ssss)............................................3

Fig. 3 CAD model of 5-LED light pipe array ..................................................4

Fig. 4 Light pipe 3-D printed using WaterSheed XC 11122 resin...................5

Fig. 5 Light pipe 3-D printed using Veroclear (note cloudy surface)..............6

Fig. 6 Light pipe after airbrushing ...................................................................7

Fig. 7 Light pipe in action ................................................................................7
v

Preface

The research reported in this document was performed in connection with 

contract/instrument W911QX-14-C-0016 with the US Army Research Laboratory. 

The views and conclusions contained in this document are those of the authors and 

should not be interpreted as presenting the official policies or position, either 

expressed or implied, of the US Army Research Laboratory or the US Government 

unless so designated by other authorized documents. Citation of manufacturer's or 

trade names does not constitute an official endorsement or approval of the use 

thereof. The US Government is authorized to reproduce and distribute reprints for 

Government purposes notwithstanding any copyright notation hereon.
vi

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1

1. Background

The mission of the US Army Research Laboratory’s (ARL’s) Guidance 

Technology Branch (GTB) includes development and implementation of diagnostic 

electronics and guidance systems for smart projectiles. These systems are typically 

installed within a confined internal volume of a projectile and are g-hardened to 

survive the acceleration loads associated with gun launch. The electronic 

components of these systems often require externally visible status indicators so 

that test personnel can efficiently determine the current operating status of the 

system. Example status messages include power on/off, GPS receiver power, 

onboard data recorder status, battery health, etc. Previously developed systems used 

light-emitting diodes (LEDs) installed into a hole drilled in the external shell of the 

projectile body and connected to internal electronics by wires. This method limits 

the number of messages that can be displayed for a given form factor and results in 

components that are cumbersome to fabricate and install. 

Light pipes, a subset of fiber optics, are capable of transmitting visible light emitted 

by an LED from the mounting location of the LED on a printed circuit board (PCB) 

to the exterior of a housing. Light pipes allow the use of robust PCB-mounted LEDs 

even if the entire circuit board is enclosed. This configuration is particularly 

advantageous for electronics systems developed by GTB because smart munitions 

typically require that electronic components be housed within an internal volume 

located along the centerline of the projectile relatively far from where indicators 

need to be viewed. 

There are 2 primary styles of light pipes, those that consist of a mirrored tube and 

those that utilize the optical phenomenon of total internal reflection (TIR). Most 

light pipes used commercially are of the TIR variety. The TIR phenomenon is a 

complete reflection of a ray of light passing through an incidence medium such as 

water or glass by the surrounding surfaces back into the medium (Fig. 1).

1 TIR 

effectively traps the light within the medium it is traveling through until it strikes a 

surface with the correct geometry to allow the light to escape. In contrast, the 

mirrored tube light pipe style uses a reflective outer coating to contain the light 

within the pipe. 
2

Fig. 1 Laser TIR in poly(methyl methacrylate)2

The theoretical performance of TIR light pipes with respect to light transmission 

efficiency can be scientifically analyzed using Snell’s law and the principle of 

Fresnel loss.1 Snell’s law describes the governing relationship of optical 

interactions occurring at the interface between 2 optical mediums. This law is 

defined by the equation

( nnii ∗ ssss ffii = nnrr ∗ ssss ffff). (1)

Snell’s law relates the angle of refraction ff of a light ray approaching an interface 

at incidence angle fi, to the index of refraction properties ni and nr of the 2 mediums 

as depicted in Fig. 2. Solving Snell’s law for a refraction angle equal to 90°

identifies the critical incidence angle required to create the TIR phenomenon. 

Angles of incidence greater than this critical angle will create light that becomes 

trapped inside the initial medium, TIR. It is apparent from Fig. 2 that if the angle 

of refraction ff is increased beyond 90°, the light ray will remain within the 

incidence medium resulting in TIR of the transmitted light. 
3

Fig. 2 Snell’s law = ( nn ∗ ss = nn ∗ ss )

1

Fresnel loss quantifies (percent) the optical intensity loss at an interface between 2 

dissimilar mediums, such as glass and air. The equation for calculation Fresnel loss 

is

100 ∗ �

nnii−nnff

nnii+nnff

�

2

, (2)

where the variables are the same as those previously used to explore Snell’s law. 

