Image forming apparatus potential surface detector

An image forming apparatus, comprising:

• an image bearing member;
• a charge unit configured to charge the image bearing member;
• an exposure unit configured to expose the image bearing member by emitting light thereto;
• a transfer unit configured to transfer an image formed on the image bearing member;
• an application unit configured to apply a voltage to the transfer unit;
• a detection unit configured to detect a current flowing between the transfer unit and the image bearing member when the voltage is applied to the transfer unit;
• and
• a control unit configured to cause the charge unit to charge the image bearing member through application of a DC voltage, and to calculate a surface potential of the image bearing member based on a detection result of the current flowing to the image bearing member detected by the detection unit when the voltage is applied to the transfer unit after the exposure unit emits a light, whose light amount value is higher than a light amount value of a light emitted to the image bearing member in an image formation, to the image bearing member,
• wherein the control unit (i) calculates a first voltage with positive polarity applied from the application unit to the transfer unit in the case where the control unit controls the application unit to apply a voltage with positive polarity from the application unit to the transfer unit and the detection unit detects a current with a predetermined current value, (ii) calculates a second voltage with negative polarity applied from the application unit to the transfer unit in the case where the control unit controls the application unit to apply a voltage with negative polarity from the application unit to the transfer unit and the detection unit detects a current with a predetermined current value, and (iii) determines a surface voltage of the image bearing member by the calculated first and second voltages,
• wherein the light amount value higher than the light amount value of the light emitted from the exposure unit to the image bearing member in the image formation comprises a light amount value that causes the surface potential of the image bearing member to become a saturation potential, and
• wherein the control unit is further configured to set a surface potential obtained by subtracting the saturation potential of the image bearing member from a surface potential of the image bearing member, which is calculated as a half of a summation of the first voltage and the second voltage as a correction value and determines the surface potential of the image bearing member.