1. A method of determining a reflectance spectrum, comprising: obtaining a normalized value from a plurality of illuminant sensor outputs, each illuminant sensor output indicating a reflectance value obtained from a target; obtaining reference data from a reference database that correlates reference spectra with a corresponding plurality of normalized illuminant sensor outputs for reference colors; and determining a spectrum &Scirc; based on the illuminant sensor outputs and the reference data, wherein the determining step comprises generating a non-linear model.