Can anyone explain to me what is a true color back - meaning what models on the market can do it if any, and what are the settings the photographer needs to use?
If you mean a three-color back that has no -- or even negligible -- color errors, I don't think there are any. If two spectra that appear different to a person match upon capture, no profile can sort that out.
A three filter back that captures colors the way the putatively normal eye sees them would have spectral responses that are a 3x3 matrix multiply away from a standard observer. I know of no camera that meets the Luther condition. There are reasons for that that go beyond the mundane practicalities of what dye sets are available. The human eye has two greatly-overlapped channels (rho and gamma) and one that's offset towards short wavelength and plays almost no part in luminance (beta). That's a good strategy if you're stuck with a simple lens that can't bring many wavelengths into simultaneous focus, like the one in our eyes. However, it's a technique that can cause chroma noise as you try to sort out the overlap.
Even
attempts to meet the Luther criterion are rare. I do have some experience with one.
When I was working at the IBM Almaden Research Laboratory in the early 90s as a color scientist, I consulted with
Fred Mintzer and his group in Yorktown Heights who developed a scanning camera with the objective that the wavelength-by-wavelength product of the camera's RGB filters, the IR-blocking filter, and the CCD's spectral response would be close to a 3x3 matrix multiply away from human tristimulus response. The
camera was used to digitize Andrew Wyeth's work, to capture artwork in the Vatican Library, and for some other projects where color fidelity was important.
A more promising route is to capture more than three color planes.
In 1992, Michael Vrehl, a student of
Joel Trussell at North Carolina State presented a paper at the SPIE Imaging Conference. I'm sorry I can't find a link to the paper itself, only one to
the abstract.
"The quality of color correction is dependent upon the filters used to scan the image. This paper introduces a method of selecting the scanning filters using a priori information about the viewing illumination. Experimental results are presented. The addition of a fourth filter produces significantly improved color correction over that obtained by three filters."
I remember being quite impressed with the improvement in color accuracy afforded by the addition of the fourth filter. The common term for cameras that have more than three filters is multispectral cameras. The ones I've seen are for scientific use. You can turn a monochromatic camera into a multispectral camera with a color wheel.
