Kodachrome Colors

[cpixip]

@npiegdon

I wouldn’t solve the kind of discrepancy you pointed out in the gray swatch by boosting the whole channel uniformly. Instead, I’d solve it with a calibrated 3D LUT. That’s exactly what that particular tool is for and it defeats the problem perfectly.

Yep, exactly. Besides, it might not even be noticable in the first place. Variances in the development process in those old days might have a much larger influence on color fidelity than this.

@npiegdon

Kodachrome calibration targets are no longer available, which makes me wonder how far off any target printed using a three-dye system would be vs. a Kodachrome target. Certainly, every three-dye film stock will exhibit the same wavy characteristics for the same reasons that Kodachrome does. So, if the same dye curves are available for other stocks, I’d be curious to see how far off they were from one another. More specifically: my calibrated 3D LUT comes from an Ektachrome target with 288 swatches. How far off would some hypothetical simulated Ektachrome curves be from your simulated Kodachrome curves?

Interesting question. Here’s a plot of the digitized data from the Kodachrome 25 data sheet,

and here’s the equivalent one for Ektachrome 100D:

Comparing the dye’s spectra separately, one gets for the yellow dyes:

This is the comparision for the magenta ones:

and finally for the cyan dyes:

There is indeed not too much difference how the dye spectra behave over the interesting wavelength range.

@npiegdon

Another detail: it’s useful to remember we’re not taking point samples on these curves. “Narrowband”–at least, until you’re using lasers or adding bandpass filters–isn’t necessarily as narrow as it sounds. Recall my measurements where each LED had a bandwidth between 30 to 40nm. Taking the extra bandwidth into account doesn’t solve the problem, but it would soften it a bit on average.

Some commercial scanners do use rather narrow interference-based bandpass filters. I am more interested on whether the Swiss setup (described in the DIASTOR-paper) does make any sense. Frankly, I think not. Here are some illumination spectra I generally play around with:

You might be interested in the yellow curve. Comparing this to the Swiss illumination choice, it’s obvious that your narrowband setup delivers power on all wavelengths. The Swiss illumination (if I used the correct numbers - need to check this again) has however an annoying dip between 610 nm and 620 nm. In other words: basically features a blind spot. No 3D-LUT can recover from this.

I do not think that this issue is too crucial, given the broad dye spectra, but this is exactly what I am interested in.

Certainly the scan quality depends on the placement of the three narrowband LEDs - that’s what I discovered when I started to look into film scanning. My original setup was very similar to the Swiss one, a later one more like your own setup, yielding better results.