Density correction system

1. A density correction system comprising:

a conveyance unit that conveys a recording medium;

a first ink ejection unit that ejects ink in a multi-drop driving system to the recording medium based on image data;

a second ink ejection unit that ejects ink in the multi-drop driving system to the recording medium based on the image data and in which a volume per one drop is smaller than a volume per one drop of the first ink ejection unit; and

a control unit that corrects a number of drops from the first ink ejection unit and a number of drops from the second ink ejection unit, which constitute a droplet amount for one pixel on the recording medium subjected to image formation,

wherein the control unit keeps constant the sum of the numbers of drops from one nozzle of the first ink ejection unit and one nozzle of the second ink ejection unit, which corresponds to one arbitrary pixel.

2. The system of claim 1 , wherein

the control unit determines whether image density of one arbitrary pixel is within a specified density range, and when the image density is not within the specified density range, the control unit corrects the number of drops from the first ink ejection unit and the number of drops from the second ink ejection unit.

3. The system of claim 2 , wherein

the control unit reads the image density of the one arbitrary pixel from the recording medium subjected to the image formation, and determines whether the image density is within the specified density range.

4. The system of claim 3 , wherein

when the control unit determines that the image density of the one arbitrary pixel is higher than the specified density range, the control unit decreases the number of drops from the first ink ejection unit by one and increases the number of drops from the second ink ejection unit by one.

5. The system of claim 3 , wherein

when the control unit determines that the image density of the one arbitrary pixel is lower than the specified density range, the control unit increases the number of drops from the first ink ejection unit by one and decreases the number of drops from the second ink ejection unit by one.

6. The system of claim 1 , wherein

the first ink ejection unit has a first nozzle line in a direction orthogonal to a conveyance direction of the recording medium, and

the second ink ejection unit has a second nozzle line in the direction orthogonal to the conveyance direction of the recording medium.

7. The system of claim 6 , wherein

the control unit pairs a nozzle in the first nozzle line with a nozzle in the second nozzle line, which are opposite to each other in the conveyance direction of the recording medium, and corrects the number of drops.

8. The system of claim 7 , comprising an input unit to input correction of image density of the recording medium subjected to the image formation.

9. The system of claim 8 , wherein

the control unit specifies, based on the input of the correction of the image density from the input unit, a pair of nozzles for which the number of drops from the first ink ejection unit and the number of drops from the second ink ejection unit are corrected.

10. The system of claim 9 , wherein

when the image density is corrected to become thick, the control unit increases the number of drops from the first ink ejection unit by one and decreases the number of drops from the second ink ejection unit by one.

11. The system of claim 9 , wherein

when the image density is corrected to become thin, the control unit decreases the number of drops from the first ink ejection unit by one and increases the number of drops from the second ink ejection unit by one.

12. A density correction method used for a system including a conveyance unit that conveys a recording medium, a first ink ejection unit of a multi-drop driving system, and a second ink ejection unit of the multi-drop driving system, in which a volume per one drop is smaller than a volume per one drop of the first ink ejection unit, the method comprising:

ejecting ink from the first ink ejection unit and the second ink ejection unit to the recording medium based on image data;

correcting number of drops ejected from the first ink ejection unit and number of drops ejected from the second ink ejection unit, which constitute a droplet amount for one pixel on the recording medium subjected to image formation; and

keeping constant the sum of the number of drops from the first ink ejection unit and the number of drops from the second ink ejection unit with respect to one arbitrary pixel.

13. The method of claim 12 , comprising:

determining whether image density of one arbitrary pixel is within a specified density range; and

correcting, when the image density is not within the specified density range, the number of drops from the first ink ejection unit and the number of drops from the second ink ejection unit.

14. The method of claim 13 , comprising:

reading the image density of the one arbitrary pixel from the recording medium subjected to the image formation, and

determining whether the image density is within the specified density range.

15. The method of claim 14 , comprising:

decreasing, when it is determined that the image density of the one arbitrary pixel is higher than the specified density range, the number of drops from the first ink ejection unit by one and increasing the number of drops from the second ink ejection unit by one.

16. The method of claim 14 , wherein

increasing, when it is determined that the image density of the one arbitrary pixel is lower than the specified density range, the number of drops from the first ink ejection unit by one and decreasing the number of drops from the second ink ejection unit by one.

17. The method of claim 12 , comprising:

pairing a nozzle in a first nozzle line provided in the first ink ejection unit with a nozzle in a second nozzle line provided in the second ink ejection unit, which are opposite to each other in a conveyance direction of the recording medium; and

correcting the number of drops.