Public Lab Research note

Make a narrow slit for a DIY spectrometer out of paper

by warren |

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Spectrometers (see #spectrometry) require a narrow slit for light to pass through, so you can get a super narrow beam of light -- this is then split by color using a prism or diffraction grating. Many Public Lab kits use slits printed onto film negatives, which is a really nice and precise way to make slits, but is not always easy for people to make themselves.

So here's a guide on making them the old fashioned way with just black paper, or even just heavier paper with black ink over it. You can also make these with two razor blades, if you want to be super precise -- here's a post comparing the two techniques under a microscope! -- but using paper is not a bad way to get started and works fine for many uses if built carefully.

Print out the design or use dark card paper

The above image nicely illustrates making one of these out of plain black paper, and you can definitely do that. We also have a PDF you can print and cut out, which can be nice for workshops.

Print out this PDF onto some thick paper (if you can!) -- using grey paper is also nice because white paper can transmit light.



Black paper would be ideal, but then the printing wouldn't show up! But obviously, if you can just follow the instructions on the screen with black paper, you don't need to have the instructions printed on the actual slit, so feel free to use black card paper.

If you can't find dark paper, try coloring the paper in with black markers on both sides before cutting it out.

If you can't print on thick paper, probably the best idea is to just use dark thick paper and don't bother printing on it.

Test to see if the paper is at all transparent by holding it up to a bright light.

Assemble the slit

Cut out both the rectangular strip and the U-shaped strip, keeping all cut lines as straight as possible.

Lay the strip over the U to form a narrow gap. Be sure it's even all the way across -- you can hold it up to the light to check -- and you can use another piece of thin card paper as a spacer to set a consistent gap.

Choosing a slit width

Basically, the narrower the slit, the higher resolution (sharper) your spectrum will be. But then less light will get through, so you'll need:

  • a more sensitive camera
  • more light, or
  • a longer exposure

Most smartphones can use a 0.2mm slit, and some even narrower. This printout shows a 0.4mm slit, which will work with less sensitive cameras.

Tape or glue the slit in place, being sure not to mess up the consistent gap you see.

Now you should be ready to use this to finish assembling your DIY spectrometer!

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Yes, this seems to be an easy solution :)

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