What is the function of a TRC/matrix tag in a scanner profile?
A. ICC permit a simple TRC/matrix to be used for scanner profile building as it is possible to build scanners in which such an approach would work – as it does for displays. However, the spectral sensitivity of most scanners is not sufficiently close to colour matching functions (effectively the sensitivity of the eye) to permit the TRC/matrix approach alone to work very well. For this reason ICC also specify the LUT approach, which permits non-linear transformations, and most profile making software uses this. However, a LUT on its own may not have spacing that is well suited to the data for reasons of precision, and adding TRCs can improve this. For this reason all profiles permit use of TRCs in addition to a LUT. Adding a separate matrix to this is not necessary for precision reasons and it can be combined with the LUT but some profile builders find it helpful to keep it separate. Note that for a TRC/matrix transform, only the colorimetric rendering intent is defined.
