Image sensor

A method of monitoring an impression medium, comprising:

• depositing printing fluid on to an impression medium received on an impression cylinder;
• determining an accumulation level of printing fluid on the impression medium;
• and
• determining if the accumulation level meets a threshold.

A non-transitory computer-readable storage medium storing instructions that, when executed by one or more processors in a printer, cause the one or more processors to:

• cause an intermediate transfer member to transfer printing fluid onto a substrate received on an impression medium;
• obtain sensor data by causing an image sensor to sense an accumulation of printing fluid on the impression medium;
• determine, based on the sensor data, an accumulation level of the printing fluid on the impression medium;
• and
• determine if the accumulation level meets a threshold.