Abstract: A method and apparatus for measuring the thickness of flap seal gum on an envelope including establishing parameters for a desired thickness of seal gum and programming such parameters into a programmable logic controller (PLC). Sensing the reflectivity intensity of radiation from the gum thickness on envelopes in a production line and comparing such sensed reflectivity intensity to the parameters for the desired reflectivity intensity as programmed into the PLC. The PLC generates indicia such as alarm signals when the reflectivity intensity sensed does not conform to the preprogrammed parameters. The indicia may include activation of an ink jet sprayer to spray ink on envelopes having gum which does not conform to the preprogrammed parameters.