There's also this which works using a purely physical process: https://www.koehlerpaper.com/en/products/Thermal-paper/TH_Bl...
There's also this which works using a purely physical process: https://www.koehlerpaper.com/en/products/Thermal-paper/TH_Bl...
Check out some results here: https://thermalprintcameras.wordpress.com/blue4est-paper/