The main design implication of Fresnel loss is that any time a ray of light goes from 

one medium to the next, there is a loss of intensity. Reducing the total number of 

instances in which a light ray passes between mediums generally improves the total 

transition performance of an optical system. 

The mirrored light pipe or light tube3 is a less common style of light pipe but has 

advantages over the TIR style for some applications. A mirrored light pipe consists

of a clear polymer, glass, or air medium for the light to travel through, surrounded 

by a reflective surface that contains the light inside the pipe. This form of light pipe 

has lower transition efficiency than TIR light due to scattering of light rays inside 

the pipe as they reflect off the outer surfaces. However, mirrored light pipes are 

significantly simpler to manufacture because light transmission can be 
4

achieved without having to meet the optical constraints for TIR as specified by 

Snell’s Law. Devices of this type are often used to transmit sunlight to the inside 

of buildings that do not have direct access to exterior windows. 

Commercial-off-the-shelf (COTS) light pipes of both types are available in limited 

geometric configurations and for a limited subset of LED packages. Because of the 

custom nature of precision munitions developed by GTB as well as the limitations 

in physical space geometry, the available COTS components are inadequate. To fill 

this gap, GTB has developed a process for creating custom-designed light pipes.

2. Technical Approach

ARL chose to pursue the design of a mirrored light pipe commonly known as a 

light tube. The main reason for this choice is that TIR light pipes must be 

surrounded by an optical medium of lower index of refraction then the incidence 

material of the light pipe, such as air surrounding glass. In general, smart projectile 

electronics enclosures are filled with potting material to support the components 

for g-hardening to ensure survivability of the electronics during gun launch. This 

potting process removes all of the air within the electronics enclosure and precludes 

the TIR phenomenon required for a light pipe of that style to function.

The light pipe prototyped by GTB was designed using computer-assisted design 

(CAD) to model of a component designed to fit directly over a PCB-mounted LED 

array (Fig. 3). A schematic drawing of the light pipe is shown in the Appendix. The 

modeled component consists of cavities to tightly enclose the board-mounted LEDs 

and rods to transmit the emitted light to the external display panel. The material 

thickness between the top of the cavities and the bottom of the transmission rods 

was minimized to reduce cross talk between the different LEDs.

Fig. 3 CAD model of 5-LED light pipe array
5

The data sheets for several material options were evaluated based on optical clarity. 

Two candidate materials were specified by the manufacturer to be extremely clear 

and thus suitable for light pipe fabrication. The first production option was Somos 

WaterSheed XC 11122 resin,4 with an index of refraction of 1.514, printed using a 

3D Systems Viper SLA machine.5 The second option was Veroclear resin6

synthesized using an Objet Connex 5007 machine. An exact index of refraction 

value for Veroclear is not given on the data sheet; however, literature on the 

manufacturer’s website suggests that the material is clear enough to produce lenses 

for eye glasses. The light pipe CAD model was 3-dimensionally (3-D) printed out 

of these 2 different materials to determine the best material choice for the light pipe 

in a practical application. Evaluation of the prototypes resulted in the selection of 

components printed using Somos WaterSheed XC 11122 resin as the best available 

option (Fig. 4). This combination outperformed parts produced using the Veroclear 

resin in terms of optical clarity and surface finish (Fig. 5). The main performance 

disadvantage of the Veroclear parts is that the parts produced by the Objet machine 

have a waxy outer layer that scatters light entering the part and creates an adhesion 

problem during application of a reflective outer finish.

Fig. 4 Light pipe 3-D printed using WaterSheed XC 11122 resin
6

Fig. 5 Light pipe 3-D printed using Veroclear (note cloudy surface)

Once the component has been 3-D printed, an opaque finish needs to be applied to 

the exterior of the plastic part to reflect transmitted light and contain it within the 

optic. An experiment was conducted with different reflective coatings to evaluate 

the light transmission, the observed cross talk, and opacity. 

Kapton tape masking was applied to the cavities where the LEDs are to be placed 

and to the rod tips where the light is to be viewed. Initially, aerosol-emitted spray 

paints in both white and black were evaluated as finish options. Black paint 

performed poorly due to large proportions of the emitted light being absorbed by 

the paint. The white paint provided significantly better light transmission. 

