An Absorbance-Based Prediction Method of Spot Color Formula for PET Film
ZHANG Yan1, 2, ZHOU Shi-sheng2*, CAO Cong-jun2, REN Peng-gang2
1. School of Mechanical and Precision Instrument Engineering, Xi’an University of Technology, Xi’an 710048, China
2. Shaanxi Provincial Key Laboratory of Printing and Packaging Engineering, Xi’an University of Technology, Xi’an 710048, China
Abstract：A model for predicting spot color ink formula for PET film printing is proposed based on the correlation between absorbance and the concentrations of primary inks. Firstly, according to the methodology with the concept of light propagation and the multiple internal reflectance of ink layer and substrate, a method for obtaining the spectral transmittance of the thin film prints with high transmission characteristic is established by making use of the reflectance of the film prints with back and white substrates. The absorption spectrum of the film prints in the visible spectrum is obtainedby the spectral transmittance. Then, the regression equation between the absorbance of spot color inks and the concentrations of a primary color ink is established to determine characteristic wavelengths with strong linear correlation. After that, according to Lambert-Beer law, the prediction model is established by using the absorbance of the spot color samples and the absorbance of the primary color samples at the characteristic wavelengths, in order to obtain the proportions of primary color inks. Finally, the deviation between the predicted formulas and the actual concentrations is analyzed. In order to evaluate the accuracy of the prediction method, the remade samples are compared with target samples in terms of chromatic difference and the spectralroot mean square error. Some dual-componentspot color samples produced on PET filmby gravureare used as experimental subjects to verify the proposed method. The analysis of spectrum shows that the reflectance on the black substrate is significantly different from the reflectance on the black substrate, but both have the same trend with the variation of primary ink concentration. The transmittance curves of spot colorsamples are situated between the curves of primary color samples and movecloser to the primary color with higher concentration. The absorption spectrum of samples increase in the regionof 400～580 nm with the decrease of concentration of primary ink A, and decrease in the region of 580～700 nm as the concentration of A decreases. Except for the region of 570～590 nm, the linear correlation coefficients R2 between absorbance of spot color inks and the concentration of primary color A are higher than 0.95 and the average value is 0.990 0, which means a strong linear correlation in the visible spectrum range. The absorbancevalues at the wavelength 520 nm (R2 of 0.994 2) and 700 nm (R2 of 0.998 5) of the primaryinks A and B are selected to predict the formulas of spot colors by using the least squares method. The results show that the 6 groups of predicted concentrations are with 2.5% deviation from the actual concentrations of target samples, which means no significant difference. The maximum chromatic difference between the targetsamples and remade samples is estimated to be 1.98, the minimum to be 0.30, and the average value to be 0.85, which satisfies the requirement of spot color reproduction. 5 of the 6 groups are smaller than 1.5, which satisfies the requirement of faithful reproduction. The maximum RMSE is 2.95%, and the minimum is 0.49%, and the average value is 1.40%, which means a high precision color reproduction in the visible spectrum. It is confirmed that the proposed method could effectively improve the printing quality and the spot color matching precision, which may provide a scientific method for the predictive study of spot color ink formula of PET film printing.
Key words：Absorbance; Spectral transmittance; PET film print; Computer color matching
张 研，周世生，曹从军，任鹏刚. 基于吸光度的PET薄膜专色配方预测方法[J]. 光谱学与光谱分析, 2019, 39(02): 415-420.
ZHANG Yan, ZHOU Shi-sheng, CAO Cong-jun, REN Peng-gang. An Absorbance-Based Prediction Method of Spot Color Formula for PET Film. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(02): 415-420.
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