I've built an optical spectrometer by following the instruction given on the Theremino website: Theremino_Spectrometer_Construction.pdf, but I'm having a little issue with it.

Everything works perfectly fine from UV to red wavelengths, but whenever I put an IR led in front of it, the light shows up at the extreme left side of the spectrum instead of the right side.

The diffraction grating I used is 1000 lines/mm and I'm expecting the IR light from the led to be around 940nm. I did remove the IR filter from the webcam lens.

I'm not sure if the issue is due to me incorrectly cutting the piece of diffraction grating, to reflections inside of the chamber or something else. I'm no expert in optics, so I hope someone with knowledge on the topic can give me a clue to what could be causing the issue.

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    $\begingroup$ Why the close vote? This is a real physics question. $\endgroup$
    – John Doty
    Jul 15, 2023 at 21:26
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    $\begingroup$ I am voting to reopen since I don't understand how an experimental physics question about an optical spectrometer can be considered "engineering", unless the close voters believe that most of what experimental physicists do is engineering. $\endgroup$ Jul 17, 2023 at 5:48
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    $\begingroup$ Matthew_R, "extreme left side" is not a very precise value, so it would help to show an actual spectrum, or at least specify what wavelength the nominal 940 nm IR appears to have? What calibrations have you done? For example, have you tried 850 nm (another common IR LED wavelength) or a red laser pointer? $\endgroup$ Jul 17, 2023 at 6:35

2 Answers 2


I did some more testing, and figured out that 940nm was simply out of the range of my spectrophotometer.

Looking at the scale in the software after calibration, it should work fine for wavelengths up to about 800nm or 810nm.

I tried a cheap red laser pointer (650nm) and ordered 730nm and 800nm leds which I'll use to further test the limit.

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    $\begingroup$ I upvoted both your question and answer, but after you do your tests with the laser pointer and IR LEDs, what do you plan to look at with your instrument? Also, if you get a cool spectrum of something, please consider adding it to your answer here! Nice work! $\endgroup$
    – Ed V
    Jul 22, 2023 at 18:58
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    $\begingroup$ Thank you. Unfortunately the spectrophotometer is quite unreliable. I've been able to get some decent spectrums for qualitative analysis a while ago but the issue is that it isn't repeatable: sometimes it works, sometimes it doesn't. It's probably due to poor construction. The hardest part is cutting and positionning the diffraction grating properly on the camera. I've still been able to observe the Rayleigh–Jeans law (made a blackbody simulator with a light bulb in a box) and measure my 365nm UV light from Amazon among a few other things. I'll post an update if I can get it to work better. $\endgroup$
    – Matthew_R
    Dec 20, 2023 at 13:09

Best spectrums I've got:

Calibration spectrum using a CFL light : Calibration with CFL (436 and 546nm mercury lines)

365nm UV lamp from Amazon: enter image description here

Rayleigh–Jeans law/blackbody radiation experiment results: enter image description here

I've tried a bunch of other light sources but unfortunately didn't save the spectrums.

The spectrophotometer itself: enter image description here


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