Image forming apparatus and control method therefor

Abstract

This invention provides an image forming apparatus which reduces degradation of the line quality in an output image by a simple method while maintaining the stability of the density characteristics of the output image, and a control method therefor. To accomplish this, in accordance with the amount of change, from a reference density value, of a density value measured from a test pattern image formed on a photosensitive drum, the image forming apparatus according to the invention creates LUT — 2 for correcting a laser output signal value corrected using γ correction LUT — 1 . The image forming apparatus performs γ correction using LUT — 1 and LUT — 2 for developing colors other than a predetermined low-brightness developing color, whereas for the predetermined low-brightness developing color, it performs γ correction using LUT — 1 and laser light amount correction instead of γ correction using LUT — 2.

1 . An image forming apparatus including an image forming unit including an image carrier, an exposure unit that exposes the image carrier in accordance with an image signal to form an electrostatic latent image on the image carrier, and a developing unit that develops the electrostatic latent image formed on the image carrier using a developing material to form a developing material image on the image carrier, and a transfer unit that transfers the developing material image formed on the image carrier onto printing paper to form an image on the printing paper, the apparatus comprising:
 a storage unit that stores correction data used in γ correction for correcting density characteristics of an image to be formed on printing paper by the image forming unit and the transfer unit, and a reference density value used when creating modification data for modifying a value of the image signal corrected using the correction data;   a pattern forming unit that forms a test pattern image on the image carrier using the image forming unit by applying the γ correction using the correction data;   a measurement unit that measures a density value of the test pattern image formed on the image carrier by said pattern forming unit;   a creation unit that creates the modification data in accordance with an amount of density change, from the reference density value stored in said storage unit, of the density value of the test pattern image that is measured by said measurement unit; and   a correction unit that, for a developing color other than a low-brightness developing color being lower in brightness than a predetermined value, performs the γ correction using the correction data and the modification data, and for the low-brightness developing color, performs the γ correction using the correction data and light amount correction for correcting an exposure amount of the exposure unit in accordance with the amount of density change.   
     
     
         2 . The apparatus according to  claim 1 , further comprising a determination unit that, when the γ correction using the correction data and the modification data is performed for the low-brightness developing color, determines whether the value of the corrected image signal corresponding to a predetermined density level serving as a criterion which determines a maximum density in the density characteristics exceeds a first threshold,
 wherein when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level exceeds the first threshold, said correction unit performs the γ correction using the correction data and the modification data for the low-brightness developing color, and 
 when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level does not exceed the first threshold, said correction unit performs the γ correction using the correction data and the light amount correction for the low-brightness developing color. 
 
     
     
         3 . The apparatus according to  claim 2 , wherein
 for the low-brightness developing color, when said determination unit determines that the value of the image signal corresponding to the predetermined density level after the γ correction using the correction data and the modification data does not exceed the first threshold, said determination unit further determines whether the value corresponding to the predetermined density level exceeds a second threshold smaller than the first threshold,   when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level does not exceed the first threshold and exceeds the second threshold, said correction unit, for the low-brightness developing color, performs the γ correction using the correction data and uses both the γ correction using the modification data and the light amount correction at a predetermined correction ratio, and   when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level does not exceed the second threshold, said correction unit performs the γ correction using the correction data and the light amount correction for the low-brightness developing color.   
     
     
         4 . The apparatus according to  claim 3 , wherein
 the predetermined correction ratio is a ratio at which a correction amount by the light amount correction of a correction amount by the γ correction and the light amount correction increases as the value of the image signal decreases when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level does not exceed the first threshold and exceeds the second threshold, and   said correction unit includes an adjustment unit that adjusts a correction amount of the γ correction and a correction amount of the light amount correction in accordance with the predetermined correction ratio when said determination unit determines that the value of the corrected image signal corresponding to the predetermined density level does not exceed the first threshold and exceeds the second threshold.   
     
     
         5 . The apparatus according to  claim 1 , wherein
 said storage unit further stores light amount correction data which makes the amount of density change and a correction amount of the exposure amount in the light amount correction correspond to each other, and   when performing the light amount correction, said correction unit performs the light amount correction by the correction amount of the exposure amount corresponding to the amount of density change using the light amount correction data stored in said storage unit.   
     
     
         6 . The apparatus according to  claim 3 , wherein the first threshold and the second threshold are determined by pre-evaluation regarding presence/absence of a jaggy in an image formed on printing paper by the image forming unit and the transfer unit when the value of the image signal corresponding to the predetermined density level is changed. 
     
     
         7 . The apparatus according to  claim 1 , wherein the low-brightness developing color is at least one of black, magenta, and cyan. 
     
     
         8 . A method for controlling an image forming apparatus including an image forming unit including an image carrier, an exposure unit that exposes the image carrier in accordance with an image signal to form an electrostatic latent image on the image carrier, and a developing unit that develops the electrostatic latent image formed on the image carrier using a developing material to form a developing material image on the image carrier, a transfer unit that transfers the developing material image formed on the image carrier onto printing paper to form an image on the printing paper, and a storage unit that stores correction data used in γ correction for correcting density characteristics of an image to be formed on printing paper by the image forming unit and the transfer unit, and a reference density value used when creating modification data for modifying a value of the image signal corrected using the correction data, the method comprising:
 causing a pattern forming unit to form a test pattern image on the image carrier using the image forming unit by applying the γ correction using the correction data; 
 causing a measurement unit to measure a density value of the test pattern image formed on the image carrier in the causing a pattern forming unit to form a test pattern image; 
 causing a creation unit to create the modification data in accordance with an amount of density change, from the reference density value stored in the storage unit, of the density value of the test pattern image that is measured in the causing a measurement unit to measure a density value; and 
 causing a correction unit to, for a developing color other than a low-brightness developing color being lower in brightness than a predetermined value, perform the γ correction using the correction data and the modification data, and for the low-brightness developing color, perform the γ correction using the correction data and light amount correction for correcting an exposure amount of the exposure unit in accordance with the amount of density change.