Hollow plastic containers provided with a narrow opening and a body having a base and at least one side wall extending longitudinally between the narrow opening and the base, such as bottles or similar, are obtained from preforms by stretch blow molding. The scanned areas of the platen are transmitted to an image sensor and a composite image constructed from the scanned areas. Both the support member and the core member are arranged between a pair of opposing membranes. This reading is stored as a zero-offset value which is then used, during normal operation of the sensor, to adjust the output signal of the sensor assembly when subsequent control linkage displacements are sensed.