Electrophotographic printer with duplex loop optical sensor

This invention provides for duplex use of densitometers. An image is produced on a substrate, that substrate is brought back through the transfer station in a duplex fashion (that is, the substrate is inverted), and a sensor senses the image on the substrate. Beneficially, the sensor senses one or more parameters such as color registration, color occlusion/interference, black separation in color REaD printing, colorimetry, and the overall transfer and fusing process; and a printing process is controlled based on the sensed parameters.