Yes, that is a good start - I would like to have curve with the relative strengths of the spectrum. But maybe it is enough to carefully calibrate the smartphone

Yes, that is a good start - I would like to have curve with the relative strengths of the spectrum. But maybe it is enough to carefully calibrate the smartphone camera on the diffraction sprectra to generate a course graph. Also, if no diffraction grating is at hand, a random audio CD usually is up to the task :)