Microspectrophotometry in Forensic Paint Analysis: The Renaissance of a Technique - Comparison of Transmittance,...

Microspectrophotometry (MSP) is a powerful analytical technique that enables objective colour measurements and provides valuable complementary information to vibrational spectroscopies such as FTIR and Raman in forensic paint analysis. In this study, the discriminating power of MSP was evaluated for the differentiation of 34 visually similar red household paints sharing the same colour code, previously investigated by Muehlethaler et al. [1]. Measurements were performed using three acquisition modes, transmittance, reflectance, and fluorescence (UV, Blue, and Green excitations), with a Tidas S MSP 800 system. Several spectral pretreatment strategies and similarity metrics were compared in order to assess the discriminating performance of each acquisition mode. The results show that reflectance provides the highest discriminating power, followed by UV and blue fluorescence, whereas transmittance and green fluorescence exhibit more limited performance. The application of a first-derivative pretreatment significantly improves discrimination for reflectance and transmittance spectra, while its effect on fluorescence data remains moderate. Principal component analysis further suggests a partial structuring of the data according to binder type (acrylic or alkyd) in the fluorescence modes, indicating a binder-related spectral contribution. On the other hand, no particular data structure was observed based on pigment composition. Overall, these results demonstrate that MSP allows a high level of discrimination between visually similar paints when optimized by acquisition mode and pretreatment. This study highlights MSP's strong potential as a key component in integrated forensic paint analysis workflows.