Here’s a behind-the-scenes follow-up to the LED test fixtures of the previous few posts: when we only need a simple circuit for a 3D-printed project, we can meld the two instead of using a formal circuit board. In this context “meld” is meant literally: the parts of the circuit can be heated up with the soldering iron so they melt into the 3D printer plastic.

When I built the dual-LED acrylic illumination test rig, I wanted the simplest circuit possible. It’s not something I need to be durable long-term and I wanted to be up and running with my tests as quickly as possible. The full length of a resistor and its wires are almost long enough to bridge the gap between the two sides of the fixture, so I tried to make that work.

When I started soldering all the wires together, I had planned to just leave everything dangling. But the close proximity of the soldering gun to the 3D printed PLA plastic started softening the plastic and I realized I can use this to my advantage. A few seconds with the soldering iron was all it took to heat up a wire so it can be melted into and embedded into the plastic. The resistors themselves took a little more effort, but I sunk them into the plastic as well. The LEDs had been held in place by their bent legs, which was sufficiently stable but had a tiny bit of wobble. Melting the plastic around LED legs gave us a much more secure placement.

Components melted into the plastic are no longer subject to flexing and eventually breaking from metal fatigue. Add a strip of electrical tape to guard against short circuiting to complete the quick and simple circuit to light up the test rig LEDs.

After running through a few acrylic test pieces looking for the best edge illumination, I decided I need a dual fixture to allow side-by-side comparison as I swap through test pieces.

Another change I made in the text fixture is to remove the aluminum foil at the bottom. While the foil may be useful to direct light, it distracts from the testing. If a particular test piece is losing light to the fixture, I don’t want that light reflected back in. I want to be able to see the failure in the form of illuminated white plastic. When there are no acrylic test pieces in the fixture, the cone of illumination is clearly visible.

The two sides aren’t exactly identical. One of the LED is slightly brighter than the other, and the two sides ended up with slightly different textures. But it should be good enough for our comparison purposes.

The first fixture implied that the cavity surround the LED is where we should focus our attention, so let’s try a few shapes. A square and a circle seems to differ only slightly in the brightness of the center top hot spot.

A triangular cavity was much more interesting – all the light has been diverted from the top center, sending them off to the side. And I tried a teardrop shape just to see what would happen. The important detail to note on the teardrop is that a lot of light was lost to the fixture instead of being sent to the edges. This tells us the cavity edges should be as small as possible to push its surface right up against the LED to reduce light loss.

The cavity sizes were then minimized for the next set, again testing for different shapes. A flat top to the cavity didn’t work as well as the cavity shape conforming to the LED shape.

But the best results came from putting a small curve in front of the point of the LED. This appears to break up the central beam and sends it to the edges like we want.

From an cost/benefit ratio perspective, this small curve is a winner. It is a very minor change to the geometry and yet it delivers significant improvement to the resulting light. When put into a larger sheet of acrylic, with greater number of internal reflections, it should do quite well. And for a little extra smoothness in illumination, we can take a piece of sandpaper and lightly roughen up the surface. Adding a frosted edge reduces the reflections somewhat, but it does help even out the overall illumination.

These experiments have been quite informative. I look forward to applying what was learned here to future acrylic projects.

After the surprising success of LED illumination in FreeNAS v2 enclosure, I wanted to spend some time experimenting with the concept. When searching for “acrylic edge illumination” on Google, everybody seems to be talking about positioning the LED at the edge of the acrylic sheet and lighting up the pattern of something engraved on the acrylic. My goal is the opposite: I want to place the LED in the middle of the acrylic, and I want the light to shine out to the edge of the acrylic sheet.

We start with the assumption that by default, a LED shining inside a piece of acrylic will only illuminate in the direction it is pointed.

Our ideal goal is to determine how to direct this light so it illuminates all the edges of the acrylic sheet, not just the direction of the LED face.

I 3D printed a small test fixture for these experiments. It has space for a 75mm x 75mm test piece of acrylic and a LED that pokes up in the middle of that space. There’s a 10mm wide border around the test piece so I can observe the pattern of illumination beyond the edge. At the outside edge of the border, a wall to observe the intensity of illumination beyond the edge. A piece of black tape covers the direction of the viewer so the LED doesn’t overwhelm the rest of the observation. A piece of aluminum foil lines the bottom of the test fixture to reflect any light back into the acrylic.

The fixture lights up as expected in the absence of any acrylic.

These two experiments tested cutting grooves in the acrylic. One set had straight grooves, a second set curved. They were successful in breaking up the center top hot spot, sending some of that light elsewhere. But the light seems too concentrated on the bottom third.

Instead of cutting grooves, this piece tested cutting entirely through the acrylic. The circular shape does seem to disperse the light fairly well.

These were interesting, but the most surprising result came from a test piece of acrylic with nothing cut in the middle. I had expected the light pattern to resemble the triangular hot spot of the LED by itself without any acrylic, but we got this:

It has the same basic trend of the other light patterns in this set of experiments, which tells us the majority of the light scattering is not done by the curved/straight grooves or the circle. The feature with the largest impact is actually the small cavity surrounding the LED itself.

The fixture has been informative, but it has one problem: it is difficult to make comparisons between different test acrylic pieces. Before proceeding with investigation, the test fixture will be expanded so there are two test pieces side by side for comparison.