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A device has been built for the rapid, automatic measurement of the third order probability density of video signals. Special cathode ray tubes are used to perform a 64-level amplitude analysis on each signal. A triple coincidence is formed among the three analyzer outputs, the number of occurrences in a frame interval being stored in an 8 Mc counter. These numbers are recorded on magnetic tape which then becomes the input to an electronic computer. The computer calculates the conditional entropy, i.e., the information generated by one of the signals when the other two are known. Examples are presented of second and third order distributions, and of entropies calculated for a variety of scenes.