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I have recently performed an old experiment using a spectrograph with a photographic film having a helium spectral lamp as a source of light. The picture attached below shows the developed photo of a helium spectrum. I am not going to lie, but I am a bit confused with the result. I want to plot a dispersion curve for helium of wavelength versus distance of spectral lines from an arbitrary zero point to use it in the future in identifying more complex atomic spectra. Unfortunately, I am not able to identify these lines to assign them corresponding wavelengths from some scientific database. I have a guess that the strong line a little bit on the right from the photo's center can be the yellow spectral line. But having a look at some helium spectra on the internet, I am not completely sure about that and other guesses. I could use some help of people who are more experienced in this sort of stuff and who can notice something what I have overlooked.

enter image description here

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    $\begingroup$ Your image is not of much use as you have not identified the direction of increasing/decreasing wavelength. $\endgroup$
    – Farcher
    Commented Nov 1, 2021 at 20:39
  • $\begingroup$ You're right. That would make the analysis much easier. $\endgroup$
    – Camillus
    Commented Nov 1, 2021 at 21:08
  • $\begingroup$ Can you do the spectrum of a neon spectral lamp or neon bulb? That will tell you which end is long wavelength since the visible neon lines are mostly toward the red end of the spectrum. $\endgroup$
    – Ed V
    Commented Nov 1, 2021 at 22:21
  • $\begingroup$ Actually, just having a long pass or short pass filter would do the trick with another run of the experiment. Interference filters are also helpful. $\endgroup$
    – Ed V
    Commented Nov 1, 2021 at 22:32
  • $\begingroup$ Have you made any more progress on this? I am curious to know how it turns out. By the way, in my answer, I show the helium echellogram. In this answer, I show the vastly more complex uranium spectrum, produced by a uranium hollow cathode lamp with argon fill gas: chemistry.stackexchange.com/a/175028/79678. Spectroscopy is always challenging! $\endgroup$
    – Ed V
    Commented Jul 18, 2023 at 1:43

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I think the heavy wide line at the left of your spectrum is the unresolved helium triplet at 587.56148, 587.56404 and 587.59963 nm. Therefore, long wavelengths are at the left side of your spectrum.

Using one of my homemade echelle spectrographs and a helium discharge tube, here is the two dimensional spectrum, called an echellogram, that I acquired for the light from a helium discharge tube:

Helium echellogram

The energized discharge tube emits light that appears yellow, to me, as expected from the helium triplet being helium’s most intense visible emission feature.

This next echellogram is annotated to show the helium line wavelengths in angstroms:

Annotated helium echellogram

Short wavelengths are at the left and, in each grating order (‘arc’), at the bottom. In this image, the helium echellogram is an overlay on a composite echellogram prepared from tungsten and deuterium lamps in the Ocean Optics DH-mini UV-VIS-NIR Lightsource. This just makes it easier to see where the spectral lines are located in the echelle grating’s diffraction orders.

The strong helium emission lines are from the standard NIST online tables

Helium NIST strong emission lines

The wavelengths in the table are in angstroms.

Hopefully, this will help you assign the rest of the lines.

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