We all understand how crucial the spectral composition of light is when scanning color negatives and slide films. The spectral characteristics of light determine how the camera sensor interprets the density of dyes in the film. Ideally, you probably want RGGB channels to operate at the same gain. However, the orange mask in most color negative films makes achieving optimal illumination particularly challenging and requiring extensive white balancing. To explore this further, I measured the spectral distribution of various light sources I had at my disposal. The results are in the PDF file you can freely download from my Google Drive. Once you review data we can discuss finding.
Very interesting to see these graphs side by side. I suppose the obvious question is whether it is possible to draw any conclusions from these as to their relative suitability for copying colour negative or positive transparencies. Also does the measured CRI have any serious bearing on that outcome.
@Harry IMHO, and what I believe everyone- who has any hands-on experience with scanning - knows two things: Sun-like spectra is good, the presence of deep reds in backing light is good, the light which spread evenly across the visible spectrum is good, for color negatives the light need to be bluish (with all above still holding). On the other side we know that purpose-built RGB lights works well if one knows how to work them properly. Whether CRI index positively identifies good light - I cannot say. I know CRI index of well suited for scanning narrow band RGB light will be horrendous.
So my first take is that one has to avoid cheap generic lights with unpredictable peaks across the spectra. I can easily imagine someone sourcing cheapest LEDs available and having them mixed and matched with intensities adjusted to hit CRI number . Remember that 99% of users use LED lights to illuminate scene with different materials , different levels of methamerism and the only criteria they use is "pleasant " colors. I can speculate that for different skin tones the optimal, natural look in photos may need different light sources - not to mention cultural preferences in re of traditional color schemas.
In our scenario we are actually doing high precision , heavy on physics and color science exercise and we use equipment designed and optimized for completely different purposes. So to overcome inherent limitations of our approach we can either completely cede control - like using Sun or electronic flash or use curated set of light sources which at least do not spoil our results too much.
