In a single-sheet multi-layer thermal imaging system images are formed due to the transformation of colorless crystalline materials to colored amorphous forms, achieved by melting or dissolution in suitable solvents. The reflection spectra of the colored forms in printed images may vary due to the presence of other components, such as thermal solvents, polymeric binders required for imaging, and can cause undesirable color combinations in printed images. Such variations may be the results of aggregation of the colored amorphous forms, or formation of different colored forms from the same colorless crystals due to a change in the mechanism of reaction. Addition of appropriate components to the imaging systems can alter the spectra of the printed images to make them suitable for improved image quality.
Fariza B. Hasan, "Alteration of reflection spectra of images from color-formers in a single-sheet thermal imaging system" in Proc. IS&T Int'l Conf. on Digital Printing Technologies and Digital Fabrication (NIP25), 2009, pp 759 - 761, https://doi.org/10.2352/ISSN.2169-4451.2009.25.1.art00095_2