I first planned to use white LEDs to illuminate the film. But now I think the ideal source is an RGB LED. Here’s why…
White LEDs are actually blue LEDs with a patch of phosphor inserted that absorbs some of the blue light and re-emits green-red light to produce a visible white color. The resulting spectral distribution of this source looks like this:
In a telecine machine, we are trying to measure the density of 3 layers of colored dyes in the processed film. For Kodachrome movie film, the spectral densities of these dyes look like this:
The CCD imager that I am using to measure the density of these dyes has a color filter array (CFA) made of colored dyes. The spectral transmittance of these dyes, combined with the spectral sensitivity of the silicon sensor, results in a sensitivity of the device that looks like this:
The peak response wavelengths of the red, green and blue pixels in the CCD almost match the peak blocking wavelengths of the red, green and blue dyes in the film as shown above, making the CCD a fine device for measuring the color on the film. The white LED spectrum doesn’t match so well, but look at the spectral output of the RGB LED…
Now we have a narrow band illuminant that matches the film dyes and the CCD pixels very nicely (though it would be nice to have a bluer blue). Aside from the obvious elegance on paper, this allows the illuminant intensity to be adjusted so that the red, green and blue pixels of the CCD will all saturate at the same exposure setting. Or, more important, it means that an exposure setting can be chosen that will nearly saturate all three colors, meaning we are using the full dynamic range of the CCD, which is, after all, somewhat limited.