It is good to see someone willing to extract spectra from this old film negative. To have captured that spectrum on film 22 years ago was an excellent achievement. I too was surprised at a response of unmeasurable. It looks like a spectrum that can be digitised. I wasn’t aware there could be non linearities but surely that could be compensated for with calibration checks, or, could relative ratios of certain wavelength pairs along the spectrum, be checked to help confirm linearity or not? [of course, one needs to identify the peaks though].
I recall reading an article which included a picture of a spectrum of a meteor imaged by Harold Ridley which said it was believed to be the first in the UK. I can’t remember if the article was in the BAA journal, or the smaller sized booklet that they used to publish and provide as an insert to the journal. I know I have that in my collection of magazines.
You’ve done an excellent job at extracting the spectra. Some differences in composition? I look forward to the findings.