GRAY-SCALE PRINTING ON PLAIN PAPER USING THERMAL INK-TRANSFER IMAGING.

A grey-scale printing method on plain paper was studied which applies the 3-level dot-pattern method (the 3-L method) to thermal ink-transfer imaging. A theoretical analysis of the gray-scale printing characteristics was carried out, based on the Yule-Nielsen effect and thermal ink-transfer characteristics. The numbers of equally-spaced gradational levels were studied, and optimum gray densities were found to maximize them. To achieve high quality printing, optimum dot-patterns were selected by the introduction of an evaluation function based on Fourier analysis. Using a thermal ink-transfer fixture with a high resolution (16 dots/mm) thermal head, this method was shown to have good performance.