Searching on-line I did find this article on negativelabpro.com about "Suggested backlight sources for scanning film with DSLR". In it the second post summarises some important points you want to take into account:
- You want a High CRI (Color Rendering Index)
- Good separation of color channels in Spectral Sensitivity Curves
- You want Even Illumination and Predictable Results
- Also consider how “collimated” or diffused the light source is in your setup
Additionally, it recommends several LED panels, including several phone displays:
Recommended LED Panels:
- Skier Sunray Copy Box II 7.6k - 97 CRI light that is bright and even, and includes film holders for a variety of sizes that keep film flat and elevated off the surface.
- Negative Supply - Basic Light and Light Source Pro 5.2k (95 and 99 CRI)
- Kaiser Slimlite Plano (CRI = 95, very> even)
- iPhone or iPad, especially newer models with OLED (must elevate film off surface!). While the reported CRI is not high, it has spectral sensitivity curves more similar to film paper, resulting in less color interference from the orange mask. It is also a more “collimated” source of light, meaning it will produce sharper results than diffuse light sources.
- Any modern Samsung Galaxy/Galaxy Note s7, s8, s9, pixel 3, etc
- Walimex Pro LED (CRI = 90)
The basic idea is that photographic paper for chemical development is not uniformly sensitive to the light spectrum, but it has specific sensitivities to red, green, blue. In particular, it has very little sensitivity to "pure" orange (590-600 nm light). This is why negative film, and only negative film but not black and white or slide film, is of orange colour.
Film is designed with the chemical paper in mind, so the best way to acquire the colours is to have sources of light which emit very little "pure orange". See following image as reference:
For example, a separate deep red, green (not a 555-560 nm green, a 530 nm green), violet-blue LED emitters would do. Another option, as it turns out, are OLED screens because their emission wavelengths are quite suitable for the purpose and emit little pure orange. LCDs are not as specific, their emissivity is more "spread out".
Using a light source with little to no orange light would result in the orange regions of the film to appear darker, which after inversion will become bright, as it is in normal chemical paper development. Having less orange will make the inversion more accurate.
So, OLED screens are the best common option for scanning negative films, unless a specific light source is available. For every other film, a white high-CRI light works.
See also for reference an explanation here.