Dye Sublimation Thermal Transfer Printing
A thermal head heats an inkpo sheet that has a coating of solid ink on a thin film, and the vaporized ink is applied to paper. The process by which a solid is changed to gas is referred to as sublimation. Three colors of sheets are sequentially stacked.
Unlike the inkjet method which relies on fine dots of ink to express a color, the sublimation method paints color over color, using 256 gradations of yellow, magenta, and cyan for each dot to form 16.77 million colors. This allows a high resolution that’s equivalent to a silver-halide print.
The final phase in the process is to protect the surface by applying a transparent film.
By covering the surface with a special layer of film, the surface is protected from fingerprints and water while withstanding ozone and light much better than other methods.
source: https://www.citizen-systems.co.jp/en/printer/special/printer_guide/