Image forming apparatus that can perform light amount calibration to adjust the density of a half tone image
An image forming apparatus includes an image forming section, an optical scanning device, an image density sensor, and a control unit. The optical scanning device scans the surface of the image carrying member with light emitted from light sources to form an electrostatic latent image. The control unit can perform light amount calibration to adjust the exposure amount of the light scanning device based on the density of a reference image detected by the image density sensor. The reference image used for light amount calibration has a dot pattern in which as many individual dots as there are light sources are arrayed at regular intervals in the main scanning direction. The dot pattern is formed repeatedly in the main and sub scanning directions. The dots forming the dot pattern are arranged at different positions in the sub scanning direction to be displaced in units of pixels from each other.
1 . An image forming apparatus comprising:
an image forming section including:
an image carrying member having a photosensitive layer formed on a surface thereof;
a charging device that electrostatically charges the surface of the image carrying member; and
a developing device having a developer carrying member that carries developer containing toner, the developing device developing an electrostatic latent image formed on the image carrying member into a toner image,
the image forming section performing image formation by using the toner;
an optical scanning device having a plurality of light sources, the optical scanning device scanning the surface of the image carrying member electrostatically charged by the charging device with light emitted from the light sources to form the electrostatic latent image with reduced electrostatic charge;
an image density sensor that detects a density of the toner image formed in the image forming section; and
a control unit that controls the image forming section and the optical scanning device,
the control unit detects the density of a reference image for image density correction formed in the image forming section with the image density sensor and, based on the result of the detection, the control unit being operable to perform image density correction including
bias calibration in which the control unit adjusts a developing voltage applied to the developer carrying member and thereby adjusts a density of a solid image and
light amount calibration in which the control unit adjusts an exposure amount of the optical scanning device and thereby adjusts a density of a halftone image,
wherein
the reference image used for the light amount calibration has a dot pattern in which individual dots of which a number is larger than a number of light sources are arrayed at regular intervals in a main scanning direction, which is a scanning direction of the optical scanning device, the dot pattern being formed repeatedly in the main scanning direction and in a sub scanning direction, which is orthogonal to the main scanning direction,
of the dots forming the dot pattern, all dots arrayed at a predetermined interval from each other in the main scanning direction are arranged at different positions from each other in the sub scanning direction so as to be displaced in units of pixels from each other, and
in the dot pattern, an area ratio between parts with low and high image densities is constant.
2 . The image forming apparatus according to claim 1 , wherein
an amount of displacement of each of the dots in the sub scanning direction is determined such that the dots of which the image density changes when the scanning positions of the light emitted from the plurality of light sources are displaced are, in the dot pattern, evenly distributed in the main scanning direction.