Technical Report, developed by TC 1-90, is a research report describing a general colour fidelity index, Rf, as a scientifically accurate measure of colour fidelity with respect to a reference illuminant. The important improvements of this measure, relative to the current colour rendering index, are the update of the colour difference calculation, in particular the object colour space, and the incorporation of 99 test-colour samples which provide a more uniform distribution of slope and curvature values as a function of wavelength and which have colour appearance values that are more widely and uniformly distributed in the three dimensions of a uniform colour space. However, Rf is not a replacement of the general colour rendering index, Ra, neither for the purpose of rating and specification of products nor for regulatory or other minimum performance requirements. Finally, relations of Rf to Ra and to luminous efficiency are discussed.