Can I use a LCD as backlight for acquiring film using a DSLR?

Question

After asking about using a DSLR for acquiring film, I am at the stage of looking for a proper backlight.

I assume a high CRI light source is required to properly reproduce each colours of the film: just like a poor CRI LED light makes reds duller to the viewer in real life, a poor CRI light would make a bad source to light a film source.

I searched for high CRI (95+, not 90+) small panels, but I cannot find many on reputable websites in Europe (I don't trust the CRI of those from China). The only one would be from Valoi but it's sold out.

What about using LCD displays, in particular those from devices which can achieve very good colour reproduction like covering 98%+ of AdobeRGB or of P3, for example recent iPads and recent iPhones? (other brands apply as well, I just remember these).

If they are so accurate in colour reproduction, are they good light source for my purpose? if the CRI of a white iPad/iPhone screen high enough?

Answer 1

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:

1. You want a High CRI (Color Rendering Index)
2. Good separation of color channels in Spectral Sensitivity Curves
3. You want Even Illumination and Predictable Results
4. 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.

Answer 2

Just because a trichromatic screen has a large coverage gamut, such as Adobe RGB, doesn't necessarily mean it also has a high CRI. Emissive screens can use three fairly narrow color bands to fool our eyes and brain into thinking we see colors they are not actually emitting.

_{Typical output of an RGB emissive display.}

Since the color filter arrays on digital camera sensors are NOT the same three colors as those used by our emissive displays, using a screen with low CRI (even having a large gamut) won't allow you to capture all of the colors in the negative (which has three dye layers that use another different set of three colors).

But I think restricting yourself to any kind of LED display is a bit of an X→Y problem. All you need is a well diffused high CRI light source.

Any high CRI light bulb behind a high diffusion screen will do the trick. A simple light baffle inside a reflective enclosure, such as a small "softbox" made to go on a camera mounted speedlight flash, should ensure even illumination on the screen at the front of such a diffuser. For that matter, a speedlight on low power works for many folks. Some even use a cyan gel to help offset the orange color of the film substrate with color films.

No fancy LCD is needed at all.