Abstract: The color management of artificially colored surfaces is necessary element of high quality production. Obtaining a given color of the surface is possible by a previously prepared ink mixture or by an autotypical printing method, i.e., by controlling the area of periodic micro-dots of four primary colors. For the development of effective measuring and control systems for these technological processes, simple and accurate methods for approximating the spectra are required. The reflection spectra are needed to calculate the color coordinates in the MKO Lab's colorimetric system, with which the international standards regulating color reproduction accuracy operate. At present, the methods of linear approximation of spectra for mixed ink systems are well studied. Information about similar studies for autotyping systems was not found in the literature. The paper discusses the variants of the leastsquares function approximation of reflection spectra of the 1944-field autotype test scale printed on a digital printing machine. Comparison is made by three criteria: to color, mean square and maximum deviations. The greatest accuracy is provided by the principal component analysis. Construction of the basis of the principal components requires a preliminary study of the autotype system used in a specific production. This is an additional operation that requires a certain time and qualifications of the personnel operating the information measuring system. Therefore, the use of principal components as bases can potentially cause difficulties. The way out of this situation is the use of pre-defined fixed functional bases for approximation. The paper considers approximation by polynomials and fixed step bases. It has been discovered, that the smoothed step type gives the best accuracy among the fixed bases considered.
Index terms: reflection spectrum, approximation, basis, principal component analysis.

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