Image forming apparatus

An image forming apparatus, comprising:

• an image bearing member;
• a developer bearing roller configured to contact the image bearing member and perform a development operation in which an electrostatic image formed on the image bearing member is developed with a developer;
• a motor configured to rotationally drive the image bearing member and the developer bearing roller respectively so that peripheral velocities of each is variable individually;
• an exposure device configured to form an electrostatic image on the image bearing member by exposing a surface of the image bearing member at a predetermined exposure amount and by forming a light-part potential from a dark-part potential on the image bearing member;
• a high-voltage power supply configured to apply a developing bias to the developer bearing roller;
• and
• a controller configured to control the developing bias and the exposure amount, wherein
• when a peripheral velocity ratio is defined as a ratio of the peripheral velocity of the developer bearing roller to the peripheral velocity of the image bearing member, the motor is configured to drive the image bearing member and the developer bearing roller at a first peripheral velocity ratio and a second peripheral velocity ratio which is larger than the first peripheral velocity ratio, and
• the controller controls a magnitude of the developing bias or the exposure amount so that, when the development operation is performed at the second peripheral velocity ratio, and when a temperature is higher than a prescribed temperature and a humidity is higher than a prescribed humidity, a second development contrast which is larger than a first development contrast when a temperature is equal to or lower than a prescribed temperature and a humidity is equal to or lower than a prescribed humidity is formed between the developing bias to the developer bearing roller and the light-part potential formed on the image bearing member.

The image forming apparatus according to claim 1, wherein

• the motor is configured to make the second peripheral velocity ratio larger than the first peripheral velocity ratio by:
• setting the peripheral velocity of the developer bearing roller lower in a case of the second peripheral velocity ratio than that in a case of the first peripheral velocity ratio;
• and
• setting the peripheral velocity of the image bearing member lower in the case of the second peripheral velocity ratio than that in the case of the first peripheral velocity ratio,
• wherein an amount of reduction of the peripheral velocity of the developer bearing roller is different from an amount of reduction of the peripheral velocity of the image bearing member.