However, aerosol-emitted paint lacked the precision to fully cover the intricate 

geometry of the component and created areas of uneven paint thickness. This 

inconsistent coating led to portions of the light escaping out the sides of the light 

pipe where paint thickness was lacking, and resulted in inadequate overall transition 

efficiency. 

Application of opaque white paint to the light pipe using an airbrush provided an 

improved surface finish. An Iwata Kustom TH airbrush8 and Wicked Colors 

Wicked Opaque White9 paint were selected. The adjustability of this applicator 

allowed for precise and even application of paint to the details of the part. Several 

thin layers of paint were applied to ensure full coverage without generating areas 

of excessive paint thickness (Fig. 6). Full coverage of all intricate surface features 

of the device led to greatly improved transition efficiency. 
7

Fig. 6 Light pipe after airbrushing

3. Results

The finished product was overlaid on an appropriately sized surface-mounted LED, 

an OSRAM blue surface-mounted diode LED,10 for testing (Fig. 7). The level of 

light transmission through the intended rod is adequate for observation in bright 

lighting conditions, and it is clear which indicator is illuminated. Some cross talk 

between rods is observed and some light leaks through the opaque coating, but this 

loss of emitted light will not affect the functionality of the light pipe for its intended 

use. 

Fig. 7 Light pipe in action
8

4. Future Work

Additional light pipe configurations are being researched, such as curricular arrays 

and more tortuous passages. Optical efficiency and sources of transition loss have 

not been explicitly evaluated and may lead to design improvements. Higher 

performance paint capable of increased reflection to contain the light inside the pipe 

may improve the brightness of the observed indicator light.
9

5. References

1. Light guide techniques for LEDs [accessed 2015 June 25]. www

.avagotech.com/docs/5988-7057EN.

2. Poly(methyl methacrylate) [accessed 2015 June 25]. http://www

polymerprocessing.com/polymers/PMMA.html.

3. Light pipes or tubes [accessed 2015 June 25]. http://www.slideshare.net 

/rizwanbasharat3/light-pipes-or-tubes.

4. Somos WaterSheed XC 11122 resin [accessed 2015 June]. http://www.dsm 

.com/products/somos/en_US/offerings/offerings-somos-water-shed.html.

5. 3D Systems Viper SLA Machine [accessed 2015 June 25].

http:www.google .com/url?sa=t&rct

=j&q=&esrc=s&source=web&cd=1&ved=0CDYQFjAA&url=http%3A%2F

%2Fwww.3dsystems.com%2Fproducts%2Fdatafiles%2Fviper%2Fdatasheets

%2FInternational%2Fviper_si2_uk.qxd.pdf&ei=8TCEVZHEDIOZgwSlj6D4

Cg&usg=AFQjCNGao_UD6caPzjndY7B3Aj8DQ9f-Lw&sig2

=XJYFgTZKcZTLjnbN1tncrA&bvm=bv.96042044,d.eXY.

6. Objet Veroclear [accessed 2015 June 25]. http://www.stratasys.com 

/materials/polyjet/transparent.

7. Objet Connex 500 [accessed 2015 June 25]. http://www.stratasys.com/3d

-printers/design-series/connex-systems.

8. Iwata Kustom TH Airbrush [accessed 2015 June]. http://www.iwata

-medea.com/products/iwata-airbrushes/kustom-series/kustom-th/.

9. Wicked colors, wicked opaque white [accessed 2015 June]. http//www 

.createxcolors.com/products/wickedcolors/wickeddetail.html#prettyPhot.

10. OSRAM Blue SMD LED – Mouser - LB T673-L2P1-25-Z [accessed 2015 

June 25]. http://www.mouser.com/ProductDetail/Osram-Opto

-Semiconductor/LB-T673-L2P1-25-Z/?qs

=%2fha2pyFadujg7AoXfk8n23aJzAqS8jh7wcJsfqlMxxXy87FYV7qT7Q%3

d%3d.
10

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11

Appendix. Light Pipe Schematic
12
13

List of Symbols, Abbreviations, and Acronyms

3-D 3-dimensional

ARL US Army Research Laboratory

CAD computer-assisted design

COTS commercial off the shelf

GTB Guidance Technology Branch

LED light-emitting diode

PCB printed circuit board

TIR total internal reflection
14

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