1. A pattern recognition system comprising: video camera means for generating a video signal representative of an area being viewed, opaque means located on a portion of said video camera means for producing on each line scan a black level video output signal corresponding to a black optical scene produced by said opaque means, means for averaging the output signal of said video camera relative to said black video signal for each line scan to generate a varying output threshold voltage, means for continuously comparing the output of said video camera means with said threshold voltage to vary the output level on each line scan relative to changes in light intensity, clock means for digitizing the compared video output signal thereby generating a clock digitized video output signal having a leading edge and adaptive to varying light intensities, and an analog comparator and in which the output of said analog comparator and said clock means are fed to a flip flop circuit for generating a change in state output in the presence of a change in the video signal as measured by the leading edge of each clock pulse.