AFAIK usually the original is photographed under a particular light (and with specific filters or cameras) that depends on the specific kind of ink, see: https

AFAIK usually the original is photographed under a particular light (and with specific filters or cameras) that depends on the specific kind of ink, see: https://activehistory.ca/2018/09/recovering-contrast-in-faded-documents/ https://activehistory.ca/2018/09/recovering-contrast-in-fade...