Analysis of Photoelectric Colorimetry and Fluorimetry of the Turin Shroud

Abstract

We analyze spectrophotometric data collected directly on the Turin Shroud in 1978. Using standard methodologies, we transform 19 visual reflectance and 16 UV-fluorescent emission spectra to CIE xyY color space coordinates. We then compare these results to those of an Italian team of investigators. Comparing x-y chromaticities, the reflectivities from clear areas (background), image, and blood-stain locations agree with corresponding data collected independently by the Italian team. In both sets of data, the clusters from image and background areas overlap strongly and are clearly separated from the blood-stain points. Data from lightly scorched areas fall within the clusters from background and image locations. Data from deeper scorches diverge significantly, as do all scorch data from the Italian team. We generate similar colorimetric data from the spectral UV-fluorescence measurements made by the Shroud of Turin Research Project (STURP). Plotted as x-y chromaticities, the clear areas group together as a cluster as do the blood data but the two clusters show a significant separation from each other. The scorch data are configured along a linear continuum advancing from the lightest clear areas to the darkest scorches. The image data bifurcate: two of the four points lie within the clear-area cluster and the two outliers fall in line with the scorch data. We offer interpretations and speculations on these findings. Quantification of the 1978 measurements establishes a procedure for future monitoring of the aging state of TS features. The measurement protocol and methodologies discussed are important for preservation of other historical art and historical relics.