A method for measuring the wall thickness of plastic containers during a container manufacturing process includes providing a plastic container, the plastic container having a longitudinal axis and at least two side walls spaced radially from the longitudinal axis. The side walls are formed of a material that absorbs light energy in a predetermined molecular absorption band. Light energy is then directed from a source through the at least two side walls of the plastic container in a plane transverse to the longitudinal axis of the plastic container. A portion of the light energy that passes through the sidewalls of the plastic container is sensed, and a signal representing a thickness of the sidewalls of the plastic container is generated from the sensed portion of the light energy.