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Chemistry LibreTexts

How to Build Spectrometer


  • A small white LED light bulb. Suggested: a Ultra-White LED 5mm
  • A power source for the light bulb. Suggested: a 3-V coin battery
  • A small visible-light grating. Suggested: A 500 line/mm linear diffraction grating, such as those sold at Rainbow Symphony
    Diffraction grating
  • A sample of colored liquid; primary colors give easily interpreted results. Suggested: Red Kool-aid, food coloring, or similar substance.
    Kool-aid Man
  • A cuvette (something to hold the liquid in). Suggested: A standard glass vial
  • A way to keep the light, cuvette, and grating stationary. Suggested: A piece of Styrofoam.
  • A camera

Putting it all together

  1. Prepare several samples of different concentrations. If using a powdered drink, measure the concentrations of the Kool-aid (in g/mL, scoops/L, or whatever other measurement system is convenient). If using food coloring, measure the number of drops that go into your sample. The important thing is to have a quantitative measurement of how much has gone into the sample.
  2. Tape the light and power source to your sample holder, and position the grating about a foot away from the sample. In order to see the spectrum, the grating will need to be positioned at an angle to the light source.
  3. Turn off as many lights in the room as you can while still being able to see what you are doing reasonably well.
  4. Position your cuvette in front of the light source so the light from the LED shines through when you look through the grating. Mark your position so that you can easily place all of your samples in the same spot.
  5. Take a 'blank' reading: fill the cuvette with clear water, place it in your sample spot and take a picture of the resulting spectrum.
  6. Take pictures of the spectra that come from each of your samples.
  7. Upload the pictures into the program, which can be downloaded here. Load the picture of your water reading by going to 'file > load reference (right) > (your directory pathway)', then do the same to your sample spectrum by going to 'file > load sample (left) > (your directory pathway)'. Depending on your camera, you may need to resize the pictures in a program such as Windows Photo Editor before you can see the entire spectrum; to do this in Photo Editor, simply import the files, select your picture, go to 'edit', choose 'resize picture' and shrink the picture to whatever percentage fits the screen.
  8. Once the spectra are uploaded, simply click on one side of the sample spectrum on your screen. A box will appear, asking you to specify if it is the red or blue end of the spectrum. Choose the correct answer and repeat the procedure at the other end of your sample. There should now be a green line connecting the red and blue ends of your spectrum. Run through the same procedure with your reference spectrum.
  9. Spend a bit of time exploring the 'Plot trace selection' portion. Try overlaying the sample and reference spectrum and compare the two.
  10. Click on the 'Make plot, compute T, compute A' button, then on the 'Generate CSV file for Excel' button. Save the Excel file anywhere that is convenient for you.
  11. Open the Excel file and make plots of the absorbance and transmittance versus the wavelength. How are the two plots related?