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On Getting Good Results From Diffraction-Grating DIY Spectrometers

imallett

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Jan 8, 2018
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See the sticky.

I'd like to report some results I got using this method:

Nominal 405nm → measured 404.70nm
Nominal 445–450nm → measured 448.00nm
Nominal 515nm → measured 516.63nm
Nominal 532nm → measured 531.99nm
Nominal 635–638nm → measured 638.11nm
Nominal 650–660nm → measured 658.25nm

Using this messy setup:



Basically, you shine a laser at a diffraction grating towards a wall. Then you measure the distance between the first forward dots. Thus, you can calculate an angle and feed it into the grating formula. You can combine both steps into one formula for more accuracy and less trig:

λ = (grating spacing)*(dot separation) / ⎷((grating standoff)²+(dot separation)²)

For example, for the 532nm test:

λ = (1.0000 μm)*(0.9980 m) / ⎷((1.5885 m)²+(0.9980 m)²)
≈ 5.3199 10⁻⁷ m
= 531.99 nm

For all measurements, I rounded to the nearest half millimeter. This should be accurate enough to get to ±0.19nm, although in-practice I doubt the walls in my apartment are aligned that precisely, etc., so the error here is probably about ±0.5–1.0nm. For the blue and violet wavelengths, I had to extend the scale with paper as shown, which reduces the accuracy even more (although I ceased the paper on the bottom, so it was at least kindof flat).

I plan to redo the experiment on a larger wall at my university soon, which should at least triple the accuracy. I may also be able to get access to a university spectrometer. In any case, I will update this thread.

Suggestions:

- Make sure the laser hits the diffraction grating perpendicularly! This is the main source of error.
- Finer diffraction gratings separate out the light more, which reduces error. There are three spacings of diffraction grating easily available on Amazon (1000 lines/mm = 1 μm spacing, 13500 lines/in ≈ 1.881 μm spacing, 500 lines/mm = 2μm spacing). Get the first one, because it has the smallest spacing.
- The farther the grating's distance to the wall, the more accurate you are.
- I aimed the center dot at an edge of the wall so that only half the dot hits. This allowed me to align the dot very accurately, and measure the distance between the two dots with only one person (myself).
- Watch out for reflections! The diffraction grating throws some modes back toward you! You can see this in the above picture (The laser is angled down a bit for show so that you get streaks showing the modes on the table. It's only 5mW, and even this energy is split among modes, so in this case one is fairly safe.).
 
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lazeristasUVISIR

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Jan 5, 2011
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The groove spacing is great in formula, but diffraction gratings are specified by numbers of grooves in mm (300, 600 etc). The computer can do the math.
 

paul1598419

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Sep 20, 2013
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I used to use diffraction gratings years ago to measure my laser's wavelength, but since I got a good spectrometer and OEM software, it hasn't been an issue. Have several B&W Tek spectrometers that are a set aside project as the software I'm using it with isn't good enough. May end up getting the OEM software for them too.
 
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