Correction Method Of Chromaticity Coordinate Offset In Cultural Relics Color Measurement

How to improve the accuracy of color measurement is one of the important research directions in the field of color.In order to improve the accuracy of color measurement,CIE(Commission Internationale de L’Eclairage)prescribes standard color measurement methods,including defining a variety of standard light sources and recommending geometric conditions for standard measurement.However,in practical application,especially in-situ testing,it is difficult to achieve color measurement under CIE standard geometry conditions.When the illumination angle changes,the chromaticity coordinates of the measured object will be significantly offset,which will affect the measurement accuracy.In the process of cultural relics color measurement,whether it is outdoor color painting of ancient building or indoor cultural relics of museum,the error of chromaticity coordinates caused by illumination angle is inevitable and has a great negative impact.At present,the research on this problem focuses on the correction of accidental errors in measurement and the improvement of the correction algorithm.There is less research on the pertinence of illumination angle to color measurement in geometric conditions,and the effect of the correction model needs to be improved.In view of the above problems,this paper takes the color measurement error caused by illumination observation geometry as the research topic,and takes the chromaticity coordinate offset error caused by different illumination angles as the breakthrough point.Aiming at the color characteristics of outdoor color painting of ancient building and traditional Chinese painting,34 RAL-K7 color cards and 26 traditional Chinese painting pigments were selected as the research objects.The scene measurement environment was simulated by experiments,and the chromaticity coordinates of the tested samples were recorded in the range of 30-60 degrees illumination angle.The prediction model of chromaticity coordinates varying with illumination angle is established by LASSO function fitting.The prediction model is modified by repeated sampling and the prediction model error is evaluated by cross validation method.Based on this prediction model,the over-determined equations are solved by SQP sequential quadratic programming algorithm based on the chromaticity coordinates at any three illumination angles of 30-60 degrees,and the chromaticity coordinates at 45 degrees illumination angle are corrected.Taking the color paintings on the east side of Liujiating in the Summer Palace as samples,the average error of the chromaticity coordinates between the measured and predicted values is 0.0005 through field experiments,which verifies the reliability of the prediction model and the correction method.Finally,the method of chromaticity coordinate offset correction in color measurement of outdoor color paintings of ancient buildings and the method of chromaticity coordinate offset correction in color measurement of indoor cultural relics of museums are established.The software of chromaticity coordinate offset correction in cultural relics color measurement is compiled by MATLAB.Input the chromaticity coordinates under 45 standard illumination angle,adjust the target illumination angle,and output the chromaticity coordinates predicted value under the target illumination angle in real time,forming a practical technological innovation.On the basis of solving the scientific problem of the influence of different illumination angles on the chromaticity coordinate offset,the proposed prediction model and the correction method based on the prediction model effectively solve the problem of chromaticity coordinate offset in the color measurement of cultural relics,which makes the data obtained from the existing monitoring methods in the color painting monitoring re-possess analytical significance,and effectively improves the accuracy of recording the color information of traditional Chinese painting.The research results provide an effective correction method for the measurement errors caused by geometric conditions in the field of color measurement.