Cardboard Spectrometer, Rainbows in Your Pocket




About: I'm an Italian freelance structural engineer, graphic designer and photographer, and I'm teaching physics in Waldorf high-schools. I always investigate electronics, robotics and science in general, I'm a pas...

Maybe you had noticed that different light sources give a wide variety of sights and sensations, especially due to colors revealed by illuminated objects. It can be true that some very nice pieces of furniture bought in a well lightened showcase, or a jewel which glittered with beautiful rainbow colors, are not so spectacular whenever you took them in your living room... let's understand together why it's so.


Step 1: What Is Light Spectrum

The spectrum of a light is the whole variety of colours which are emitted from that light source. Rays emitted can also be invisible to human eye, some can be heat, others can be different types of electromagnetic radiations. But for now we only wish to see juicy tints from any light source we run into, and wonder about how much variety of colours is in a single lamp. This could also make us thinking deeply about lighting choices for our cities and our house.

The spectrum you see in this instructable's first step comes from a fluorescent neon light, and indeed you can distinguish coloured lines detached one each other. Looking into an incandescent light, you will see that things are different, and also solar light, LED for plants growing, sodium lamps, and so on.

Step 2: Spectrometers Types

Spectrometers have different working principles. You can see the spectrum with a glass prism, with a diffraction grate, but also with a vynil record or a Compact Disc. If you look a light while reflecting on a CD you will see some beautiful colours of the rainbow. This is already a cool toy, but it needs some better setup to become a semi-professional spectrometer.

Step 3: Materials

We will actually use a CD which I hope you already have in your scrap. You also need some cardboard tubes, but pvc pipes are also good. Personally I love the surface of these cardboard tubes I found somewhere, it's smooth and can be easily sanded with sandpaper.
Then keep close at hand some adhesive tape and a piece of iron wire.
My project expect you to design and 3D print covers for the tube, but if you don't own a 3D printer don't worry, you as a good maker will have no difficult to adapt some plastic caps.

Step 4: Viewfinder and Reflector

You will be surprised to see how much simple this project is.
The viewfinder is nothing more than an hole distant about one inch from the edge. Use a drill if the cardboard is quite thick, but maybe you can manage it with a different tool. Then make it rounded and smooth with a little roll of sandpaper.
The reflector is the piece of CD-rom, just cut it like a slice of cake. The width shall be just a bit more than inner diameter of the tube. Embed this CD slice inside the tube, just under the hole, with the reflecting surface looking the hole, and an angle of about 35-40°, you will now the right angle when you will see a rainbow while looking into the hole. When you are pretty sure you can glue the piece of CD in place.

Step 5: The Stop Ring

The cap will have to rotate freely but also to stay in place. To obtain that, shape a piece of iron wire on the cardboard tube so to make a big ring, then fix it at about 1cm from the edge at the opposite side of the viewfinder, with some amount of well thighten electric tape.

Step 6: The 3D Printed Caps

Those are anything special... just a pair of caps, one with a slit. You can obtain then maybe from milk bottles, just cut a 1mm wide slit in center of one. Then adjust the iron ring and number of tape turns around the ring to obtain a perfect joint. You can also glue them in place but I don't suggest it. The rear caps is simpler to draw since it doesn't need a slot even a slit.
Anyway, if you own a 3D printer it's time to make something cool. Use a 3D software to design caps like in the pictures, with right diameters to get stuck around the ring and the tape. I used Rhinoceros but any software will be fine since it's a very simple shape. Please refer to other instructables to know something more about slicers and printing parameters.

When you have both caps you can simply embed them in place. The front top also rotates so you can see different effects when light comes directly through the slit, just point the tube toward the light and look into the hole.

Colors of the Rainbow Contest

This is an entry in the
Colors of the Rainbow Contest



    • Colors of the Rainbow Contest

      Colors of the Rainbow Contest
    • Classroom Science Contest

      Classroom Science Contest
    • Gardening Contest

      Gardening Contest

    11 Discussions


    8 days ago

    Great project! You deserve extra credit because it is built without Arduinos, Laser cutters, 3D printers or CNS machines. (not that I have anything against them!) Spectroscopy is so fundamental in understanding the world around us, and by making it accessible to all, you help enlighten all of us!


    9 days ago

    Very nice! You can actually do some science with your project.

    Check out and They have a spectrometer of similar design to yours, but theirs is made of folded paper and is very flimsy, that is, it's very easy to damage, while yours is very robust as it can take some beating in the field.

    In, there's a web app (and python stand-alone one as well) that can process the spectrum you photographed for analysis. Not only can you tell the composition of glowing gasses, such as neon, argon, xenon, sodium, mercury, sun, etc, but you can also identify non-gaseous materials by analyzing the reflected or transmitted light from the material. Imagine filling a glass vial with a material, say a drop or two of dish soap in water, then you shine sunlight through it recording and analyzing the spectrum you obtained from the material. You would now have a spectral profile of that dish soap in your database.

    You can amass a database of spectrum profiles for later use. So if you ever encounter an unknown substance that you analyze with your spectrometer, you might identify what it is from comparing it's spectrum with the data you already collected.

    Don't you love this sciency nerdy stuff?

    I voted for your project.

    1 reply
    andrea biffialcurb

    Reply 8 days ago

    Yes! It's really awesome! And kids love this too, they could learn amazing topics with very easy diy stuff! :-)
    I'm actually holding physics classes in Waldorf high-schools, and these projects are an essential part of teaching!


    9 days ago

    Hi andrea!

    Take a bow!!
    It is such a wonderful project and I voted for you!!

    With many greetings from Hamburg ;-))
    Yours Aeon Junophor

    2 replies

    Question 9 days ago

    i dont understand where to put the cd. please make a diagram or sketch.

    2 answers
    andrea biffiegon1990

    Answer 8 days ago

    You're right... I couldn't take a good picture but today I will add a sketch!

    andrea biffijessyratfink

    Reply 10 days ago

    Thanks Jessy, it's a tool set I built for my optics physics class, we observed many different lamps, it was great.