A method for measuring the wall thickness of plastic containers (16) during a container manufacturing process includes providing a plastic container (16), the plastic container (16) 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 unit (32) through the at least two side walls of the plastic container (16) in a plane transverse to the longitudinal axis of the plastic container (16). A portion of the light energy that passes through the sidewalls of the container (16) is sensed by a sensor (34), and a signal representing a thickness of the sidewalls of the plastic container is generated from the sensed portion of the energy by a computer (56).