an apparatus and method for sensing gaps between labels adhered to a backing material in longitudinally spaced - apart positions as the backing material travels along the print path of a thermal printer . an array of light - emitting diodes oriented transverse to the print path illuminates the backing material . a fiber optic detects the light transmitted through the backing material and passes the light to a photodiode . the photodiode produces an analog electrical signal indicative of the amount of light transmitted , which varies depending on whether the light has passed through only the backing material or through both the backing material and the print media it carries . the analog signal is converted to a digital signal which is processed by a microprocessor to monitor and control the advancement of the print media and backing material through the printer .