Abstract: An image forming system adopting a dual-component developing system-for forming on an image carrier a toner image. The image forming system includes a charge unit for uniformly charging a surface of the image carrier, a developing magnet roll for giving a bias potential to toner, a switching device, such as a bipolar transistor, connected to a PWM image signal, a charge accumulating device, connected to the switching device, for accumulating charges, a total charge amount detection circuit for determining the total amount of charges, and a prediction device for predicting the total consumption of toner depending on the total amount of determined charges.

Abstract: An image forming system adopting a dual-component inversion developing system for forming on an image carrier a toner patch image used for detecting an image concentration and controlling an image forming condition such as toner concentration based on the concentration of the toner patch image. The image forming system includes a charge unit for uniformly charging a surface of the image carrier, a developing magnet roll supplied with a developing bias, and a patch forming unit for selecting voltage application conditions to the charge unit and the developing magnet roll so as to reverse the bias potential relationship between the developing magnet roll and a charge potential of the image carrier and forming a first toner patch image at a contrast potential between the charge potential of the image carrier and the bias potential.