JPS63264358A - Color image forming apparatus - Google Patents

Abstract

PURPOSE:To enhance saturation and color reproducibility, in a color image forming apparatus having printing heads of yellow (Y), magenta (M), cyan (C) and black (K), by making the dot diameter of K smaller than that of each of other colors and making the dot diameter of Y larger than that of each of the residual colors. CONSTITUTION:For example, at the time of pixel density of 300 dpi, the printing dot diameter of an ink droplet is set to 120mum in all of colors Y, M, C, K heretofore but, in this case, the dot diameter of K-ink is made small by 10% to be set to about 108 mum and the dot diameter of Y-ink is made large by 10% to be set to about 132 mum. As mentioned above, by reducing the dot diameter of the K-ink, the lowering in saturation is suppressed and image quality is enhanced. Further, by increasing the dot diameter of the Y-ink, the density difference with the other inks is corrected and the color reproducibility of color mixture is enhanced. As a means reducing or increasing a dot diameter, heads having a nozzle diameters respectively obtaining required dot diameters are used or the pressure of the pump 2 feeding ink to each head under pressure may be changed.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to yellow (Y), magenta (M), cyan (
The present invention relates to a color image forming apparatus having printing heads for each color of C) and black (K).

(Prior Art) When forming a color image, color correction processing as shown in FIG. 3 is generally performed.

That is, instead of printing K by overlapping the three colors Y, M, and C, as shown in Fig. 4, the signal of the common part of Y, M, and C is deleted, a separate signal is provided, and the K ink is printed. These are full black black plate generation (BP) and under color removal (UCR) processing for printing, and masking processing for correcting the spectral characteristics of the ink. However, although the BP/UCR processing has the effect of increasing the contrast and sharpness of an image, depending on the reproduced colors, the saturation and brightness may be lower than those of the original, resulting in poor single-color reproducibility. Also, as shown in Figure 5, the maximum density of Y ink is about 1.0, while the other three colors have a maximum density of 1.5 to 1.2.
The color reproducibility was not sufficient because the a degree characteristics were different.

Conventionally, the dot diameters of Y, M, and C inks are the same, and for example, in the case of a pixel density of 300 dpi, the dot pitch is 85 μm, but in consideration of overlapping during solid painting,
Printing is performed with a dot diameter of about 120 μm, which is 85 μm multiplied by f7.

Therefore, a proposal has been made to improve the image quality by making only the dot diameter of K ink larger than the dot diameter of other colors (
(See Japanese Patent Application Laid-Open No. 60-466461).

However, the purpose of this method is to increase the contrast of the image, and it is actually a method that goes against the grain in terms of color reproducibility.

(Object of the Invention) The present invention aims to improve the problems of the prior art, and provides a color image forming apparatus that improves color reproducibility and improves image quality.

(Structure of the Invention) In order to achieve the above object, the dot diameter of K is made smaller than the dot diameter of other colors, or the dot diameter of K is made smaller than the dot diameter of other colors, and the dot diameter of K is made smaller than the dot diameter of other colors. At least one dot diameter, especially Y, is made larger than the dot diameters of the remaining colors.

According to the above configuration, saturation is increased and color reproducibility is improved.

(Example) Embodiments will be described in detail below with reference to the drawings.

FIG. 1 shows an inkjet recording apparatus according to an embodiment of the present invention. Reference numeral 1 denotes an inkjet head, to which ink 3 pressurized by a pump 2 is supplied and ejected from a nozzle 1a. At the same time, vibrations are applied to the ink by an electromechanical transducer 4 driven by an excitation circuit (not shown).

The ejected ink is formed into droplets in synchronization with the vibration. The ink droplets are selectively charged by a charging electrode 5 according to image information, and the charged ink droplets are deflected by a deflection electrode 6 and collide with a recording paper 8 set on a platen 7, thereby being printed. On the other hand, uncharged ink droplets travel straight into the gutter 9 and are collected in an ink tank (not shown). Since color printing is performed here, inkjet heads having the above configuration are provided for each of the colors Y, M, and C.

