MEASUREMENT OF COLOR USING A NONDESTRUCTIVE METHOD FOR THE STUDY OF PAINTED WORKS-OF-ART

Art historians, archaeologists and also restorers of art works are cal led upon, within the scope of research and study of painted works of a rt, to determine and describe with accuracy the colours used by each a rtist. On the other hand, scientists who work on data banks of painted works of art are called upon to acquire very-high-resolution images c ombined with accurate colorimetric registration for archiving purposes . An extended study oriented towards colour measurement parameters det ermination is then necessary. From the current available relevant lite rature it is obvious that the research concerns primarily colorimetric fidelity without providing any details as far as mathematical process ing of colour measurement data is concerned. In the present study ther e is suggested a non-destructive methodology for colour measurement th at is much cheaper than the one that is applied if non-destructive spe ctrophotometry is used for this purpose. This is performed in a compar ative manner with the help of a three-charge-coupled device or a mono- charge-coupled device colour detector five times less expensive than t he first one, on two reference panels containing a total of 34 colour samples. Aiming at the uniquely and generally accepted determination o f colour, the R, G and B values are first properly corrected and then transformed to x, y (chromaticity) and Y values in accordance with the CIE 1931 standard colorimetric XYZ system. This procedure is performe d with a linear mathematical transformation followed by a least-square s method in order to determine the best values of the matrix elements and minimize the error between measured and computed values. The so-ob tained values are then compared with the respective x, y and Y values provided by the spectrophotometer on the same reference panels. The me an relative difference between the chromaticity x and y values provide d by the set of the two detectors and the values coming from the spect rophotometer is approximately 1.7%. The collected data expressed in tr i-stimulus values are finally converted into CIELAB notation (La*b*) for data evaluation. An average colour accuracy of about three units i n the CMC uniform color space was achieved. These results offer a very promising attempt at elaborating a protocol of colour measurement in a reproducible way.