1. Field of the Invention
The present invention relates to an image formation apparatus which performs the correction of the density unevenness due to the fluctuation of the discharging amount and the like per nozzle in a recording head. Particularly, the invention is effectively applicable to an image formation apparatus using a thermal ink jet recording head with an arrangement of a plurality of nozzles or a color image recording apparatus using a plurality of recording heads.
2. Related Background Art
Traditionally, image formation apparatuses which perform recordings on a sheet, OHP sheet, or other recording media (hereinafter referred to as a recording sheet or simply as a sheet) have been proposed in a mode to mount a recording head thereon according to various types of recording methods. For such a recording head, there are a wire dot type, thermo-sensitive type, thermal transfer type, ink jet type, and the like. Particularly, the ink jet type recording head is constructed in such a way that ink is ejected directly upon a recording sheet. This type of recording, therefore, attracts the user's attention as a quiet recording method and also as a method the running cost of which is inexpensive.
In order to eliminate the density variations and the generation of the density unevenness among the discharging performances, the following method is adapted for an ink jet recording apparatus to implement the stabilization of its discharging characteristics with respect to the discharging velocity and the orientational capability (impact precision) and discharging amount V DROP (p1/dot).
1. Method for Controlling Discharging Amounts
This is a separated pulse width modulation method (PWM control method) such as proposed by the applicant hereof in the specification of U.S. patent application Ser. No. 821,773 (filed on Jan. 16, 1992), in which the pulse width of a prepulse is varied in accordance with the temperatures of a recording head, thus controlling the fluctuation of the discharging amount due to the temperature variation.
2. Method for Correcting Density Unevenness
This is a method whereby to read the density unevenness of a test pattern printed by a recording head for the correction of the density signal per nozzle (discharging port), the so-called head shading method: an HS method.
For the method in the paragraph 1, an averaged discharging amount of a head is controlled in a serial printing system. It is therefore possible to eliminate the density variations due to the temperatures varying in a page or between pages. However, it is impossible to correct the density unevenness possessed by the head itself (the unevenness present in the connecting direction by the serial printing system), that is, the unevenness resulting from the discharging amount per nozzle of the head.
The uneven discharging amount per nozzle results from difficulties in manufacturing the nozzles of recording head, recording elements, and the like due to the uneveness of properties brought about by the manufacturing processes of recording heads or from the property unevenness of the materials with which the heads are constructed. Thus, because of such unevenness existing in the nozzles, recording elements, and the like, the sizes and densities of dots become uneven; hence generating the density unevenness in a recording image.
Because of this, it is difficult to eliminate the density unevenness for the nozzles of the head completely. Therefore, in the serial printing system, there occurs the density unevenness having the serial connections as its pitches and particularly, in an image of an even tone, the unevenness becomes conspicuously noticeable.
The method in the paragraph 2 is a method for overcoming the drawbacks existing in the one in the paragraph 1. This is the method proposed by the applicant hereof in U.S. Pat. No. 5,038,208 (filed on Nov. 15, 1988), U.S. patent application Ser. No. 480,041 (filed on Feb. 14, 1990) and Ser. No. 516,129 (filed on Apr. 27, 1990), in which the unevenness correction (HS method) is made for a certain predetermined output pattern (density signals being constant), and the uneven discharging amounts between nozzles of a head are also reduced to a certain extent. For this method, the corrections are made for a given pattern (the pattern determined by a combination of specific nozzles). Accordingly, it is possible to eliminate the density unevenness completely in the vicinity of the location where the density of such a pattern is present by use of a correction table 1 (the table representing the distributions of the discharging amounts of a recording head, that is, a table showing the characteristics of discharging amount per nozzle) and a correction table 2 (the table for correcting the density signals in accordance with the selected characteristic). However, if the correction is only for one determined density, the combination of nozzles used for the head is automatically confined whereas the combinations of nozzles are caused to change from moment to moment in service when a printing ratio varies. As a result, there still occurs a state of the density unevenness when the printing ratio changes rapidly or a low printing ratio or a high printing ratio is required. Accordingly, the correction table 2 which is available only for linear corrections cannot cope with the situation and then density unevenness occurs or the state of density unevenness varies according to the recording agent (ink or the like) or the colors thereof. Hence, it becomes necessary to provide a method for correcting the density unevenness for each color for the entire area ranging from the low to high densities.
Also, when the above-mentioned methods are applied to the entire area, the density unevenness or streaks occur with respect to the correction table 1 on the contrary particularly if the output of an even density is attempted in the low density area.
Therefore, when an ink jet recording apparatus is operated by use of image signals (multi-valued data) through external equipment such as a character reader, the resultant density streaks are generated in the printing in a low density area if pictorial color image or the like is printed. When a printing is performed in such a state, the full color images formed by the four colors, cyan, Magenta, yellow, and black, are caused to generate density streaks repeated at the boundaries in the serially connected portions; thus lowering the image quality significantly. Further, since the corrections for the density unevenness from the low to high density portions are given exceedingly or insufficiently, the color balancing is locally out of order for the area of an even tone such as a blue sky, sunset sky, or the skin of an object person. As a result, coloring tone is varied to cause the appearance of color unevenness or color reproducibility is lowered (that is, difference in color is more enhanced); hence degrading the image quality. Also, there is a problem among others that the density unevenness takes place even in a monochromatic image of black, red, blue, green, and the like.