Conventionally, for example, when the pixel density is 300 dpi, Y.

The printing dot diameter of the ink droplet for each color of M, C, and Y was set to 120 μm, but in this example, the diameter of the ink dot was reduced by 10% to approximately 108 μm, and for Y.
The ink dot diameter was increased by 10% to approximately 132 μm.

As means for reducing or enlarging the dot diameter, a head having a nozzle diameter capable of obtaining the desired dot diameter is used. Alternatively, the pressure of the pump 2 that pumps the ink to the head is changed (for example, in a constant flow pump using a piston cylinder, the pressure is changed by changing the driving frequency of a step motor that drives the piston cylinder).

FIG. 2 shows the color reproducibility of the conventional example and this example.

M, C, Red (R), Green (G), Blue (B)
.

Skin color is expressed using the L"a"b" color system. This L"a"b" color system is based on the isohue line ( The distance from the dotted line in Figure 2 represents the hue, and the distance from the origin represents the saturation.
Color bias is legible. In the figure, the mark Δ is the one in which BP-OCR is not performed, and the symbol * is the one in which BP-UCR is performed (conventional example). When B P-UCR is performed, it is found that there are colors whose saturation decreases depending on the reproduced colors, such as G and B, compared to the original. This shows that printing with K ink has the effect of lowering saturation. However, BP-UCR
It is clear from the printed samples that by doing this, contrast and sharpness are obtained and the quality of one image is improved.

Therefore, in the present invention, BP-UCR is performed, but on the other hand, K
By reducing the diameter of the ink dots, reduction in saturation is suppressed and image quality is improved. Furthermore, the dot diameter of the Y ink is increased to correct the density difference with other inks and improve the color reproducibility of mixed colors. The 0 mark is the result of this example, and it can be seen that the saturation is increased in B and skin colors. Further, the table below shows the color differences of seven colors.

As can be seen from the above table, the color differences in 6 out of 7 colors are small, which is effective in improving color reproducibility.

In the example, only the two colors K and Y are shown with different dot diameters, but it is expected that the color reproducibility will be further improved by changing the dot diameters of the other two colors and combining them.

(Effects of the Invention) As described above, according to the present invention, color reproducibility is improved without changing the color processing circuit, and therefore, image quality is improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an inkjet recording apparatus according to an embodiment of the present invention, and Fig. 2 shows the color reproducibility of the conventional example and this embodiment.
A diagram expressed in the L"a"b* color system. Figure 3 is a diagram showing color correction processing, Figure 4 is a diagram showing BP/UC
FIG. 5, which is a diagram showing the R processing, is a density characteristic diagram for Y, M, and C. 1... Inkjet head, 1a... Nozzle, 2... Pump, 5... Charging electrode, 6...
Deflection electrode, 7... platen, 8... recording paper. Patent applicant Ricoh Co., Ltd. No. 1 Fig. 1 Inkjet head Ia - nozzle / 2'° hop 3... Ink 5... Load tr electrode 6°° pole Fig. 2 b*

Claims (4)

[Claims] (1) Equipped with print heads for each color of yellow, magenta, cyan, and black, and the black dot diameter is smaller than the dot diameter of other colors, or the black dot diameter is smaller than the dot diameter of other colors. A color image forming apparatus further comprising means for making at least one dot diameter of another color larger than dot diameters of the remaining colors. (2) The color image forming apparatus according to claim (1), wherein the color for increasing the dot diameter is yellow. (3) The color image forming apparatus according to claim (1), wherein the printing head is an inkjet head, and the nozzle diameter is changed as means for changing the dot diameter. (4) The color image forming apparatus according to claim (1), wherein the printing head is an inkjet head, and the means for changing the dot diameter is changing the pressure of a pump that supplies ink to the head.