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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.

David Bailey
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Matthew_R
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2 Answers2

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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.

Matthew_R
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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

Matthew_R
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