Typewriting machines – Typing by other than type-face or type-die – Thermal
Reexamination Certificate
2001-04-02
2002-12-17
Hilten, John S. (Department: 2854)
Typewriting machines
Typing by other than type-face or type-die
Thermal
C347S195000
Reexamination Certificate
active
06494629
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a data processing method for correcting heating data for a thermal head of a thermal printer, to eliminate influence of heat energy accumulated in the thermal head such that print quality may not be degraded by the heat accumulation. The present invention relates also to a data processing apparatus for this method.
2. Background Arts
There are thermosensitive recording type thermal printers and thermal transfer type thermal printers. The former heats a thermosensitive recording sheet directly with a thermal head, to cause the sheet to develop color. The latter heats the back of an ink ribbon placed upon a recording sheet to transfer ink to the recording sheet. The thermal printer has a thermal head which has an array of heating elements arranged on a ceramic substrate. The array of heating elements correspond to a line of pixels, and the heating elements are each individually driven to record a dot at a time, as the recording sheet is conveyed in a perpendicular direction to the heating element array. Thus an image is printed line by line on the recording sheet. Hereinafter, the direction along which the heating element array extends will be called a main scan direction, whereas the direction along which the recording sheet is conveyed will be called a sub scan direction.
In the thermosensitive recording type and sublimation ink transfer type thermal printing, one dot constitutes one pixel of the printed image, and has a variable density including a zero level, that is designated by image data for each pixel. However, where the heating elements are driven simply in accordance with the image data, the densities of the dots can deviate from the designated densities because of heat accumulation in the thermal head.
Most of heat energy generated from the heating elements is used for recording, but the rest stays unused or dissipates. The unused heat energy is mainly accumulated in a glazing layer which is formed between the heating elements and ceramic substrate. Part of the accumulated heat energy is transmitted from the glazing layer to the ceramic substrate and is accumulated therein, or partly transmitted further to an aluminum plate supporting the substrate and is accumulated therein. From the aluminum plate, the heat energy is partly transmitted to a radiation plate, and radiates from the radiation plate. Hereinafter, the layers disposed under the heating elements will be referred to as heat accumulating layers. The heat energy accumulated in this way is partly transmitted back to the heating elements and added to the heat energy presently generated from the heating elements, thereby rising the densities of the dots from the designated values. Where density should change steeply from high to low, the change becomes gentler in the printed image, so the contour or edge of the printed image becomes vague. Since the heat accumulation in the thermal head gradually increases as the recording proceeds, it results a gradual density change overall the printed image, called shading. That is, the density is generally low at the start of recording, and as the recording proceeds, it becomes higher as a whole.
In order to prevent the above mentioned degrading of the printed image caused by the heat accumulation, U.S. Pat. No. 5,800,075 suggests a data processing method, wherein the heat accumulation in the respective heat accumulating layers of the thermal head is estimated on the basis of input image data, and the input image data is corrected with correction data determined for each pixel based on the estimated heat accumulation.
According to this method, because the heat energy values transmitted to the heat accumulating layers are calculated by linear conversion of the generated heat energy values, if a high density area, e.g. black characters or lines, is included in a middle density area, peripheral portions around the high density area have higher densities than expected, and the image quality is degraded.
The thermal printers also have a problem that the environmental temperature has influence on the densities of the printed dots because the heat energy generated from the heating elements of the thermal head is transmitted to and diffused in the ink ribbon, the recording sheet, a platen roller, a platen drum or the like. That is, if the environmental temperature is pretty high or low, the dot densities get higher or lower than expected, even while the heat energy generated from the heating elements is proper. As a solution for this problem, it is known in the art to adjust voltage applied to the thermal head depending upon the temperature of the thermal head and the environmental temperature.
Since the above data processing method is on the presumption that the head drive voltage applied to the thermal head is kept unchanged, this method rather causes a problem where the head drive voltage is changed according to the temperature of the thermal head and the environmental temperature. That is, the dot densities would deviate from the designated values as the recording proceeds.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a data processing method for correcting heating data for a thermal head and an apparatus therefor, whereby the influence of heat accumulation in the thermal head on the recording density is well eliminated taking account of head drive voltage applied to the thermal head.
Another object of the present invention is to provide a data processing method for correcting heating data for a thermal head to eliminate influence of heat accumulation in the thermal head on recording density, and an apparatus therefor, whereby conditions of heat energy accumulated in the respective heat accumulating layers are estimated with high accuracy, taking account of heat transmission properties between the heat accumulating layers, and thus the influence of heat accumulation on the recording density is well eliminated.
On the assumption that a thermal head has an array of heating elements arranged in a line and first to Nth heat accumulating layers disposed under the heating elements in this order from the side of heating elements, and the heating elements are driven in accordance with corrected heating data, to print one line after another on a recording sheet, one pixel of each line corresponding to one heating element of the array in regular sequence, and that a drive voltage to be applied across the heating elements is determined according to a temperature of the Nth heat accumulating layer and an environmental temperature around the thermal head, a data processing method of the present invention comprising the steps of:
A. determining first to (N−1)th coefficients for the first to (N−1)th heat accumulating layers based on heat transmission properties between the first to Nth heat accumulating layers, and a Nth coefficient for the Nth heat accumulating layer based on the drive voltage for the thermal head and on the heat transmission properties between the first to Nth heat accumulating layers;
B. obtaining first to Nth correction data for each pixel of a subject line to print, by multiplying first to Nth heat accumulation data by the first to Nth coefficients respectively, the first to Nth heat accumulation data being representative of heat accumulation amounts in the first to Nth heat accumulation layers respectively, and previously stored in relation to each heating element of the array;
C. obtaining corrected heating data for each pixel of the subject line, from basic heating data representative of a heat energy value to be applied to the recording sheet for recording the pixel and the first to Nth correction data for the pixel;
D. obtaining a new series of first heat accumulation data by multiplying the corrected heating data of the subject line by a coefficient, multiplying the previously stored first heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence;
E. obtaining a ne
Enomoto Hisashi
Hayashi Shinji
Feggins K.
Fuji Photo Film Co. , Ltd.
Hilten John S.
Sughrue & Mion, PLLC
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