DE2747450

A light pen is scanned across a bar coded label to produce a reflected light signal having a modulated intensity. The average peak-to-peak value of the plus and minus peaks of the return signal is continuously generated to provide a reference for detecting each subsequent positive and negative transistion. The instantaneous average peak-to-peak reference permits the positive transistions to be distinguished from the negative transistions in spite of large deviations in the amplitude modulation and large shifts in the signal to ambient light level ratio of the return signal. Initially the light pen is operated in a low power pulsed mode. The initial transistions in the return signal cause the pen to temporarily illuminate the bar coded label with a continuous light, and activate plus and minus peak detectors.