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.

IMG_5158

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.

[caption id="attachment_8537" align="alignnone" width="4000"]IMG_5159 Test fixture #2 illuminated without acrylic test pieces.[/caption]

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.

[caption id="attachment_8544" align="alignnone" width="4000"]IMG_5161 Square LED cavity (left) and circular LED cavity (right)[/caption]

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.

[caption id="attachment_8550" align="alignnone" width="4000"]IMG_5162 Triangular (left) and teardrop (right)[/caption]

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.

[caption id="attachment_8557" align="alignnone" width="4000"]IMG_5163 Flat top cavity (left) and conforming curve cavity (right)[/caption]

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.

LED scatter curve

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.

[caption id="attachment_8565" align="alignnone" width="4000"]IMG_5165 Best illumination to date with the small curve (left) which can be further enhanced by a frosted edge (right)[/caption]

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