# Prototyping With Light PIpes

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## Introduction: Prototyping With Light PIpes

The advent of 3D-printable transparent materials has opened doors to new possibilities with rapid prototyping for optical applications. Now, we can consider how to use and manipulate light in 3D printed parts.

Light pipes, sometimes referred to as light guides or light tubes, are physical components used to purposefully redirect or transport light from a source to a desired location. Applications of this concept range from the redirection of natural light into homes and physical structures without direct access to sunlight to providing remote LED illumination in electronic assemblies.

In this guide, we’ll consider a practical application of 3D printed optics: the LED light pipe. We’ll look at a common application of light pipes in consumer products, which is a “mini” light pipe used to channel the light from a standard 5mm LED mounted on a printed circuit board to a user interface.

To investigate the best material to use for 3D printed light pipes, we’re going to print light pipes using 3 different material samples:

1. Transparent material, unfinished.
2. Transparent material surface finished with a 2-part polyurethane clear coat
3. Acura 60

First, let’s go over some important considerations around optics and design.

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## Step 1: Optics

For an angle of incidence greater than the critical angle, total internal reflection occurs within the glass as the light would reflect against the internal surface of the medium.

As total internal reflection occurs along the trajectory of light through a light pipe, light can be moved from the source of photons to a desired target, where it is finally refracted out and used in your application.

Note that total internal reflection occurs when the light travels from a medium with a higher optical density to a medium with a respectively lower optical density.

In our example, we’ll demonstrate 3D printed plastic light pipes in air. Since the plastic we’re using in our light pipes has a higher optical density than air, we can expect some total internal reflection to occur and for our light pipes to work!

## Step 2: Design Considerations

When designing a system that utilizes light pipes, your goal typically will be to use light provided by your source — in this case, an LED — as efficiently as possible. In other words, you want to minimize the light loss that will occur between your source and the viewer.

To do this, first make sure you’re capturing as much of the source light as possible in your application. One method for accomplishing this task with minimal cost is to enclose your source in the light pipe structure so that as much of the light leaving the source as possible is ‘captured’ by the light pipe.

A stronger but perhaps less practical method would be to focus the source-emitted light into the light pipe structure using a lens.

Next, you’ll want to make sure that the “pipe” section of your light pipe is as reflective as possible. This is because you want to maximize the total internal reflection of the light along its path, allowing as little light loss through the medium as possible along its trajectory.

Naturally, you’ll want to deliver as much light as possible into the air at the end of the light pipe. To do this, you want to minimize the amount of internally reflected light at the tip of the light pipe.

This can be achieved by selectively abrasing the exit surface. This will create a multitude of randomly oriented critical angles relative to the internally reflected rays, increasing the amount of light that will “escape” diffusely from the light pipe.

Think of the exit as a ‘selective leak’ of light, where you actually want to minimize internal reflection.

## Step 3: Materials: Transparent - Unfinished

The first material sample we tested was the transparent unfinished material light pipe.

The transparent material printed a bit cloudy, causing less internal reflection than the other two samples. Though cloudier than the other two samples, this sample still provided a bright, even glow when lit with our red LED.

## Step 4: Materials: Transparent - Finished

The second material sample tested consisted of the same transparent material, but finished with a two-part polyurethane coating.

The coating caused a more even diffusion pattern, providing an even softer illumination effect than observed in the unfinished component. This is because the finish creates an optical boundary that causes light to diffuse more evenly across the coated surface as it exits the light pipe.

## Step 5: Materials: Acura 60

The final sample tested in the series was the Acura 60 material, which printed quite clearly.

As a result, when lit with our LED the Acura 60 light pipe provided a more specular glow, creating a halo-like caustic pattern as the light from the light pipe reflected off of nearby surfaces. This unique effect was not observed in the other two transparent material samples.

## Step 6: Summary of Results

We observed unique results for all three samples. The greatest differentiation in results was observed between the unfinished transparent and the Acura 60 samples. The finished transparent sample provided final results somewhat in-between both of the Acura 60 and the unfinished Transparent material.

As previously mentioned, the total internal reflection phenomenon is dependent upon the internal surfaces of a light pipe being smooth and non-diffusing so that internal incident light from the LED is fully reflected. In fact, the diffusion of light is precisely what allows it to exit the light pipe, as incident rays refract at random angles less than the critical angle.

Since our 3D printed parts are created in layers, the internal surfaces of our light pipes are not quite as smooth and crystalline as they would be if manufactured in or by casting urethane parts, which might be the case for final production parts.

Based on our results, we suggest using the transparent material with a polyurethane-finished surface for creating light pipes in your prototypes. Thought the result wasn’t as spectacular as the Acura 60, this is a much more cost effective way to prototype light pipes as the Acura 60 material is quite expensive.

If you really need a sharper, more specular appearance and are willing to spend the extra cash, go with Acura 60.

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