1. Field of the Present Disclosure
The present disclosure relates to an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copy machine, a facsimile machine or a multi function peripheral, calibration is performed as a density correction technique for image forming. In the calibration, light is irradiated to a photo conductor drum by an exposure device and consequently an electrostatic latent image of test patches (an image for density correction) is formed on the photo conductor drum; a toner image is formed by supplying toner to the electrostatic latent image from a development device; and after transferring the toner image to an intermediate transfer drum, densities of the image is optically measured using an optical sensor or the like, exposure light intensity of the exposure device, a surface voltage of the photo conductor drum, a development bias of the development device and so forth are adjusted on the basis of the measured density of the image for correcting densities of image forming.
In an image forming apparatus that includes a development roller arranged in a development device in order to supply toner to a photo conductor drum, if development characteristic is not uniform to rotation angles of the development roller due to eccentricity of a rotation shaft, unevenness of a roller surface of the development roller and so forth, then developer is ununiformly distributed on the outer surface of the development roller, toner does not precisely adhere from the development roller to a photo conductor drum by an expected toner amount, and consequently a whole area of a test patch may not have an uniform density on the photo conductor drum. In such a case, the precise calibration may not be performed.
In an image forming apparatus, in order to reduce density unevenness of a test patch, a test patch is used of which a length on a photo conductor drum is larger than a periphery length of a development roller, and an average value of densities within the test patch is measured.
In the aforementioned image forming apparatus, the length of the test patch is larger than the periphery length of the development roller, and therefore if the periphery length of the development roller is large then the length of the test patch must be large. A large length of the test patch results in a large toner consumption amount on the test patch and a long time required for the calibration.
Further, for example, it may be considered that two pattern images that are two sets of test patches corresponding to densities in gradation are formed and a measured density corresponding to a density setting value is calculated from a measured density of a test patch of the first set and a measured density of a test patch of the second set. However, as shown in FIG. 6A and FIG. 6B, if a position of the test patch of the first set and a position of the test patch of the second set within a fluctuation pitch (i.e. a fluctuation period) of density unevenness are close to each other, the density unevenness still affects the measured densities of the both test patches and consequently influence of the density unevenness is not reduced.