In direct thermal print media where image dyes are formed in a layer structure, edge sharpness is observed to be dependent on density, and to be related to internal scattering and reflection. To study the effect quantitatively in various digital print media, a one-step scanner-based methodology was developed to measure MTF and SQF sharpness as a function of density and contrast on a single continuous density edge target. This was used to confirm that a previously described multi-resolution algorithm successfully corrected the density-dependent sharpness loss in direct thermal media.
Dirk Hertel, "On the Density-dependence of Sharpness in Thermal Print Media" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP21), 2005, pp 207 - 210, https://doi.org/10.2352/ISSN.2169-4451.2005.21.1.art00059_1