A 3-level dot-pattern method (the 3-L method) has been developed in order to achieve stable and many gradational-level thermal printing with minimal loss in print resolution. This 3-L method employs dot-pattern matrices which consist of three density level dots; black (saturation density of thermal paper), gray (half-black density), and white (paper brightness) dots. The matrices are assigned to picture elements of continuous tone images according to their gradational levels. A theoretical analysis of the gradational printing characteristics was carried out on the basis of a modified Yule-Nielsen equation and an equation describing the color change of thermal paper. Using these equations, the numbers of equally spaced gradational levels were studied, and an optimum gray density for maximizing them was found. To further ensure high quality printing, optimum dot patterns were selected by the introduction of an evaluation function based on a discrete Fourier transform. The 3-L method was demonstrated to have good performance in tests with a thermal printing fixture employing a high-resolution (16 dots/mm) thermal head.
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