The Video Differential Planimeter

Abstract

The video differential planimeter is an instrument which measures variations in the projected area of any remote object with the aid of a flying spot scanner or television camera system. The composite video signal, caused by the scanning of the object and its contrasting background, is shaped to yield a sequence of constant amplitude rectangular pulses that are negative going during a scan of the object, and positive going at all other times. The average of this sequence of pulses is obtained from a point on a bleeder resistance returned to the negative supply, and is fed into an integrator. The net integration voltage is read off and the integrator is returned to zero at the end of each field. If the object's projected area remains fixed, the sum of the positive and negative pulse areas entering the integrator will be constant. Hence, the change in integration voltage from field to field will be zero. If the object's projected area varies, the sum of the positive and negative pulses entering the integrator will change, and the readout voltage of the integrator will vary proportionately. The V.D.P. is applied here to the measurement of the vibration of a speaker cone, and to the measurement of the human respiration and plethysmogram.