Patent ID: 8180236

Claim:
An image forming apparatus comprising: a rotatable photosensitive member; a charging member for electrically charging said photosensitive member; an applying device configured to apply to said charging member a charging bias in the form of a DC voltage biased with an AC voltage; a forming device configured to form a toner image on said photosensitive member; a detecting device configured to detect a current passing between said charging member and said photosensitive member; an image forming executing device configured to selectively execute a first image forming mode in which the charging bias in the form of the DC voltage biased with the AC voltage of a first frequency is applied to said charging member and the image is formed on said photosensitive member and a second image forming mode in which the charging bias in the form of the DC voltage biased with the AC voltage of a second frequency is applied to said charging member and the image is formed on said photosensitive member; a test mode executing device for selectively executing a first test mode in which both of a first operation for applying to said charging member a plurality of test biases different in voltage values at a first frequency and detecting the passing current by said detecting device and a second operation for applying to said charging member a plurality of test biases different in voltage values at a second frequency and detecting the passing current by said detecting device are performed, and a second test mode in which a third operation for applying to said charging member a plurality of test biases, smaller in the number of test biases than the number of test biases in the first test mode, at the second frequency and detecting the passing current by said detecting device is performed; a calculating device for calculating associated information from a current value detected by said detecting device in the first operation and a current value detected by said detecting device in the second operation; and a correcting device for correcting, when the image is formed in the second image forming mode, the AC voltage applied to said charging member, wherein said correcting device corrects the AC voltage applied to said charging member on the basis of the associated information calculated by said calculating device and a current value detected by said detecting device in the third operation.