Two image patterns projected onto the ITB are used in process control to sample the machine's current ability to create quality images. These are:
- Gradation Pattern. This pattern is used in the first adjustment for potential (Process Control 1: Potential Control) described later in this section.
- Interval Patten. This pattern is used in the second adjustment for potential (Process Control 2: Toner Supply Control) described later in this section. Process control is performed by projecting a pattern onto the ITB at the unexposed intervals between sheets of paper on the belt during printing (Default: 1 pattern per 20 A4 LEF sheets).
These two phases of process control that project images onto the image transfer belt are exclusively for process control sampling; paper on the image transfer belt remains unaffected. After the ID sensor reads these image patterns projected onto the belt, they are removed by first the ITB cleaning unit and then the PTR cleaning unit and then discarded as used toner.