Image forming apparatus that can perform light amount calibration to adjust the density of a half tone image

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.