Portal Test Chamber Sign and Light Guide Panel Generator

Heads-up: for a less personal intro to this subject, check out the Hackaday post.

Have you ever wondered how laptops and flatscreen TVs illuminate themselves? Well, wonder no longer! I too was enchanted by how delightfully compact these illuminated setups can be, and I wondered if I could somehow recreate their backlight source from first principle with “makerspace”-friendly tools.

Backlit Backstory

This project has a bit of a backstory behind it. Back in 2016, I snagged myself a Silhouette Cameo 2 vinyl cutter, a purchase that augmented my sticker collection overnight. Vinyl cutters are fantastically versatile tools that will readily churn out not just stickers, but “hand-written” pen-drawings and gaskets too. I’ve even loaded it with a ball-point pen to “pre-crease” the folding pattern for some more complicated origami pieces.

Armed with my new sticker-making powers, I found myself hooked on the idea of making a “half-scale” portal sign as a bedroom prop. I constructed a frame from 2020 aluminum extrusion. For the front and back, I had a piece of 1/8-in. frosted white acrylic sheet and a piece of 1/16-in black ABS sheet cut to size at my local Tap Plastics shop. For aesthetics, I cut 1-inch aluminum L-channel to form a border around the front. This frame also held the acrylic sheet in place. Finally, for illumination, I taped aluminum foil to the back interior and tucked an LED strip along the interior.

The result certainly had the right feel, but the poor backlighting left a lot to be desired. Then I wondered: my laptop has no problem with this. How does industry do it–and do it at scale? I tabled this idle curiosity, got distracted, built a laser cutter, started a PhD, and left with a masters.

Light Guide Research and Rough Attempts

Six years later, the thought came back–and this time I had the chance to play with it. Since the original 48-in. sign has been a huge ordeal to lug around, traveling from California to Washington in my car, I decided I’d just make a ~14in. version this time.

With a bit of research, I found that LCD displays are generally edge-lit. The light travels through a thin clear sheet sandwiched under the LCD layer. This sheet is bespeckled with a dot pattern that lets light escaped in a controlled fashion, transforming an edge-light source into a creamy, evenly-light source from the front. (For more details on the theory behind this, see the Hackaday Post.)

To mimic this effect, I tried laser-etching a grid pattern into a piece of clear acrylic with my laser cutter. After adding another acrylic sheet on top to act as a diffuser and covering the back with a reflective one-way window tint film, I stumbled on a decent starting point.

Admittedly, the first try yielded horrific light intensity drop-off in the middle. But it kicked me down a path towards writing a Python script to generate a custom diffusion layer.

In short, I cobbled together a script to produce a density pattern that followed a second order B-spline. Inside the jupyter notebook, you could play with some slider bars to change the density function.

When you were satisfied with the preview, the script would output a 2D panel of custom dimensions with the pattern included. At that point, you simply upload to the laser cutter and etch!


Admittedly, I picked the lazy-way out, which was calibration “by eye” since this project was just a prop. I told myself that if I didn’t see noticeable improvements after 3-or-4 test pieces, I would need another strategy, but calibration “by eye” worked out quite well! In the future, I could probably take images with my camera on fixed settings and try to extract the resulting light intensity along the diffusion path, but that’s a bit more work than I need.

After about 6 tries cutting small pieces, I settled on some convincing settings and I cut out the entire back plate. Note, laser etching is slow, so the cutting process took about 30 minutes. Nessie, faithful as ever, was a huge help here though.

Here’s a thin cross-section of the final plate with both sides illuminated.

Portal Sign Mark 2

For the slimmed-down mini version, I came up with a very different construction recipe. The edge was instead made from 1/2-in. u-channel from Online Metals. This construction let me tuck two light bars into each of the long edges. To make the sign features I simply printed them out onto an overhead transparency with an inkjet printer.

The resulting sign was about half as thick as the original.

Along the way, I sourced a few key ingredients from Aliexpress. Not only did I get two “backlight” bars here, I also added a layer of backlight diffuser film (LGT075J) under the white acrylic sheet. Aliexpress also provided me with some black Kapton tape to attach the backlight bars to the edge of the light guide panel without adding any unwanted color.

Slimmed Down Light Box Sandwich Recipe

The final stackup is as follows.

  • overhead transparency with the portal sign pattern printed onto it.
  • 1/8-in frosted white acrylic sheet
  • LGT075J backlight diffuser film
  • light guide panel with custom pattern etched on it and two backlight bars taped on both long sides with black Kapton tape
  • 1mm aluminum backing plate, made to order from sendcutsend.com
  • 1/2-in. aluminum u-channel, cut to size to form a frame around the border.

I’m thrilled with how this turned out. In fact, I’ve been tempted to poke my city’s nerdiest coffee shops and cafes to see if they’re interested in adding some decoration to their space. (Hmm, with 18 signs left, this could make for a great scavenger hunt.)

A Light Snack?

Living in Seattle these days means coming to terms with 4-5 months of gloom. From December through February, the Pacific Northwest is plunged into darkness with 4:30 sunsets and cloud coverage that just won’t budge. This time of year feels “too dark,” like something in the back of my head is starving for more light. Having spent all my prior life in California, I never imagined having an empty “light bucket.” But I certainly do; it was simply always topped off back then.

This project was an after-hours obsession through some of these dark months. When I finally had something convincing, I recall feeling this joyful brain rush when I lit up the panel for the first time. Part of me wonders: would I have been so obsessive over the details had I not been so light hungry?