An inkjet printer for printing on a continuous web of recording media includes a receiver to receive a beam of collimated light transmitted across the surface of a continuous web of recording media supported by a roller. The intensity of the transmitted beam received by the receiver can be used to determine the presence of a defect in the continuous web or in the roller. A controller is configured to reduce the speed of or stop movement of the web through the printer and to retract one or more printheads from a printing location to prevent the defect from damaging the printhead(s).