Light pipes, also referred to as light guides or light tubes, are commonly used in electronic devices to transmit light from LEDs on a circuit board to indicator symbols or buttons. Light is directed through the light pipe by means of internal reflection on the inner surfaces, so a light pipe should be designed to minimize loss of light over the length of the pipe.
Most light pipes are made from optical-grade acrylic resin, with a mirror-like finish on the outside walls to maximize internal reflections. The Form 1’s Clear resin is a clear acrylate, but it is not optical-grade. Without polishing, even the highest resolution stereolithography parts will not match the mirror surface finish of molded parts.
However, you can produce perfectly functional light pipes on the Form 1. These light pipes can be used for functional prototypes, and depending on the part and the quantity required, they may even be a cost-effective production method.
This guide aims to summarize our findings and offer tips and design guidelines for designing your own light pipes on the Form 1.
Two differently-shaped light guides were constructed to test the transmission across two 90° bends; an S-shape and a Z-shape. These are the two main styles of rigid light pipes.
In the S-shape, the light ray reflections follow the smooth contour of the bend without loss, while the Z-shape uses 45° angle prismatic surfaces to bounce light across sharper corners. Of the shapes tested, the Z-shape pipe more effectively transmitted light to the end. The S-shape pipe lost a lot of light by the second bend.
The S-shape pipe was designed following guidelines for injection molding parts: it has a constant wall thickness, t, an inner radius of 0.5t, and an outer radius of 1.5t. The excellent application brief, Light Guide Techniques by Avago Technologies, recommends a minimum-bend radius that is twice the diameter of the light guide (2t).
Follow-up experiments should follow this 2t recommendation to determine whether there any differences in transmission between a prism design and a properly designed curved pipe of equal length. Other research has shown that equiangular spirals provide lossless bends for light pipes, so this shape is another possible curve to use in the design of contoured light pipes. Light pipes can also be made with any cross-sectional shape.
Light Guide Techniques recommends 0.5 mm fillets added to all edges, although the light pipes tested used sharp corners without issue. Advanced light pipe shapes can even include branching, such that one LED can drive multiple “lights”, or conversely, multiple LEDs can drive a single indicator location. Figure 26 in Light Guide Techniques shows an example wherein a single status indicator light can be red, yellow, or green using a single forked lightpipe.
Coating the LED end of the light pipe should try to capture as much of the light emitted from the diode as possible. Testing was done using a 5mm red LED, which fit into a simple cut-revolve in the model. FIG 2 shows the profile of the cut revolve with 0.1mm of radial clearance around the LED, a recommended clearance for press fitting parts on the Form 1. The connection between LED and light pipe was further improved with a drop of clear mineral oil, which allowed the same LED to be used in every light pipe.
Optical-grade epoxy can be used if a rigid bond is needed. Other LED shapes (such as surface-mount technology) require different shape cut-outs in the light pipe to maximize optical transmission (see pages 4-6 of Light Guide Techniques). The “light output end” of the light pipe should have a diffuse exit surface to enhance the probability of light escaping.
Form 1 light pipes work as-is, but all light pipes tested were sanded with 220 grit sandpaper on their output surface, which produced a smooth matte finish and enhanced their function.
Printing Lightpipes with the Form 1
While light pipes are fully functional right out of the printer (after the required wash in IPA), to maximize the transmission due to internal reflection, clear gloss acrylic spray should be used. Two thin coats fill in the fine surface ridges that result from layer thickness or raster fill paths the same way Scotch tape can render frosted glass transparent. The possible downside to these lightpipes is leakage along their lengths (i.e. the lightsaber effect). Glossy white paint is sometimes used to block escaping light and increase internal reflectivity. This experiment used glossy white Krylon fusion spray paint, which reduced the end transmission and still showed a red glow along the length.
When a second coating of grey primer was added on top of the white, this effect worsened. Using a metallic silver spray paint made the light appear darker and reduced end transmission (though much less than the white paint), but it effectively contained the light emission to the output end. The silver paint used was Krylon High Heat & Radiator paint, which is not particularly reflective. A reflective chrome or mirror paint may be an even better choice.
A variety of finished and part geometries were prototyped, and appear below:
1. For tight 90 degree bends, Z-shape (prismatic surfaces) performed better than S-shape (smooth contours). For curved light pipes, minimum bend radius of 2x the wall thickness is recommended.
2. Light-entering end should couple as closely as possible to the LED to maximize flux transmission.
3. Light-emitting end should be sanded to a diffuse surface finish (220 grit).
4. End transmission results from brightest to dimmest; clear acrylic gloss spray, metallic silver, raw (Form 1 wash procedure only), white gloss + gray primer Coating the outside of the pipe with silver paint makes the light "darker" and dimmer but effectively contains the light (eliminating the “lightsaber effect”).
5. Coating glossy pipe with an outer coat of silver shows no advantage over using only metallic silver.
6. All light pipes printed were oriented to avoid support structure touching the LED contact surface and then had supports generated with the default settings. No sanding was done except to the light emitting end. Sanding all surfaces smooth before spraying with acrylic may improve performance, but is not necessary. Ideal printing orientation would place support only on the light emitting surface.