UCLA

Indigo, a blue colorant extracted from the leaves of several plants found throughout the world, has a long history of use, stretching back to the third millennium BCE. In the last decades, its identification on cultural heritage materials was achieved through Raman spectroscopy and fiber optics reflectance spectroscopy (FORS). In recent years, an imaging method called multiband reflectance imaging subtraction (MBRIS) has been introduced to the cultural heritage field to identify and map indigo. As with FORS, this method takes into consideration the electronic properties of indigo and its optical behavior in the visible range. More specifically, this method is based upon the subtraction of two broadband images, one taken at the maximum absorption and a second one at the inflection point of the steep reflectance. This MA research reexamines the potential for the unbiased characterization of indigo using multiband reflectance imaging, reflectance and fluorescence spectroscopy, and hyperspectral imaging (HSI). Reference blue colorants as well as archaeological and ethnographic objects were analyzed using the aforementioned techniques. This MA research critically reviews the strengths of the techniques currently used for the characterization of indigo and compares how data are affected by factors such as concentration of the analyte and substrate. The extent to which the fluorescence of indigo can be used for its differentiation amongst other blue colorants for materials characterization is also explored. While FORS was able to unequivocally differentiate between indigo and the other blue colorants, the MBRIS method was not. Case studies revealed that the MBRIS can provide false-positives and may not be well-equipped to map indigo present in high concentrations. The bandpass filters used in the MBRIS method are too broad to provide unbiased identification of colorants. Greater differentiation of powdered pigments in powder form was achieved with a new method of generating false-color images that utilizes principles of MBRIS. HSI spectroscopy, combined with trichromatic false-color imaging processing has shown promising results in terms of data visualization. The fluorescence of indigo could not be detected in solid samples with the spectroscopy setups used in this study; this suggests the emission is likely too weak to be captured by luminescence imaging techniques used in the cultural heritage field.