Well, so do I. And I do not yet have a good answer.
If you are working with standard film stock, there are for example IT8 targets and software available which do a decent job in making sure that the colors you have in your film stock are properly transferred into the digital domain. But even than, most digital displays fall rather short of the dynamical range classical film stock paired with an old projection device is able to deliver. But that’s another story.
But, what do you do if you have another film stock to scan, quite possibly one out of production for a long time, like Agfa Moviechrome? Your IT8-calibration with the currently available film stock might get you close, but you do not know how close. For sure, there is no way to manufacture a IT8 calibration target for that old film stock.
Furthermore, you might not even want to stay close to the original footage. I give you two examples from my experiences working with Super-9 color reversal footage for that claim. One example is composed of little ducks swimming in a white bathtub. The lighting was a combination of incandescent bulbs and fluorescent tubes. This resulted in a dramatic color shift of the footage. Here’s a rather faithful scan (color-wise) of a single frame:
This is how the film stock would appear to you when placed on a perfectly white background (which gives your brain the “white” reference).
However, when the original footage is projected in a dark room, things change dramatically. Missing the white background reference, your brain is going to adjust your color perception. The projected image as perceived by the viewers would look more like this:
The issue here is especially present in Super-8 film stock, as the cameras had only two color temperatures to work with; the film stock was fixed at tungsten 3400 K type A (indoor use), and you normally worked with the build-in “daylight” filter outdoors. In many situations however (like the one discribed above) the illumination situation was more complex, creating color cast. They are easily noticable in the digitized versions, but were much less noticable in any projection setting.
The second example is connected to the small length of a typical Super-8 reel, featuring only 15 m of film stock (a few minutes of film). So you typically worked with serval different rolls in single movie project.
Occationally, the final rolls got developed differently, so color shifts are noticable between them. Here’s an example. This frame with decent colors
is followed a few seconds later in the movie with footage that looks like this:
The color difference is due to the fact that this footage comes from from another roll of film, obviously developed somehow differently than the previous one.
Note again here, the greenish tint in the later footage would probably not be noticed by the viewer in a darkened projection environment; the brain would “white-balance” the difference out. But the difference is there and noticable in the digital version.
So - the question arises here: what should be the goal of the archival scanning? Keep every color variations present as seen by the scanner? Or even out these color variations, in order to approximate more closely the viewer experience during projection?
My current approach to this is the following: I am aiming for two different output channels. The first one is the original scan, preferably with a high dynamic range to be able to counteract color variation for the second output channel - for that second output channel, I aim at the highest viewing experience, both in color definition as well as image sharpness. Clearly, there is a lot of artistic license (and manual work) present in the second case…
Coming back to the other topic - what’s better, narrow-band LEDs, “white-light” LEDs or broad-band illumination. Or even a mixture of all? I do not yet have a definite answer, but let’s start with the obvious.
As the film emulsions of the old days were trimmed to be viewed during projection by human visual systems, the broad-band illumination supplied by a halogen lamp of the appropriate color temperature is probably the optimal illumination. Such an illumination comes close to a black-body radiator of the appropriate color temperature. Also, the human observer’s eye and brain might be approximated closely with present-day digital hard- and software. So that combination might be the optimal one.
As remarked above, it is close to impossible to obtain color calibration targets for old film stock, so there’s no way to improve this further. This is probably as close as you can get in terms of color fidelity - at least in my use case of home movies.
One step back is probably using a white-light LED. Nowadays, the quality of the top-level LEDs have remarkably increased. But if you look at the spectral distribution of the Kinetta white-light LED, as listed in the paper above,
you notice two prominent peaks. The narrow one, at the blue end of the spectrum, is actually the peak of the blue LED used in these white-light LEDs; the other, much broader peak is due to fluorescent dyes. Together, they trick the human visual system in seeing that light as “white”. But the interaction between such a spectral distribution and the camera’s filter characteristics could be stranger. Nonetheless, newer white-light LEDs have a better spectral curve, and it’s probably a feasible option.
Yet another step back from the reference case (human observer in a darkened room) will be a light source composed of different LEDs for red, green and blue. Such a combination might have advantages in terms of color separation, but might yield funny color grading. In my own scanner, I use this combination, but I had to change the prominent wavelength of one LED because of such false colors occurring. Also, I needed to modify the camera filtering (replacing the IR-filter) in order to improve the color definition in the yellow-to-red color sector. But I am still working on these issues, with no good solution yet.
Finally, we come to - in my opinion - the worst lighting option: the combination of separate color LEDs with a white light LED. You’ll likely end up with a spectrum that mimics something like a fluorescent tube (which is not a good idea, even so some flat bed scanners use such things). Of course, this is a “fluorescent tube” you can adjust to your personal taste by varying the current through the individual LEDs. If your goal is directly to achieve a great viewing experience (as discussed above), such a combination could still have some merit.
In concluding this post, I will describe my current setup: the illumination is using separate LEDs for red, green and blue. The amplitudes of these LEDs are so adjusted that in the raw images all four color channels have a similar, high amplitude. In this way, the number of exposures needed to digitize the high dynamical range of color-reversal film is minimized. The whitebalance of the camera is fixed in such a way that white image parts stay white. The camera itself was modified with a better IR-filter, which improved the color definition of yellow-red tones (skin tones are in this range). It still remains to be seen if that was a good idea, that is a point I am working on. The final “archival” scan is composed of five different exposures which are exposure-fused and stored as 16-bit per channel data. In a second processing step, the “archival” copy is color-graded and resolution enhanced; this is the final “viewing” version of the scan.