It’s an old technology, originally designed for copiers and fax machines, that utilizes chemically coated paper. It has since been transformed into a highly successful technology for bar coding. The direct thermal printhead consists of a long, linear array of tiny resistive heating elements (about 100 to 300 per inch) that are arranged perpendicular to the paper flow. Each printhead element locally heats an area directly below it on the chemically coated paper. This produces a chemical reaction that causes a black dot to form in that area. The image is built by rows of dots that are formed as the media passes beneath the active edge of the printhead.