Conversion of a digital camera into a non-contact colorimeter for use in stone cultural heritage: the application case to Spanish granites

1 In this study, a digital CMOS camera was calibrated for use as a non-contact 2 colorimeter for measuring the color of granite artworks. The low chroma values of the 3 granite, which yield similar stimulation of the three color channels of the camera, 4 proved to be the most challenging aspect of the task. The appropriate parameters for 5 converting the device-dependent RGB color space into a device-independent color 6 space were established. For this purpose, the color of a large number of Munsell 7 samples (corresponding to the previously defined color gamut of granite) was measured 8 with a digital camera and with a spectrophotomer (reference instrument). The color data 9 were then compared using the CIELAB color formulae. The best correlations between 10 measurements were obtained when the camera works to 10-bits and the 11 spectrophotometric measures in SCI mode. Finally, the calibrated instrument was used 12 successfully to measure the color of six commercial varieties of Spanish granite. 13 14