Acrylic Tea Light Holder (that Is Also a Data Visualization)

I wanted to brighten these dark days by creating something with light. So why not make an acrylic tea light holder? ‘Acrylic’ in the sense that it is made from Plexiglas®, a type of plastic that looks like glass but is much lighter (in weight) and can be cut with a laser cutter.

Oh and the design is 'data-driven', which will be explained in the next step...

Step 1: The Design

Since in my day job I am a data scientist, I wanted the design to be 'data driven', i.e. the shape of the tea light holder should be determined by data. More precisely: weather data. The tea light holder should reflect the hours of sunshine and daylight of every day in the past year.

The idea is to make a stack of discs, 12 in total, one for each month. Each disc as a hole in it (so it is actually a ring), to make space for the tea light, the flame and the hot air. Each ring has as many teeth (is that the correct word?) as there are days in the month it represents. The teeth are arranged clockwise, starting with the first day of the month and ending with the last day of the month. The length of each tooth depends on the number of daylight hours (i.e. sunset - sunrise). The inside of the ring is not regular: each tooth is made 'hollow', the hollowness depending on the number of hours of sunshine during that day. The image above will hopefully make it clear.

The idea is that by making a tooth hollow (representing a sunny day), there will pass more light through the tooth creating a bright dot in the tea light holder. Less sunny days will be represented by darker teeth.

Step 2: Getting Data

Data is everywhere and it becomes more and more easy to get it digitally, thanks to all kinds of 'open data' initiatives. For my design I needed weather data which has been made available for my country by an organisation called 'KNMI', which is a meteorological non-profit institute (comparable with the NOAA's weather service in the US).

I downloaded a file with, among others, the number of hours of sunshine for each day in 2014. In addition, I got a file with the exact sunset and sunrise times, again for each day in 2014. It required a little programming to read the files, calculate the number of hours of daylight per day, and combine the two.

Gathering data, combining different sources (in different formats), and getting it in the format you want can be a real nuisance and taking up almost half of a project's time, but in this case it was not too complicated.

Step 3: Getting the Material

The material I use is 'Polymethyl methacrylate (PMMA)', also known as acrylic glass, or under one of its trademarked names Plexiglas or Perspex. It is a transparent type of plastic, that can be cut by a laser. It is available in plates of different thickness, typically ranging from 1 mm up to 20 mm.

I use 5 mm translucent plexiglass, with a 30% light throughput. (They also sell the more transparent version with 70% light throughput, or the completely transparent type). I hope to get the effect that when it was a sunny day, the corresponding 'tooth' in the tea light holder will be brighter than the ones representing cloudy days.

I was told there are two types of plexiglass: cast and extruded. The former is more expensive but has less 'built in stress' making it more suitable for machine treatment. I choose the latter one (owing it to the Dutch culture of always picking the cheapest option ;-) ). I had no problems laser cutting it, but I noticed that it breaks easily which became a problem for some of the very thin teeth on some rings.

Step 4: Programming

I built a program in Processing, a programming language based on Java, that makes building visible stuff easy.

The program reads the data and generates a ring for each month. Processing allows for building 3D models. The library Proscene makes it easy to navigate around the 3D models (zooming, panning, rotating, etc.). The second library I used was Processing's DXF export library. This allows to export any drawing made with Processing in the DXF format, which is one of the formats the laser cutter can handle.

One of the images above shows a little sample program that draws a rectangle and exports it to DXF format when the user presses the 'r' key. I made 12 of those DXF files, one for each ring. These are going to be imported in the laser cutter software in the next step.

Step 5: Laser Cutting

I do not own a laser cutter unfortunately. They are still too expensive (for my budget), too big (for my small apartment), and they need proper ventilation equipment. Fortunately there are places where you can rent these machines. I went to iFabrica, which is based in Amsterdam, but I am sure that there are similar places all over the world.

The laser cutter comes with an application (LaserGrav) in which you can draw something and have the laser cutter cut it out, or engrave it. The application also allows for importing files created by other applications (typically vector formats like DXF and CDR).

The images above show how the simple rectangle generated in Processing is imported in the LaserGrav software. Note that the size of the original drawing in pixels, becomes the size in millimeters on the laser cutter. This is very convenient since you can generate drawings in the exact size that you want, without complicated transformations. When you want the final result to have a certain size in millimeters, give the drawing in Processing the same size in pixels and everything will work out fine.

You need to tweak a little with the laser cutting's settings: setting the speed of the laser (in mm per second) and its power (i.e. how forceful its beam should be). This all depends on the material used. When using thin plywood you can use higher speeds and lower power then when using plexiglass or other 'tougher' material. It's just a question of trial and error: you draw a few simple shapes (e.g. circles) and vary the speed and power until you figured out at what settings the laser cuts through the material. In my case (5 mm acrylic glass), I had to put the speed down to 3mm per second. All in all, it took quite some time to laser cut all 12 rings...

Plexiglas normally comes with a protection foil. I removed that foil after laser cutting to protect the plexiglass from getting burning spots from the laser.

Step 6: Assemble

This step needs some more work from my side. When stacking the rings, I did the alignment by hand. There are probably some more precise ways to do that, for instance by punching two tiny holes at each side of each ring and sticking a pin trough all forcing the holes (and thus the rings) to align perfectly.

To glue the rings together I used 'Acrifix', a liquid glue that comes in a small bottle with a brush.

Unfortunately, when assembling, it turned out that the laser cutter did not go through the material completely at some of the rings. When trying to press them out of the plate, I broke a couple. So our tea light holder will miss a few months...

Step 7: The End-result (...or Is It?)

Well there it is, the end result, our acrylic, data-drive, tea light holder!

To be honest, I have mixed feelings. I do like the design, but fortunately some rings are missing (see previous step). And the fact that the design is based on data does not really show. Also: there is hardly any difference in brightness of teeth representing sunny days and those representing cloudy ones.

But that is what designing is all about: you try and learn and retry and learn until you are satisfied. This is just version zero-dot-one. To be continued!