Thermal transfer printer and method of controlling print density in thermal transfer printing using the same

A method of controlling printed density in thermal transfer printing using a thermal printer head with heating elements and a temperature detector, comprising the steps of:

• preparing in advance a density table representing a relation between energy pulses applied to heating elements of the head and actual density printed by the energy pulses in a state in which there is no temperature gradient between said heating elements and said temperature detector for various uniform temperatures of the thermal head, and storing the table in a memory;
• calculating density correcting amount for printing by using said table;
• determining, when printing is continued by repetitively applying constant pulse energy to said heating elements for n times, based on relation between the number of repetition n of energy pulses and amount of increase of the actual printed density during n times of repetition of the energy pulses, a temperature gradient coefficient for estimating temperature gradient between the hearing elements and the temperature detector, dependent on the number of repetition;
• calculating said density correcting amount from a ratio of the temperature gradient coefficient after repetition of said energy pulses for more than a predetermined number of times;
• applying pulse energy to said heating elements, which pulse energy is provided by subtracting energy corresponding to said density correcting amount from pulse energy corresponding to a prescribed printed density calculated from said density table corresponding to the temperature measured by the temperature detector, whereby desired printed density can be accurately provided.

A thermal transfer printer comprising:

• a thermal head including a plurality of heating elements;
• temperature detector detecting temperature of said thermal head;
• a density table ROM for storing a density table representing a relation between energy pulses applied to the heating elements and actual density printed by the energy pulses when there is no temperature gradient between said heating elements and the temperature detector, for various uniform temperatures;
• a density correcting circuit for correcting data by determining, when printing is continued by repetitively applying a constant pulse energy for n times to said heating elements, based on relation between said number of repetition n of the energy pulses and amount of increase of the actual printed density during n times of repetition of the energy pulses, determining a temperature gradient coefficient for estimating a temperature gradient between said heating elements and said temperature detector, dependent on the number of repetition n of said energy pulses, and calculating density correcting amount from a ratio of said temperature gradient coefficient after said n times of repetition to said temperature gradient coefficient after repetition of said energy pulses for more than a predetermined number of times;
• and
• an energy pulse controller for determining energy pulses to be applied to the heating elements of said thermal head based on said density table for a temperature detected by said temperature detector on said thermal head, based on corrected data applied from said density correcting circuit, and for applying the energy pulses to said thermal head.