The short answer: the LEDs I am using are from the Osram Oslon 10 mm Square type, about 1W power. You can purchase them in various wavelengths and I tried out different combinations. I am still in the process of optimizing them (more on this later in this post), but here are the ones I currently use:
blue channel: “blue”, wavelength 470 nm
green channel: “green”, wavelength 538 nm
red channel: “red”, wavelength 623 nm
I tried other, more extreme wavelengths, for example “hyper-red” = 660 nm for the red channel, but got mixed results depending on the film stock and Super-8 camera used.
The reason is the (complex) interaction between the response curve of the film stock, the response curve of the camera and the small band of wavelengths a single color LED is actually sampling. The old film stock was optimized for projection by the continous spectrum created by the projection lamp, and any single color LED samples just a tiny part of that spectrum. So during experiments, with one film stock/LED combination, the outline of the daylight filter of the camera showed up in scans as a faint red shadow around the perimeter of the image. Switching to another red wavelength solved this problem.
On the other hand, turning to “white light” LEDs did not solve my problem either, as their spectra are typically very different from a classical light bulb. They usually have a big power spike in the blue color range. Also, I wanted to be able to adjust the color temperature of the light source. So I stayed with the separate LED setup.
Well, the above mentioned wavelength-combination gives me currently the best results, but I am still optimizing on that point.
Initially, I started out with a larger integrating sphere which was easier to design, but the power output was not sufficient. Reducing the diameter to 50 mm solved this. I am able to image the darkest shadows of Kodachrome 25 film stock with an exposure setting of 1/32 sec @ ISO 100. The camera I am using is a standard Raspberry Pi v1-camera. (I tried other camera setups, but this turned out to be the best option for me so far - long story…)
With respect to the color balance: I am using a fixed color-balance on the camera, as the Raspberry Pi camera (or rather the processing pipeline on the Raspberry) has some issues with highly saturated colors (think of a bright blue sky on Kodachrome for example).
To calibrate the LEDs, the image of the empty film gate is used in conjunction with a histogram display. Here’s how that looks in my software:
On the top, there are three sliders for the power of the color channels. Before adjustment of the color balance the exposure time of the camera is reduced so that the film gates intensities are within the histogram display range. Then the three color sliders are moved until the curves in the histogram (bottom part) overlap optimally. In the situation captured above, I still would reduce the power in the red and blue channel a little bit, until all color curves overlap perfectly.
I am actually able to switch the camera into raw-mode to do this with the four raw color channels (R/G1/G2/B). But, normally, just the usual RGB-stream is sufficient and substantially faster to adjust than the raw camera stream. The LEDs need some time to heat up, about 15 min is usually sufficient.
That is all the color adjustment I am currently using. I can get away with this because my whole image processing is done in HDR-space, so I have plenty of headroom to adjust for good color in the post-processing pipeline.
Film capturing typically consists in my setup of taking 5 exposures of each film frame, ranging from 1/1000 sec (full intensity of the film gate maps to a bright “white”) to 1/32 sec (even the darkest shadows of Kodachrome 25 film stock show structure in this image, most other stuff is grossly overexposed). These 5 exposures for each frame are stored on disk for later processing (which consists of: exposure fusion/sprocket registration/cropping/gamma adjustments/color grading).
Another approach to color balancing that worked for me is to adjust the LED powers directly to the material at hand. Super-8 film stock (Kodak/Fuij/Agfa/etc) is very different, colorwise. But one can adjust the LED color balance directly to the film stock at hand, provided some part of a frame can be identified as pure gray/white surface.
However: that approach is very difficult and unreliable if you have no good reference of a colorless surface.
For some Super-8 movies I made in the 80s I actually filmed a color chart at the beginning of the filming - never did I expect that this would turn out to be helpful nearly 40 years later!